Almanac A1C

Sardine Fasting: A Protein-Sparing Ketogenic Bridge Between Fasting and Feeding for Metabolic Optimization

Posted

by

dr. Karunia Valeriani Japar

Table of Contents
Keywords: , , , ,

Introduction

Metabolic diseases including type 2 diabetes, obesity, dyslipidemia and non-alcoholic fatty liver disease have reached epidemic levels globally, driving unprecedented rates of premature morbidity and mortality. Recent estimates from the United States demonstrate that the prevalence of metabolic syndrome among adults has surged to nearly 42%, with similar increases documented in Europe and Asia. In the United Kingdom alone, projections for 2025 indicate that over one in three adults will be affected, placing immense strain not only on personal health but also on healthcare systems and national economies. Thie “silent” epidemic profoundly elevates risks for heart disease, stroke, certain cancers, and reduces both quality and length of life, underscoring the urgent need for innovative, evidence-based approaches to prevention and management.

Traditional dietary strategies and pharmacologic intervention have yielded modest success in reversing the underlying drivers of metabolic dysfunction, prompting renewed interest in metabolic therapies, including fasting and nutritional ketosis for disease modification and healthy aging. The scientific understanding of fasting continues to evolve, now encompassing water fasting, intermittent fasting, time-restricted eating, protein-sparing modified fasts (PSMF), and more recently, fasting-mimicking diets (FMD). These approaches capitalize on the body’s ability to switch metabolic fuels (from glucose to fatty acids and ketones), activate cellular stress resistance pathways, and promote autophagy and tissue repair, while minimizing negative consequences associated with chronic energy restriction.

Against this backdrop, sardine fasting has emerged as a novel hybrid protocol, synthesizing principles of fasting, ketogenic nutrition, and nutrient density. By providing high-quality protein and essential fat within a framework of caloric restriction, sardine fasting aims to deliver the metabolic, cellular and longevity benefits of fasting, while actively protecting lean mass and offering unique cardiometabolic advantages. This article explores the scientific basis, practical applications, risks, and benefits of sardine fasting for individuals seeking potent metabolic interventions in pursuit of longevity.

What is Sardine Fasting

Sardine fasting is a specialized dietary protocol involving the exclusive consumption of sardines over a defined period, typically ranging from 1 to 3 days as a means to harness the metabolic benefits of fasting while providing strategic nutritional support. The practical protocol usually entails consuming approximately six cans of sardines daily (totaling about 600-800grams), with water or non-caloric beverages permitted and all other foods excluded. Most practitioners recommend choosing bone-in, water packed sardines for optimal nutrient delivery, though versions in olive oil may be used to increase healthy fat intake if desired. This sardine-centric regimens target a caloric intake in the range of 900-1,200 kilocalories per day and supplies around 100-120 grams of high-quality protein, 60-80 grams of fat (primarily omega-3 fatty acids), and minimal carbohydrates thus achieving deep ketosis and metabolic switching while sparing muscle catabolism [1,2,3,4].

The historical genesis of sardine fasting traces back to a confluence of ketogenic and fasting-based metabolic therapies, particularly those employed in clinical research on cancer and neurological disorders. Dr Dominic D’Agostino popularized sardine fasting after observing robust metabolic and performance outcomes in cancer patient protocols designed by Dr. Fred Hatfield, who would periodically undertake week-long sardine fast outlined by George Blackburn and George Cahill whereby dietary protein was administered without carbohydrates to blunt insulin response and preserve lean tissue, the sardine fasting protocol evolved as a whole-food adaptation with added benefits from the unique nutritional matrix of sardines. Recent years have seen the approach popularized with longevity forums and metabolic health communities, often used as a rapid metabolic reset or alternative to water fasting for those seeking muscle protection, satiety, and enhanced cardiometabolic support [1,4,5,6].

In comparison to other minimalist or single-food fast, sardine fasting is distinguished by its clinical rationale and nutrient profile. Single-food fasting protocols such as egg fasts, potato fasts, or steak only fast, may provide monotrophic dietary simplicity but typically lack the balance amino acid composition, essential fatty acids, and micronutrients (calcium, vitamin D, selenium, taurine) intrinsic to sardine-based regimens. Fat fasting, for instance, relies on extremely high fat intake (80-90% of diet) with minimal protein, which rapidly induces ketosis but carries the risk of muscle protein breakdown and limited micronutrient availability. In contrast, the sardine fast provides both ketogenic stimulus and robust protein for muscle preservation, along with additional metabolic and longevity, promoting compounds found in this unique fish, positioning it as a pragmatic, nutrient-dense evolution of both fasting and minimalist eating paradigms. This blend of deep ketosis, autophagy activation, protein sparing, and comprehensive micronutrient support sets sardine fasting apart as a scientifically-grounded, whole-food intervention for metabolic healthy and vitality [1,2,4,5,7].

Mechanistic Rationale and Theoretical Basis

How Sardine Fasting Bridges Fasting and Feeding

Sardine fasting occupies a unique metabolic niche between complete fasting and conventional feeding, simultaneously activating cellular pathways characteristic of energy deprivation while providing substrates necessary for anabolic processes. This dual activation distinguishes it from traditional binary nutritional states and represents an intentional exploitation of what might be termed “metabolic opportunism”, harnessing adaptive stress responses without triggering catabolic pathways that compromise lean tissue [4,5,6,8,9].

During a complete fast, the body rapidly transitions through distinct metabolic phases: initial glycogenolysis (the breakdown of hepatic and muscle glycogen stores), subsequent reliance on gluconeogenesis from amino acids and glycerol, and finally, establishment of sustained ketogenesis with mobilization of adipose triglycerides. This adaptive sequence ensures survival by providing alternative fuel substrates to glucose-dependent tissues, particularly the central nervous system. However, prolonged fasting inevitably entails proteolysis, the breakdown of skeletal muscle protein to supply amino acids for gluconeogenesis and maintain essential physiological functions. In contrast, the fed state is characterized by insulin secretion, activation of anabolic pathways including the mechanistic target of rapamycin (mTOR), suppression of lipolysis and ketogenesis, and promotion of protein synthesis [8,9,10,11,12,13].

Sardine fasting strategically bridges these states by maintaining caloric restriction sufficient to induce a fasting-like hormonal milieu, low insulin, elevated glucagon, increased catecholamines, while simultaneously providing high-quality protein and essential fatty acids that activate anabolic signaling cascades. The minimal carbohydrate content (typically less than 5-10 grams daily) ensures negligible insulin response, thereby disinhibiting hormone-sensitive lipase in adipose tissue and carnitine palmitoyltransferase-1 (CPT-1) in hepatic mitochondria, facilitating fatty acids mobilization and oxidation. Concurrently, the provision of approximately 100-120 grams of protein delivers essential and branched-chain amino acids that stimulate mTOR signaling and maintain muscle protein synthesis, despite the overall energy deficit. This configuration creates a metabolic paradox: the body perceives energy scarcity (triggering adaptive fasting responses including autophagy, mitochondrial biogenesis, and cellular stress resistance), yet receives nutrient signals (amino acids, omega-3 fatty acids) that prevent excessive catabolism and support tissue maintenance [6,8,10,11,14,15].

The Metabolic Switch: Glycemic Control, Ketosis, and Metabolic Flexibility

The central metabolic transformation induced by sardine fasting involves what has been termed “flipping the metabolic switch}, the transition from glucose-centered to fatty acid ketone centered bioenergetics. This metabolic switch represents a fundamental reprogramming of cellular fuel utilization that confers multiple physiological advantages relevant to metabolic health and longevity [8,16,17].

