Should you eat before you exercise? Pick your session and see if fasted works
Should you eat before you exercise? For most sessions of an hour or less, training fasted or fed makes little difference, and reviews find similar weight and body-composition changes either way. Longer aerobic sessions tend to go better with a snack first.
Pick your session, set how long it lasts and how you feel on an empty stomach, and see a simple rule of thumb built from three systematic reviews. Not medical advice: ask a doctor first if you have a condition, take medication or have an eating-disorder history.
You wake up, it is not yet breakfast, and you have a walk, a run or a gym session planned. Should you eat first? Plenty of people say training fasted (before eating, often after the overnight gap) burns more fat. Others say you will run out of fuel. The research is calmer than either camp, and for most beginners the answer is: it depends on how long and how hard, and on how you feel.
Try it: fasted or fed for your session?
For most sessions under an hour, fasted or fed makes little difference. Past an hour, eating first tends to help.
What the research says
Three reviews cover most of what we know. A 2016 review in the British Journal of Nutrition pooled 27 studies and found that low-to-moderate intensity aerobic exercise done fasted used more fat as fuel during the session. That sounds like a win, but it measures what the body burns during the workout, not how much body fat you lose over weeks. The authors looked only at those short-term effects.
The longer-term question was covered by a 2017 review of five studies (96 people). Fasted and fed exercise led to similar small changes in weight and body composition. The authors warn that so few trials means caution is needed. Your total intake and activity across the week matter far more than the clock.
For performance, a 2018 review of 46 studies found that eating beforehand improved longer aerobic exercise but not shorter efforts. Our rule of thumb of "about an hour" is a simple reading of that, not a hard line.
Who should be careful
This is general information, not medical advice. If you have a medical condition, are pregnant or breastfeeding, have or had an eating disorder, or take medication (especially for diabetes or blood pressure), talk to a doctor before combining fasting with exercise. Both can change blood sugar and blood pressure. If you have not exercised for some time, the NHS advises speaking to your GP first. Stop if you feel dizzy, faint or unwell.
A simple way to decide
- Short and easy (a walk, a gentle ride): go fasted if you like, or eat first if it feels better.
- Around an hour or longer: eat something beforehand and drink water.
- Hard intervals: try both once and keep what feels better.
- Your goal: the usual adult target is at least 150 minutes of moderate activity a week plus muscle-strengthening work on 2 days. Whether any one session is fasted is a detail.
Whatever you choose, eat a balanced meal afterwards, with protein and some carbohydrate, and drink water.
Sources
- Vieira AF et al. (2016). Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. British Journal of Nutrition 116(7).
- Hackett D, Hagstrom A (2017). Effect of overnight fasted exercise on weight loss and body composition: a systematic review and meta-analysis. Journal of Functional Morphology and Kinesiology 2(4).
- Aird TP, Davies RW, Carson BP (2018). Effects of fasted vs fed-state exercise on performance and post-exercise metabolism. Scandinavian Journal of Medicine and Science in Sports 28(5).
- NHS. Physical activity guidelines for adults aged 19 to 64.