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Autophagy, Metabolic Switching, and Circadian Alignment as Converging Mechanisms Through Which Structured Fasting Promotes Longevity and Cardiometabolic Resilience

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dr Karunia Valeriani Japar

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Keywords: Fasting, Autophagy, Longevity, Time-Restrictive Eating, Precision Medicine

Introduction

Intermittent and periodic fasting have rapidly transitioned from primarily religious observances and ancestral “feast–famine” survival patterns into central strategies within contemporary longevity and biohacking paradigms. A growing body of basic and clinical research indicates that, beyond inducing weight loss, structured fasting cycles modulate conserved nutrient‑sensing pathways, enhance cellular repair, and attenuate risk factors for multiple age‑related diseases. In model organisms ranging from yeast and C.elegans to flies, rodents, and non‑human primates, dietary restriction and fasting regimens extend lifespan and delay multimorbidity, largely via suppression of pro‑growth signalling (for example GH–IGF‑1 and mTORC1) and activation of stress‑resistance and autophagy programs. In humans, intermittent fasting and fasting‑mimicking protocols improve metabolic health, cardiovascular parameters, and inflammatory profiles, suggesting a biologically plausible pathway to extend health span, even though definitive data on human lifespan extension are not yet available. This article aims to synthesize current mechanistic and clinical evidence on what fasting can do for longevity and to discuss how these insights can be operationalized into clinically responsible, AI‑enabled, personalized fasting frameworks within preventive and longevity‑focused practice

Fasting Concepts and Regimens

Fasting sits within the broader category of dietary restriction strategies, which encompass chronic caloric restriction, protein restriction, and targeted macronutrient manipulations that modulate nutrient‑sensing pathways without necessarily inducing malnutrition. Within this spectrum, intermittent fasting (IF) refers to eating patterns that alternate predefined periods of substantial energy restriction (typically 16–48 hours) with periods of normal or ad libitum intake, thereby creating recurrent metabolic transitions between the fed and fasting states. Canonical IF protocols include alternate‑day fasting (ADF; 24 hours of water‑only or very‑low‑calorie intake alternating with 24 hours of unrestricted eating), modified ADF with ~70–75% energy restriction on “fast” days, the 5:2 regimen (500–700 kcal on two non‑consecutive days per week with usual intake on the remaining days), and time‑restricted feeding/eating (TRF/TRE), in which all daily calories are consumed within a limited window of approximately 6–12 hours [1].

In contrast, periodic fasting (PF) involves less frequent but more prolonged and severe restriction, usually lasting ≥2–7 days per cycle in humans, followed by extended refeeding intervals of at least one week. PF can be implemented as water‑only fasting or via fasting‑mimicking diets (FMDs), which are low‑protein, low‑sugar, relatively high‑unsaturated‑fat regimens that provide 30–50% of usual calories while closely reproducing the hormonal and metabolic milieu of complete fasting, including reductions in IGF‑1 and glucose and increases in ketone bodies. These PF/FMD cycles are typically performed a few times per year in otherwise healthy individuals, or more frequently under clinical supervision for patients with specific conditions, to balance potential benefits on aging and disease risk with the burden and safety concerns of prolonged restriction [1].

Among IF patterns, time -restricted eating  has gained particular traction because it is behaviourally simple and leverages circadian biology by confining intake to a consistent 6–10 hour daytime window and extending the nocturnal fasting period. Experimental and clinical TRE protocols often start with 12‑hour eating windows and progressively narrow to 8–10 hours, with early daytime windows (e.g., 8:00–16:00) showing favourable effects on glycemic control, blood pressure, and markers of oxidative stress even in the absence of major weight loss. From a longevity perspective, these various regimens differ in their intensity, frequency, and safety profile water-only PF at the extreme, TRE at the more moderate, daily-life-compatible end, but they converge mechanistically on repeated cycles of nutrient deprivation, metabolic switching (toward fatty acid oxidation and ketogenesis), and subsequent refeeding‑driven anabolic and regenerative responses. It is increasingly recognized that both the fasting and refeeding phases contribute to health span benefits, with short, well‑defined restriction intervals interspersed with adequate nourishment appearing to induce cellular repair and remodelling programs that are less pronounced during chronic, continuous restriction [1].

Evolutionary and Systems-Level Rationale

From an evolutionary perspective, humans developed in environments characterized by recurrent periods of food scarcity interspersed with episodic abundance, making intermittent caloric deprivation an expected, rather than exceptional, physiological state. In this context, fasting can be viewed as an ancient stress signal that activates adaptive survival pathways, enhancing metabolic flexibility, stress resistance, and repair, rather than representing mere energy deficit. The adaptive “starvation response” evolved to preserve critical organ function and support foraging and cognitive performance under low‑energy conditions, and many of the molecular programs engaged during fasting are now recognized as central to aging biology and resilience to chronic disease. Modern environments of continuous food availability and circadian disruption may therefore represent a mismatch with our evolved fasting–feeding cycles, contributing to obesity, insulin resistance, and cardiometabolic disease that structured fasting patterns aim to counterbalance [2,3,4,5,6].

