Does fasted exercise burn more body fat?
Fasted exercise increases fat use during the workout itself. But studies that measured body fat over several weeks have not found an advantage over exercising after a meal. Those are different outcomes, and the distinction matters.
What researchers measure during a workout
Muscles use both fat and carbohydrate for fuel. The mix changes depending on whether you have eaten. A meta-analysis of 27 studies involving 273 adults found that aerobic exercise in a fasted state oxidized an average of 3.08 g more fat during the session than exercise after eating. These studies measured metabolism during exercise lasting up to two hours, not changes in body fat.
A reading taken during one run therefore cannot tell you how much body fat someone will have a month later. That requires comparing people who train for weeks and measuring their body composition.
What happens to body fat over several weeks
In a randomized trial of 20 young women, both groups did an hour of aerobic exercise three times a week and followed diets designed to create an energy deficit. One group exercised after an overnight fast; the other consumed a meal before exercise. After four weeks, body weight and fat mass had fallen in both groups, with no significant difference between them.
A review of five studies with 96 participants also found no advantage for fasted exercise in changes to body weight or composition. Its authors caution that the evidence base is small. A difference not found in a small sample does not prove that one could never exist.
When eating before exercise can help
A workout may have another goal: covering a distance, keeping a pace, or simply feeling comfortable while moving. A review of 46 studies found that eating before prolonged aerobic exercise improved performance, while eating before shorter sessions did not. It did not establish that either approach produces more fat loss.
If you feel better exercising after breakfast, there is no fat-loss reason to skip it. If a short morning workout feels fine without food, you need not eat first for the sake of a fuel-use measurement. See our guide to eating before exercise for more on meal timing.
Who should discuss meal timing with a clinician
With diabetes, particularly when using insulin or medication that increases insulin release, exercise can cause hypoglycemia. The American Diabetes Association position statement calls for exercise, glucose monitoring, food and medication plans tailored to the individual. A general article is not a basis for trying fasted workouts in this situation.
If skipping food is driven by a need to “make up for” something you ate, rather than by a convenient workout time, meal timing is no longer the main question. If food or exercise brings persistent anxiety, speak with a qualified professional.
How to choose whether to eat first
Use the way you feel and the purpose of the session. Eating beforehand may help performance during prolonged exercise; for a short session, either schedule may work. Changes in body fat are assessed over weeks, not from the fat used in one workout. See also how exercise calorie estimates relate to weight change.
Frequently asked questions
- Does fasted exercise burn more fat?
- Fat oxidation is higher on average during fasted aerobic exercise. This is measured during the session and is not the same as losing more body fat over several weeks.
- Does working out before breakfast lead to faster fat loss?
- Small studies comparing fasted and fed exercise have not found an advantage in body fat loss. The evidence is too limited to make a universal claim about every type of workout.
- Do I need breakfast before a short morning workout?
- Choose according to how you feel. Research has not found a performance advantage from eating before shorter aerobic sessions or a body fat loss advantage from skipping breakfast.
- What about a long workout?
- A review found that eating before prolonged aerobic exercise improved performance compared with exercising fasted. Choose meal timing and size according to what you tolerate.
Read next
- What to eat before a workout — The question "what to eat before a workout" is almost always actually a question of "how long before.
- How many calories are burned during a workout: running, cycling, swimming, gym — An hour of running at a moderate pace for a 70 kg person is about 550–700 kcal, cycling at a leisurely pace is 350–450, swimming is 400–600, and strength training is 200–350.
- Calorie deficit: how to calculate it and how to hold it — A calorie deficit is the one thing without which weight does not come down.
This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.
Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.