Figure 1.Summary of The Major Metabolic Pathways Involved in The Metabolic switch and Responses of Excitable Cells to The Ketone b- Hydroxybutyrate (b-OHB)[8]

Under normal dietary conditions, hepatic glycogen stores contain approximately 100 grams of glucose, sufficient to maintain blood glucose for roughly 12-24 hours of fasting. Muscle glycogen, totaling approximately 400grams, serves primarily as a local energy reserve for muscle contraction rather than contributing to systemic glucose homeostasis. Once hepatic glycogen becomes depleted, typically within the first 24 hours of carbohydrate restriction or fasting, the liver upregulates gluconeogenesis to maintain blood glucose at approximately 3.5-4.5 mmol/L , the minimum required to support obligate glucose- consuming tissues such as red blood cells and portions of the renal medulla [9,11].

Simultaneously, the depletion of glycogen stores and reduction in insulin levels trigger a cascade of transcriptional and enzymatic changes that activate fatty acid oxidation and ketogenesis. The insulin-to-glucagon ratio decreases dramatically during fasting or carbohydrate restriction, leading to activation of hormone-sensitive lipase in adipocytes, which hydrolyzes stored triglycerides into free fatty acids (FFAs) and glycerol. These FFAs enter systemic circulation and are taken up by peripheral tissues, including skeletal muscle and cardiac muscle, where they undergo mitochondrial b-oxidation to generate acetyl-CoA and ultimately, ATP [8,9,11].

In the liver, the influx of fatty acids exceeds the capacity of the tricarboxylic acid (TCA) cycle, particularly as oxaloacetate becomes preferentially diverted toward gluconeogenesis. This metabolic bottleneck results in accumulation of acetyl-CoA, which is then shunted into the ketogenic pathway. The enzyme 3-hydroxy-3-methylglytaryl-CoA (HMG-CoA) synthase 2 whose expression is dramatically upregulated during fasting through activation of peroxisome proliferator-activated receptor alpha (PPARa) and fork head box protein A2 (FOXA2), catalyzes the rate-limiting step in ketogenesis. The resulting ketone bodies-primarily b-hydroxybutyrate and acetoacetate are released into circulation, where they reach concentrations of 0.5-3.0 mmol/L during nutritional ketosis (and higher during prolonged fasting or diabetic ketoacidosis) [8,9,11,18].

Ketone bodies serve as efficient alternative fuel substrates, particularly for the brain, which can derive up to 60-70% of tis energy requirements from ketones during sustained fasting. This metabolic adaptation is critical for survival during periods of food scarcity, as it reduces the brain glucose requirement from approximately 120grams daily to as little as 40 grams, thereby minimizing the need for gluconeogenesis from muscle-derived amino acids. Beyond their role as metabolic fuels, ketone bodies function as signaling molecules that modulate gene expression, reduce oxidative stress, enhance mitochondrial function and promote cellular resilience [8,9,11,17,19,20].

The transcriptional programs underlying the metabolic switch are orchestrated by several key regulatory factors including PPARa, sirtuin 1 (SIRT1), sirtuin 3 (SIRT3), AMP-activated protein kinase (AMPK), and cAMP response element-binding protein (CREB). SIRT1, a NAD+- dependent deacetylase activated during the metabolic switch, suppresses hepatic glucose production through inhibition of CREB-regulated transcription coactivator 2 (CRTC2)-mediated gluconeogenesis. Time course analyses demonstrate that SIRT1 activation coincides with the transition from glycogenolysis to ketone production. Directly regulating the acetylation state and activity of mitochondrial enzymes involved in the metabolic switch, including acetyl-CoA synthetase 2, long-chain acyl-CoA dehydrogenase, ornithine transcarbamylase and complex I of the electron transport chain [8,9,17].

In skeletal muscle, the metabolic switch involves coordinated upregulation of genes encoding proteins that mediate fatty acid import and oxidation. The fatty acid transporter CD36 and fatty acid binding proteins (FABPs) are dynamically recruited to the plasma membrane during fasting, analogous to the insulin-stimulated translocation of glucose transporter type 4 (GLUT4) in the fed state. Pyruvate dehydrogenase kinase 4 (PDK4), which phosphorylates and inactivates pyruvate dehydrogenase (the rate-limiting enzyme for glucose oxidation), is dramatically upregulated during fasting, increasing by as much as 13-fold in some studies, thereby actively suppressing carbohydrate oxidation and reinforcing the metabolic switch toward lipid utilization [8,9,21].

The metabolic flexibility conferred by sardine fasting defined as the capacity to efficiently transition between glucose and fatty acid oxidation based on substrate availability and metabolic demand represents a hallmark of metabolic health. Impaired metabolic switching, conversely, is a characteristic feature of metabolic syndrome, type 2 diabetes, and obesity. Studies demonstrate that repeated cycles of fasting or time-restricted feeding enhance the efficiency and rapidity of the metabolic switch; individuals adapted to ketogenic interventions exhibit significantly lower glucose-ketone index (GKI) values and achieve ketosis more rapidly upon carbohydrate restriction compared to metabolically inflexible individuals. This adaptation likely involves epigenetic modifications, increased expression of rate-limiting enzymes in fatty acid oxidation and ketogenesis, and enhanced mitochondrial capacity [8,9,16,22].

Sardine fasting optimizes these adaptations by combining carbohydrate restriction (to trigger the metabolic switch) with provision of omega-3 polyunsaturated fatty acids (which enhance mitochondrial fatty acid oxidation capacity and PPARa activation) and high-quality protein (which maintains metabolic rate and preserves insulin sensitivity). The result is metabolic environment characterized by stable blood glucose in the physiological range (typically 70-90mg/dL), robust ketone production, minimal insulin secretion, enhanced fat oxidation, and preservation of lean tissue, collectively representing an optimal metabolic state for cellular maintenance, repair, and longevity [1,4,5,8,10].

Muscle Preservation: The Protein-Sparing Effect versus Traditional Fasting

The preservation of lean body mass during energy restriction represents one of the most clinically significant advantages of sardines fasting and constitutes its primary distinction from traditional water fasting. This “protein-sparing” effect derives from a constellation of metabolic, hormonal cellular mechanisms that collectively minimize muscle protein breakdown while maintaining or even enhancing protein synthetic capacity [4,6,10].

During complete fasting, skeletal muscle serves as the body’s primary reservoir of amino acids, which are mobilized to support gluconeogenesis, maintain plasma protein concentrations, and sustain synthesis of essential proteins in organs such as the liver, immune system and gastrointestinal tract.  Muscle protein catabolism is mediated primarily through two intracellular proteolytic systems: the ubiquitin-proteasome pathway (which degrades the majority of cellular proteins) and autophagy-lysosomal pathways (which degrade organelles and protein aggregates). While autophagy activation during fasting serves beneficial functions, including removal of damaged mitochondria, clearance of protein aggregates, and cellular quality control, excessive or prolonged autophagy in the absence of adequate protein intake can lead to progressive muscle atrophy and functional decline [6,12,23,24].

The seminal work of George Blackburn and George Cahill in the 1970s established the physiological basis for protein-sparing during fasting. These investigators demonstrated that administration of amino acids or protein (approximately 1.201.5 grams per kilogram of ideal body weight) in the absence of significant carbohydrate resulted in a dramatically smaller insulin response compared to combined protein-carbohydrate administration. Because insulin exerts potent antilipolytic effects, suppressing hormone sensitive lipase and thereby inhibiting mobilization of adipose triglycerides-minimizing insulin secretion is essential for maintaining fatty acid availability and sustaining ketogenesis. The protein sparing modified fast thus achieves a metabolic configuration in which amino acids are provided to support essential protein synthesis without triggering the insulin mediated suppression of lipolysis that would compromise ketone production and fat oxidation [6,12,23,24].

