Fasting Game Plan

This page is about translating the biology of fasting into a practical approach you can use. The science from Fasting Basics and Ketosis tells us what’s happening; this page tells us how to think about applying it, given your specific goals, life stage, and circumstances. The detailed protocols for each fasting variant live in the Cheatsheet; this page is about matching the right protocol to the right person at the right time.

The Default Position

For most metabolically healthy adults, the baseline fasting approach is straightforward: eat within a roughly 8-12 hour window during daylight hours, stop eating 2-3 hours before bedtime, and don’t snack between meals. This pattern, sometimes called time-restricted eating (TRE), produces most of the metabolic benefits of more intensive fasting protocols without requiring anything beyond a slight shift in eating timing.

The mechanistic case for this baseline draws from Satchidananda Panda’s lab at the Salk Institute, which has done substantial primary research on time-restricted eating in mice and humans. The findings:

• The body’s metabolic and cellular machinery operates on circadian cycles, with different processes upregulated at different times of day
• Eating during the body’s expected feeding window (broadly: daylight hours for diurnal species like ours) aligns with these cycles
• Eating outside this window (particularly late at night, but also extreme early morning eating in some people) disrupts the alignment and produces metabolic dysfunction over time
• The cycling between fed and fasted states across a 24-hour day appears to be the design rather than perpetual feeding

In Panda’s mouse studies, animals fed the same total calories of the same diet showed dramatically different metabolic outcomes depending on whether they ate freely across 24 hours or during a restricted window. The time-restricted animals were leaner, had better insulin sensitivity, lower inflammation, and longer lifespans. The window timing was also important: eating during the active phase (night for mice, day for humans) produced better outcomes than eating during the rest phase.

Human studies are smaller and shorter but generally directionally similar. The Sutton/Peterson 2018 trial at the Pennington Biomedical Research Center showed that early time-restricted feeding (6-hour eating window, finishing dinner before 3 PM) produced substantial improvements in insulin sensitivity, blood pressure, and oxidative stress in pre-diabetic men, even without weight loss. The protocol was experimentally extreme; the implication is that the timing of eating matters independently of how much you eat.

The everyday version of this for most people: stop snacking, eat your meals within a reasonable daytime window (perhaps 8-10 hours), don’t eat within 2-3 hours of bed, and let the body do what it’s evolved to do during the overnight fast. This isn’t a “fasting protocol” so much as the return to a default eating pattern that humans throughout history have largely maintained until very recently.

The Beginner’s Path

Most popular fasting content jumps directly into specific protocols (16:8 leangains, OMAD, alternate-day fasting). This is often counterproductive for people new to fasting because aggressive protocols on a metabolically unflexible system produce miserable initial experiences and high quit rates.

A more sensible progression:

• Step 1 – Stop snacking: Just this. Three meals a day, no snacks, no caloric beverages between meals. Two weeks. This alone produces measurable metabolic improvements and prepares the body for longer fasted periods. Many people discover they need fewer meals than they thought they did once they stop grazing.
• Step 2 – 12-hour overnight fast: Finish dinner by 8 PM, don’t eat until 8 AM. Most people do this anyway without thinking about it. Two weeks at this baseline before extending.
• Step 3 – 14:10: Finish dinner by 7 PM, don’t eat until 9 AM. (Or whatever 14-10 split fits your schedule.) Two to four weeks.
• Step 4 – 16:8: Finish dinner by 6 or 7 PM, don’t eat until 10 or 11 AM. This is the popular “16:8 leangains” protocol popularised by Martin Berkhan. Two to four weeks.
• Step 5 – Extend or settle: From 16:8, you can extend toward 18:6 or 20:4 (the “Warrior Diet” framing), settle into 16:8 as your long-term default, or begin experimenting with periodic longer fasts (24-hour, 36-hour) while maintaining a daily TRE baseline.

