Almanac A1C

D3 + K2 Synergy: Why 5000 IU Vitamin D Needs Vitamin K2 Backup

Posted

by

A1C Medical Team

Table of Contents
Keywords: , , , , , ,

Introduction

Vitamin D deficiency has emerged as a highly prevalent, yet modifiable, risk factor across the globe, particularly among older adults and individuals with metabolic disorders. Multiple population-based surveys demonstrate that a substantial proportion of adults fail to achieve serum 25-hydroxyvitamin D concentrations associated with optimal musculoskeletal and cardiometabolic health, with prevalence estimates of insufficiency often exceeding 40-50% in many regions. At the same time, habitual intake of vitamin K2 remains low in most Westernized dietary patterns, with limited consumption of fermented foods and specific animal products that are natural sources of menaquinones. This combined pattern of inadequate vitamin D3 and K2 status is especially concerning in aging and metabolically at-risk populations, in whom changes in body composition, reduced sun exposure, polypharmacy, and chronic low-grade inflammation further compromise nutrient status and tissue level function.

Beyond isolated roles of each nutrient, accumulating basic science and clinical data support the concept of vitamin D3 and K2 as a coupled regulatory axis for calcium handling, bone remodeling, vascular integrity, and immune modulation. Vitamin D3 enhances intestinal calcium absorption and upregulates the expression of vitamin K-dependent proteins such as osteocalcin and matric Gia protein, while vitamin K2 is required to carboxylate and activate these proteins so they can effectively direct calcium into bone and away from soft tissues. This coordinated interaction provides a compelling biological rationale for combined D3+K2 supplementation, particularly when higher-dose vitamin d is used in individuals at risk for osteoporosis, vascular calcification, or immune dysregulation. Co-supplementation may therefore support skeletal strength, reduce the propensity for pathological calcification, and contribute to broader aspects of healthy aging.

The present article aims to synthesize current evidence on vitamin D3 and K2 from a preventive aging and metabolic wellness perspective. first, the physiology, metabolism, and mechanistic interplay of vitamin D3 and k2 will be reviewed, with an emphasis on their roles in bone, vascular, and immune health. Second, clinical data on the benefits and potential risks of supplementation will be discussed, including an examination of contemporary dosing strategies for adults, the emerging practice of using doses at or above 5000 IU/day of vitamin D3, and practical considerations for pairing these regimens with vitamin K2. Finally, the article will address safety concerns and controversies, including the documented statistical error in the original estimation of the Recommended Dietary Allowance (RDA) for vitamin D, and will outline implications for future guidelines and personalized supplementation in aging and metabolically vulnerable populations.

Physiology of Vitamin D3

Vitamin D3 (cholecalciferol) is obtained through two primary pathways: endogenous cutaneous synthesis following exposure to ultraviolet B radiation and exogenous intake from dietary sources. The dominant source for most individuals is derma photoproduction, a process that begins when 7-dehydrocholesterol, a cholesterol precursor present in the epidermis and dermis, absorbs ultraviolet B radiation in the wavelength range of 290-315nm, with peak efficiency at approximately 295-297nm. This proteolytic reaction induces cleavage of the B-ring of 7-dehydrocholesterol to form previtamin d3, which subsequently undergoes a temperature-dependent, non-enzymatic isomerization over several hours to yield vitamin D3. Once formed, vitamin D3 enters the dermal capillaries and is transported in the circulation bound primarily to vitamin D-binding protein. Dietary sources of vitamin D3 include fatty fish, egg yolks, and fortified foods, though these typically contribute a smaller proportion of total vitamin D status in sun-exposed populations [1,2,3,4,5].

The biological activity of vitamin D3 requires two sequential hydroxylation steps. The first occurs predominantly in the liver, where vitamin D3 is hydroxylated at the C-25 position by the microsomal cytochrome P450 enzyme CYP2R1, yielding 25-hydroxyvitamin D [25(OH)D]. CYP2R1 has been identified as the major, though not exclusive, 25-hydroxylase through both in vitro expression studies and knockout mouse models; other enzymes, including CYP27A1, contribute to hepatic 25-hydroxylation, but CYP2R1 appears to be the principal enzyme under physiological conditions. Serum 25(OH)D is the most abundant circulating vitamin d metabolite and serves as the clinical biomarker of vitamin D status, with a half-life approximately two to three weeks. The second activation steps take place primarily in the proximal tubule of the kidney, where 25(OH)D undergoes 1a-hydroxylation by the mitochondrial enzyme CYP27B1 to produce the hormonally active form, 1,25-dihydroxyvitamin D [1,25(OH)2D, also known as calcitriol]. CYP27B1 is tightly regulated by parathyroid hormone, fibroblast growth factor 23, serum calcium, and phosphate levels, ensuring precise control of circulating 1,25 (OH)2D concentrations. Importantly, extrarenal expression of CYP27B1 has been documented in numerous tissues, including bone, immune cells, and the intestine, allowing for local production of 1,25(OH)2D and autocrine or paracrine signaling [6,7,8,9,10].

The biological actions of 1,25(OH)2D are mediated through the vitamin D receptor (VDR), a member of the nuclear hormone receptor superfamily that functions as a ligand-activated transcription factor. Upon binding 1,25(OH)2D, VDR heterodimerizes with the retinoid X receptor (RXR) and translocate to the nucleus, where the VDR-RXR complex binds to vitamin d response elements (VDREs) in the promoter regions of target genes. Through recruitment of coactivator or corepressor complexes, the activated VDR modulates the transcription of genes involved in calcium and phosphate homeostasis, including those encoding epithelial calcium channels (TRPV6 in the intestine, TRPV5 in the kidney), calcium-binding proteins (calbindin-D9k and calbindin-D28k), and calcium ATPases. The critical role of VDR in regulating intestinal calcium absorption is underscored by genetic studies in humans with hereditary vitamin D-resistant rickets due to inactivating VDR mutations, as well as by VDR knockout mice, which exhibit severe hypocalcemia, secondary hyperparathyroidism, and rickets or osteomalacia that can be rescued by high-calcium diets. Beyond its classical role in mineral metabolism, VDR is expressed in over 38 tissue types, and genomic analyses have identified hundreds of VDR-responsive genes involved in cellular proliferation, differentiation, apoptosis, immune modulation, and innate antimicrobial defence, reflecting the pleiotropic effects of vitamin D signalling [9,11,12,13].

Vitamin D status is influenced by a constellation of physiological, environmental, and lifestyle factors that vary across the lifespan. Geographic latitude and season are major determinants of cutaneous vitamin D3 synthesis, as the zenith angle of the sun governs the penetration of UVB radiation through the atmosphere; at latitudes above approximately 35 degrees north or south, insufficient UVB reaches the earth’s surface during winter months to support meaningful vitamin D production. Skin pigmentation is another critical factor, as melanin acts as a natural sunscreen, absorbing UVB radiation and reducing the efficiency of previtamin D3 formation; individuals with darker skin phenotypes require substantially longer sun exposure to synthesize equivalent amounts of vitamin D3 compared to those with lighter skin. Adiposity and obesity are associated with lower circulating 25(OH)D concentrations, likely due to sequestration of the lipophilic vitamin D in adipose tissue and volumetric dilution effects. Aging is accompanied by a progressive decline in the dermal content of 7-dehydrocholesterol, reduced cutaneous synthesis capacity, and often diminished sun exposure due to mobility limitations and lifestyle changes, rendering older adults particularly vulnerable to vitamin D insufficiency. Finally, contemporary lifestyle factors  including widespread use of sunscreen, indoor occupations, clothing that covers most body surface area, and limited dietary intake of vitamin D-rich foods have contributed to the high global prevalence of suboptimal vitamin D status, even in sun-replete regions [2,5,14,15]

Physiology of K2

Vitamin K exists as a family of fat-soluble vitamers sharing a 2-methyl-1,4-naphthoquinone ring structure but differing in their side chains, which determine their pharmacokinetics and tissue distribution. Phylloquinone (vitamin K1) is the predominant dietary form, abundant in green leafy vegetables and plant oils, and is preferentially taken up by the liver to support hepatic synthesis of coagulation factors. Menaquinones (vitamin K2) comprise a series of homologues (MK-n) with varying lengths of isoprenoid side chains, produced by bacterial fermentation and present in foods such as natto, certain cheeses, and animal products. Among these, MK‑4 (a short-chain menaquinone) and MK‑7 (a long-chain menaquinone) are the most extensively studied in human health. Intestinal absorption of vitamin K1 and K2 occurs in the proximal small intestine via micelle formation, followed by incorporation into chylomicrons and secretion into the lymphatic system. After absorption, K1 and MK‑4 are rapidly cleared from the circulation, predominantly distributed to and retained in the liver, whereas longer-chain menaquinones such as MK‑7 and MK‑9 exhibit longer half-lives and are transported mainly in low-density lipoproteins, allowing wider distribution to extrahepatic tissues including bone and vasculature. Animal and tracer studies further indicate that diverse dietary vitamin K forms can be converted to MK‑4 in specific tissues, suggesting a conserved role for MK‑4 as a local vitamin K reservoir [16,17,18,19,20].