Metabolically, fasting induces a progressive shift from exogenous glucose utilization to increased fatty acid oxidation and hepatic ketogenesis once hepatic glycogen stores are substantially depleted, typically after approximately 12–24 hours without caloric intake in humans, depending on prior diet and activity. During this “metabolic switch,” adipose tissue lipolysis increases, releasing free fatty acids that are converted in the liver to ketone bodies (acetoacetate and β‑hydroxybutyrate), which then serve as an alternative oxidative substrate for skeletal muscle, heart, and, during more prolonged fasting, the brain. Human studies and clinical observations suggest that circulating ketone concentrations often rise into the range of roughly 1–2 mmol/L by around 48 hours of continuous fasting in metabolically healthy individuals, although there is wide inter‑individual variability. These fuel shifts are accompanied by upregulation of antioxidant defenses, increased mitochondrial biogenesis, and activation of DNA repair pathways, changes that collectively enhance cellular stress resistance and may contribute to slower accumulation of age‑related molecular damage [7,8,9].

At the systems level, fasting exerts coordinated hormonal and adipokine effects that reorient the organism from growth and storage toward maintenance and repair. With progressive fasting, circulating insulin and glucose decline, growth hormone rises, and insulin‑like growth factor‑1 (IGF‑1) bioactivity is reduced, leading to decreased mTORC1 signalling and promotion of autophagy and cellular recycling processes. Leptin levels fall while adiponectin tends to increase, a pattern associated with improved insulin sensitivity, enhanced fatty acid oxidation, and favourable cardiometabolic risk modulation. Concurrent increases in circulating ketone bodies not only provide an efficient energy substrate but also act as signalling metabolites that influence inflammation, oxidative stress, and gene expression in multiple tissues, including brain and vascular endothelium. This integrated “metabolic re‑programming” is thought to underpin many of the downstream cardiovascular, metabolic, neurocognitive, and immune benefits of intermittent and periodic fasting described in preclinical models and human studies, positioning fasting as a systems‑level intervention that taps into deeply conserved survival circuitry relevant to longevity [6,7,9,10,11].

Molecular Mechanisms Linking Fasting and Longevity

Fasting impacts longevity primarily through coordinated modulation of conserved nutrient-sensing  pathways that couple energy availability to cellular growth, repair, and stress resistance. Reduced availability of glucose and amino acids during fasting lowers circulating insulin and IGF‑1, leading to decreased activation of the insulin–IGF‑1/PI3K–AKT axis and downstream inhibition of mTORC1–S6K signalling, while simultaneously activating AMP‑activated protein kinase (AMPK), sirtuins, and FOXO transcription factors. In model organisms, genetic or pharmacologic downregulation of IIS and mTORC1, or activation of AMPK, sirtuins, and FOXO, recapitulates many of the lifespan‑extending effects of dietary restriction, underscoring these pathways as core mediators of fasting-induced longevity benefits. Human and animal data indicate that intermittent or periodic fasting cycles repeatedly engage these “pro‑longevity” pathways, shifting cellular programs away from anabolic growth and toward maintenance, repair, and enhanced stress resilience [7,12,13,14,15].

Figure 1. Dietary restriction modulates multiple systemic, neural, and cellular mechanisms that improve health and combat the diseases of aging [12]

A central effector of these upstream signals is autophagy, a lysosome‑dependent degradation process that clears damaged proteins, organelles, and dysfunctional mitochondria to maintain proteostasis and organelle quality control. Suppression of mTORC1 and activation of AMPK during fasting converge on autophagy initiation complexes, while FOXO and sirtuin‑regulated transcriptional programs upregulate genes involved in autophagosome formation, antioxidant defense, and chaperone‑mediated protein folding. In yeast, worms, flies, and mice, genetic enhancement of autophagy extends lifespan, whereas autophagy deficiency abrogates the longevity benefits of dietary restriction and fasting, indicating that intact autophagic machinery is required for fasting‑mediated lifespan extension. By facilitating the removal of oxidized proteins, aggregated species, and damaged mitochondria, fasting‑induced autophagy reduces reactive oxygen species burden, stabilizes mitochondrial function, and supports tissue homeostasis, particularly in metabolically active organs such as liver, heart, skeletal muscle, and brain [7,12,13,15].

Fasting also exerts important epigenetic and transcriptional effects that link metabolic state to long‑term gene expression patterns relevant to aging. The ketone body β‑hydroxybutyrate (BHB), which rises during prolonged fasting, has been identified as an endogenous inhibitor of class I histone deacetylases, increasing histone acetylation at specific loci and promoting the expression of genes involved in antioxidant defense, anti‑inflammatory responses, and metabolic regulation in multiple tissues. In parallel, fasting increases NAD⁺ availability and activates sirtuins (such as SIRT1), which deacetylate and modulate FOXO proteins and PGC‑1α, thereby promoting mitochondrial biogenesis, enhanced oxidative metabolism, and stress‑resistance gene expression. Human data show that intermittent fasting can upregulate SIRT1 and FOXO3 expression, with positive correlations between SIRT1 and FOXO3 in peripheral blood cells, supporting the concept that fasting-induced activation of this axis functions as an anti‑aging biomarker pathway. Through these epigenetic and transcriptional mechanisms, fasting not only acutely reprograms cellular metabolism, but may also induce longer‑lasting shifts in gene networks that favour health span [15,16,17,18,19].