The progression should be slower if you have substantial metabolic dysfunction (insulin resistance, type 2 diabetes, obesity, chronic inflammation), faster if you’re already metabolically healthy and adaptable. The point isn’t to optimise the progression timeline; it’s to give your body time to develop metabolic flexibility before challenging it with longer fasts.

Matching Protocol to Goal

Different fasting protocols suit different objectives. Protocols that work brilliantly for one goal may be counterproductive for another.

General health and metabolic baseline

• 16:8 daily time-restricted eating is the default. Stops snacking, keeps insulin baseline lower, produces overnight ketosis, allows daily MMC cycling for gut health, and is sustainable indefinitely without major lifestyle disruption. Suitable for the majority of adults indefinitely.
• No need for longer protocols unless you have specific goals beyond general health.

Building metabolic flexibility

• 16:8 baseline, plus periodic longer fasts to develop deeper fat-oxidation capacity. A 24-36 hour fast every 1-2 weeks accelerates keto-adaptation and develops the capacity to function well without food for extended periods. Once metabolically flexible, you can dial back to the 16:8 baseline.
• A 3-5 day fast or fasting-mimicking diet protocol once or twice a year extends the adaptation further and accesses the stronger therapeutic effects of prolonged fasting.

Fat loss

The relationship between fasting and fat loss is quite complex. Fasting can support fat loss, but doesn’t automatically cause it. The mechanism is straightforward: fasting reduces insulin, which removes a major brake on lipolysis (fat mobilisation from adipose tissue). But mobilising fat from storage doesn’t equate to net fat loss unless overall energy balance is also negative.

What fasting does well for fat loss:

• Makes caloric deficit easier by reducing meal frequency without willpower-intensive portion control
• Improves insulin sensitivity over time, making the body more responsive to satiety signals
• Reduces baseline insulin, which improves the metabolic environment for fat mobilisation
• Tends to preserve lean mass better than continuous caloric restriction (especially in keto-adapted individuals)
• Develops metabolic flexibility, reducing the dependence on regular meals to maintain energy

What fasting doesn’t reliably do for fat loss:

• Override caloric balance. People who fast and then overeat during eating windows don’t lose fat
• Produce automatic weight loss without attention to food quality and quantity
• Work indefinitely without plateaus. Body composition is sensitive to multiple factors

Practical fat loss protocols:

• 16:8 daily TRE with modest caloric deficit during eating windows
• OMAD (one meal a day) for people who do well with single meals
• Periodic 24-36 hour fasts (1-2 per week) for additional deficit without daily restriction
• 5-7 day fasting-mimicking diet protocols 1-2 times per year for substantial fat loss in shorter timeframes

A note on body composition: fasting protocols tend to work better for people with substantial fat to lose. The leaner you are, the harder fasting becomes for further fat loss, and the more important attention to protein, training, and recovery becomes. Athletes and lean individuals often do better with less aggressive fasting and more attention to overall eating quality.

Longevity

The longevity protocols draw most heavily on Valter Longo’s research at USC. The general approach:

• Daily 12-14 hour TRE as baseline
• Periodic 3-5 day fasting-mimicking diet (FMD) 2-4 times per year
• Plant-forward eating during eating windows with regular fish consumption
• Mediterranean dietary pattern as the eating-window default

The mechanistic case combines time-restricted eating (circadian alignment, daily autophagy), periodic prolonged fasting (deeper autophagy, immune system regeneration, IGF-1 suppression), and dietary quality (anti-inflammatory food matrix, micronutrient density). Longo’s clinical trial data on FMD shows sustained improvements in cardiovascular and metabolic risk markers, with effects persisting weeks to months after the protocol concludes.

Cognitive performance and brain health

Mark Mattson’s work suggests the combination of regular time-restricted eating with periodic longer fasts produces the most substantial cognitive benefits, through the BDNF elevation, ketone provision, and neuroprotective mechanisms covered in Fasting Basics. 