The core biochemical function of vitamin K2 is to act as an essential cofactor for the microsomal enzyme γ‑glutamyl carboxylase, which catalyses the post-translational conversion of specific glutamate (Glu) residues to γ‑carboxyglutamate (Gla) in vitamin K–dependent proteins. In this reaction, reduced vitamin K (vitamin K hydroquinone) donates electrons that drive the carboxylation process, becoming oxidized to vitamin K epoxide; vitamin K epoxide is then recycled back to the reduced form by vitamin K epoxide reductase (VKORC1), constituting the vitamin K cycle that enables repeated catalytic use of the same K molecule. γ‑Carboxylation imparts strong calcium-binding properties to target proteins by enabling bidentate coordination of calcium ions through the Gla residues, which is critical for their conformational activation and biological function. Although this carboxylation system was first characterized in hepatic coagulation factors, many extrahepatic vitamin K–dependent proteins are preferentially carboxylated by K2, reflecting the superior extrahepatic bioavailability of long-chain menaquinones such as MK‑7. Experimental models manipulating γ‑glutamyl carboxylase or VKORC1 expression confirm that impaired vitamin K–dependent carboxylation leads to the accumulation of inactive (undercarboxylated) proteins and is associated with disordered mineral metabolism, ectopic calcification, and metabolic dysfunction [20,21,22,23,24,25,26].

Several vitamin K2–dependent proteins play central roles in skeletal integrity and vascular health. Osteocalcin, synthesized by osteoblasts, undergoes vitamin K–dependent γ‑carboxylation at multiple Glu residues, enabling high-affinity binding to hydroxyapatite crystals within the bone matrix and contributing to appropriate mineralization and bone quality. Undercarboxylated osteocalcin is commonly used as a biomarker of subclinical vitamin K insufficiency in bone and is reduced by K2 supplementation, indicating improved carboxylation status. Matrix Gla protein (MGP), expressed predominantly by vascular smooth muscle cells and chondrocytes, is another key K2-dependent protein that functions as a potent inhibitor of vascular and soft-tissue calcification. Carboxylated MGP binds calcium ions and hydroxyapatite crystals and interferes with pro-calcific signalling, whereas inactive, undercarboxylated MGP is strongly associated with increased arterial stiffness, valvular and arterial calcification, and higher cardiovascular risk. Experimental MGP knockout models develop fulminant aortic and coronary calcification leading to early death, underscoring the non-redundant role of fully carboxylated MGP in maintaining vascular elasticity. Additional vitamin K–dependent proteins, such as Gla-rich protein (GRP), also contribute to the bone–vascular axis by inhibiting ectopic calcification in cartilage and vascular tissues, further highlighting vitamin K2 as a critical modulator of calcium distribution between bone and the vasculature [22,23,26,27,28,29,30].

Synergistic Interplay Between Vitamin D3 and K2

The molecular partnership between vitamin D3 and K2 is rooted in the capacity of 1,25-dihydroxyvitamin D to transcriptionally upregulate several key vitamin K–dependent proteins, which then require K2-mediated carboxylation to achieve full biological activity. In osteoblasts, activation of the vitamin D receptor (VDR) by 1,25(OH)₂D stimulates the expression of osteocalcin (BLGAP) a bone matrix protein that undergoes vitamin K–dependent γ-carboxylation at three glutamate residues. Studies in human osteoblasts demonstrate that 1,25(OH)₂D increases osteocalcin mRNA and protein levels in a VDR-dependent manner, and that this upregulation is coupled to enhanced osteoblast differentiation and mineralization. Similarly, vitamin D signalling in vascular smooth muscle cells and other cell types modulates the expression of matrix Gla protein (MGP), another critical vitamin K–dependent inhibitor of soft-tissue calcification. Functional cooperation between VDR and the osteogenic transcription factor Runx2 has been shown to be necessary for vascular calcification responses to vitamin D₃, underscoring the importance of downstream K-dependent protein activation in mediating the biological consequences of vitamin D action. This transcriptional link establishes a regulatory cascade wherein vitamin D increases the synthesis of K-dependent proteins, but those proteins remain inactive in their undercarboxylated form unless adequate vitamin K2 is available to complete the post-translational carboxylation step [31,32,33,34,35,36].

The physiological synergy between vitamins D3 and K2 is perhaps most evident in their coordinated regulation of calcium flux and distribution. Vitamin D3 enhances intestinal calcium absorption through VDR-mediated upregulation of calcium transport proteins in the duodenum, increasing the availability of circulating calcium for skeletal mineralization. However, the mere elevation of serum calcium does not ensure appropriate deposition into bone; in the absence of sufficient vitamin K2, absorbed calcium may preferentially accumulate in vascular smooth muscle and other soft tissues, contributing to pathological calcification. Vitamin K2 directs calcium homeostasis by activating osteocalcin, which binds calcium ions and facilitates their incorporation into the hydroxyapatite matrix of bone, and by activating MGP, which prevents calcium precipitation in arterial walls and cardiac valves. This dual action, enhancing bone mineralization while inhibiting vascular calcification creates a functional partnership in which D3 supplies calcium and upregulates the regulatory machinery, while K2 ensures that calcium is trafficked to skeletal sites and excluded from the cardiovascular system. The clinical relevance of this interplay is illustrated by the “calcium paradox,” wherein individuals with osteoporosis often exhibit concomitant vascular calcification, a pattern consistent with inadequate activation of K-dependent proteins that normally segregate calcium between bone and soft tissue [9,36,37,38,39].

Emerging clinical evidence supports the hypothesis that combined D3+K2 supplementation yields superior outcomes for bone mineralization and vascular health compared to vitamin D alone. In postmenopausal women, a three-year randomized trial found that combined supplementation with 1000 μg/day vitamin K₁ plus 320 IU vitamin D maintained carotid artery vessel wall characteristics, whereas both the control group and the vitamin D-only group experienced significant deterioration over the same period, suggesting a protective vascular effect of the D+K combination. In patients with chronic kidney disease, a population at high risk for both bone loss and vascular calcification, a nine-month trial demonstrated that the addition of 90 μg menaquinone-7 (MK-7) to 10 μg vitamin D significantly reduced the increase in carotid intima-media thickness compared to vitamin D supplementation alone. However, not all trials have confirmed benefits; a large 12-month randomized controlled trial in CKD stage 3b–4 patients found that 400 μg/day vitamin K₂ (MK-7) did not improve pulse wave velocity or other measures of vascular stiffness, and a two-year trial in men with aortic valve calcification showed no significant difference in calcification progression with combined MK-7 (720 μg/day) plus vitamin D (25 μg/day) versus placebo, despite a marked reduction in undercarboxylated MGP. These mixed findings may reflect differences in baseline vitamin K status, disease severity, treatment duration, and the reversibility of established calcification, and suggest that K2 supplementation may be more effective as a preventive strategy or when initiated earlier in the disease process. Nonetheless, the mechanistic rationale and supportive data from several intervention studies provide a compelling case that adequate vitamin K2 is essential to optimize the skeletal benefits and mitigate potential vascular risks of higher-dose vitamin D3 supplementation, particularly in aging and metabolically at-risk populations [28,32,37,40,41,42].

Clinical Roles in Aging and Metabolic Health

Bone health deteriorates with advancing age through mechanisms including reduced osteoblastic activity, increased osteoclast-mediated resorption, hormonal changes, and diminished nutrient absorption, culminating in osteopenia, osteoporosis, and elevated fracture risk in older adults. Sarcopenia, the progressive loss of skeletal muscle mass and function, frequently coexists with bone fragility, contributing to falls and functional decline. Epidemiological evidence links low vitamin D status and inadequate vitamin K intake with reduced bone mineral density and increased fracture incidence across diverse populations. Several observational studies have demonstrated that lower circulating 25(OH)D concentrations and reduced dietary vitamin K intake are associated with higher rates of hip and vertebral fractures in elderly men and women, independent of calcium intake. Mechanistically, vitamin D deficiency impairs intestinal calcium absorption and promotes secondary hyperparathyroidism, leading to accelerated bone turnover and cortical bone loss, while vitamin K insufficiency results in undercarboxylation of osteocalcin and diminished incorporation of calcium into the bone matrix. Meta-analyses and systematic reviews have shown that vitamin K2 supplementation, particularly at doses of 45 mg/day of menaquinone-4, significantly reduces vertebral and clinical fracture risk in postmenopausal women with osteoporosis, with more modest effects on bone mineral density. Combined therapy with vitamin D3 and K2 has been shown in some trials to produce additive or synergistic benefits; for example, a Japanese study found that continuous combined therapy with vitamin K2 (menaquinone-4, 45 mg/day) and vitamin D3 (750 IU/day) for two years in postmenopausal women with decreased bone mass resulted in greater maintenance of lumbar spine bone mineral density and reduced bone turnover markers compared to monotherapy or control. The effect of vitamin K2 on fracture reduction appears clinically more significant than its impact on BMD, suggesting that K2 improves bone quality and structural integrity beyond changes detectable by densitometry alone [43,44,45,46,47,48,49].