Emerging work highlights the polyamine-hypusination axis, particularly spermidine‑mediated autophagy, as another conserved metabolic hub linking fasting to longevity. Distinct fasting and caloric restriction regimens increase endogenous spermidine levels in yeast, flies, mice, and human volunteers, and genetic or pharmacologic inhibition of spermidine synthesis blunts fasting‑induced autophagy and abolishes lifespan‑ and health span‑extending effects in multiple species. Mechanistically, spermidine promotes hypusination of the translation factor eIF5A, which is required for efficient translation of autophagy‑related and stress‑response proteins; disruption of this polyamine–hypusination axis impairs autophagy induction and eliminates fasting‑mediated cardioprotective and anti‑arthritic benefits in animal models. These findings position spermidine and its downstream hypusination pathway as phylogenetically conserved integrators of fasting signals, autophagy enhancement, and longevity, and suggest that fasting may act, in part, by orchestrating a coordinated network of nutrient‑sensing, autophagic, and epigenetic mechanisms that collectively slow the pace of biological aging [13,20].

Evidence from Model Organisms

Intermittent and periodic fasting have been extensively studied in model organisms, where they consistently modulate lifespan and the onset of age‑associated phenotypes. In classic work by Goodrick and colleagues, rats exposed to alternate‑day fasting (ADF) starting early in life (around 5 weeks of age) exhibited increases in average lifespan of up to approximately 80% compared with ad libitum‑fed controls, although later studies showed smaller, strain‑ and age‑dependent effects, ranging from modest benefit to neutral or even detrimental outcomes. Subsequent experiments have confirmed that various intermittent feeding paradigms can extend median and maximal lifespan in rodents, but the magnitude and direction of effect depend critically on species, genetic background, sex, diet composition, and the age at which the regimen is initiated, highlighting the importance of biological context in interpreting fasting–longevity data [7,21,22,23].

In mice, time‑restricted feeding (TRF) on obesogenic high‑fat diets has provided a particularly compelling demonstration that feeding–fasting cycles influence health and aging beyond calorie quantity alone. When food access is restricted to a 8–10‑hour window during the active (dark) phase, high‑fat‑fed mice are protected from excessive weight gain, insulin resistance, hepatic steatosis, dyslipidemia, and age‑related decrements in motor coordination, despite consuming an equivalent number of calories to ad libitum high‑fat controls. TRF in these models normalizes respiratory exchange ratio rhythms, reduces hepatic lipid accumulation, and enhances cellular defenses against metabolic stress, suggesting that restoration of a robust feeding–fasting rhythm can counteract the metabolic and functional deterioration typically induced by a Western‑style diet [24,25,26].

Periodic fasting‑mimicking diet (FMD) regimens further extend this paradigm by introducing multi‑day, low‑calorie, low‑protein, high‑unsaturated‑fat cycles at intervals across the lifespan. In C57BL/6 mice, 4‑day FMD cycles administered twice per month starting at middle age increased median lifespan by around 11%, reduced tumor incidence by roughly 45–50%, decreased inflammatory disease burden, and improved late‑life cognitive performance relative to controls fed a standard diet. FMD‑treated mice also showed reductions in visceral adiposity and multi‑system regeneration, with hematopoietic and immune profiles partially rejuvenated toward more youthful lymphoid‑dominant patterns, indicating that periodic, intense fasting‑like states can induce systemic remodelling rather than merely delaying pathology [7,27].

Across these diverse models, from yeast and nematodes to flies, rodents, and other species, the beneficial effects of fasting and fasting‑mimicking paradigms map closely onto conserved molecular pathways implicated in aging. Lifespan extension is consistently associated with suppression of insulin–IGF‑1 and mTOR signalling, enhanced activation of AMPK and sirtuins, and robust induction of autophagy and mitophagy, which together improve proteostasis, mitochondrial function, and cellular stress resistance. In parallel, fasting regimens help preserve stem and progenitor cell pools in tissues such as the hematopoietic system and intestine, supporting better regenerative capacity and organ function at advanced ages. While extrapolation to humans requires caution, these convergent findings across phylogenetically distant organisms provide strong mechanistic support for fasting as a modulator of fundamental aging processes, rather than as a simple modifier of body weight alone [7,12,13,27,].

Human Data: Health Span, Not Just Weight Loss

Human trials of intermittent fasting (IF) and time‑restricted eating (TRE) increasingly show that their benefits extend beyond simple weight reduction, with consistent improvements in metabolic and vascular risk markers even when weight loss is modest or equivalent to control diets. In randomized and controlled studies of adults with overweight, obesity, or insulin resistance, alternate‑day fasting (ADF) produces substantial reductions in fasting insulin and insulin resistance, often in the range of ~40-60% that are greater than those seen with isocaloric daily caloric restriction, despite similar changes in body weight and fat mass. Meta‑analyses and clinical trials further report that ADF can reduce body weight, improve markers of dyslipidemia, and lower indices of non‑alcoholic fatty liver disease, while also enhancing glycemic control in individuals with elevated baseline metabolic risk. These data support the concept that intermittent fasting paradigms modulate insulin signalling and metabolic flexibility in ways that are not fully captured by weight‑centric metrics alone [1,28,29,30,31,32].