The seminal work of George Blackburn and George Cahill in the 1970s established the physiological basis for protein-sparing during fasting. These investigators demonstrated that administration of amino acids or protein (approximately 1.2–1.5 grams per kilogram of ideal body weight) in the absence of significant carbohydrate resulted in a dramatically smaller insulin response compared to combined protein-carbohydrate administration. Because insulin exerts potent antilipolytic effects suppressing hormone-sensitive lipase and thereby inhibiting mobilization of adipose triglycerides minimizing insulin secretion is essential for maintaining fatty acid availability and sustaining ketogenesis. The protein-sparing modified fast thus achieves a metabolic configuration in which amino acids are provided to support essential protein synthesis without triggering the insulin-mediated suppression of lipolysis that would compromise ketone production and fat oxidation [6,10,11].

Ketone bodies themselves exert direct protein-sparing effects independent of exogenous protein provision. b-hydroxybutyrate and acetoacetate serve as alternative fuel substrates for the central nervous system, myocardium, and skeletal muscle, thereby reducing the body’s glucose requirement and, consequently, the demand for gluconeogenesis from amino acids. Studies fasting humans demonstrate that as ketone body concentrations increase 9typically reaching 3-5 mmol/L after several days of complete fasting), there is a progressive decline in muscle amino acid release, with the largest reduction observed in alanine-the primary gluconeogenetic amino acid. This metabolic adaptation reflects the brain’s increasing reliance on ketones for energy, which diminishes hepatic glucose production requirements from approximately 180 grams daily to as little as 40-80 grams daily [6,9,13].

Growth hormone (GH) represents another critical mediator of muscle preservation during fasting. Fasting induces dramatic increases in GH secretion with elevations of 300–1250% reported after 5–40 days of water-only fasting mediated by ghrelin signalling, reduced insulin and insulin-like growth factor 1 (IGF-1) feedback, and increased hypothalamic GH-releasing hormone. GH exerts direct anabolic effects on skeletal muscle, including stimulation of amino acid uptake, enhancement of protein synthesis, and inhibition of protein degradation. Importantly, studies of GH administration in individuals following restricted diets demonstrate significant reductions in lean body mass loss, even in the absence of changes in fat loss, confirming GH’s specific protein-sparing action [25,26].

Sardine fasting amplifies these protein-sparing mechanisms through provision of high-quality, leucine-rich protein. Leucine, the most abundant branched-chain amino acid in sardines, serves as a potent activator of mTOR complex 1 (mTORC1), the master regulator of protein synthesis. Leucine activates mTORC1 through multiple mechanisms, including binding to Sestrin2 (relieving its inhibition of mTORC1), promoting translocation of mTORC1 to the lysosomal surface (where it encounters its activator Rheb), and enhancing phosphorylation of downstream targets including ribosomal protein S6 kinase and eukaryotic translation initiation factor 4E-binding protein 1. These molecular events culminate in increased translation initiation and elongation, enhanced ribosome biogenesis, and net increases in muscle protein synthesis rates [1,10,14,15,27].

Critically, the relationship between leucine, mTOR, and protein synthesis is concentration- and timing-dependent. Studies demonstrate that muscle protein synthesis is maximally stimulated by leucine concentrations achieved through consumption of approximately 20–30 grams of high-quality protein per meal, with diminishing returns at higher doses. However, the muscle anabolic response to leucine and protein consumption appears to be enhanced during states of energy deficit and elevated catecholamine activity precisely the hormonal milieu created by sardine fasting. Furthermore, the continuous or frequent provision of amino acids (as occurs when consuming multiple cans of sardines throughout the day) may sustain mTOR activation and protein synthesis more effectively than intermittent bolus feeding, particularly in the context of overall caloric restriction [10,14,28].

The protein-sparing modified fast administered via continuous enteral nutrition which most closely approximates the amino acid delivery pattern of frequent sardine consumption has been shown to preserve lean body mass more effectively than the same protein intake provided as discrete meals. This advantage appears to derive from sustained amino acid availability preventing nocturnal muscle catabolism, continuous activation of anabolic signalling pathways (including mTOR and protein kinase B/Akt), and inhibition of AMPK which, despite its beneficial metabolic effects in some contexts, can suppress protein synthesis when excessively activated. Studies comparing continuous versus intermittent amino acid administration confirm that protein synthesis is inhibited and returns to baseline 90–210 minutes after protein ingestion in intermittent protocols, whereas continuous delivery maintains elevated synthesis rates [10].

The whey protein content of sardines merits particular attention in the context of protein sparing. Sardines are rich in rapidly absorbed, highly bioavailable proteins with exceptional amino acid scores, similar to the whey proteins extensively studied in PSMF research. Whey proteins stimulate human muscle protein synthesis more effectively than casein or soy proteins, mediated through rapid amino acid absorption kinetics, high leucine content, and preferential activation of the Akt-mTOR signalling cascade. In the complete absence of carbohydrates as occurs during sardine fasting, the diminished insulin response shifts metabolic balance toward lipid oxidation and inhibits triglyceride synthesis while simultaneously activating triglyceride lipase through glucagon influence. This hormonal environment permits robust fat mobilization and ketogenesis while the continuous amino acid supply supports muscle anabolism [1,10].

Comparative studies of very-low-calorie ketogenic diets versus traditional caloric restriction consistently demonstrate superior lean mass preservation with protein-rich ketogenic approaches. In adolescents with severe obesity, protein-sparing modified fasts achieved significant weight loss without life-threatening side effects and, critically, without the muscle loss typically observed with standard very-low-calorie diets. The continuous enteral nutrition variant (ProMoFasT) demonstrated even greater efficacy, with patients maintaining or increasing lean body mass despite consuming only 600–800 kilocalories daily for extended periods. These outcomes confirm that adequate protein provision, particularly when combined with ketosis, effectively decouples fat loss from muscle loss, an achievement with profound implications for metabolic health, physical function, and longevity [6,10,29,30].

In hibernating mammals, nature’s masters of protein sparing, similar mechanisms operate during months-long periods without food intake. Black bears, for example, experience only 15% protein loss in muscle tissue during the first month of denning, with no further loss occurring over subsequent months despite continued fasting. This remarkable muscle preservation involves dramatic reduction in protein turnover (decreased rates of both synthesis and breakdown), maintenance of oxidative capacity, preservation of slow oxidative muscle fibers, strategic nitrogen recycling, and elevation of protective factors including growth hormone and specific amino acid transporters. While sardine fasting obviously differs from hibernation in duration and physiological context, the principle remains consistent: metabolic adaptations that reduce energy expenditure, provide alternative fuel substrates (ketones), and strategically manage nitrogen economy can preserve functional lean tissue even during substantial energy deficit [6,12].

The net result of these integrated mechanisms, ketone-mediated reduction in gluconeogenesis, growth hormone-stimulated anabolism, leucine-activated mTOR signalling, continuous amino acid provision, and the unique metabolic milieu of low insulin with adequate protein, is that sardine fasting preserves, and potentially enhances, lean body mass while simultaneously inducing robust fat loss, ketosis, and activation of longevity-promoting cellular pathways. This combination represents a significant advance over traditional fasting protocols and positions sardine fasting as a scientifically rational intervention for individuals seeking metabolic optimization without compromising functional capacity or physical performance [4,6,10].