Cardiovascular aging is characterized by progressive arterial stiffening, endothelial dysfunction, and pathological vascular and valvular calcification, processes that are accelerated in the presence of metabolic disease, chronic kidney disease, and diabetes. Vitamin K2 has emerged as a potentially protective nutrient in this context due to its essential role in activating matrix Gla protein, the most potent endogenous inhibitor of vascular calcification. Elevated levels of inactive, dephosphorylated-undercarboxylated MGP (dp-ucMGP) serve as a biomarker of vitamin K insufficiency and are strongly associated with increased arterial stiffness, coronary artery calcification, heart failure, and cardiovascular mortality. Higher dietary intake of vitamin K2, particularly from menaquinone-rich foods, has been associated in prospective cohort studies with reduced coronary heart disease and all-cause mortality, whereas vitamin K1 intake showed weaker or no associations, consistent with the superior extrahepatic bioavailability of K2. In the context of higher-dose vitamin D3 supplementation, adequate vitamin K2 status may be particularly important to mitigate potential risks of ectopic calcification, as vitamin D increases intestinal calcium absorption and upregulates osteocalcin and MGP, both of which require K2-dependent carboxylation to function properly. However, clinical trial data on vascular outcomes remain mixed. A randomized controlled trial in chronic kidney disease patients showed that the addition of menaquinone-7 (90 μg/day) to vitamin D significantly attenuated progression of carotid intima-media thickness compared to vitamin D alone, suggesting vascular protection. Conversely, a large two-year trial in elderly men with aortic valve calcification found that high-dose MK-7 (720 μg/day) plus vitamin D (25 μg/day) did not slow progression of aortic valve or coronary artery calcification despite a marked reduction in dp-ucMGP, indicating biochemical activation of MGP without detectable clinical benefit in established calcific disease. These disparate findings may reflect differences in disease stage, baseline vitamin K status, calcification reversibility, and trial duration, and suggest that vitamin K2 may be more effective as a preventive strategy in earlier stages of vascular aging rather than as a treatment for advanced calcification [29,37,38,42].

Beyond skeletal and cardiovascular health, vitamin D3 and K2 are increasingly recognized for their roles in metabolic regulation and immune resilience, both critical to healthy longevity. Vitamin D deficiency is strongly associated with insulin resistance, impaired glucose tolerance, and increased risk of type 2 diabetes in observational studies, and the vitamin D receptor is expressed in pancreatic β-cells, skeletal muscle, liver, and adipose tissue. Mechanistically, 1,25(OH)₂D enhances insulin secretion by binding directly to the VDR on β-cells and regulating the transcription of the insulin gene, improves peripheral insulin sensitivity by upregulating glucose transporter-4 expression and facilitating glucose uptake in muscle and adipocytes, and reduces systemic inflammation by downregulating nuclear factor κB and proinflammatory cytokines such as TNF-α, IL-1, and IL-6. Vitamin D supplementation has been shown in some intervention trials to improve insulin sensitivity, reduce HOMA-IR (homeostatic model assessment of insulin resistance), lower fasting glucose, and promote weight loss in individuals with obesity and vitamin D deficiency, though results have been inconsistent across studies and may depend on baseline vitamin D status, dosage, and metabolic phenotype. Emerging evidence also links vitamin K2 to improved insulin sensitivity and reduced risk of diabetes, potentially mediated through activation of osteocalcin, which in its undercarboxylated form acts as a hormone that stimulates adiponectin secretion and enhances insulin sensitivity in peripheral tissues. Vitamin D plays a pivotal role in immune modulation, regulating both innate and adaptive immunity through effects on macrophages, dendritic cells, T cells, and B cells. Vitamin D enhances antimicrobial peptide production, modulates T helper cell balance (shifting away from pro-inflammatory Th1 and Th17 responses), and suppresses autoimmune inflammation, with epidemiological evidence suggesting that vitamin D deficiency during immune system development increases the risk of autoimmune diseases including type 1 diabetes, multiple sclerosis, rheumatoid arthritis, and systemic lupus erythematosus. Together, these pleiotropic effects of vitamin D3, coupled with the potential metabolic and anti-inflammatory contributions of vitamin K2, position the D3+K2 axis as a key target for interventions aimed at preserving metabolic health, immune competence, and functional independence in aging populations [29,50,51,52,53,54].

Current Intake Recommendations and Their Limitations

Current dietary reference intakes for vitamin D and vitamin K are based primarily on their classical roles in skeletal health and hemostasis, respectively, and were established through expert consensus processes that relied on limited data and bone-centric endpoints. For vitamin D, the Institute of Medicine (IOM) in 2010 established Recommended Dietary Allowances (RDAs) of 600 IU/day for adults aged 19–70 years and 800 IU/day for those aged 71 years and older, along with an upper intake level (UL) of 4000 IU/day for adults. These values were derived from dose-response analyses of vitamin D intake and serum 25(OH)D concentrations, with the explicit goal of ensuring that at least 97.5% of the population would achieve a serum 25(OH)D concentration of 20 ng/mL (50 nmol/L), the threshold deemed sufficient to prevent rickets, osteomalacia, and support normal calcium metabolism in healthy individuals. The IOM committee concluded that a serum 25(OH)D level of 20 ng/mL was adequate for bone health based on a reanalysis of randomized controlled trials assessing fracture and bone mineral density outcomes, and explicitly rejected higher target thresholds such as 30 ng/mL (75 nmol/L), despite recommendations from other expert bodies, including the Endocrine Society, which suggested that levels above 30 ng/mL may be necessary to optimize bone health, muscle function, and extraskeletal outcomes. For vitamin K, adequate intakes (AIs) rather than RDAs were set due to insufficient data to establish an Estimated Average Requirement (EAR); the AI is 120 µg/day for adult men and 90 µg/day for adult women, values based on median dietary phylloquinone (vitamin K1) intakes and designed to maintain normal coagulation. Importantly, no separate recommendations exist for vitamin K2 (menaquinones), and the AI values do not account for the emerging evidence of vitamin K’s role in bone and vascular health beyond hemostasis [55,56,57,58].

Population-based studies conducted in diverse geographic settings reveal a persistent and substantial discrepancy between recommended vitamin D intakes and the 25(OH)D concentrations actually observed in real-world cohorts, with higher latitudes and older adults disproportionately affected. A global meta-analysis of vitamin D deficiency found that the prevalence of serum 25(OH)D below 30 nmol/L ranged from 5.9% at low latitudes (0–20 degrees) to 14.9% at 40–60 degrees north latitude, with the prevalence of insufficiency (25(OH)D <50 nmol/L) reaching 57.4% in populations living at 60–80 degrees north latitude. Even in sun-rich regions, vitamin D insufficiency remains common; for example, in São Paulo, Brazil (latitude 23°S), community-dwelling older adults had mean 25(OH)D levels of 79 nmol/L in summer, but those in nursing homes averaged only 42 nmol/L, with 50% below 25 nmol/L. In northern European countries, such as the Netherlands (latitude 52°N), 26–27% of healthy adults had 25(OH)D concentrations below 50 nmol/L, despite relatively high dietary and supplemental vitamin D intakes in some subgroups. Older adults are at particularly high risk due to age-related declines in dermal 7-dehydrocholesterol content, reduced renal 1α-hydroxylase activity, increased adiposity leading to volumetric dilution of vitamin D, and lifestyle factors such as limited outdoor activity and institutionalization. A consensus statement on vitamin D in the older population emphasized that severe vitamin D deficiency (25(OH)D <25–30 nmol/L) should be avoided in elderly individuals, as it is associated with secondary hyperparathyroidism, bone loss, osteomalacia, and increased fall and fracture risk, yet baseline serum 25(OH)D in many intervention trials among older populations frequently falls below 40 nmol/L, indicating widespread inadequacy despite existing recommendations. Furthermore, several controlled trials have demonstrated that intakes of 400–600 IU/day are often insufficient to achieve serum 25(OH)D concentrations of 30 ng/mL or higher, particularly in overweight or obese individuals, leading expert groups such as the Endocrine Society to suggest that 1500–2000 IU/day may be necessary to consistently raise 25(OH)D above this threshold. The discordance between RDA levels and the intakes required to prevent insufficiency in a substantial proportion of the population raises questions about the adequacy of the current guidelines, particularly for individuals with obesity, darker skin pigmentation, limited sun exposure, or chronic disease [

The absence of formal vitamin K2-specific recommendations is a notable gap in current dietary guidance, especially in light of accumulating mechanistic and clinical evidence linking menaquinone status to vascular and skeletal endpoints beyond coagulation. The existing adequate intake values for total vitamin K (90–120 µg/day) are based solely on the amount of phylloquinone (K1) needed to maintain normal coagulation factor synthesis in the liver and do not reflect the extrahepatic roles of vitamin K2 in bone mineralization and vascular calcification inhibition. Observational studies have shown that higher dietary menaquinone intake but not phylloquinone intake is associated with reduced coronary artery calcification, lower risk of coronary heart disease mortality, and decreased aortic calcification in prospective cohorts, suggesting that K2 has unique cardiovascular benefits not captured by K1. For example, in the Rotterdam Study, individuals in the highest tertile of menaquinone intake (>32.7 µg/day) had a 57% lower risk of coronary heart disease mortality compared to those in the lowest tertile (<21.6 µg/day). Intervention trials using vitamin K2 (menaquinone-7) at doses ranging from 45 µg/day to 720 µg/day have demonstrated reductions in undercarboxylated osteocalcin, improvements in bone mineral density, and decreased progression of arterial stiffness in some populations, though results have been inconsistent across trials. Recent expert syntheses suggest that a daily intake of 100–180 µg of vitamin K2 (MK-7) may be more appropriate to optimize bone and cardiovascular health, far exceeding the contribution of menaquinones in typical Western diets and underscoring the potential need for targeted supplementation. Until regulatory agencies establish separate, evidence-based recommendations for vitamin K2 that account for its pleiotropic extrahepatic functions, clinicians and patients must rely on emerging research and expert opinion to guide supplementation strategies, particularly in aging populations and those receiving higher-dose vitamin D therapy where the need for adequate K2-dependent protein carboxylation is most critical [55,56,57,59,60,61].