Early time‑restricted feeding (eTRF), in which all caloric intake is confined to a 6–8 hour window earlier in the day, provides a clear example of health span‑relevant benefits independent of major weight change. In a supervised crossover trial of men with prediabetes, eTRF (e.g., 8:00–14:00) improved insulin sensitivity and β‑cell responsiveness, lowered 24‑hour glucose levels, reduced blood pressure by approximately 11/10 mmHg, and decreased markers of oxidative stress, despite no significant differences in energy intake or body weight versus a control schedule with extended eating periods. Mechanistic analyses in these participants indicated reductions in insulin and IGF‑1 signalling, increased indices of antioxidant defenses and autophagy, and modulation of aging‑related pathways such as SIRT1, BDNF, and mTOR, suggesting that aligning feeding–fasting cycles with circadian biology can reprogram cardiometabolic and cellular aging markers. Participants also reported reduced evening appetite and acceptable feasibility, indicating that TRE/eTRF may be a practical approach for long‑term risk reduction [33,34].

Randomized phase 2 trials of the fasting‑mimicking diet (FMD) in generally healthy or mildly at‑risk adults extend these findings to more periodic, multi‑day fasting‑like interventions. Three monthly 5‑day FMD cycles have been shown to reduce body weight, waist circumference, and particularly trunk and total body fat; lower systolic blood pressure; and decrease circulating IGF‑1, a key pro‑aging growth factor, with an acceptable safety profile. Post hoc analyses reveal that participants with elevated baseline risk factors (high BMI, blood pressure, fasting glucose, triglycerides, cholesterol, C‑reactive protein, or IGF‑1) experience the largest absolute improvements, including clinically meaningful reductions in fasting glucose, triglycerides, LDL cholesterol, and CRP. Follow‑up data suggest that some of these metabolic and inflammatory benefits persist for months after completing FMD cycles, indicating durable effects on health span‑related pathways rather than transient metabolic perturbations [27,35].

Taken together, these human studies indicate that IF, TRE/eTRF, and periodic FMD can improve insulin sensitivity, atherogenic lipids, blood pressure, inflammatory markers, and molecular indicators of aging, often in ways that cannot be fully explained by weight loss alone. However, the current evidence base primarily addresses intermediate health span outcomes and disease risk factors, not hard endpoints such as all‑cause mortality or maximum lifespan. Existing trials are relatively short in duration, involve modest sample sizes, and are heterogeneous in protocol design, limiting definitive conclusions about long‑term safety and longevity impact. As such, the hypothesis that fasting extends human lifespan remains inferential, supported by mechanistic, animal, and surrogate endpoint data will require large, long-term and rigorously controlled studies to be directly confirmed [1,28,29,32,].

Cardiometabolic and Endocrine Benefits

Cardiometabolic risk reduction is among the most consistently demonstrated clinical benefits of intermittent fasting (IF) and time-restricted eating (TRE). Systematic reviews, meta‑analyses, and randomized trials show that IF protocols (including alternate‑day fasting, 5:2 regimens, and TRE) typically induce mild to moderate weight loss, often in the range of roughly 1-10% of baseline body weight, together with significant improvements in fasting glucose, insulin resistance indices (HOMA‑IR), HbA1c, LDL cholesterol, and triglycerides in individuals with overweight, prediabetes, or metabolic syndrome. A recent meta‑analysis reported that fasting interventions led to meaningful reductions in fasting blood glucose, HbA1c, HOMA‑IR, LDL‑c, and inflammatory markers such as interleukin‑6, supporting a broad cardiometabolic benefit profile that is directionally consistent across diverse fasting protocols [28,36,37].

Importantly, several trials indicate that the favourable effects of IF and TRE on cardiometabolic markers cannot be fully attributed to weight loss alone. In controlled studies where IF or TRE is compared with continuous calorie restriction producing similar weight loss, intermittent patterns often yield equal or superior improvements in fasting insulin, blood pressure, and lipid parameters, suggesting additional pathway‑level effects on vascular function, hepatic lipid handling, oxidative stress, and autonomic tone. For example, TRE interventions that restrict eating to 6–10 hours per day have been associated with reductions in fat mass, fasting insulin, total and LDL cholesterol, and modest decreases in blood pressure, even when total caloric intake and weight loss are modest. Mechanistic analyses from these trials point toward enhanced fatty acid oxidation, reductions in hepatic triglyceride accumulation and VLDL production, improved endothelial function, and favourable shifts in autonomic balance, providing a biologically plausible link between fasting patterns and reduced cardiometabolic risk [29,32,37,38,39].

Observational data from religious fasting cohorts provide complementary, albeit less definitive, evidence that routine periodic fasting may protect against cardiometabolic disease. In a large cohort including members of the Latter‑day Saints community, individuals who reported routine periodic fasting (typically one day per month) had significantly lower odds of coronary artery disease and lower prevalence of diabetes compared with non‑fasters, even after adjustment for traditional risk factors and other lifestyle behaviours. More recent systematic reviews of Ramadan and Orthodox Christian fasting suggest modest reductions in systolic blood pressure and total cholesterol, respectively, and overall improvements in some cardiometabolic biomarkers, although heterogeneity in dietary content, sleep, and activity patterns complicates causal attribution. These religious fasting models nonetheless support the concept that long‑term integration of structured fasting into habitual lifestyle may contribute to lower cardiometabolic burden at the population level [40,41,42].

