Running Fasted: Does It Work? What the Studies Say

Running fasted: the short answer

For a healthy runner, running fasted in the morning, meaning after the night and before breakfast, is generally not harmful if the run is short (up to 45-60 minutes) and easy. But it is not a shortcut to weight loss: studies show you burn a little more fat during the run, and that in the end, with the same diet and training, weight and fat loss do not differ significantly. For long runs, intervals and races, starting on an empty stomach is almost always the worse choice: less energy, lower quality, more risk of hitting the wall.

In this guide you get what happens in the body, the numbers from the three studies that really matter (with a small calorie conversion that puts many headlines in perspective), when it makes sense and when it does not, how to proceed if you want to try, what to eat if you prefer not to run empty, who should talk to a doctor first and the most common mistakes. For the rest of fuelling around running there is the article what to eat before and after running: this guide is the piece on fasting.

What happens in your body when you run fasted

After a night without food, your liver glycogen stores are lower and blood glucose is stable but not high. When you start running, the body leans a bit more on fat as fuel: that is the idea behind fasted running. It is true, and measurable: fat oxidation during effort goes up. The question is by how much, and what it means for your body outside the run.

The second effect is talked about less: without available carbohydrate, at higher intensities the body cannot replace sugar quickly. So as soon as you pick up the pace, perceived effort rises, pace drops and you may get dizziness, nausea or empty legs. That is why the same run at the same pace is often harder when fasted.

Does running fasted make you lose weight? Numbers from three studies

1. Vieira et al., British Journal of Nutrition, 2016. A systematic review and meta-analysis of 27 studies and 273 participants, on aerobic exercise lasting up to 120 minutes. Result: during fasted exercise fat oxidation is higher, on average by about 3 grams (difference -3.08 g, 95% confidence interval -5.38 to -0.79). Three grams of fat equal about 28 kcal (9 kcal per gram): a real but tiny difference against a run that burns 500-700. The authors conclude that more fat is oxidised during fasted exercise, not that you lose more weight.

2. Schoenfeld et al., Journal of the International Society of Sports Nutrition, 2014. Twenty healthy young women, split into two groups, for four weeks: one hour of aerobic exercise three times a week, fasted or after a meal, both on a reduced-calorie diet. Both groups lost weight and fat mass, with no significant difference between them. It is a small, short study, but the design is clean.

3. Bachman et al., Journal of Nutrition and Metabolism, 2016. Twelve young trained men ran one hour at 60% of VO2max, fasted or after breakfast. On the fasted day total 24-hour energy intake was lower (about 15,300 kJ against 19,200 kJ) and fat oxidation higher. Note: in the comparison the breakfast is counted in the calories of the breakfast group, so much of the gap is simply the skipped meal. The authors themselves speak of possible implications for weight management, not proof.

How to read it all: running fasted is not magic and is not a danger for people in good health. If it is convenient and makes you eat less over the rest of the day, it may help; if it leaves you ravenous and you eat double at breakfast, it cancels out. For weight what counts is the balance over a week, not over a morning. If you want to see how many calories you burn, the cycling calories guide applies the same reasoning to rides.

When running fasted is fine and when it is not

It can be fine if: 1. The run is short (30-60 minutes) and easy, meaning you can talk. 2. You are healthy and used to running. 3. It is cool: heat increases fluid loss and worsens how you feel. 4. You feel good that way, with no dizziness or nausea.

It is better to eat first if: 1. The run lasts more than 75-90 minutes: long runs are building sessions, not sacrifice. 2. You have intervals, race pace or a race planned: quality depends on carbohydrate. For the types of sessions read running intervals and types of training. 3. It is hot or humid. 4. You are already running a lot (above 50-60 km a week) and recovery is at its limit: see how many miles a week should I run. 5. You slept badly or are recovering from illness.

How to do it well if you want to try

A simple routine for easy morning runs: 1. Get up and drink a glass of water (200-300 ml). Coffee is fine if you tolerate it. 2. Start slowly: the first 10 minutes are slower than usual. 3. Stay in the easy zone (you can talk). To keep pace under control use heart rate zones. 4. Carry a plan B: a gel or a few sugar cubes, useful if a sudden slump appears. On products see the guide to gels and electrolytes. 5. After the run eat a breakfast with carbohydrate and protein within an hour, and drink. For recovery there is running recovery in the first 24-48 hours.

Increase duration by no more than 10-15 minutes at a time and do it no more than 2-3 times a week. If after 2 weeks fasted runs leave you wrecked for the rest of the day, you have your answer: it is not for you, and there is nothing wrong with that.

If you prefer not to run empty: what to eat and how much

Sports nutrition guidelines (ACSM, Academy of Nutrition and Dietetics and Dietitians of Canada, 2016) indicate for demanding exercise about 1-4 g of carbohydrate per kg of body weight in the 1-4 hours before. For an easy one-hour run in the morning you do not need to get there: something small 30-60 minutes before is enough.

Examples for a 70 kg runner, indicative: a banana (about 25 g of carbohydrate), two rusks with jam (about 30 g), a slice of bread with honey (about 30-35 g), a piece of fruit and a spoonful of yoghurt. If you only have 15-20 minutes, pick something liquid or very simple, such as half a banana or diluted juice. Avoid large amounts of fibre, fat and protein right before running: they slow digestion. Always test in training, never for the first time in a race.

Running fasted, performance and races

Races should be tackled with full stores: breakfast 2-3 hours before, with simple carbohydrate and little else, and during the race, beyond an hour and a half, carbohydrate intake. Training fasted on easy days does not take away the habit of eating on the run, but you must also train the gut to do it on long runs. For a full plan see the 16-week marathon plan.

Some elite athletes use fasting in a targeted way in some sessions (the so-called train low strategy), but the evidence for race benefits is mixed and it is not a method to copy without a coach. For an amateur, the first step that really improves performance is sleeping, eating regularly and staying consistent.

Who should talk to a doctor first

If you have diabetes or take drugs that lower blood sugar, if you are pregnant, have or have had eating disorders, or have heart problems or low blood pressure, do not try fasting without a doctor's advice. Stop and get symptoms checked if you get strong dizziness, palpitations, cold sweat, confusion or fainting.

One last thing not to ignore: training a lot while eating too little for long periods can lead to a chronic energy deficit, with fatigue, repeated injuries and, in women, menstrual changes. If you recognise these signs, it is not a willpower problem: talk to a sports doctor or a nutritionist.

Common mistakes

1. Running fasted to lose weight and then rewarding yourself with a huge breakfast. 2. Doing intervals or tempo runs fasted too: quality drops and risk rises. 3. Stretching the run beyond 60-75 minutes without fuelling. 4. Neglecting water, especially in summer. 5. Trying a new method in a race. 6. Ignoring your body because it works for the people you follow online. 7. Forgetting recovery: when fasted the post-run meal matters even more.

Sources and notes

Vieira AF et al., Br J Nutr 2016;116(7):1153-1164 (systematic review and meta-analysis). Schoenfeld BJ et al., J Int Soc Sports Nutr 2014;11:54. Bachman JL et al., J Nutr Metab 2016, article 1984198. Guidelines on carbohydrate before exercise: Thomas DT, Erdman KA, Burke LM, Nutrition and Athletic Performance, joint position statement of ACSM, AND and DC, 2016. Food quantities are indicative examples and do not replace the advice of a doctor or nutritionist. To find where to run, open Velunite's running routes. Velunite Editorial, updated 3 November 2026.