The Statistical Error in Vitamin D RDA Estimation

The Institute of Medicine’s 2011 dietary reference intakes for vitamin D established a Recommended Dietary Allowance (RDA) of 600 IU per day for adults aged 1–70 years and 800 IU per day for those over 70 years, with the explicit goal of ensuring that 97.5% of healthy individuals would achieve serum 25-hydroxyvitamin D [25(OH)D] concentrations of at least 50 nmol/L (20 ng/mL), the threshold deemed sufficient to support bone health and prevent rickets and osteomalacia. To derive this RDA, the IOM panel aggregated data from 10 controlled supplementation studies conducted during winter months at latitudes above 50 degrees north to minimize confounding from cutaneous vitamin D synthesis, yielding 32 study-level mean values of serum 25(OH)D at varying supplementation doses. The IOM regressed these 32 study means against vitamin D intake to establish a dose-response relationship and calculated a 95% confidence prediction interval based on the standard deviation of the study averages. From this analysis, the IOM estimated that an intake of 600 IU per day would produce a mean 25(OH)D level of 63 nmol/L, with a lower 95% prediction limit (representing the 2.5th percentile) of 56 nmol/L, which was conservatively rounded down to 50 nmol/L to accommodate uncertainty. Based on this lower prediction limit, the IOM concluded that 600 IU per day would be sufficient to ensure that 97.5% of individuals, not just study groups that would achieve serum25(OH)D levels of 50 nmol/L or higher [55,56,57,58].

In 2014, Veugelers and Ekwaru published a critical re-analysis demonstrating that the IOM had committed a fundamental statistical error by confusing prediction limits for group means with prediction limits for individuals. The authors pointed out that the correct interpretation of the IOM’s lower 95% prediction limit is that 97.5% of study averages are expected to exceed this limit, not that 97.5% of individual participants will exceed it. This distinction is crucial because the variability among individuals within studies is much larger than the variability among study means; therefore, a prediction interval constructed from study-level data dramatically underestimates the dose required to achieve a given serum concentration in nearly all individuals. To determine the true intake needed to ensure that 97.5% of individuals reach 25(OH)D ≥50 nmol/L, Veugelers and Ekwaru reviewed the same 10 studies used by the IOM, focusing on the eight studies that reported both mean and standard deviation of serum 25(OH)D at each dose. For each of the 23 dose groups in these eight studies, they calculated the individual-level 2.5th percentile by subtracting two standard deviations from the group mean, and then regressed these 2.5th-percentile values against vitamin D intake to generate an individual-level lower prediction limit. This regression revealed that 600 IU per day would achieve serum 25(OH)D levels above 50 nmol/L in only a small fraction of individuals; in fact, at 600 IU per day, the estimated 2.5th percentile of individual values was approximately 26.8 nmol/L, far below the target of 50 nmol/L. Conversely, to ensure that 97.5% of individuals would reach or exceed 50 nmol/L, the re-analysis estimated that a daily intake of approximately 8895 IU of vitamin D would be required, with the caveat that this value required extrapolation beyond the range of doses examined in the original studies (which did not exceed 2400 IU/day) and thus should be interpreted with caution [55,56,59,60,61].

The re-estimated intake requirement of approximately 8000–9000 IU per day to achieve ≥50 nmol/L in 97.5% of individuals has profound implications for public health guidelines and clinical practice. This value is more than tenfold higher than the current RDA of 600 IU/day and exceeds the IOM-established tolerable upper intake level (UL) of 4000 IU/day for adults, raising questions about the adequacy and safety margins embedded in current recommendations. Real-world evidence supports the contention that the existing RDA is insufficient for a substantial proportion of the population. In Canadian cohort studies conducted at northern latitudes where cutaneous synthesis is limited, 10–15% of individuals taking vitamin D supplements of 400 IU or more (total intake ≥632 IU/day when including dietary sources) still had serum 25(OH)D levels below 50 nmol/L, a prevalence far exceeding the 2.5% failure rate the RDA is intended to permit. A subsequent independent validation by Heaney and colleagues, using data from the GrassrootsHealth cohort that included individuals with vitamin D intakes ranging from zero to over 10,000 IU per day, confirmed the magnitude of the error and supported the conclusion that doses substantially higher than the current RDA are necessary to prevent insufficiency in the majority of the population. The statistical error has been characterized by some commentators as one of the most significant miscalculations in modern nutritional policy, and has led to calls for urgent reconsideration of the vitamin D RDA to align public health targets with the actual physiological requirements of individuals rather than population averages. For clinicians working in preventive and aging medicine, these findings underscore the need for individualized vitamin D dosing informed by baseline 25(OH)D levels, body weight, comorbidities, and lifestyle factors, often necessitating intakes in the range of 2000–5000 IU per day or higher to achieve and maintain optimal status, particularly in populations at high risk for insufficiency [55,56,57,59,60,61].

Rationale for a Minimum of 5000 IU Vitamin D3

The positioning of 5000 IU per day as a pragmatic and often necessary daily intake for many adults arises from the convergence of the statistical re-analysis demonstrating inadequacy of the current 600 IU RDA, real-world population data showing persistent vitamin D insufficiency despite adherence to conventional recommendations, and accumulating safety data from long-term supplementation trials. For individuals residing at higher latitudes, those with limited sun exposure, older adults, and persons with obesity or metabolic disease, intakes at or above 5000 IU per day may be required to achieve and maintain serum 25(OH)D concentrations in the range of 30–40 ng/mL (75–100 nmol/L), a target increasingly endorsed by clinical endocrinology guidelines for optimizing skeletal, muscular, and potentially extraskeletal health outcomes. Several prospective safety studies have documented that daily supplementation with 5000 IU of vitamin D3 for periods ranging from six months to seven years is well tolerated, does not result in hypercalcemia or vitamin D toxicity (defined as serum 25(OH)D >150 ng/mL), and produces stable serum 25(OH)D concentrations typically in the range of 40–70 ng/mL, well below the threshold associated with adverse effects. In a four-year longitudinal study of 14 patients with various chronic diseases taking daily oral 5000 IU vitamin D3, no subject exceeded a serum 25(OH)D concentration of 100 ng/mL, and the treatment was considered safe, with the increase in 25(OH)D levels influenced by baseline disease state and adherence. Similarly, a seven-year observational study of long-term hospitalized patients receiving vitamin D3 doses ranging from 5000 to 50,000 IU per day found that the regimen was well tolerated, with average 25(OH)D levels stabilizing around 12 months at approximately 50–60 ng/mL in those receiving 5000 IU per day, and intact parathyroid hormone levels appropriately suppressed compared to non-supplemented controls. A recent randomized controlled trial of 5000 IU per day vitamin D3 in healthcare workers for prevention of influenza-like illness during the COVID-19 pandemic reported no safety concerns, with excellent adherence and no symptoms of hypercalcemia or nephrolithiasis in 255 participants followed for up to nine months. These findings provide strong empirical support for 5000 IU per day as a safe and effective dose to prevent or correct vitamin D insufficiency in the general adult population, particularly in groups at elevated risk for low vitamin D status [55,60,62,63,64,65,66].