Taken together, the available data support the use of structured IF and TRE as non‑pharmacological adjuncts for the prevention and treatment of cardiometabolic disease, particularly in individuals with overweight, insulin resistance, hypertension, dyslipidemia, or metabolic syndrome. However, the magnitude of benefit varies by protocol, population, and adherence, and some effects, especially on blood pressure appear more robust in the context of at least modest weight loss. Moreover, most studies are of relatively short duration and may not capture long‑term safety, sustainability, or rare adverse events, emphasizing the need for individualized risk–benefit assessment that accounts for comorbidities, medications (e.g., glucose‑lowering and antihypertensive agents), nutritional status, and potential contraindications. Within a clinically supervised framework, IF and TRE can thus be positioned as part of a comprehensive cardiometabolic risk reduction strategy that includes diet quality, physical activity, sleep, and, when appropriate, pharmacotherapy, rather than as standalone or universally applicable interventions [28,32,36,37,43,44,45].

Neurocognitive, Immune, and Cancer-Related Effects

Preclinical and clinical data increasingly support a role for fasting as a modulator of neurocognitive function, immune homeostasis, and oncogenic processes, acting through convergent metabolic and signalling pathways. In the central nervous system, intermittent fasting (IF) induces a metabolic switch to ketone utilization and activates molecular programs that enhance neuronal stress resistance, synaptic plasticity, and adult hippocampal neurogenesis. Rodent studies demonstrate that IF upregulates brain‑derived neurotrophic factor (BDNF), CREB signalling, and Notch pathway activity, increases markers of long‑term potentiation and postsynaptic density (e.g., PSD‑95), and augments hippocampal stem cell markers, collectively supporting improved learning and memory capacity. In transgenic mouse models of Alzheimer’s disease and other neurodegenerative conditions, intermittent fasting reduces amyloid‑β plaque accumulation, attenuates oxidative damage, and slows cognitive decline, effects that appear to be mediated by enhanced autophagy, mitochondrial quality control, and ketone‑mediated signalling through β‑hydroxybutyrate. Early human data, though limited, suggest that prolonged IF or fasting‑mimicking interventions may reduce oxidative stress and improve selected cognitive indices in older adults or individuals with mild cognitive impairment, aligning with these mechanistic insights [46,47,48,49,50].

Fasting also exerts systemic immune and microbiome effects that are increasingly recognized as relevant to both metabolic and autoimmune disease. hort‑term, supervised fasting followed by structured dietary refeeding in patients with metabolic syndrome has been shown to induce marked shifts in gut microbiota composition, including enrichment of butyrate‑producing taxa such as Faecalibacterium prausnitzii and an increase in microbial pathways related to short‑chain fatty acid and propionate production, changes that coincide with sustained reductions in blood pressure and body weight. In experimental autoimmune encephalomyelitis (EAE) models of multiple sclerosis (MS), intermittent fasting alters the gut microbiome toward increased diversity and enrichment of  Lactobacillaceae, Bacteroidaceae, Prevotellaceae, and other taxa linked to antioxidative and anti‑inflammatory pathways, resulting in reduced disease severity and a shift in immune cell profiles toward more anti‑inflammatory phenotypes. Small clinical studies in MS patients similarly suggest that intermittent fasting can increase beneficial butyrate‑producing bacteria (e.g., Faecalibacterium and Blautia), elevate adiponectin levels, and improve some patient‑reported outcomes and inflammatory markers, although larger trials are needed to define clinical efficacy. These data collectively point to fasting as a means of remodelling the gut–immune axis in ways that may mitigate systemic inflammation and autoimmune activity [1,51,52,53].

In oncology, fasting and fasting‑mimicking diets (FMDs) have shown compelling antitumor and chemosensitizing effects in preclinical models, with emerging translational evidence in humans. In tumor‑bearing mice, short‑term fasting or low‑calorie, low‑protein FMDs inhibit tumor growth, sensitize malignant cells to chemotherapy, and reduce treatment‑related toxicity, while simultaneously protecting normal tissues, a phenomenon often termed “differential stress resistance.” Mechanistically, fasting deprives cancer cells of growth factors and nutrients, drives them into a metabolically vulnerable state, and promotes antitumor immunity by enhancing T‑cell–mediated cytotoxicity, altering NK‑cell function, and inducing immunogenic cell death via autophagy. A randomized phase II trial (DIRECT) in women with HER2‑negative early breast cancer showed that peri‑chemotherapy FMD was safe and improved radiological and pathological tumor responses compared with standard care, supporting the feasibility of integrating FMD as an adjunct to neoadjuvant chemotherapy. Additional trials report that FMD during chemotherapy is well tolerated, reduces chemotherapy‑related toxicity, and favourably modulates metabolic parameters, though definitive data on progression‑free or overall survival are still lacking [1,54,55,56].

