Educational content — not medical advice. Ravenous does not track, measure, or verify any fasting activity.
Fasting means voluntarily abstaining from food for a defined period. It ranges from short daily eating windows (like 16 hours without food) to extended fasts lasting 24 hours or longer. People fast for different reasons: metabolic health, weight management, mental clarity, religious or personal practice, and interest in autophagy — a cellular process discussed below. This guide covers what happens in your body during a fast, what the research does and does not support, and the real risks involved.
These timeframes are general estimates based on available research. Individual response varies significantly based on your metabolism, recent food intake, activity level, and body composition. This is not a measurement of what is happening in your specific body — it is general information about fasting physiology.
Autophagy is a cellular process where your body breaks down and recycles damaged or dysfunctional cell components. It is a normal, ongoing process in the body, not something that switches on only during fasting. Fasting research suggests autophagy activity may increase during extended periods without food, based largely on animal studies. Much of the strongest evidence for fasting-induced autophagy comes from research in mice and other organisms, not controlled human studies. What is less certain in humans: • The exact fasting duration required to meaningfully increase autophagy • How much autophagy actually increases during a typical extended fast • Whether the benefits observed in animal studies translate directly to human health outcomes This is an active area of ongoing research. Claims that a specific fasting duration "triggers" or "activates" autophagy at a precise hour should be treated as approximations, not established fact.
This section is educational only. Ravenous does not measure or verify autophagy activity in your body. No fasting duration display in this app is a claim about what is actually happening in your cells.
Extended fasting — beyond a typical overnight or daily eating window — carries real health risks and is not appropriate for everyone. Do not attempt extended fasting if you: • Are pregnant or breastfeeding • Have type 1 diabetes or take insulin • Take medications for blood sugar (sulfonylureas or similar) without physician guidance • Have a history of an eating disorder • Are underweight or have a history of malnutrition • Are under 18 • Have a chronic illness affecting metabolism, kidney function, or heart function • Are currently taking medications that require food for proper absorption or safety If any of these apply to you, extended fasting should only be considered under direct medical supervision, if at all.
Fasting beyond 24 hours carries risks that increase the longer the fast continues: • Hypoglycemia (dangerously low blood sugar) — can cause confusion, fainting, seizure • Electrolyte imbalance — the leading cause of serious complications during extended fasting, can affect heart rhythm • Dehydration — often overlooked, worsens all other risks • Dizziness, fainting, and falls • Impaired judgment and concentration • Muscle loss if fasting is prolonged or frequent without adequate refeeding Warning signs to stop fasting immediately and eat: • Dizziness or fainting • Heart palpitations or irregular heartbeat • Confusion or difficulty concentrating • Severe weakness • Nausea or vomiting If you experience any of these — break your fast and eat. If symptoms are severe or do not resolve after eating, seek medical attention.
Exercising during a fast — especially an extended one — adds risk. Training with depleted glycogen stores and reduced hydration can increase the risk of dizziness, poor performance, and muscle breakdown. If you train while fasted: • Keep intensity low to moderate • Stay well hydrated • Stop immediately if you feel dizzy, weak, or unusually fatigued • Avoid heavy or high-intensity training during fasts longer than 24 hours See the Training Safety guide for information on rhabdomyolysis risk, which training while depleted can increase.
All food is eaten inside an 8-hour window, with roughly 16 hours — mostly overnight — without calories. The framing here is circadian: insulin sensitivity, glucose handling and many digestive processes follow a daily rhythm that runs strongest earlier in the day, so an earlier window is studied differently from a late one. In a 5-week controlled crossover trial in men with prediabetes, an early 6-hour eating window (all food before mid-afternoon) was associated with improved insulin sensitivity, lower blood pressure and reduced oxidative stress compared with a 12-hour window, and those changes appeared without weight loss (Sutton et al., Cell Metabolism, 2018). Research on the wider 16:8 pattern is more mixed: a 12-week randomised trial of 16:8 alone found modest weight change and no significant advantage over unrestricted timing (Lowe et al., JAMA Internal Medicine, 2020). What that means practically: research on early time-restricted eating found metabolic marker changes independent of weight, while research on a late-shifted 16:8 window has been less consistent. Neither result predicts what any individual reader will experience.
Normal eating on five days, with two non-consecutive days restricted to roughly 500–600 kcal. It is a weekly-calorie approach rather than a daily-window one, which some people find easier to hold than continuous restriction. In a 6-month randomised trial in overweight women, two restricted days per week was associated with weight loss and insulin-sensitivity changes comparable to daily continuous energy restriction (Harvie et al., International Journal of Obesity, 2011). Later trials have generally reached the same conclusion — comparable to daily restriction, not superior to it (Trepanowski et al., JAMA Internal Medicine, 2017). Restricted days are still eating days. The contraindications listed above apply here too, and anyone taking blood-sugar medication should not attempt restricted days without physician guidance.
Water only, for one to three days. This should be done with clinical supervision. That framing is not boilerplate. The largest published safety series of extended fasting — 1,422 people fasting 4–21 days — was conducted in a residential medical programme with daily clinical monitoring (Wilhelmi de Toledo et al., PLOS ONE, 2019). Electrolyte disturbance is the leading cause of serious complications in extended fasting, and it is not something a person can detect reliably in themselves. Beyond 24 hours, the risk profile in the KNOW THE RISKS section applies in full, and refeeding needs to be managed deliberately. Ravenous does not track, time, or verify fasts, and nothing here is a recommendation to attempt one.