Potential Benefits: Metabolic and Longevity Outcomes

Sardine fasting uniquely combines nutritional ketosis, targeted protein intake, and micronutrient synergy, resulting in a range of metabolic and potential longevity benefits. The described protocol leverages these pathways to promote autophagy and maintain muscle mass, while providing omega-3s, taurine, and minerals that may further support cardiometabolic and neurocognitive health [1,4,5,7].

Ketosis, Autophagy, and Cellular Repair

The caloric restriction and near-complete carbohydrate elimination in sardine fasting rapidly engage hepatic ketogenesis, elevating circulating ketone bodies (primarily β-hydroxybutyrate) to levels typically seen in nutritional or short-term water fasting (0.5–3.0 mmol/L). This metabolic state favours fat oxidation, reduces reliance on glucose, and supports cellular processes associated with stress resilience, including upregulation of antioxidant defences and anti-inflammatory gene networks. Importantly, low glucose and insulin coupled with amino acid provision from sardines help sustain muscle while still permitting autophagy, a process whereby cells degrade and recycle damaged proteins and organelles. This “self-eating” mechanism is crucial for health span, as it limits accumulation of dysfunctional cell components and enhances repair, with translational evidence linking intermittent fasting and mild ketosis to markers of cellular rejuvenation and possibly reduction in age-related pathologies [8,11,17,23,33,35].

Effects on Body Composition, Hepatic Fat, and Insulin Sensitivity

Sardine fasting promotes rapid weight loss predominantly from fat stores, with relative preservation of lean mass due to the high-protein, leucine-rich nature of sardines. This contrasts with water-only or fat-only fasting, which are more likely to result in muscle breakdown, particularly with more extended protocols. Studies of protein-sparing and ketogenic diets show significant reductions in total and visceral fat, with pronounced improvement in hepatic steatosis. In very-low-calorie ketogenic diet (VLCKD) interventions, liver fat fraction decreased by over 4% in two months, independent of overall weight loss attributable to increased hepatic fatty acid oxidation and ketone body production. Improved insulin sensitivity is another consistent outcome; short-term ketogenic or PSMF regimens lower fasting glucose and insulin while stabilizing glycemic variability, supporting reversal or remission of metabolic syndrome features [6,10,29,32].

Omega-3, Taurine, Micronutrients: Cardiovascular and Cognitive Health

Canned bone-in sardines offer a distinct portfolio of long-chain omega-3 fatty acids (EPA, DHA), taurine, calcium, vitamin D, selenium, and other micronutrients rarely matched by single-food fasting protocols. EPA and DHA improve lipid metabolism, lower inflammatory cytokines, reduce triglyceride and LDL cholesterol, and can modestly raise HDL cholesterol, collectively reducing the risk of cardiovascular events. Omega-3s are also critical for maintaining neural plasticity, supporting neurotrophic factors, and exerting anti-inflammatory effects within the CNS links that may partially explain observed cognitive and mood benefits in epidemiological and intervention studies. Taurine, abundant in sardines, has antihypertensive, antioxidative, and neuroprotective properties, contributes to calcium homeostasis, and may reduce the risk of cardiometabolic disease through improvement of endothelial function and mitigation of obesity-induced inflammation. Calcium and vitamin D from sardine bones strengthen skeletal integrity and may help blunt bone loss during caloric deficits [1,7,34,36,37,38].

Appetite Regulation: Role of GLP-1 and Satiety Pathways

Sardine protein consumption has been shown in animal and human studies to increase circulating GLP-1 (glucagon-like peptide-1), a gut-derived incretin hormone involved in appetite regulation and satiety. GLP-1 promotes insulin secretion, delays gastric emptying, and activates central pathways that suppress hunger, mechanisms exploited by GLP-1 agonist drugs for weight management and diabetes control. Enhanced satiety during sardine fasting is also attributed to the high protein load, which stimulates peptide YY and cholecystokinin, and to the stable blood ketone levels, which are themselves appetite-suppressive. This results in sustained adherence to caloric restriction and reduced risk of rebound hyperphagia during and after the protocol [5,8,32,39,40,41].

Sardine Fasting vs Fat Fasting and Other Modalities

ParameterSardine FastingFat FastingPSMFFMDWater Fasting
Duration (typical)1-7 days2-5 days6-12 weeks (up to 6 months)5 days monthly1-7+ days
Daily Calories900-1,2001,000-1,200600-800750-1,100 (day 1: ~1,100; days 2-5:~750)0
Protein (g/day)100-12010-20100-150 (1.2-1.5 g/kg LBM)20-400
Fat (g/day)60-80100-12015-3045-650 (endogenous only)
Carbohydrate (g/day)<5-10<5<20-3040-900
Macronutrient Ratio40% P/ 55% F/5% C10% p/ 85% F/ 5% C60% P/ 20% F/20% C10% P/ 56% F/34%CN/A
Primary Food SourcesSardines (bone-in, water or olive oil packed)Oils, butter, fatty meats, avocado, mayonnaiseLean meat, poultry, fish, eggs, limited vegetablesPlant-based: nuts, olives, vegetables, soups, teasWater, non-caloric beverages only
Ketosis DepthModerate-High (0.5-3.0 mmol/L)Very high (2.0-5.0 mmol/L)Moderate (1.0-3.0 mmol/L)Moderate-high (3.0-8.0 mmol/L)Very high (5.0-8.0 + mmol/L)
Insulin ResponseMinimalMinimalLow-moderateLowMinimal (near zero)
Muscle PreservationExcellentPoorVery goodGoodPoor (progressive loss)
Autophagy ActivationModerate-highHighModerateVery highVery High
GH ElevationHigh (300-1250% increase)HighModerate-highVery highVery High (up to 1250%)
IGF-1 ReductionModerate (30-40%)High (40-50%)Moderate-highVery highVery High (45050%)
Metabolic FlexibilityEnhancedVariableEnhancedEnhancedVariable (impaired if prolonged)
Micronutrient DensityVery high (omega-3. Ca, D, Se, taurine)LowModerate (requires supplementation)Moderate -high (plant polyphenols)None
Adherence/PalatabilityModerateLow-moderateModerate highModerate-highLow (hunger, fatigue)
Primary Clinical UseMetabolic reset, muscle preservation, rapid ketosisBreaking weight plateau, rapid ketosis inductionRapid weight loss, obesity treatment, pre-surgicalMulti-system regeneration, longevity, cellular renewalSpiritual practice, autophagy maximization, metabolic reset
ContraindicationsGout, hyperuricemia, fish allergyEating disorders, gallbladder diseasePregnancy, T1DM, severe organ disease, eating disordersPregnancy, underweight, eating disorders, certain medicationPregnancy, underweight, T1D, eating disorders, elderly
Key Reference[1,4,5][2][10,30,31,32][33][10,13,21,33]

Abbbreviations: P=Protein; F= Fat, C= Carbohydrate; PSMF= Protein-Sparing Modified Fast; FMD = Fasting Mimicking Diet; LBM= Lean Body Mass; T1DM= Type 1 Diabetes Mellitus; GH = Growth Hormone, IGF-1= Insulin like growth factor 1; Ca= Calcium; D= Vitamin ; Se= Selenium

This comparative analysis demonstrates that sardine fasting occupies a unique metabolic position, combining the ketogenic and autophagy-activating properties of traditional fasting modalities with superior muscle preservation through high-quality protein provision and exceptional micronutrient density through whole-food sardine consumption. Unlike fat fasting, which prioritizes rapid ketosis at the expense of muscle tissue, sardine fasting provides leucine-rich protein that activates mTOR signalling and maintains anabolic capacity. Compared to PSMF, sardine fasting offers a simpler whole-food approach with enhanced omega-3 fatty acid content and cardiovascular benefits. While FMD and water fasting may produce more profound IGF-1 suppression and autophagy activation, sardine fasting provides a practical middle path for individuals seeking metabolic benefits without compromising lean mass or tolerating complete food deprivation.