Taken together, the neurocognitive, immune, and cancer‑related findings position fasting as a systemic modifier of key pathways, such as autophagy, mitochondrial function, ketone signalling, gut–microbiome–immune interactions, and tumor immunosurveillance that intersect with the biology of aging. While preclinical models provide robust mechanistic support and early‑phase clinical studies are promising, the translation of these effects into routine clinical practice and quantifiable impacts on neurodegenerative disease progression, autoimmune activity, and cancer survival will require larger, longer, and more rigorously controlled human trials [46,49,54,56,57].

Circadian Alignment and Time-Restricted Eating

Circadian timing appears to be a critical determinant of how fasting and feeding impact metabolic health, potentially as important as the overall duration or degree of energy restriction. The human circadian system is organized around a central pacemaker in the suprachiasmatic nucleus (SCN), which is primarily entrained by the light–dark cycle, and multiple peripheral clocks in metabolic organs such as liver, muscle, adipose tissue, and intestine, which are strongly influenced by the timing of food intake. Epidemiological and experimental data show that misalignment between these central and peripheral clocks as commonly occurs in shift work, irregular schedules, and late‑night eating is associated with increased risk of obesity, type 2 diabetes, hypertension, and cardiovascular disease, likely via adverse effects on glucose tolerance, insulin sensitivity, blood pressure regulation, and autonomic function [38,58,59,60,61].

Within this framework, time-restricted eating (TRE) aims to restore alignment between feeding–fasting cycles and endogenous circadian rhythms by confining food intake to a consistent 8–10‑hour window, usually during the biological daytime. Early TRE (eTRF), which front‑loads caloric intake into the morning and early afternoon (e.g., 8:00–16:00), has shown particular promise: controlled trials report reductions in appetite and evening hunger, improvements in 24‑hour glucose profiles and insulin sensitivity, and decreases in blood pressure and oxidative stress markers, sometimes even without significant weight loss. In older adults with metabolic syndrome or overweight, TRE has been associated with improved cardiometabolic risk profiles and enhanced sleep satisfaction, supporting the idea that realigning eating patterns with biological day–night cycles can reduce metabolic burden beyond caloric effects alone. At the same time, recent isocaloric studies in women with overweight suggest that TRE primarily shifts internal circadian phase and clock gene expression without uniformly improving traditional cardiometabolic markers, highlighting that protocol details (early vs late window, population, duration) critically shape outcomes [38,58,62,63,64,65].

Mechanistically, feeding–fasting cycles act as potent “zeitgebers” (time cues) for peripheral clocks, regulating rhythmic expression of clock genes and downstream metabolic genes in liver, adipose tissue, muscle, and intestine. Animal and human data indicate that restricting feeding to the active phase enhances the amplitude and synchrony of circadian oscillations in pathways governing glucose uptake, glycogen and lipid metabolism, mitochondrial function, and oxidative substrate selection (carbohydrate vs lipid oxidation), thereby improving metabolic flexibility across the day. Time‑restricted feeding in rodent models aligned with the activity phase increases activation of PGC‑1α, promotes mitochondrial biogenesis and fatty acid oxidation, reduces visceral and subcutaneous adiposity, and improves glucose tolerance, LDL/HDL ratios, and cardiovascular risk markers. Conversely, eating during the usual rest phase, as in night‑shift patterns, blunts these beneficial rhythms, leading to impaired glucose handling, higher blood pressure, and adverse autonomic and endocrine profiles [58,59,60,61,66].

For longevity-oriented practice, these findings suggest that the temporal structure of fasting is as important as its frequency or intensity. Aligning eating windows with daylight hours and maintaining consistent sleep–wake cycles appear to maximize the benefits of fasting on glucose regulation, blood pressure, lipid handling, and oxidative stress, while minimizing circadian stress and the downstream risk of cardiometabolic and possibly oncologic disease. Practically, this supports prioritizing early or mid‑day TRE, avoiding large late‑evening meals, and minimizing rotating shift work where possible in individuals engaging in fasting for health span and longevity purposes [38,58,63,67,68].

Risks, Limitations, and Heterogeneity of Response

Despite growing enthusiasm for fasting as a longevity and metabolic intervention, its effects are heterogeneous and context dependent, with clear potential for harm when applied indiscriminately. In rodents, the impact of intermittent fasting (IF) on lifespan ranges from neutral to strongly positive depending on strain, sex, diet composition, and the age at which fasting is initiated, and some protocols show little benefit or even adverse effects when severe restriction is imposed late in life or in frail animals. Large multi‑strain mouse studies indicate that while caloric restriction and IF can extend lifespan on average, the magnitude and direction of effect vary widely across genotypes, underscoring that fasting is not a universally beneficial intervention even within a single species. These data caution against assuming uniform benefits of aggressive fasting in older or biologically vulnerable individuals [23,69,70,71].

In humans, very prolonged or frequently repeated fasts, especially water-only regimens lasting many days to weeks carry non-trivial medical risks and typically require specialized supervision. Reports from water‑only fasting programs and clinical reviews describe episodes of symptomatic hypotension, dehydration, electrolyte disturbances, uric acid elevations, arrhythmias, and, in extreme cases, the need for emergent medical care, particularly when baseline comorbidities, medications (e.g., antihypertensives, insulin, sulfonylureas), or inadequate refeeding are not carefully managed. While short, supervised water‑only fasts can improve blood pressure and some metabolic markers, these potential benefits must be weighed against the risks of malnutrition, sarcopenia, and hemodynamic instability, especially in lean, elderly, or multi‑morbid patients [72,73].