A five-day, plant-based, low-protein, low-sugar eating pattern designed to reproduce some of the nutrient-sensing changes of water fasting while a person still eats. It was developed and studied in the Longo lab at USC and is run in cycles — typically monthly for three cycles in the published trials — rather than continuously (Wei et al., Science Translational Medicine, 2017). It is a research protocol, not a diet to improvise. The detail below is included because the numbers get misquoted constantly, not as instructions to follow.
Macronutrient composition used in the published trials: • Protein: 9–11% of calories (deliberately low) • Complex carbohydrates: 43–47% • Fat: 44–46% • Roughly 700–1,100 kcal per day — higher on day 1, lower on days 2–5 The composition matters as much as the calorie figure. Protein is held low on purpose: protein and amino acids — leucine in particular — are the primary signal to the mTOR nutrient-sensing pathway, so a low-protein, low-sugar intake keeps that signalling low even though food is being eaten (Wei et al., Science Translational Medicine, 2017; Brandhorst et al., Cell Metabolism, 2015). The depletion-phase mechanism, as described in that research: • mTOR and IGF-1 signalling are suppressed — the pathways that read "nutrients are plentiful, grow" • AMPK activity rises — the sensor that reads low cellular energy • The body shifts toward ketosis as glycogen falls and fat becomes the dominant fuel • Autophagy activity increases in this state in the animal models — with the human caveats set out in the autophagy section above Cyclic, not chronic. The protocol was designed around short cycles followed by normal refeeding, and the refeeding phase is part of the design rather than the end of it. Chronic low-protein eating is a different thing entirely and is not what this research studied — particularly for anyone training hard, where protein intake sits at the centre of everything else in this app.
Biological age is an estimate of how a body is functioning relative to its years lived. There is no single agreed measurement — there are four broad families of method, and they do not always agree with each other. 1. Telomere length — the protective caps on chromosomes, which shorten with cell division. Widely measured, but individual variation and assay differences make it a weak marker at the level of one person (Aviv et al., Nature Reviews Cardiology, 2013). 2. DNA methylation / epigenetic clocks — chemical marks on DNA read as an age estimate. The Horvath clock (Genome Biology, 2013) estimates chronological age; GrimAge (Lu et al., Aging, 2019) was trained on mortality and disease risk; DunedinPACE (Belsky et al., eLife, 2022) estimates the current pace of aging rather than a fixed age. These are the most studied clocks and require a lab test. 3. Metabolic age / PhenoAge — an age estimate built from routine blood chemistry: albumin, creatinine, glucose, inflammation markers, liver enzymes, white cell measures (Levine et al., Aging, 2018). This is the family used in the FMD findings below. 4. Functional measures — what the body can actually do. Grip strength, VO2 max and gait speed each track with age-related outcomes in large cohort studies (Bohannon, Journal of Geriatric Physical Therapy, 2019; Mandsager et al., JAMA Network Open, 2018; Studenski et al., JAMA, 2011). That fourth family is the one already in this app. The Longevity Tests and the Additional Metrics section cover grip strength, VO2 max and several of the same functional domains — those are self-recorded functional measures, not a lab clock and not a biological-age result. Ravenous does not estimate anyone's biological age.
A 2024 analysis from the Longo lab, pooling data from two randomised clinical trials, reported that participants who completed three monthly FMD cycles showed a median reduction of approximately 2.5 years in biological age — measured with blood-chemistry-based clocks (PhenoAge-type), relative to their own baseline (Brandhorst et al., Nature Communications, 2024). Insulin resistance and hepatic fat markers also shifted in that analysis. The risk-reduction percentages that circulate from this work — figures quoted for cardiovascular disease, cancer and diabetes risk — are MODELED ESTIMATES derived from the observed biomarker shifts using established risk equations. They are not directly observed trial outcomes. No participant was followed for decades to see whether those events occurred. A biomarker moving in a favourable direction and a disease being prevented are two different claims, and only the first one was measured. The distinction is the whole point: research on this protocol found biomarker and clock changes across a group over three cycles. That is not a prediction about any individual reader, and no number here should be read as an outcome someone will personally experience. Results in trials are group medians with wide individual spread, participants were screened and supervised, and the follow-up was short.
How you end a fast matters, especially after 24+ hours without food. Eating a large meal too quickly after an extended fast can cause refeeding syndrome — a potentially serious shift in fluid and electrolyte balance. General guidance for breaking a longer fast: • Start with something small and easy to digest • Prioritize protein and healthy fats over a large carbohydrate load initially • Eat slowly and stop when comfortably full — not stuffed • Rehydrate gradually • If you fasted longer than 48 hours, consider consulting a physician before and after, particularly if you have any underlying health conditions
This guide is for educational purposes only and is not medical advice. Fasting practices vary widely in safety depending on individual health status. Consult a qualified healthcare professional before beginning any fasting practice, particularly extended fasts beyond 24 hours. Ravenous does not track or verify fasting activity, and no content in this guide should be treated as a personalized recommendation.