Screening and clinical monitoring are critical components for the safe and effective implementation of sardine fasting, especially given its very-low-calorie, ketogenic, and protein focused nature. Although sardine fasting offers metabolic benefits and muscle preservation advantages, it remains a potent nutritional intervention that can pose risk if applied indiscriminately or without proper oversight.

Screening Recommendations

Before initiating sardine fasting, comprehensive clinical screening should identify contraindications and individual risk factors. Absolute contraindications include pregnancy and lactation, type 1 diabetes mellitus, active eating disorders, severe hepatic or renal impairment, and critical illness. Patients with known fish allergies or a history of gout or hyperuricemia require careful evaluation due to the purine content and potential for uric acid elevation with prolonged sardine consumption. Elderly or frail individuals warrant additional assessment for sarcopenia risk, electrolyte imbalances, and polypharmacy interactions [30,31,32,34].

Pre-fasting evaluation should include:

  • Detailed medical history emphasizing, renal, hepatic, cardiovascular, and musculoskeletal status.
  • Baseline laboratory test: serum electrolytes, renal, and liver function panels, uric acid, complete blood count, fasting glucose, lipid profile, and ketone levels if possible
  • Nutritional assessment to determine baseline micronutrient status and nutritional reserve
  • Medication review to identify agents that may require dose adjustments or cessation during fasting, most notably insulin, sulfonylureas, antihypertensives, and diuretics [30.31].

Clinical Monitoring During Fasting

Continuous monitoring during the sardine fasting period is essential to detect early signs of adverse effects and to ensure metabolic stability. Depending on the protocol duration and patient risk profile, monitoring strategies include:

  • Regular assessment of hydration status and electrolyte balance, with particular attention to sodium, potassium, magnesium, and calcium
  • Daily or every other day ketosis measurement (blood b-hydroxybutyrate) to ensure target metabolic state while avoiding extreme ketosis or ketoacidosis.
  • Symptom surveillance for “keto flu” manifestations such as headache, dizziness, fatigue, constipation, or orthostatic hypotension.
  • Monitoring for signs of muscle cramps, weakness, or neuropathy that may indicate micronutrient deficiencies, particularly magnesium or potassium depletion [1,32].
  • Close observation for uric acid elevation or gout attacks in susceptible individuals
  • Telehealth or in-person check-ins to adjust hydration, electrolyte supplementation, and dietary intake if necessary

For longer duration or repeated cycles, periodic re-evaluation of renal and liver function, as well as muscle mass (e.g., by bioimpedance or functional testing), is advisable to detect any cumulative adverse effects [10,30].

Post-Fasting Refeeding and Follow Up

Refeeding after sardine fasting is a critical phase requiring careful clinical guidance to avoid refeeding syndrome and sustain metabolic benefits. Gradual reintroduction of carbohydrates, balanced protein intake, and adequate hydration and electrolyte replenishment are recommended. Clinical follow-up should assess weight stability, glycemic control, lipid profile, renal function, and musculoskeletal health [6,33].

Individualized plans for integration of sardine fasting within broader nutritional and lifestyle interventions optimize safety and long-term metabolic improvements. Education on recognizing symptoms warranting medical attention and adherence support enhances successful implementation [31,32].

In summary, stringent screening coupled with diligent clinical monitoring and tailored refeeding protocols underpin the safe application of sardine fasting, ensuring metabolic advantages are maximized while minimizing risks [4,5,30,31,32].

Risks, Barriers, and Adverse Effects

While sardine fasting offers distinct metabolic and nutritional advantages over traditional fasting protocols, it is not without relevant risks, barriers, and adverse effects, particularly if misapplied or subjected to prolonged restriction. Careful attention to implementation and clinical context is required to mitigate these concerns and safeguard participant well-being [5,7,10,32].

Risk of Muscle Loss and Inadequate Protein Intake

A foundational rationale for sardine fasting is its capacity to spare muscle through ample, high-quality protein intake. However, if the protocol’s protein or energy provision is insufficient whether due to reduced intake, extended duration, or intercurrent illness, there remains a tangible risk of skeletal muscle catabolism. This risk is exaggerated in frail, sarcopenic, or elderly populations, whose baseline muscle mass is already compromised and who may be more susceptible to anabolic resistance. Extended restriction (beyond 7 days, or repeated shortened cycles without proper refeeding) can exceed the muscle-sparing capacity of dietary protein, leading to progressive lean mass reduction similar to that observed in water fasting, ultimately impairing strength, metabolic rate, and immune function. These concerns underscore the importance of tailoring fasting duration to an individual’s baseline nutritional reserve and actively monitoring functional status and anthropometrics [6,10,30].

Electrolyte Disturbances and Micronutrient Depletion

Sardine fasting, though superior to water-only protocols regarding micronutrient content (notably for calcium, vitamin D, selenium, and omega-3s), is still a very-low-calorie intervention and is prone to precipitating shifts in electrolytes. This is especially pertinent for sodium, potassium, and magnesium, which may be depleted through increased renal excretion during nutritional ketosis or inadequate intake if fluid balance is not deliberately managed. Clinical symptoms such as muscle cramps, palpitations, dizziness, or orthostatic hypotension often signal subclinical disturbances and should prompt immediate assessment and correction. Electrolyte imbalances can be life-threatening in those with underlying cardiac, kidney, or adrenal disorders, thus necessitating baseline and follow-up laboratory monitoring in at-risk groups [1,7,30,32].

Purine Load, Uric Acid and Gout Exacerbation

One unique concern with sardine fasting is the elevated purine content inherent to oily fish, which is metabolized into uric acid. In susceptible individuals, those with a prior history of gout, hyperuricemia, chronic kidney disease, or on certain diuretics, this purine load may precipitate an acute gout flare or worsen existing uric acid burden. Though emerging data suggest that animal protein (particularly fish) may not be as goutogenic as previously feared when consumed within a broader diet, the concentrated exposure over consecutive days warrants caution, and alternative fasting protocols should be considered for such patients [7,34].

Adherence, Palatability, and Psychological Barriers

Sardine fasting, by virtue of its monotrophic design, presents substantial adherence challenges for many participants. The repetitive consumption of a single food (sardines) can induce taste fatigue, nausea, gastrointestinal discomfort, or even outright aversion, ultimately limiting the acceptability and duration of the dietary intervention. Adverse gastrointestinal effects such as bloating, loose stools, or fishy aftertaste have been occasionally reported, particularly with high daily intakes. These palatability barriers may compromise not only short-term adherence but also willingness to repeat the protocol in a cyclical schedule, thereby blunting its putative benefits [1,5].

“Keto Flu” and Other Transient Symptoms

The rapid metabolic transition into ketosis, especially for those accustomed to carbohydrate-rich diets, can precipitate the syndrome colloquially termed “keto flu”. Characterized by headache, fatigue, dizziness, irritability, muscle aches, and mild nausea, this constellation arises from acute shifts in carbohydrate availability, water loss, and sodium depletion. While most symptoms resolve within 3–5 days with adequate hydration and electrolyte supplementation, their intensity can discourage adherence and prompt premature cessation of the fast. Preventive strategies include gradual carbohydrate tapering pre-protocol, increased salt and fluid intake, and magnesium or potassium supplementation as indicated [8,32].