Epidemiological evidence also raises caution about certain behavioural patterns that superficially resemble fasting. Skipping breakfast, a common method to prolong the overnight fast is consistently associated in prospective cohorts with higher cardiometabolic risk and increased cardiovascular and all‑cause mortality, although residual confounding (smoking, lower overall diet quality, socioeconomic factors) limits causal inference. Meta‑analyses including more than 200,000 participants report that habitual breakfast skippers have approximately 20–30% higher risk of cardiovascular disease and all‑cause death than regular breakfast eaters, suggesting that simply extending fasting windows by omitting morning intake may not reproduce the benefits observed in structured, nutritionally adequate IF protocols. Similarly, repeated cycles of marked weight loss and regain (“weight cycling” or “yo‑yo dieting”) have been linked to adverse metabolic outcomes and possibly higher rates of type 2 diabetes and cardiovascular events, and animal models of weight cycling show increased adiposity and hyperinsulinemia after weight regain. These findings raise concern that aggressive, unsupervised fasting in individuals prone to weight cycling might exacerbate, rather than mitigate, long‑term cardiometabolic risk [74,75,76,77].

Psychological and behavioural risks must also be considered, particularly in younger or vulnerable populations. Cross‑sectional and cohort data indicate that intermittent fasting is common among adolescents and young adults and is associated with a higher likelihood of disordered eating behaviours, including binge eating, compulsive exercise, laxative use, and self‑induced vomiting, with the strongest associations observed in women and gender‑diverse individuals. Commentary from eating‑disorder specialists emphasizes that fasting may uniquely reinforce rigid food rules, heightened preoccupation with eating, and weight‑ and shape‑focused cognitions, potentially triggering or worsening eating disorders in predisposed individuals. These observations support the need for careful psychological screening, explicit exclusion criteria (e.g., active or past eating disorders, severe body image disturbance), and ongoing monitoring when integrating fasting into clinical or biohacking programs [78].

Overall, the heterogeneity of response to fasting, across species, genotypes, life stages and psychological contexts argues for a nuanced, individualized approach rather than universal prescriptions. While intermittent and periodic fasting can meaningfully improve metabolic and aging‑related biomarkers in many settings, the same interventions can be ineffective or risky in others, particularly when extreme, unsupervised, or overlaid onto weight‑cycling or disordered‑eating patterns. For clinicians and practitioners, this underscores the importance of structured risk–benefit assessment, attention to age, frailty, comorbidities, medications, and mental health, and favouring moderate, sustainable protocols over prolonged, severe, or highly restrictive regimens when aiming to harness fasting for health span and longevity [70,77,78].

From Generic Protocols to AI-Enabled Precision Fasting

Inter‑individual variability in biological aging and metabolic regulation makes it unlikely that a single fasting regimen will optimize health span or longevity for all people. Genetic background, baseline insulin sensitivity, body composition, age, sex, gut microbiome features, circadian chronotype, medications, and comorbidities all modulate glycemic, lipid, inflammatory, and hormonal responses to specific fasting patterns, as suggested by heterogeneous effects across fasting trials and multi‑strain animal studies. Recent multi‑omic profiling work in humans undergoing intermittent fasting shows coordinated but individually variable changes across metabolome, proteome, immune markers, and microbiota, underscoring the need for personalized fasting prescriptions rather than uniform protocols [23,70,79].

AI‑driven and multi‑omics approaches offer a framework to operationalize precision fasting by integrating diverse, longitudinal data streams. Candidate inputs include continuous or intermittent biomarker measurements (glucose, ketones, lipids, inflammatory markers, organ‑specific enzymes, biological age clocks), wearable‑derived signals (sleep staging, heart rate variability, resting heart rate, activity and sedentary patterns), and gut microbiome or metagenomic profiles that influence nutrient handling and fasting tolerance. Proof‑of‑concept studies have already used machine‑learning models trained on continuous glucose monitoring and accelerometry data to detect fasting versus feeding states with high accuracy and to quantify adherence to intermittent fasting protocols, demonstrating that sensor‑based, time‑series data can be leveraged to characterize real‑world fasting behaviour objectively. In parallel, AI‑enabled digital health programs combining CGM with behavioural tracking and adaptive feedback have improved glycemic control and weight in people with early metabolic dysfunction, illustrating how algorithmic personalization can support lifestyle interventions at scale [80,81,82,83,84].

These advances lay the foundation for modelling intermittent fasting as a non-pharmacologic, dose-responsive intervention whose effects on chronic disease risk and aging biology can be optimized using predictive analytics. Machine‑learning models trained on large clinical and real‑world datasets could, in principle, estimate the expected cardiometabolic and inflammatory response to different fasting schedules (e.g., early vs late TRE, 5:2 IF, monthly FMD) for a given individual, based on their baseline omics, microbiome profile, and wearable‑derived behaviour patterns, and then iteratively refine these predictions as new data accumulate. In an AI‑health tech setting, this enables closed‑loop fasting programs in which algorithms dynamically adjust fasting windows, macronutrient composition, refeeding strategies, and supportive interventions (such as exercise timing or microbiome‑targeted fibers) in response to short‑term metrics (daily glucose and HRV patterns) and longer‑term trajectories in health span biomarkers and biological age estimates. Conceptually, such systems extend the logic of AI‑driven closed‑loop insulin delivery to lifestyle and fasting prescriptions, aiming to keep individuals within “target ranges” of metabolic and inflammatory control while minimizing adverse effects and maximizing adherence [80,82,83,84,85,86].