• 16:8 TRE daily
• One or two 24-36 hour fasts per month
• Occasional 3-5 day protocols (Longo’s FMD or equivalent) 1-2 times per year
• Adequate protein during eating windows to support neurological function
• Periodic ketogenic phases to access ketone neuroprotection if not contraindicated

Therapeutic applications (type 2 diabetes, metabolic syndrome, autoimmune conditions)

These warrant medical guidance rather than self-experimentation. Jason Fung’s clinical work with type 2 diabetes patients in Toronto has demonstrated that intermittent fasting protocols can reverse type 2 diabetes in many cases. The protocols are aggressive (often involving multiple 24-72 hour fasts per week alongside ketogenic eating) and require careful monitoring of medications. The Hallberg/Virta Health approach (covered in Ketosis) uses continuous remote care with a ketogenic diet rather than fasting per se, but achieves similar therapeutic outcomes.

For autoimmune conditions, fasting protocols have shown promise in several small clinical trials, including for multiple sclerosis (Longo’s group), rheumatoid arthritis, and inflammatory bowel disease. The mechanisms involve reduced inflammation, immune cell turnover, and the metabolic shifts that affect autoimmune signalling. Anyone with autoimmune conditions should work with practitioners experienced in fasting protocols rather than self-prescribing.

Sex Differences and Hormonal Considerations

The fasting literature has historically been dominated by male subjects and male researchers, and the resulting protocols haven’t always translated cleanly to women’s physiology. 

Reproductive hormones are more sensitive in women: The female reproductive system is highly responsive to energy availability signals. Substantial caloric restriction or extended fasting can disrupt menstrual cycles, suppress ovulation, and reduce fertility in women in ways that don’t have direct parallels in men. The mechanism involves the hypothalamic-pituitary-gonadal axis sensing low energy availability and downregulating reproductive function to preserve energy for survival.

This isn’t unique to women (the female athlete triad and Relative Energy Deficiency in Sport affect both sexes, as covered in Energy Systems), but the threshold for hormonal disruption appears to be lower in women, particularly women who are already lean, athletic, or under substantial life stress.

Cycle phase affects fasting tolerance: Women’s hormonal responses to fasting vary across the menstrual cycle. The follicular phase (first half of the cycle, lower estrogen and progesterone) tends to produce better fasting tolerance, with more stable energy and better cognitive performance during fasting periods. The luteal phase (second half, higher progesterone) often produces increased hunger, lower fasting tolerance, and more pronounced effects on mood and energy during fasted periods. Many women find that adjusting fasting intensity across the cycle (more aggressive in the follicular phase, less aggressive in the luteal phase) produces better results than maintaining the same protocol throughout.

Stacy Sims’ framework: Stacy Sims, drawing on her primary research and the broader literature, has been the most influential voice articulating how fasting protocols should be modified for women. Her general position: women can benefit substantially from fasting and metabolic flexibility work, but the protocols that work for men (aggressive 16:8 or longer, sustained ketogenic eating, frequent extended fasts) often don’t translate well to women’s physiology. Modified approaches typically work better:

• Shorter daily fasting windows (12-14 hours often work better than 16+ for many women)
• Less frequent extended fasts (one or two 24-hour fasts per month rather than weekly)
• Cycling fasting intensity across the menstrual cycle
• Maintaining adequate carbohydrate intake (women’s bodies respond more poorly to chronic low-carb than men’s on average)
• Particular attention to protein adequacy

These are general patterns rather than universal rules. Individual variation is substantial, and some women do exceptionally well on aggressive fasting protocols. The point isn’t that women shouldn’t fast; it’s that the default protocols developed primarily on male subjects shouldn’t be applied uncritically.

The framework’s position: Women should generally start with shorter, less aggressive protocols than the popular literature suggests, pay attention to menstrual cycle effects, and adjust based on individual response. Loss of menstrual cycle, persistent fatigue, sleep disruption, mood changes, or other signs of HPA-axis dysfunction are signals to dial back rather than push through.