Despite recommendations for 600–1000 IU per day for most adults, population-level data demonstrate that these intakes are insufficient to prevent vitamin D insufficiency in a substantial proportion of individuals, particularly those living at higher latitudes, older adults, and persons with darker skin pigmentation or elevated body mass index. In the Canadian Health Measures Survey, 10–15% of adults consuming more than 600 IU per day from diet and supplements still exhibited serum 25(OH)D levels below 50 nmol/L, a prevalence far exceeding the 2.5% failure rate intended by the RDA framework. Similarly, intervention trials in older adults have consistently shown that 400–800 IU per day of vitamin D3 often fails to raise serum 25(OH)D above 30 ng/mL, the threshold associated with reduced fracture risk and optimal muscle function. For example, a meta-analysis of randomized controlled trials found that higher-dose vitamin D supplementation (700–1000 IU/day or more) was necessary to achieve antifracture efficacy, with benefits beginning at serum 25(OH)D levels of at least 30 ng/mL (75 nmol/L), whereas lower doses (400 IU/day) were insufficient to reduce falls or fractures. In nursing home residents, supplementation with 800 IU per day reduced fall risk by 72% compared to placebo or lower doses, but achieving this benefit required consistently raising 25(OH)D to at least 24 ng/mL, a target often unmet with lower intakes. The Endocrine Society’s clinical practice guidelines explicitly acknowledge that raising serum 25(OH)D above 30 ng/mL may require at least 1500–2000 IU per day for most adults, and recommend higher doses for individuals with risk factors for vitamin D deficiency. Taken together, these data indicate that the conventional RDA of 600–800 IU per day represents a minimal, rather than optimal, intake, and that doses of 2000–5000 IU per day are necessary for many adults to attain and sustain serum 25(OH)D concentrations associated with maximal health benefits across skeletal, muscular, immune, and potentially metabolic systems [55,60,61,64,66,67,68].

Individualization of vitamin D dosing is critical, as the dose-response relationship between supplementation and serum 25(OH)D is strongly modified by body weight, adiposity, baseline vitamin D status, ethnicity, and metabolic disease burden, all of which influence vitamin D absorption, distribution, metabolism, and bioavailability. Obesity is one of the most important modifiers of vitamin D pharmacokinetics; adipose tissue sequesters the lipophilic vitamin D molecule, reducing circulating 25(OH)D concentrations through volumetric dilution across an expanded body mass and potentially through downregulation of hepatic CYP2R1 activity in obesity. In the VITAL trial, a large randomized controlled trial of 2000 IU per day vitamin D3 versus placebo in older US adults, the increase in serum 25(OH)D at two years was significantly attenuated in participants with higher body mass index; those with obesity (BMI ≥30 kg/m²) exhibited a blunted rise in total 25(OH)D, free vitamin D, and bioavailable vitamin D compared to normal-weight individuals, even after adjusting for potential confounders. A large observational study of over 17,000 healthy volunteers confirmed that obese individuals had serum 25(OH)D levels that were on average 19.8 nmol/L lower than normal-weight subjects at any given level of vitamin D supplementation, and overweight individuals had levels 8.0 nmol/L lower, independent of age, sex, and season. Based on dose-response modelling, the authors recommended that vitamin D supplementation be two to three times higher for obese subjects and 1.5 times higher for overweight subjects relative to normal-weight individuals to achieve the same target 25(OH)D concentration, consistent with the Endocrine Society’s guideline that obese individuals may require two to three times more vitamin D. Baseline vitamin D status also influences dose requirements; individuals with severe deficiency (25(OH)D <20 ng/mL) often require loading doses of 50,000 IU weekly for 8 weeks, or the equivalent of 6000–7000 IU per day, to rapidly normalize serum levels, followed by maintenance therapy with 1500–2000 IU per day or higher. Ethnicity is another key determinant, as individuals with darker skin pigmentation synthesize vitamin D less efficiently and often present with lower baseline 25(OH)D levels; African American children receiving 2000 IU per day for 16 weeks achieved significantly higher serum 25(OH)D levels and lower arterial stiffness compared to those receiving 400 IU per day, suggesting that higher doses are necessary to overcome constitutive deficits in this population. Finally, individuals with chronic kidney disease, malabsorption syndromes, or taking medications that accelerate vitamin D catabolism (such as anticonvulsants or glucocorticoids) may require even higher doses, often 4000–10,000 IU per day, to maintain adequate vitamin D status. These considerations underscore the importance of measuring baseline serum 25(OH)D, assessing individual risk factors, and titrating vitamin D3 supplementation to achieve personalized target levels, with 5000 IU per day serving as a reasonable starting point for many at-risk adults, subject to clinical monitoring and dose adjustment based on response [64,66,69,70,71,72,73].

Safety Profile and Potential Adverse Effects of High-Dose Vitamin D

Vitamin D toxicity is uncommon and usually arises from excessive supplemental intake rather than sunlight, and is defined biochemically by marked elevation of serum 25(OH)D (typically >150 ng/mL), hypercalcemia, and hypercalciuria. Clinically, patients present with nonspecific symptoms driven by hypercalcemia, including nausea, vomiting, constipation, polyuria, polydipsia, dehydration, weakness, confusion, and, in severe cases, nephrolithiasis, nephrocalcinosis, and renal insufficiency. Laboratory features characteristically include elevated total and ionized calcium, suppressed parathyroid hormone, raised 25(OH)D, and often impaired renal function, highlighting the need for prompt recognition and withdrawal of vitamin D and calcium sources when toxicity is suspected [57,73,74].

Randomized trials using long-term doses of 3200–4000 IU/day have generally confirmed a good safety profile but identified a small, statistically significant increase in hypercalcemia and related adverse events in a minority of participants. A recent meta-analysis of such trials reported approximately 4 additional cases of hypercalcemia per 1000 individuals receiving 3200–4000 IU/day compared with lower doses or placebo, with most events mild and reversible. Observational data with higher regimens (e.g., 5000–10,000 IU/day or 50,000–100,000 IU/week) also show low rates of toxicity when dosing is individualized and guided by serum 25(OH)D monitoring, although concerns persist about potential U-shaped relationships between very high 25(OH)D levels and skeletal or fall outcomes, reinforcing the need to avoid chronic supraphysiological concentrations [57,62,63,71,73,75,76].

The risk of adverse effects from higher-dose vitamin D is strongly modified by renal function, baseline calcium and vitamin D status, concomitant medications, and cofactor adequacy. Chronic kidney disease, granulomatous disorders, or lymphomas can amplify the calcemic response to vitamin D, while thiazide diuretics and high-dose calcium supplements increase the propensity for hypercalcemia, and enzyme-inducing drugs may necessitate higher doses to achieve target 25(OH)D. Baseline and follow-up monitoring of serum 25(OH)D, calcium, and creatinine (eGFR) is therefore recommended in older adults and patients with CKD or polypharmacy when using doses ≥2000–4000 IU/day. Adequate vitamin K2 and magnesium may mitigate some risks by optimizing calcium handling and vitamin D metabolism: K2 activates matrix Gla protein and osteocalcin to direct calcium into bone and away from soft tissues, while magnesium serves as a cofactor for vitamin D–activating enzymes and improves the biochemical response to supplementation. A pragmatic strategy is to combine moderate-to-high-dose vitamin D (often 2000–5000 IU/day) with attention to K2 and magnesium status and periodic biochemical monitoring, thereby maximizing benefits while maintaining a favourable safety margin in aging and metabolically at-risk populations [29,37,40,54,57,64,71,73].

Vitamin K2 Safety and Dosing Concentrations

Vitamin K2 (as menaquinone-7 or menaquinone-4) has an excellent safety profile at physiologic supplemental doses typically used for bone and cardiovascular prevention, with no upper intake level defined and no evidence of toxicity in humans at intakes up to at least 360–720 µg/day for MK‑7 and 45 mg/day for MK‑4 in long-term trials. Common adverse effects in studies are minimal and nonspecific (e.g., mild gastrointestinal complaints), and there are no reports of hypercoagulability or other serious events in participants without anticoagulant therapy, reflecting the fact that K2 primarily supports γ-carboxylation of vitamin K–dependent proteins rather than driving excessive coagulation in the setting of normal hepatic regulation [18,20,26].

The principal safety concern with vitamin K2 relates to pharmacodynamic interactions with vitamin K antagonist anticoagulants such as warfarin and acenocoumarol, which inhibit vitamin K epoxide reductase (VKORC1) to reduce hepatic γ-carboxylation of clotting factors. Supplemental K2 can partially counteract this effect, leading to reduced international normalized ratio (INR) values and potential loss of anticoagulant control, even at relatively low doses (e.g., 50–100 µg/day), and therefore should generally be avoided or introduced only under close clinical supervision with frequent INR monitoring and dose adjustment of the anticoagulant. In contrast, K2 supplementation appears safe in patients receiving direct oral anticoagulants (DOACs), which do not target the vitamin K cycle, although robust trial data remain limited and individualized risk–benefit assessment is still advised [25,26,29,57].

In clinical practice, practical dosing ranges for vitamin K2 depend on the formulation and therapeutic goal. For bone health and primary cardiovascular prevention in adults, MK‑7 doses between 90 and 200 µg/day are commonly used and have been shown to improve surrogate markers of vitamin K status, such as reducing dephosphorylated-undercarboxylated matrix Gla protein (dp-ucMGP) and undercarboxylated osteocalcin, with some trials reporting improvements in bone mineral density and attenuation of age-related arterial stiffening. In Japan, pharmacologic doses of MK‑4 at 45 mg/day are licensed for osteoporosis treatment and have demonstrated fracture risk reduction, often in combination with vitamin D. When vitamin K2 is combined with higher-dose vitamin D3 (e.g., 2000–5000 IU/day) for bone and vascular prevention, many experts consider 100–200 µg/day of MK‑7 or 1.5–5 mg/day of MK‑4 as rational ranges to support optimal carboxylation of osteocalcin and matrix Gla protein, particularly in aging or metabolically at-risk populations, provided that anticoagulant use is carefully evaluated and monitored [29,37,40,44].