Practical Implementation and Protocol Variations

Typical Schedules: 1-3 Days, Extended Up To 7, and Cyclical Applications

Sardine fasting protocols exhibit substantial flexibility, with duration and frequency tailored to individual goals, metabolic state, and tolerance. The most commonly described approach involves a short term protocol of 1 to 3 days, wherein participants consume approximately six cans of sardines daily (totalling 900–1,200 kilocalories) alongside water and non-caloric beverages such as black coffee or tea, without any designated eating window or time restriction. This brief intervention is designed to rapidly induce nutritional ketosis, mobilize hepatic and visceral fat stores, and “reset” metabolic flexibility without causing significant muscle catabolism or electrolyte disturbances. Such short-duration protocols are particularly suited to individuals seeking an acute metabolic intervention, for example, to overcome a weight-loss plateau, accelerate transition into ketosis after dietary deviation, or facilitate recovery from minor illness or surgical procedures [5].

Extended protocol of 5 to 7 days have been described in specific clinical contexts, most notably by Dr. Dominic D’Agostino referencing Dr. Fred Hatfield’s experience with metastatic cancer, wherein one to two cans of sardines per day were consumed for up to one week at a time. These longer interventions align more closely with fasting-mimicking diet (FMD) paradigms and may confer deeper autophagy activation, more pronounced reductions in insulin-like growth factor-1 (IGF-1), and enhanced cellular repair processes. However, extended sardine fasting necessitates closer clinical monitoring due to cumulative risks of electrolyte depletion, micronutrient insufficiency, adherence fatigue, and though mitigated by protein intake, potential erosion of lean mass if baseline nutritional reserve is limited [5,10,30,33,42]..

Cyclical or periodic applications represent an emerging and potentially optimal strategy for integrating sardine fasting into long-term metabolic health and longevity frameworks. Dr. Hatfield reportedly implemented week-long sardine fasts once monthly while maintaining a ketogenic baseline diet between cycles. This monthly cyclical pattern mirrors the structure of Valter Longo’s fasting-mimicking diet, which involves five consecutive days of caloric restriction (750–1,100 kilocalories) each month, with ad libitum feeding during the intervening weeks. Evidence from rodent and human FMD studies demonstrates that periodic cycles induce multi-system regeneration, selectively reduce visceral fat, preserve or increase lean mass, improve insulin sensitivity, reduce biological age markers, and extend predicted lifespan, effects that continuous caloric restriction does not replicate. Applying this framework to sardine fasting, a practical protocol might involve 3-day sardine fasts performed monthly or quarterly, integrated within a broader ketogenic or low-carbohydrate dietary pattern. Such cyclical interventions capitalize on the refeeding phase to stimulate stem cell proliferation, tissue repair, and anabolic rebound while limiting cumulative metabolic adaptation or adherence burden [4,33,42].

Guidance on Sardine Selection, Preparation, and Palatability Strategies

Successful implementation of sardine fasting depends substantially on appropriate food selection and palatability optimization, given that monotrophic diets inherently challenge adherence. Sardine selection should prioritize nutritional density, sustainability, and individual tolerance. Bone-in sardines are strongly preferred, as the edible bones provide exceptional bioavailable calcium (approximately 350 milligrams per 100 grams), phosphorus, and magnesium nutrients critical for bone health, electrolyte balance, and mitochondrial function during caloric restriction. The bones are sufficiently small and soft to be consumed without awareness or choking risk, and discarding them substantially diminishes the nutritional value of the protocol [1,7].

Wild-caught versus farmed sardines present a nuanced consideration. Wild-caught sardines generally contain higher concentrations of omega-3 fatty acids (EPA and DHA), lower levels of omega-6 polyunsaturated fats, and reduced exposure to contaminants such as polychlorinated biphenyls (PCBs) compared to some farmed varieties. However, species, geographic origin, and specific aquaculture practices exert greater influence on nutrient composition than wild versus farmed status alone. For example, sardines from certain regions or farmed in land-based systems may exhibit omega-3 levels comparable to or exceeding wild-caught counterparts. Pragmatically, selecting sustainably sourced canned sardines from reputable producers, whether wild-caught or responsibly farmed, ensures adequate omega-3 content while minimizing heavy metal exposure, as sardines’ position low on the food chain confers intrinsically low mercury bioaccumulation [34,43].

Preparation format influences both nutrient delivery and palatability. Sardines packed in water minimize caloric density and emphasize protein, making them ideal for individuals prioritizing maximal fat loss and deepest ketosis. Sardines packed in olive oil provide additional monounsaturated fatty acids, enhance fat-soluble vitamin (A, D, E, K) absorption, and may improve palatability for some individuals. While fresh sardines prepared by grilling, baking, or steaming offer superior taste and are less “fishy” than canned varieties, they require access, preparation time, and culinary skill that may be impractical during intensive fasting protocols. Thermal processing studies indicate that baking or steaming best preserves sardines’ nutritional value, including omega-3 fatty acids and protein quality, compared to frying, though differences are modest [1,5,43].

Palatability strategies are essential for sustaining adherence beyond the first 24 hours. Common challenges include taste fatigue, aversion to the “fishy” odour or texture, and gastrointestinal discomfort from repetitive consumption. Evidence-based strategies include [5,44]:

  • Flavour enhancement with zero-or low-carbohydrate condiments: mustard, black pepper, hot sauce (carbohydrate-free variants such as Nando’s or Tabasco), apple cider vinegar, lemon juice, and fresh herbs (parsley, cilantro, dill) can substantially improve palatability without compromising ketosis.
  • Avoiding prolonged visual exposure: anecdotal reports suggest that extended eye contact with sardines during consumption increases nausea; consuming sardines rapidly or without detailed visual inspection may enhance tolerance.
  • Temperature variation: some individuals find chilled sardines more palatable than room-temperature preparations, while others prefer briefly warming them to reduce the “canned” taste.
  • Diversification of sardine brands and packing media: rotating between different sardine products (e.g., tomato-based, olive oil, water-packed) within a single fast may reduce monotony.
  • Strategic meal timing: consuming sardines in response to genuine hunger rather than on a fixed schedule leverages appetite suppression from protein and ketones, reducing overall intake requirements and enhancing satiety

Despite these strategies, a subset of individuals will find sardine fasting intolerable due to intrinsic taste preferences or gastrointestinal sensitivity. In such cases, alternative protein-sparing modified fast (PSMF) approaches using lean meats, eggs, or other fatty fish may represent more sustainable options [10].

Integration With Other Dietary and Lifestyle Interventions

Sardine fasting is most effective when integrated within a broader dietary and lifestyle regimen rather than used in isolation. The protocol can be layered onto ketogenic or low-carbohydrate diets, where individuals already adapted to fat metabolism transition more easily into sardine fasting and achieve more rapid ketosis and appetite suppression. Combining sardine fasts with periodic or cyclical fasting routines (monthly or quarterly) amplifies the metabolic benefits while minimizing adaptation and adherence barriers, drawing from evidence in fasting-mimicking diet (FMD) studies [33,45,46].

Physical activity, particularly resistance-based exercise, remains important during sardine fasting. Maintaining moderate-intensity resistance training supports muscle protein synthesis, counters the catabolic effects of calorie deficit, and preserves lean mass. Lower-intensity aerobic movement and active recovery techniques, such as yoga or walking, further promote cardiovascular and metabolic health without risking fatigue or injury [47,48].

After completing the fast, gradual refeeding with complex carbohydrates, fiber-rich vegetables, and phytonutrient-dense whole foods is recommended to sustain the metabolic improvements and avoid nutrient gaps. Supplementing with sodium, potassium, and magnesium throughout and after fasting helps address electrolyte shifts and “keto flu” symptoms [8,32,33].