The Lifecycle Question

Childhood and adolescence

Children and adolescents generally shouldn’t engage in deliberate fasting protocols. Growing bodies have substantially different metabolic demands than adult bodies, and the long-term effects of imposing fasting on a developing system are insufficiently studied. The specific applications of ketogenic dietary protocols for refractory pediatric epilepsy are well-established and medically supervised; routine fasting for general health benefits in children isn’t.

The reasonable approach for children: regular meal timing, no constant snacking, no caloric beverages between meals, and the natural overnight fast. This produces appropriate metabolic cycling without imposing structured fasting. Children who eat balanced meals at reasonable times and don’t constantly snack are getting the relevant benefits without needing protocol design.

We don’t know enough about deliberate fasting protocols in children to recommend them outside specific medical applications.

Pregnancy

The evidence base for fasting during pregnancy is limited, and the available evidence suggests caution. Research on Ramadan fasting during pregnancy (a substantial natural experiment given hundreds of millions of pregnant Muslim women fasting during Ramadan each year) has produced mixed findings on birth outcomes, with some studies showing increased risk of low birth weight, others showing no effect, and substantial heterogeneity by trimester and individual context.

We don’t know enough about the safety of fasting during pregnancy to recommend it. The substantial metabolic and developmental demands of fetal growth, the changes in maternal metabolism throughout pregnancy, and the limited research base together justify caution. The natural overnight fast (12-14 hours) that most pregnant women experience anyway is probably fine; deliberate extension into longer fasting protocols isn’t well-supported.

Women trying to conceive should also approach aggressive fasting cautiously. Conception requires adequate energy availability; substantial caloric deficit or aggressive fasting can suppress ovulation.

Breastfeeding

Similar caution applies. Milk production requires substantial caloric and nutritional input. Aggressive fasting during breastfeeding can reduce milk supply and may affect milk composition. Limited research is available; the conservative position is to avoid extended fasting protocols during active breastfeeding and to maintain adequate caloric intake.

Modest time-restricted eating (12-14 hours) probably doesn’t disrupt breastfeeding in most women. Longer fasts (24+ hours) likely shouldn’t be attempted during active breastfeeding without specific medical guidance.

Adult years (roughly 18-60)

The standard adult population is where most fasting research has been conducted and where the protocols translate most readily. The protocols covered above (16:8 TRE, periodic longer fasts, FMD, therapeutic protocols for specific conditions) are generally suitable for this population with appropriate individual modification.

Older adults (60+)

Older adults can fast safely, but should pay particular attention to several considerations:

• Protein adequacy. Sarcopenia (age-related muscle loss) accelerates substantially after age 60. Fasting protocols that reduce overall protein intake can exacerbate sarcopenia. Older adults practising fasting should ensure adequate protein during eating windows, with the per-meal protein threshold higher than for younger adults (30-40g per meal rather than 20-30g) to overcome age-related anabolic resistance, as covered in The Longevity Program.
• Medication interactions. Older adults are more likely to be on medications that interact with fasting (diabetes medications, blood pressure medications, certain psychiatric medications). Medical guidance is essential.
• Hydration and electrolytes. Older adults are more vulnerable to dehydration and electrolyte imbalance. Adequate sodium intake during fasting is particularly important.
• Bone health. Some evidence suggests that aggressive caloric restriction in older adults can accelerate bone loss. Adequate calcium, vitamin D, vitamin K2, and weight-bearing exercise become particularly important.
• Less aggressive protocols. Older adults generally do better with moderate TRE (12-14 hours) rather than aggressive 16:8 or longer. Periodic longer fasts can still be valuable, but should be approached more conservatively.

The substantial longevity benefits of fasting are particularly relevant for older adults; the practical implementation should be gentler than what younger adults can tolerate.