Clinical Implementation and Practical Considerations

Clinical implementation of fasting requires a stratified , patient‑centered approach that accounts for individual risk, therapeutic goals, and preferences rather than applying uniform protocols. For metabolically unhealthy but otherwise stable adults, short daily time‑restricted eating (TRE) aligned with circadian rhythms, for example, beginning with a 12:12 pattern (12 hours eating, 12 hours fasting) and gradually progressing to 14:10 is a pragmatic and generally well‑tolerated entry point that can improve glycemic control, blood pressure, and lipid markers without drastic dietary overhauls. Expert guidance and scoping reviews emphasize incremental tightening of the eating window (reducing by 1–2 hours every few days or weeks) to allow physiological adaptation and to maintain adherence, particularly in individuals with metabolic syndrome or non‑alcoholic fatty liver disease [87,88,89,90].

More intensive regimens, such as 16:8 TRE, 5:2 intermittent fasting, or monthly 5‑day fasting‑mimicking diet (FMD) cycles may be appropriate for selected patients with obesity, prediabetes, or high cardiovascular risk when implemented with structured monitoring. Clinical trials of 5:2 IF and related protocols show clinically meaningful weight loss and improvements in insulin resistance, HbA1c, and atherogenic lipids, while FMD cycles in generally healthy or at‑risk adults reduce trunk fat, blood pressure, IGF‑1, fasting glucose, triglycerides, and C‑reactive protein, with the largest benefits seen in those with elevated baseline risk factors. In such patients, regular assessment of body weight, waist circumference, blood pressure, fasting glucose or CGM data, lipids, and adherence is recommended, alongside medication review to adjust glucose‑ and blood‑pressure‑lowering therapies and avoid hypoglycemia or hypotension during fasting days [27,35,87,90,91].

By contrast, prolonged water‑only fasts extending beyond about 3–5 days carry substantially higher risk and should generally be limited to specialized, medically supervised settings, particularly in lean, elderly, or comorbid individuals. Narrative reviews and fasting‑clinic data report that long water-only fasts can produce short‑term improvements in weight and blood pressure but are associated with adverse events such as metabolic acidosis, electrolyte disturbances, hypotension, headaches, and insomnia, with many benefits dissipating within months after refeeding. Safety guidance from clinical and expert sources underscores the importance of pre‑fast evaluation (including electrolytes, renal function, and medication review), daily monitoring, careful refeeding to avoid refeeding syndrome, and caution or avoidance in those with low BMI, advanced age, cardiovascular disease, renal impairment, pregnancy, or a history of eating disorders [73,92].

Across all fasting regimens, attention to foundational lifestyle factors is critical to translate biological benefits into sustainable improvements in health span. Reviews and clinical recommendations stress that fasting should be combined with nutrient‑dense dietary patterns (adequate protein intake, high micronutrient density, limited ultraprocessed foods and added sugars) to preserve lean mass and prevent micronutrient deficiencies, especially in individuals engaging in repeated fasting cycles. Resistance training and regular physical activity are essential adjuncts to prevent sarcopenia, support insulin sensitivity, and improve sleep quality and fatigue profiles when caloric intake is intermittently reduced. Sleep hygiene and circadian alignment (consistent bedtimes, limiting late‑night eating, prioritizing daytime TRE) further enhance metabolic and cardiovascular benefits, while psychological support and screening for disordered eating help ensure that fasting practices remain health‑promoting rather than harmful. In sum, clinically responsible fasting implementation hinges on individualized protocol selection, structured monitoring, and integration with high‑quality nutrition, exercise, sleep, and mental‑health support [36,58,78,87,90,93].

Conclusion

Current evidence supports fasting, particularly intermittent and periodic regimens, as a potent intervention to modulate several key hallmarks of aging, including deregulated nutrient sensing, impaired proteostasis, mitochondrial dysfunction, and chronic low‑grade inflammation. In preclinical models, these dietary strategies consistently extend lifespan and delay the onset of age‑related diseases, while in humans they reliably improve cardiometabolic and inflammatory risk profiles and show emerging neuroprotective and anticancer potential, even though definitive data on human lifespan extension remain incomplete.

At the same time, the very stress‑response and autophagy pathways that mediate fasting’s benefits can become maladaptive when excessively activated or applied to vulnerable populations, emphasizing the importance of individualized, medically supervised implementation rather than universal prescriptions. As AI‑driven analytics, multi‑omic profiling, and continuous wearable data streams mature, fasting can be reframed from a generic wellness trend into a precision, data‑informed longevity intervention that is dynamically tailored and integrated with nutrition, exercise, sleep, and pharmacologic therapies. For clinicians operating at the intersection of preventive medicine, biohacking, and digital health, fasting therefore represents a compelling yet nuanced lever, not only to potentially influence lifespan, but more concretely to expand the proportion of life lived in robust health and functional independence.

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