Refeeding and Long-Term Sustainability

After completing a sardine fast, refeeding should be gradual to prevent blood sugar spikes, digestive issues, and electrolyte imbalances. Start by reintroducing low-glycemic foods like vegetables, lean proteins, and healthy fats, increasing calories and carbohydrates slowly over several days. Keeping protein intake moderate helps build and maintain muscle, especially important after calorie restriction. Ensuring hydration and supplementing with electrolytes (sodium, potassium, magnesium) supports recovery and prevents cramps or refeeding symptoms [8,32,33].

Long-term, sardine fasting is best used as a periodic reset, rather than as a frequent or sole strategy. Integrating it seasonally or monthly into a balanced, whole-food diet rich in protein, healthy fats, and vegetables allows for ongoing metabolic benefits, while limiting the monotony and risks of prolonged restriction. Regular exercise, especially resistance training, and periodic health monitoring (blood tests, body composition checks) help sustain benefits and catch any nutrient gaps or adverse effects early [30,33].

Conclusion

Sardine fasting represents a novel, evidence-informed approach to metabolic health and longevity, strategically combining the physiological benefits of fasting with the unique nutrient density of whole sardines. This regimen leverages a high-quality protein and omega-3-rich, low-carbohydrate matrix to induce ketosis, promote autophagy, preserve lean muscle, and deliver cardiovascular and cognitive benefits unlikely to be matched by other minimalist or restrictive fasts. Mechanistically, sardine fasting bridges the adaptive stress responses of fasting with ongoing anabolic support, optimizing both metabolic switching and cellular repair without excessive muscle loss.

Importantly, sardine fasting’s safety and efficacy depend on individualized screening, attentive clinical monitoring, and prudent refeeding to prevent muscle catabolism, electrolyte disturbances, or adverse metabolic shifts. Its practical implementation requires thoughtful consideration of schedule, sardine quality, palatability, and integration with broader dietary and lifestyle habits, including resistance training and periodic protocol cycling. When used as an adjunct within a sustainable health framework, sardine fasting can serve as a potent tool for improving body composition, reducing hepatic and visceral fat, enhancing insulin sensitivity, and supporting long-term resilience.

In summary, while not without risks, sardine fasting offers a scientifically grounded, whole-food intervention for those seeking metabolic reset and longevity benefits, provided it is applied in a clinically informed and personalized manner.