Seasonal Feast-Famine Cycling

One of the more interesting framings of fasting comes from looking at how traditional populations actually ate before industrialisation removed seasonality from the food supply. Hunter-gatherer and traditional agricultural populations didn’t fast as a discrete optimisation protocol; they cycled through periods of abundance and scarcity that the seasons imposed.

The pattern in traditional populations: Weston Price’s documentation of traditional populations in the 1930s revealed a striking pattern across cultures: regular cycling between feast and famine that mapped onto seasonal food availability. 

• Northern populations (Inuit, Sami, Native American Plains tribes) ate predominantly fatty animal foods through long winters with limited plant availability, then took advantage of seasonal plant foods, berries, and fish runs during shorter summers
• European agricultural populations followed seasonal cycles tied to harvest periods: feast at harvest, gradually leaner foods through autumn and winter, lean spring as winter stores depleted before the new growing season, then abundance returning in summer and early autumn
• Tropical populations cycled with monsoon and dry seasons, with major variations in plant food availability
• Hunter-gatherer populations everywhere experienced unpredictable cycles depending on hunt success, with regular days or weeks of leaner eating between successful hunts

The cycling was imposed by the environment. The human body developed in this context of regular cycling between abundance and scarcity, fat-rich and lean periods, feast and fast.

What changed: Industrial agriculture, refrigeration, and global food distribution removed seasonality from the food supply for most modern populations. Whatever the season, whatever the local climate, you can buy any food at any time. The cycling that the body evolved to expect has been smoothed into a constant abundance that doesn’t exist anywhere in our evolutionary history.

This may contribute to several of the metabolic disorders that have become prevalent. Constant abundance produces constant insulin elevation, constant inflammation, and the elimination of the seasonal lean periods during which the body would have been preferentially burning stored fat, doing repair work, and accessing the adaptations that fasting triggers.

A modern protocol that approximates seasonal cycling: Dr. Daniel Pompa’s 5-1-1 protocol is one practitioner approach that approximates the pattern within a weekly framework:

• Five days per week: lower-carbohydrate, possibly ketogenic eating (mimicking lean season)
• One day per week: 24-hour fast (mimicking acute scarcity)
• One day per week: feast day with higher carbohydrates and varied food (mimicking abundance)

Variations on this pattern (4-2-1, 2-2-3) work similarly. The point isn’t precise adherence to a specific pattern but cycling between higher-fat, lower-carb periods, fasted periods, and refeeding periods.

A seasonal protocol following the actual seasons: A more ambitious approach maps eating to local seasonality:

• Spring: Lighter eating, increased fasting, leveraging the natural shift from winter stores to the new growing season. Fresh greens, sprouts, early vegetables, and less meat.
• Summer: Abundance of plant foods, berries, fruits. More carbohydrate tolerance because of the metabolic state and the natural availability of carb-rich foods. Less fasting, more eating windows. This is when traditional populations would have been refeeding from winter leanness.
• Autumn: Transition season. Increasing fat-rich foods (tubers, squash, nuts, fattier meat) as plants finish their growing cycle. Beginning to shift toward heavier eating in anticipation of winter.
• Winter: Higher fat, lower carbohydrate eating dominated by stored foods (root vegetables, fermented vegetables, meat, dairy). More extended fasting periods. Less variety. Closer to ketogenic eating.

This approach connects fasting practice to the broader seasonal eating framework covered in Macronutrient & Hydration Basics and The “Natural” Diet. It treats fasting not as an isolated optimisation protocol but as part of a coherent seasonal relationship with food that connects to how humans actually lived for most of our species’ existence.

The practical implementation depends substantially on your local climate. People in temperate northern climates have the clearest seasonal differentiation. People in tropical climates have less seasonal variation in food availability but may still benefit from cycling between higher-carb and lower-carb periods to maintain metabolic flexibility. The principle is cycling between abundance and scarcity rather than rigid adherence to specific seasonal foods.

