Anyone who fasts for more than a day arrives at the same question. Can you still train? The evidence gives a split answer, and the split runs along the line between strength and endurance rather than between hard and easy. The clearest data come from a study of 13 adults, seven men and six women, fasting seven days on water while being tested throughout. Maximal isometric and isokinetic strength was unchanged across the week. Over the same seven days, peak oxygen uptake fell by 13% and muscle glycogen dropped to roughly half its starting level (Kolnes et al., Nature Communications, 2025). Those two findings sitting side by side are the useful part. Strength and endurance capacity did not move together. The ability to produce force held while the ability to sustain high output did not. The mechanism is about fuel routing. In the same study, PDK4 protein rose 13-fold, which restrains carbohydrate oxidation and pushes the body toward fat as its fuel (Kolnes et al., 2025). Carbohydrate is what powers high-intensity, glycolytic work. Restrain access to it and the sessions that depend on it get harder, while a heavy set of five, which draws on a different energy system, is far less affected. That is a small sample of 13 people over one extreme protocol, so it maps best onto multi-day fasting rather than an ordinary overnight fast. Still, the direction is consistent with what shows up elsewhere. Which brings us to the claim that gets repeated most often: that training fasted burns more fat. Inside a single session, that holds up. Exercising in a fasted state raises fat oxidation during the workout itself (Vieira et al., British Journal of Nutrition, 2016). The measurement is straightforward and the effect is consistent. Across weeks, it stops holding. A four-week trial that matched calorie intake between groups found no body-composition advantage to doing cardio fasted rather than fed (Schoenfeld et al., Journal of the International Society of Sports Nutrition, 2014). A systematic review pooling five studies and 96 participants reached the same place, reporting trivial differences in body mass and body fat between overnight-fasted and fed exercise (Hackett and Hagstrom, Journal of Functional Morphology and Kinesiology, 2017). So both things are true at once. The session-level effect is measurable, and it does not carry through to the month-level result. What gets burned inside an hour turns out to be a smaller lever than what gets eaten across a week. The largest body of evidence on training while fasted does not come from a lab. It comes from Ramadan, where millions of people train while abstaining from food and fluid during daylight for a month. A systematic review of soccer players across that month found that short-term maximal performances, including jumps, sprints, maximal voluntary contractions and skill measures, were largely preserved when training load, nutrition and sleep were maintained. Repeated-sprint work, 30-second maximal efforts and long-duration endurance were the measures that more often declined (Chtourou et al., Tunisie Medicale, 2019). That literature carries two caveats worth stating rather than glossing. It is heavily male: one body-composition review across 12 studies included 183 athletes, of whom 12 were women (Aloui et al., Tunisie Medicale, 2019). And Ramadan bundles fasting together with sleep disruption and a shifted daily schedule, so any decline observed during it cannot be attributed cleanly to the absence of food. Putting it together gives a workable placement rather than a rule. Lifting appears to hold up well through a fast. Low-intensity work fits comfortably, since it runs largely on fat anyway. Hard interval and repeated-sprint sessions are the ones that sit better on fed days, when the fuel they depend on is available. One last note, and it applies to all of this. These findings are averages from small groups, and several of them come from young or predominantly male samples. Research describes central tendencies; you are the one running the experiment on yourself. Worth noticing alongside the physiology: whether the training belongs where you are putting it, or whether it is there to earn the fast.
Safety
Educational only, not medical advice. Training during an extended fast carries added risk of dizziness, low blood pressure on standing, and falls. Extended fasts past a few days warrant medical supervision. Talk to your doctor before combining fasting with training, particularly if you take medication affecting blood sugar or blood pressure. If you are struggling with your relationship to food or exercise, the National Alliance for Eating Disorders helpline can help.
References
Kolnes KJ et al., Nat Commun, 2025 (PMID 39747857). Vieira AF et al., Br J Nutr, 2016;116(7):1153-1164. Schoenfeld BJ et al., J Int Soc Sports Nutr, 2014;11:54. Hackett DA and Hagstrom AD, J Funct Morphol Kinesiol, 2017;2(4):43. Chtourou H et al., Tunis Med, 2019;97(10):1114-1131 (PMID 31691939). Aloui A et al., Tunis Med, 2019;97(10):1087-1094 (PMID 31691936).