References

  1. Santos HO, May TL, Bueno AA. Eating more sardines instead of fish oil supplementation: Beyond omega-3 polyunsaturated fatty acids, a matrix of nutrients with cardiovascular benefits. Frontiers in Nutrition. 2023 Apr 14;10.
  2. West H. What Is Fat Fasting, and Is It Good for You? [Internet]. Healthline. Healthline Media; 2019. Available from: https://www.healthline.com/nutrition/keto-fat-fast
  3. Dabbs R. Sardine Fasting Is the Latest Health Trend – But Experts Warn It’s a Terrible Idea [Internet]. Men’s Health. 2025 [cited 2025 Nov 17]. Available from: https://www.menshealth.com/uk/nutrition/food-drink/a69014131/sardine-fasting-diet/
  4. Norwitz N. Sardine Fasting. The Ultimate Health Hack [Internet]. Substack.com. StayCurious Metabolism; 2025 [cited 2025 Nov 17]. Available from: https://staycuriousmetabolism.substack.com/p/sardine-fasting-the-ultimate-health
  5. Dabbs R. Sardine Fasting Is the Latest Health Trend – But Experts Warn It’s a Terrible Idea [Internet]. Men’s Health. 2025 [cited 2025 Nov 17]. Available from: https://www.menshealth.com/uk/nutrition/food-drink/a69014131/sardine-fasting-diet/
  6. Thomas DD, Istfan NW, Bistrian BR, Apovian CM. Protein Sparing Therapies in Acute Illness and Obesity: A Review of George Blackburn’s Contributions to Nutrition Science. Metabolism: clinical and experimental [Internet]. 2018 Feb 1 [cited 2020 Aug 11];79:83–96. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5809291/
  7. Sardines Benefits: 5 Surprising Ways These Tiny Fish Can Revolutionize Your Health Game – GBC Health [Internet]. GBC Health. 2025. Available from: https://www.gbchealth.org/blog/sardines-benefits/
  8. Anton SD, Moehl K, Donahoo WT, Marosi K, Lee SA, Mainous AG, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring, Md). 2018;26(2):254–68.
  9. Fazeli PK, Steinhauser ML. A critical assessment of fasting to promote metabolic health and longevity. Endocrine Reviews [Internet]. 2025 Jul 23; Available from: https://academic.oup.com/edrv/advance-article/doi/10.1210/endrev/bnaf021/8211151
  10. Formisano E, Schiavetti I, Raffaella Gradaschi, Gardella P, Romeo C, Pisciotta L, et al. The Real-Life Use of a Protein-Sparing Modified Fast Diet by Nasogastric Tube (ProMoFasT) in Adults with Obesity: An Open-Label Randomized Controlled Trial. Nutrients [Internet]. 2023 Nov 17;15(22):4822–2. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10674249/
  11. Dhillon KK, Gupta S. Biochemistry, Ketogenesis [Internet]. National Library of Medicine. StatPearls Publishing; 2023. Available from: https://www.ncbi.nlm.nih.gov/books/NBK493179/
  12. Fabrice Bertile, Habold C, Yvon Le Maho, Giroud S. Body Protein Sparing in Hibernators: A Source for Biomedical Innovation. Frontiers in Physiology. 2021 Feb 18;12.
  13. Casale J, Huecker MR. Physiology, Fasting [Internet]. PubMed. Treasure Island (FL): StatPearls Publishing; 2020. Available from: https://www.ncbi.nlm.nih.gov/books/NBK534877/
  14. Drummond MJ, Rasmussen BB. Leucine-enriched Nutrients and the Regulation of Mammalian Target of Rapamycin Signalling and Human Skeletal Muscle Protein Synthesis. Current Opinion in Clinical Nutrition and Metabolic Care [Internet]. 2008 May;11(3):222–6. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5096790/
  15. Saif ur Rehman, Rahmat A, Zhang H, Muhammad Mubashar Zafar, Wang M. Research progress in the role and mechanism of Leucine in regulating animal growth and development. Frontiers in Physiology. 2023 Nov 17;14.
  16. Hernandez A, Truckenbrod L, Federico Q, Campos K, Moon B, Ferekides N, et al. Metabolic switching is impaired by aging and facilitated by ketosis independent of glycogen. Aging (Albany NY) [Internet]. 2020 May 5 [cited 2022 Mar 28];12(9):7963–84. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7244089/
  17. Mattson MP, Moehl K, Ghena N, Schmaedick M, Cheng A. Intermittent metabolic switching, neuroplasticity and brain health. Nature Reviews Neuroscience [Internet]. 2018 Jan 11;19(2):80–0. Available from: https://www.nature.com/articles/nrn.2017.156
  18. The Nutrition Source. Diet Review: Ketogenic Diet for Weight Loss [Internet]. The Nutrition Source. 2018. Available from: https://nutritionsource.hsph.harvard.edu/healthy-weight/diet-reviews/ketogenic-diet/
  19. Casale J, Huecker MR. Physiology, Fasting [Internet]. PubMed. Treasure Island (FL): StatPearls Publishing; 2020. Available from: https://www.ncbi.nlm.nih.gov/books/NBK534877/
  20. Zhu H, Bi D, Zhang Y, Kong C, Du J, Wu X, et al. Ketogenic diet for human diseases: the underlying mechanisms and potential for clinical implementations. Signal Transduction and Targeted Therapy [Internet]. 2022 Jan 17;7(1). Available from: https://www.nature.com/articles/s41392-021-00831-w
  21. Kolnes KJ, Nilsen F, Steffen Brufladt, Meadows AM, Jeppesen PB, Skattebo Ø, et al. Effects of seven days’ fasting on physical performance and metabolic adaptation during exercise in humans. Nature Communications [Internet]. 2025 Jan 2;16(1). Available from: https://www.nature.com/articles/s41467-024-55418-0#Sec12
  22. Smith RL, Soeters MR, Wüst RCI, Houtkooper RH. Metabolic Flexibility as an Adaptation to Energy Resources and Requirements in Health and Disease. Endocrine Reviews. 2018 Apr 24;39(4):489–517.
  23. Grumati P, Bonaldo P. Autophagy in Skeletal Muscle Homeostasis and in Muscular Dystrophies. Cells. 2012 Jul 26;1(3):325–45.
  24. Casale J, Huecker MR. Physiology, Fasting [Internet]. PubMed. Treasure Island (FL): StatPearls Publishing; 2020. Available from: https://www.ncbi.nlm.nih.gov/books/NBK534877/
  25. Clemmons DR, Snyder DK, Williams R, Underwood LE. Growth Hormone Administration Conserves Lean Body Mass during Dietary Restriction in Obese Subjects*. ˜The œJournal of clinical endocrinology and metabolism/Journal of clinical endocrinology & metabolism. 1987 May 1;64(5):878–83.
  26. Horne BD, Anderson JL, May HT, Bair TL, Le VT, Iverson L, et al. Weight loss-independent changes in human growth hormone during water-only fasting: a secondary evaluation of a randomized controlled trial. Frontiers in endocrinology [Internet]. 2025 Jul;15:1401780. Available from: https://pubmed.ncbi.nlm.nih.gov/39991046/
  27. Dodd KM, Tee AR. Leucine and mTORC1: a complex relationship. American Journal of Physiology-Endocrinology and Metabolism. 2012 Jun 1;302(11):E1329–42.
  28. Rai P. Role of Essential Amino Acids in Protein Synthesis and Muscle Growth. Journal of Biochemistry Research [Internet]. 2023 Aug 31;6(4):1–4. Available from: https://www.openaccessjournals.com/articles/role-of-essential-amino-acids-in-protein-synthesis-and-muscle-growth-16798.html
  29. Cunha GM, Guzman G, Correa De Mello LL, Trein B, Spina L, Bussade I, et al. Efficacy of a 2-Month Very Low-Calorie Ketogenic Diet (VLCKD) Compared to a Standard Low-Calorie Diet in Reducing Visceral and Liver Fat Accumulation in Patients With Obesity. Frontiers in Endocrinology. 2020 Sep 14;11.
  30. Bakhach M, Shah V, Harwood T, Lappe S, Bhesania N, Mansoor S, et al. The Protein-Sparing Modified Fast Diet. Global Pediatric Health. 2016 Jan 22;3:2333794X1562324.
  31. Protein-Sparing Modified Fast Diet Program [Internet]. Cleveland Clinic Abu Dhabi. 2021 [cited 2025 Nov 17]. Available from: https://www.clevelandclinicabudhabi.ae/en/health-hub/health-resource/treatments-and-procedures/protein-sparing-modified-fast-diet-program
  32. Clinic C. What To Know About the Protein-Sparing Modified Fast Diet [Internet]. Cleveland Clinic. 2022. Available from: https://health.clevelandclinic.org/protein-sparing-modified-fast-diet
  33. Brandhorst S, Choi IY, Wei M, Cheng CW, Sedrakyan S, Navarrete G, et al. A Periodic Diet that Mimics Fasting Promotes Multi-System Regeneration, Enhanced Cognitive Performance, and Healthspan. Cell metabolism [Internet]. 2015;22(1):86–99. Available from: https://www.ncbi.nlm.nih.gov/pubmed/26094889?dopt=Abstract
  34. Are sardines good for you? Nutritional benefits and more [Internet]. www.medicalnewstoday.com. 2020. Available from: https://www.medicalnewstoday.com/articles/are-sardines-good-for-you
  35. Hofer SJ, Ioanna Daskalaki, Bergmann M, Jasna Friščić, Zimmermann A, Mueller MI, et al. Spermidine is essential for fasting-mediated autophagy and longevity. Nature Cell Biology [Internet]. 2024 Aug 8; Available from: https://www.nature.com/articles/s41556-024-01468-x
  36. Banaszak M, Małgorzata Dobrzyńska, Kawka A, Górna I, Dagmara Woźniak, Juliusz Przysławski, et al. Role of Omega-3 fatty acids eicosapentaenoic (EPA) and docosahexaenoic (DHA) as modulatory and anti-inflammatory agents in noncommunicable diet-related diseases – Reports from the last 10 years. Clinical Nutrition ESPEN. 2024 Oct 1;63:240–58.
  37. Qaradakhi T, Gadanec LK, McSweeney KR, Abraham JR, Apostolopoulos V, Zulli A. The Anti-Inflammatory Effect of Taurine on Cardiovascular Disease. Nutrients. 2020 Sep 17;12(9):2847.
  38. Xu YJ, Arneja AS, Tappia PS, Dhalla NS. The potential health benefits of taurine in cardiovascular disease. Experimental & Clinical Cardiology [Internet]. 2021;13(2):57. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC2586397/
  39. Dailey MJ, Moran TH. Glucagon-like peptide 1 and appetite. Trends in Endocrinology & Metabolism [Internet]. 2013 Feb;24(2):85–91. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3594872/
  40. Zheng Z, Zong Y, Ma Y, Tian Y, Pang Y, Zhang C, et al. Glucagon-like peptide-1 receptor: Mechanisms and Advances in Therapy. Signal Transduction and Targeted Therapy. 2024 Sep 18;9(1):1–29.
  41. Cleveland Clinic. GLP-1 agonists [Internet]. Cleveland Clinic. 2023. Available from: https://my.clevelandclinic.org/health/treatments/13901-glp-1-agonists
  42. Longo VD, Di Tano M, Mattson MP, Guidi N. Intermittent and periodic fasting, longevity and disease. Nature Aging [Internet]. 2021 Jan;1(1):47–59. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8932957/
  43. Macri I. How To Eat Sardines & Why You Should Start Today [Internet]. Cooked & Loved. 2020. Available from: https://www.cookedandloved.com/recipes/how-to-eat-sardines-why-you-should-start-today/
  44. Day IA. In A Day Naturopathy [Internet]. In A Day Naturopathy. 2021 [cited 2025 Nov 17]. Available from: https://www.inadaynaturopathy.com.au/recipes/sardines-little-is-size-big-in-nutrients
  45. Lichtash C, Fung J, Ostoich KC, Ramos M. Therapeutic Use of Intermittent Fasting and Ketogenic Diet as an Alternative Treatment for Type 2 Diabetes in a Normal Weight woman: a 14-month Case Study. BMJ Case Reports. 2020 Jul;13(7).
  46. Kubala J. Intermittent fasting and keto: Should you combine the two? [Internet]. Healthline. 2018. Available from: https://www.healthline.com/nutrition/intermittent-fasting-and-keto
  47. Centers for Disease Control and Prevention. Physical Activity and Your Weight and Health [Internet]. Healthy Weight and Growth. CDC; 2024. Available from: https://www.cdc.gov/healthy-weight-growth/physical-activity/index.html
  48. Anthonius T, Van DW, Phillips SM. The impact and utility of very low-calorie diets: The role of exercise and protein in preserving skeletal muscle mass. Current Opinion in Clinical Nutrition and Metabolic Care [Internet]. 2023 Sep 8;26(6). Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC10552824/