Training and Fasting

The intersection of fasting and exercise is complex and worth treating in some detail. The substantial framework lives in Training Specificity and Testing under the training-fasted section; the high-level synthesis for matching fasting protocols to training goals lives here.

The general principles:

• Low-intensity aerobic work (walking, easy cycling, zone 1-2 training) is generally fine and often beneficial when done fasted, particularly once keto-adapted. Fat oxidation is the primary fuel; fasting accesses fat stores more readily than fed-state training.
• High-intensity work (sprinting, heavy lifting, intervals at or near VO2 max) is generally better fed, particularly for performance. The phosphocreatine and glycolytic energy systems used in high-intensity work don’t draw on fat stores meaningfully; ketones can’t replace glucose in anaerobic conditions; performance typically suffers when fasted.
• Resistance training in the morning before breaking the fast is workable but produces somewhat reduced performance for most people, particularly in the early stages of fasting practice. The Targeted Intermittent Fasting protocol (covered in detail in the Cheatsheet) can resolve this by providing protein during the workout to mitigate catabolism.
• The cyclical relationship between training and refeeding is what produces results. Train hard, refeed appropriately, fast during recovery, train hard again. The cycling between catabolic stimulus (training, fasting) and anabolic response (refeeding) drives adaptation in ways that constant feeding doesn’t.

The matching of training to the fasting state depends substantially on the goal. Endurance athletes building mitochondrial density benefit from periodic fasted training. Power and strength athletes typically perform and adapt better when fed. Recreational exercisers can do either depending on personal preference and observed response.

Common Pitfalls

The patterns that produce poor outcomes with fasting:

• Overeating during eating windows: The most common failure mode. Restricting eating to a window then dramatically overeating during it produces no metabolic benefit and can produce worse outcomes than regular eating. The metabolic effects of fasting depend on cumulative time in the fasted state and on not overwhelming the body with massive refeeding.
• Drinking calories during the fasting window: Bulletproof coffee with butter and MCT oil produces some legitimate effects (sustained energy, reduced hunger, partial fat-burning maintenance) but isn’t actually fasting. It breaks autophagy, raises insulin moderately, and prevents access to the deeper fasted-state adaptations. If you’re using bulletproof coffee, recognise you’re not fasting in the strict sense; you’re doing a low-insulin breakfast.
• Inadequate electrolyte management: Sodium, magnesium, and potassium losses during fasting (substantial because of reduced insulin’s effect on kidney sodium retention) produce most of the “keto flu” symptoms that make fasting miserable. Adequate sodium intake (often 2-3 grams above typical) during fasting periods substantially improves the experience. The Cheatsheet covers electrolyte protocols in detail.
• Combining aggressive fasting with aggressive training: Both impose substantial physiological stress. Combining them aggressively (multiple high-intensity training sessions per week alongside extended fasts) often produces overtraining, immune dysfunction, and hormonal disruption. The cycling matters: train hard when fed, fast when recovering, alternate.
• Combining fasting with chronic life stress: Fasting is itself a hormetic stressor. Adding it to an already high-stress baseline (chronic work stress, sleep deprivation, relationship turmoil, financial pressure) can push the HPA axis into dysfunction. The “fasting makes me feel terrible” experience often traces to this combination rather than to fasting per se.
• Treating fasting as a license to eat poorly: Some people use fasting to compensate for poor food quality during eating windows. This is partially counterproductive. The metabolic benefits of fasting are substantially enhanced by good eating quality during the fed state.
• Sustained aggressive fasting without breaks: Even good interventions become counterproductive when sustained too long without recovery. Continuous 16:8 daily plus weekly 24-36 hour fasts plus periodic 5-day fasts plus aggressive caloric restriction during eating windows is too much. The body needs feast periods to refeed properly and to access the anabolic processes (muscle protein synthesis, hormonal restoration, tissue repair) that fasting suppresses.
• Ignoring negative signals: Persistent fatigue, mood disturbance, sleep disruption, libido loss, menstrual cycle disruption, hair shedding, cold intolerance, and persistent gastrointestinal symptoms are signals that fasting isn’t working in your current context. Adjusting protocols (less aggressive, fewer extended fasts, different timing, more refeeding) usually resolves these. Pushing through often produces worse outcomes than backing off.