Future Directions in Research

Future research on the vitamin D3–K2 axis should prioritize large, adequately powered randomized controlled trials that evaluate combined supplementation with hard clinical endpoints across cardiovascular, metabolic, and functional aging domains. Existing studies largely focus on surrogate markers such as bone mineral density, arterial stiffness, or dp-ucMGP, and have yielded mixed results regarding vascular calcification and fracture outcomes, underscoring the need for long-term trials that assess incident fractures, major adverse cardiovascular events, progression of coronary and valvular calcification, incident diabetes, disability, and mortality in diverse aging and metabolically at-risk populations. Such trials should include stratification by baseline vitamin D and K status, comorbidities (e.g., CKD, diabetes), and concomitant therapies (e.g., calcium, anticoagulants) to clarify which subgroups derive the greatest benefit from D3+K2 interventions and to distinguish preventive from therapeutic effects in established disease [29,37,40,42].

Parallel efforts are needed to refine individualized dosing models that integrate genetic, phenotypic, and digital health data to optimize vitamin D3 and K2 supplementation on a personalized basis. Genome-wide association studies and candidate gene analyses have identified polymorphisms in genes related to vitamin D binding protein (GC) hepatic 25-hydroxylase (CYP2R1)renal 1α-hydroxylase (CYP27B1) and the VDR itself that influence baseline 25(OH)D levels and response to supplementation, while variants in vitamin K–related genes such as VKORC1 and GGCX affect carboxylation efficiency of vitamin K–dependent proteins. Body composition, particularly BMI and fat mass, substantially modifies the dose–response relationship between vitamin D intake and serum 25(OH)D, with overweight and obese individuals often requiring 1.5–3-fold higher doses to achieve the same circulating levels as normal-weight individuals. Digital health tools, including wearable devices, smartphone-based sun-exposure tracking, and home testing kits for 25(OH)D and dp-ucMGP, offer opportunities to implement adaptive dosing algorithms that adjust supplementation based on real-time biomarker feedback, environmental exposure, and behavioural data, but require validation in pragmatic clinical trials [12,13,24,25,60,69,70,72,77,78].

Finally, the documented statistical error in the original RDA estimation for vitamin D and the accumulating evidence for D3+K2 synergy call for a comprehensive re-evaluation of current guideline frameworks. The Veugelers and Ekwaru re-analysis demonstrated that the IOM’s 600 IU/day RDA, derived from prediction limits on study means rather than individuals, substantially underestimates the intake required to ensure that 97.5% of individuals achieve serum 25(OH)D ≥50 nmol/L, with corrected estimates in the 8000–9000 IU/day range based on individual-level variability. Concurrently, observational and interventional data suggest that standard vitamin D–centric guidelines, which do not explicitly address vitamin K2 status, may insufficiently account for the risk of ectopic calcification in the context of higher calcium and vitamin D intakes, particularly in older adults and patients with CKD or cardiovascular disease. Future consensus statements will need to integrate improved statistical modelling of dose–response relationships, revised target ranges for 25(OH)D informed by both bone and extraskeletal outcomes, and explicit consideration of vitamin K2 as a cofactor in safe calcium handling, moving from a narrow bone-centric paradigm to a systems-level approach to skeletal, vascular, and metabolic health across the lifespan [29,37,40,42,49,55,56,57,60].

Conclusion

The converging evidence reviewed in this article supports a central role for the vitamin D3–K2 axis as an integrated regulator of calcium metabolism, skeletal integrity, and vascular health across the lifespan. Vitamin D3 ensures adequate intestinal calcium absorption and upregulates key vitamin K–dependent proteins, while vitamin K2 activates these proteins through γ-carboxylation, thereby directing calcium into bone and away from soft tissues. This coordinated system underpins not only bone mineralization and fracture resistance but also the prevention of vascular and valvular calcification, with downstream implications for cardiometabolic health and functional aging.

At the same time, current dietary reference intakes for vitamin D and vitamin K remain grounded in legacy bone- and coagulation-centric models that inadequately reflect individual variability, extraskeletal targets, and modern lifestyle constraints. The statistical error in the IOM’s RDA estimation, together with large-scale epidemiologic and clinical data, indicates that intakes of 600–800 IU/day are insufficient for many adults to achieve and maintain serum 25(OH)D concentrations associated with optimal musculoskeletal and systemic outcomes, particularly in older, obese, dark-skinned, or low-sunlight populations. In this context, daily doses at or above 5000 IU of vitamin D3, coupled with physiologic K2 supplementation (e.g., 90–200 µg/day MK‑7 or equivalent MK‑4) and adequate magnesium, are increasingly defensible as a pragmatic strategy in preventive and aging medicine, provided that therapy is individualized and anchored by periodic monitoring of 25(OH)D, calcium, and renal function.

Going forward, clinical practice and policy will need to evolve from a narrow, deficiency-prevention paradigm to a more nuanced, lifespan-oriented framework that explicitly incorporates updated statistical analyses, mechanistic insights into the D3–K2 interaction, and the realities of contemporary metabolic and demographic risk profiles. This will require revision of guideline targets for vitamin D and vitamin K, formal recognition of vitamin K2’s role in bone–vascular crosstalk, and the integration of individualized dosing models that account for body composition, genetics, comorbidities, and digital biomarker monitoring. Aligning public health recommendations and clinical practice with this emerging evidence base offers a substantial opportunity to reduce the burden of osteoporotic fractures, vascular calcification, and age-related functional decline, and to reposition the D3–K2 axis as a core pillar of preventive metabolic and longevity-focused care.