Who Shouldn’t Fast

Absolute contraindications (avoid fasting):

• Active eating disorder or recent eating disorder history
• Type 1 diabetes (extreme caution required if attempted; medical supervision essential)
• Pregnancy
• Breastfeeding (extended fasts; brief overnight fasts are fine)
• Children under 18 (outside specific medical applications)
• Severe medical conditions where caloric or nutrient intake is medically essential

Relative contraindications (proceed cautiously, ideally with medical guidance):

• Type 2 diabetes on medications (medications need adjustment)
• Adrenal insufficiency or severe HPA-axis dysfunction
• Hypoglycemic tendency
• Chronic kidney disease
• History of gallbladder disease
• Underweight or BMI under 20
• Significant chronic stress or major life crisis
• Active infection or recent illness
• Recovery from surgery
• Substantial sleep deprivation
• High-volume athletic training

Conditions warranting careful consideration:

• Thyroid dysfunction (fasting can affect thyroid function)
• Mood disorders (effects vary; some people improve substantially, others worsen)
• Female reproductive disorders (PCOS often improves; amenorrhea can worsen)
• Heart conditions (medication interactions, electrolyte concerns)
• Older adults at risk of sarcopenia (modified protocols only)

If you have a chronic medical condition, are on medications, or have substantial physiological stress in your life, work with practitioners experienced in fasting rather than self-prescribing aggressive protocols. 

Summary

For the general adult population without specific health concerns:

• Stop snacking. Three meals or fewer per day.
• Maintain a 12-14 hour overnight fast as the baseline.
• Don’t eat within 2-3 hours of bed.
• Extend toward 16:8 if it suits your schedule and you tolerate it well.
• Consider an occasional 24-hour fast (once a month or so) to maintain metabolic flexibility.
• Consider a 3-5 day fasting-mimicking diet 1-2 times per year for deeper adaptations.
• Adjust to your life stage, sex-specific considerations, and individual response.
• Track how you feel rather than rigidly adhering to protocols.

For specific goals:

• Fat loss: 16:8 with modest caloric awareness during eating windows, possibly OMAD if it suits you, periodic 24-36 hour fasts.
• Longevity: 12-14 hour TRE baseline, periodic 3-5 day FMD protocols, attention to dietary quality.
• Cognitive performance: 16:8 baseline, occasional longer fasts, ketogenic phases for direct ketone effects.
• Therapeutic applications: Work with practitioners experienced in fasting; don’t self-prescribe aggressive protocols.

For life stages:

• Pregnancy and breastfeeding: Avoid extended fasting; natural overnight fasts only.
• Children: No deliberate fasting protocols outside medical applications.
• Older adults: Gentler protocols, particular attention to protein and electrolytes, work with practitioners.
• Athletes: Match fasting to training periodisation rather than imposing both simultaneously.

Final word

Fasting works because cycling between fed and fasted states is the metabolic pattern your body evolved to expect. Modern food abundance has eliminated this cycling for most people; deliberately reintroducing fasting restores something the body anticipates rather than imposing something foreign.

The optimal fasting protocol for you depends on your goals, life stage, sex, training, stress level, medical conditions, and individual physiology. The popular “16:8 is the answer” framing oversimplifies; different people benefit from substantially different approaches at different points in their lives.

The body benefits from regular cycling between fed and fasted states; the body benefits from periodic deeper fasting; the body benefits from coherent timing rather than constant grazing. The specific implementation is yours to figure out.