Reference

  1. Lehmann B, Genehr T, Knuschke P, Pietzsch P, Meurer M. UVB-induced conversion of 7-dehydrocholesterol to 1alpha,25-dihydroxyvitamin D3 in an in vitro human skin equivalent model. The Journal of investigative dermatology [Internet]. 2001 Nov 1;117(5). Available from: https://pubmed.ncbi.nlm.nih.gov/11710930/
  2. Chen TC, Chimeh F, Lu Z, Mathieu J, Person KS, Zhang A, et al. Factors that influence the cutaneous synthesis and dietary sources of vitamin D. Archives of Biochemistry and Biophysics. 2007 Apr;460(2):213–7.
  3. Sun Y, Alessandroni L, Angeloni S, Del Bianco E, Sagratini G. From 7-dehydrocholesterol to vitamin D3: Optimization of UV conversion procedures toward the valorization of fish waste matrices. Food Chemistry: X [Internet]. 2024 Apr 9;22:101373. Available from: https://www.sciencedirect.com/science/article/pii/S2590157524002608
  4. Cutaneous Biosynthesis of Vitamin D | Center for Academic Research and Training in Anthropogeny (CARTA) [Internet]. Anthropogeny.org. 2025 [cited 2025 Dec 17]. Available from: https://carta.anthropogeny.org/moca/topics/cutaneous-biosynthesis-vitamin-d
  5. Wacker M, Holick MF. Sunlight and Vitamin D. Dermato-Endocrinology. 2013 Jan 1;5(1):51–108.
  6. Zhu JG, Ochalek JT, Kaufmann M, Jones G, DeLuca HF. CYP2R1 is a major, but not exclusive, contributor to 25-hydroxyvitamin D production in vivo. Proceedings of the National Academy of Sciences of the United States of America [Internet]. 2013 Sep 24;110(39):15650–5. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3785760/
  7. Urbschat A, Paulus P, von Quernheim QF, Brück P, Badenhoop K, Zeuzem S, et al. Vitamin D hydroxylases CYP2R1, CYP27B1 and CYP24A1 in renal cell carcinoma. European Journal of Clinical Investigation. 2013 Oct 12;43(12):1282–90.
  8. Bikle Daniel D. Vitamin D Metabolism, Mechanism of Action, and Clinical Applications. Chemistry & Biology [Internet]. 2014 Mar;21(3):319–29. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3968073/
  9. Fleet JC. The Role of Vitamin D in the Endocrinology Controlling Calcium Homeostasis. Molecular and Cellular Endocrinology [Internet]. 2017 Sep;453(1):36–45. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5529228/
  10. Cheng JB, Levine MA, Bell NH, Mangelsdorf DJ, Russell DW. Genetic evidence that the human CYP2R1 enzyme is a key vitamin D 25-hydroxylase. Proceedings of the National Academy of Sciences. 2004 May 5;101(20):7711–5.
  11. Haussler MR, Jurutka PW, Mizwicki M, Norman AW. Vitamin D receptor (VDR)-mediated actions of 1α,25(OH)2vitamin D3: Genomic and non-genomic mechanisms. Best Practice & Research Clinical Endocrinology & Metabolism. 2011 Aug;25(4):543–59.
  12. Ferrari S, Bonjour JP, Rizzoli R. The Vitamin D Receptor Gene and Calcium Metabolism. Trends in Endocrinology & Metabolism. 1998 Sep;9(7):259–65.
  13. Kommagani R, Payal V, Kadakia MP. Differential Regulation of Vitamin D Receptor (VDR) by the p53 Family. Journal of Biological Chemistry. 2007 Oct;282(41):29847–54.
  14. Oktawia Borecka, Dutton JJ, Tang JY, Fraser WD, Rhodes LE. P04 Skin concentration of 7-dehydrocholesterol is not a limiting factor for vitamin D3 synthesis in older versus younger adults, and a similar response occurs to UVR in these age groups. British Journal of Dermatology [Internet]. 2023 Jul 1 [cited 2025 Dec 17];189(1):e15–5. Available from: https://academic.oup.com/bjd/article-abstract/189/1/e15/7221217?login=false
  15. Neville JJ, Palmieri T, Young AR. Physical Determinants of Vitamin D Photosynthesis: A Review. JBMR Plus. 2021 Jan;5(1).
  16. Sharifa AlBlooshi. Vitamin K and women’s health: a review. Frontiers in Global Women s Health. 2025 Jul 11;6.
  17. EXCERPTED FROM: Vitamin and Mineral Safety 4th Edition (2025) Council for Responsible Nutrition (CRN) https://www.crnusa.org/resources/vitamin-mineral-safety Vitamin K2 -Menaquinone-7 bw body weight CNS Chinese Nutrition Society CRN Council for Responsible Nutrition DRI dietary reference intake EC SCF European Commission Scientific Committee on Food EFSA European Food Safety Authority EVM Expert Group on Vitamins and Minerals HOI highest observed intake level [Internet]. [cited 2025 Dec 17]. Available from: https://crnusa.org/sites/default/files/pdfs/09.2-CRNVMS4-VITAMINK2_MK7_FINAL-EHedits.pdf
  18. Sato T, Schurgers LJ, Uenishi K. Comparison of menaquinone-4 and menaquinone-7 bioavailability in healthy women. Nutrition Journal. 2012 Nov 12;11(1).
  19. Ellis JL, Fu X, Karl JP, Hernandez CJ, Mason JB, DeBose-Boyd RA, et al. Multiple Dietary Vitamin K Forms Are Converted to Tissue Menaquinone-4 in Mice. The Journal of Nutrition. 2021 Nov 9;152(4):981–93.
  20. Jadhav N, Ajgaonkar S, Saha P, Gurav P, Pandey A, Basudkar V, et al. Molecular Pathways and Roles for Vitamin K2-7 as a Health-Beneficial Nutraceutical: Challenges and Opportunities. Frontiers in Pharmacology [Internet]. 2022 Jun 14;13. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9237441/#:~:text=The mechanistic basis for the disease-modulating effects of
  21. Lacombe J, Ferron M. Gamma-carboxylation regulates osteocalcin function. Oncotarget. 2015 Aug 11;6(24).
  22. SCHURGERS LJ, SPRONK HMH, SKEPPER JN, HACKENG TM, SHANAHAN CM, VERMEER C, et al. Post-translational modifications regulate matrix Gla protein function: importance for inhibition of vascular smooth muscle cell calcification. Journal of Thrombosis and Haemostasis. 2007 Dec;5(12):2503–11.
  23. Tie JK ., Stafford DW. Structural and functional insights into enzymes of the vitamin K cycle. Journal of Thrombosis and Haemostasis. 2016 Jan 29;14(2):236–47.
  24. Hao Z, Jin D, Chen X, Schurgers LJ, Stafford DW, Tie JK. Gamma-glutamyl carboxylase mutations differentially affect the biological function of vitamin K-dependent proteins. Blood. 2020 Aug 12;
  25. Berkner KL, Runge KW. Vitamin K-Dependent Protein Activation: Normal Gamma-Glutamyl Carboxylation and Disruption in Disease. International Journal of Molecular Sciences [Internet]. 2022 Jan 1 [cited 2024 Apr 3];23(10):5759. Available from: https://www.mdpi.com/1422-0067/23/10/5759
  26. El Asmar M, Naoum J, Arbid E. Vitamin K Dependent Proteins and the Role of Vitamin K2 in the Modulation of Vascular Calcification: A Review. Oman Medical Journal [Internet]. 2014 May 15;29(3):172–7. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4052396/
  27. gov. 2025 [cited 2025 Dec 17]. Available from: https://clinicaltrials.gov/study/NCT01407601
  28. Gancheva S, Kitanova M, Ghenev P, Zhelyazkova-Savova M. Experimental Model of Subclinical Vitamin K Deficiency. Folia Medica. 2020 Jun 30;62(2):378–84.
  29. Hariri E, Kassis N, Iskandar JP, Schurgers LJ, Saad A, Abdelfattah O, et al. Vitamin K2—a neglected player in cardiovascular health: a narrative review. Open Heart. 2021 Nov;8(2):e001715.
  30. Merra G, Dominici F, Gualtieri P, Capacci A, Cenname G, Esposito E, et al. Role of vitamin K2 in bone-vascular crosstalk. International Journal for Vitamin and Nutrition Research. 2022 Aug 30;
  31. van Driel M, van Leeuwen JPTM. Vitamin D endocrine system and osteoblasts. BoneKEy Reports. 2014 Feb 5;3.
  32. Caus M, Alonso-Montes C, Fernandez-Martin JL, Marti-Antonio M, Bozic M, Valdivielso JM. Vitamin D Receptor From VSMCs Regulates Vascular Calcification During CKD: A Potential Role for miR-145a. Arteriosclerosis, thrombosis, and vascular biology [Internet]. 2023 Aug;43(8):1533–48. Available from: https://pubmed.ncbi.nlm.nih.gov/37381989/
  33. Shearer MJ. The roles of vitamins D and K in bone health and osteoporosis prevention. Proceedings of the Nutrition Society. 1997 Nov;56(3):915–37.
  34. Breen EC, Wijnen van, Lian JB, Stein GS, Stein JL. In vivo occupancy of the vitamin D responsive element in the osteocalcin gene supports vitamin D-dependent transcriptional upregulation in intact cells. Proceedings of the National Academy of Sciences. 1994 Dec 20;91(26):12902–6.
  35. Han MS, Che X, Cho G, Park HR, Lim KE, Park NR, et al. Functional Cooperation between Vitamin D Receptor and Runx2 in Vitamin D-Induced Vascular Calcification. Makishima M, editor. PLoS ONE. 2013 Dec 12;8(12):e83584.
  36. van de Peppel J, van Leeuwen JPTM. Vitamin D and gene networks in human osteoblasts. Frontiers in Physiology. 2014 Apr 9;5.
  37. van Ballegooijen AJ, Pilz S, Tomaschitz A, Grübler MR, Verheyen N. The Synergistic Interplay between Vitamins D and K for Bone and Cardiovascular Health: a Narrative Review. International Journal of Endocrinology [Internet]. 2017;2017(1):1–12. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5613455/
  38. Hasific S, Øvrehus KA, Hosbond S, Lambrechtsen J, Kumarathurai P, Mejldal A, et al. Effects of vitamins K2 and D3 supplementation in patients with severe coronary artery calcification: a study protocol for a randomised controlled trial. BMJ Open [Internet]. 2023 Jul 1;13(7):e073233. Available from: https://bmjopen.bmj.com/content/13/7/e073233#:~:text=Abstract
  39. Louati K. Vitamins D3, K2, and Calcium | LifeMD [Internet]. Lifemd.com. LifeMD Inc.; 2025 [cited 2025 Dec 17]. Available from: https://lifemd.com/learn/benefits-of-combining-vitamins-d3-k2-calcium
  40. Witham MD, Lees JS, White M, Band M, Bell S, Chantler DJ, et al. Vitamin K Supplementation to Improve Vascular Stiffness in CKD: The K4Kidneys Randomized Controlled Trial. Journal of the American Society of Nephrology. 2020 Aug 13;31(10):2434–45.
  41. Bartstra JW, Draaisma F, Zwakenberg SR, Lessmann N, Wolterink JM, van der Schouw YT, et al. Six months vitamin K treatment does not affect systemic arterial calcification or bone mineral density in diabetes mellitus 2. European Journal of Nutrition. 2020 Oct 17;60(3):1691–9.
  42. Diederichsen ACP, Lindholt JS, Möller S, Øvrehus KA, Auscher S, Lambrechtsen J, et al. Vitamin K2 and D in Patients With Aortic Valve Calcification: A Randomized Double-Blinded Clinical Trial. Circulation. 2022 May 3;145(18):1387–97.
  43. Na S, Ahmad SS, Karim S, Ibrahim IM, Alkreathy HM, Alsieni M, et al. Effect of Vitamin K on Bone Mineral Density and Fracture Risk in Adults: Systematic Review and Meta-Analysis. Biomedicines. 2022 May 1;10(5):1048.
  44. Ushiroyama T, Ikeda A, Ueki M. Effect of continuous combined therapy with vitamin K2 and vitamin D3 on bone mineral density and coagulofibrinolysis function in postmenopausal women. Maturitas. 2002 Mar;41(3):211–21.
  45. Guo J, Zhou Z, Gong J, Hu W, Liu Y. Vitamin K intake levels are associated with bone health in people aged over 50 years: a NHANES-based survey. Frontiers in Medicine [Internet]. 2024 Nov 25;11. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC11625553/
  46. Capozzi A, Scambia G, Lello S. Calcium, vitamin D, vitamin K2, and magnesium supplementation and skeletal health. Maturitas. 2020 Oct;140(140):55–63.
  47. Cockayne S, Adamson J, Lanham-New S, Shearer MJ, Gilbody S, Torgerson DJ. Vitamin K and the Prevention of Fractures. Archives of Internal Medicine [Internet]. 2006 Jun 26;166(12):1256. Available from: https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/410550
  48. Graff IE, Øyen J, Kjellevold M, Frøyland L, Gjesdal CG, Almås B, et al. Reduced bone resorption by intake of dietary vitamin D and K from tailor-made Atlantic salmon: a randomized intervention trial. Oncotarget. 2016 Aug 14;7(43).
  49. Papadopoulou SK, Papadimitriou K, Voulgaridou G, Georgaki E, Tsotidou E, Zantidou O, et al. Exercise and Nutrition Impact on Osteoporosis and Sarcopenia—The Incidence of Osteosarcopenia: A Narrative Review. Nutrients [Internet]. 2021 Dec 16;13(12):4499. Available from: https://www.mdpi.com/2072-6643/13/12/4499
  50. Sung CC, Liao MT, Lu KC, Wu CC. Role of Vitamin D in Insulin Resistance. Journal of Biomedicine and Biotechnology. 2012;2012:1–11.
  51. Argano C, Mirarchi L, Amodeo S, Orlando V, Torres A, Corrao S. The role of vitamin D and its molecular bases in insulin resistance, diabetes, metabolic syndrome, and cardiovascular disease: State of the art. International Journal of Molecular Sciences. 2023 Oct 23;24(20):15485–5.
  52. Hanafy AS, Elkatawy HA. Beneficial Effects of Vitamin D on Insulin Sensitivity, Blood Pressure, Abdominal Subcutaneous Fat Thickness, and Weight Loss in Refractory Obesity. Clinical Diabetes. 2018 Jul 1;36(3):217–25.
  53. Garbossa SG, Folli F. Vitamin D, sub-inflammation and insulin resistance. A window on a potential role for the interaction between bone and glucose metabolism. Reviews in Endocrine and Metabolic Disorders [Internet]. 2017 Apr 13;18(2):243–58. Available from: https://link.springer.com/article/10.1007/s11154-017-9423-2
  54. Mohd Ghozali N, Giribabu N, Salleh N. Mechanisms Linking Vitamin D Deficiency to Impaired Metabolism: An Overview. Xie Z, editor. International Journal of Endocrinology. 2022 Jul 6;2022:1–16.
  55. Veugelers P, Ekwaru J. A Statistical Error in the Estimation of the Recommended Dietary Allowance for Vitamin D. Nutrients. 2014 Oct 20;6(10):4472–5.
  56. The IOM Miscalculated Its RDA For Vitamin D – GrassrootsHealth [Internet]. GrassrootsHealth. 2024 [cited 2025 Dec 17]. Available from: https://www.grassrootshealth.net/blog/iom-miscalculated-rda-vitamin-d/
  57. National Institutes of Health. Vitamin D [Internet]. National Institutes of Health. 2025. Available from: https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
  58. US) M, A Catharine Ross, Taylor CL, Yaktine AL, B H. Dietary Reference Intakes for Adequacy: Calcium and Vitamin D [Internet]. Nih.gov. National Academies Press (US); 2011. Available from: https://www.ncbi.nlm.nih.gov/books/NBK56056/
  59. Papadimitriou DT. The Big Vitamin D Mistake. Journal of preventive medicine and public health = Yebang Uihakhoe chi [Internet]. 2017 [cited 2019 Oct 12];50(4):278–81. Available from: https://www.ncbi.nlm.nih.gov/m/pubmed/28768407
  60. Giustina A, Bouillon R, Dawson-Hughes B, Ebeling PR, Lazaretti-Castro M, Lips P, et al. Vitamin D in the older population: a consensus statement. Endocrine. 2022 Oct 26;79(1).
  61. Cui A, Zhang T, Xiao P, Fan Z, Wang H, Zhuang Y. Global and regional prevalence of vitamin D deficiency in population-based studies from 2000 to 2022: A pooled analysis of 7.9 million participants. Frontiers in Nutrition [Internet]. 2023 Mar 17;10(1). Available from: https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2023.1070808/full
  62. Sapporo Medical Journal [Internet]. Sapporo Medical Journal. 2020. Available from: https://www.maejournal.com/article/safety-of-5000-international-units-daily-oral-dosing-of-vitamin-d3-for-four-years-in-various-diseases
  63. McCullough PJ, Lehrer DS, Amend J. Daily oral dosing of vitamin D3 using 5000 TO 50,000 international units a day in long-term hospitalized patients: Insights from a seven year experience. The Journal of Steroid Biochemistry and Molecular Biology. 2019 May;189:228–39.
  64. Williams SE. Vitamin D supplementation: Pearls for practicing clinicians. Cleveland Clinic Journal of Medicine. 2022 Mar;89(3):154–60.
  65. van Helmond N, Brobyn TL, LaRiccia PJ, Cafaro T, Hunter K, Roy S, et al. Vitamin D3 Supplementation at 5000 IU Daily for the Prevention of Influenza-like Illness in Healthcare Workers: A Pragmatic Randomized Clinical Trial. Nutrients. 2022 Dec 30;15(1):180.
  66. Glueck C, Jetty V, Wang P, Shah P, Prince M, Lee K, et al. Safety of 50,000-100,000 units of vitamin D3/week in vitamin D-deficient, hypercholesterolemic patients with reversible statin intolerance. North American Journal of Medical Sciences. 2016;8(3):156.
  67. Rahme M, Sharara SL, Baddoura R, Habib RH, Halaby G, Arabi A, et al. Impact of Calcium and Two Doses of Vitamin D on Bone Metabolism in the Elderly: A Randomized Controlled Trial. Journal of Bone and Mineral Research. 2017 May 18;32(7):1486–95.
  68. Omolbanin Motamed Rezaei, Sharifi F, Mitra Moodi, Asghar Zarban, Reza Tahergorabi, Zoya Tahergorabi. The prevalence and determinants of vitamin D status among older adults: Data from a longitudinal aging study. International Journal of Preventive Medicine. 2023 Jan 1;14(1):27–7.
  69. Tobias DK, Luttmann-Gibson H, Mora S, Danik J, Bubes V, Copeland T, et al. Association of Body Weight With Response to Vitamin D Supplementation and Metabolism. JAMA Network Open [Internet]. 2023 Jan 17;6(1):e2250681. Available from: https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2800490
  70. Pereda CA, Nishishinya MB. Optimal dosage of vitamin D supplementation in obese patients with low serum levels of 25-Hydroxyvitamin D. A systematic review. Obesity Medicine. 2022 Jan;29:100381.
  71. Rizzoli R. Vitamin D supplementation: upper limit for safety revisited? Aging Clinical and Experimental Research. 2020 Aug 28;
  72. Ekwaru JP, Zwicker JD, Holick MF, Giovannucci E, Veugelers PJ. The Importance of Body Weight for the Dose Response Relationship of Oral Vitamin D Supplementation and Serum 25-Hydroxyvitamin D in Healthy Volunteers. Harvey N, editor. PLoS ONE [Internet]. 2014 Nov 5 [cited 2019 Oct 31];9(11):e111265. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4220998/
  73. Sizar O, Khare S, Goyal A, Bansal P, Givler A. Vitamin D Deficiency [Internet]. PubMed. Treasure Island (FL): StatPearls Publishing; 2023. Available from: https://www.ncbi.nlm.nih.gov/books/NBK532266/
  74. Asif A, Farooq N. Vitamin D Toxicity [Internet]. PubMed. Treasure Island (FL): StatPearls Publishing; 2021. Available from: https://www.ncbi.nlm.nih.gov/books/NBK557876/
  75. Armin Zittermann, Trummer C, Theiler‐Schwetz V, Pilz S. Long-term supplementation with 3200 to 4000 IU of vitamin D daily and adverse events: a systematic review and meta-analysis of randomized controlled trials. European Journal of Nutrition [Internet]. 2023 Feb 28;62(4):1833–44. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10195747/
  76. Bouillon R. Safety of High-Dose Vitamin D Supplementation. The Journal of Clinical Endocrinology & Metabolism. 2019 Dec 20;105(4).
  77. Zhu JG, Ochalek JT, Kaufmann M, Jones G, DeLuca HF. CYP2R1 is a major, but not exclusive, contributor to 25-hydroxyvitamin D production in vivo. Proceedings of the National Academy of Sciences of the United States of America [Internet]. 2013 Sep 24;110(39):15650–5. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3785760/
  78. Cheng JB, Levine MA, Bell NH, Mangelsdorf DJ, Russell DW. Genetic evidence that the human CYP2R1 enzyme is a key vitamin D 25-hydroxylase. Proceedings of the National Academy of Sciences. 2004 May 5;101(20):7711–5.

‌