In 30 seconds
- During the session, yes: across 27 studies, fasted aerobic exercise used about 3 g more fat for energy than exercise after eating.
- After a few weeks, the result is the same: in 5 studies lasting 4-6 weeks, exercising fasted or fed gave the same weight and fat loss.
- What decides how much fat you lose is your calorie deficit over the day and the week, not what time you have breakfast.
- If the session is long, eat beforehand: across 46 studies, eating first improved performance in prolonged aerobic exercise.
During the session, yes: when you are fasted, your body uses a little more fat for energy. But after a few weeks, fat loss is the same as if you train after breakfast. What decides the result is your calorie deficit, not the time you eat.
What fasted cardio is
It means doing aerobic exercise in the morning, before breakfast, after the hours without food overnight: brisk walking, running, cycling, the cross-trainer.
The idea is simple. With no recent food, insulin is low and the body draws more on its fat stores. The question is whether that translates into losing more fat.
During the session
Fasted, you use a little more fat, but not much
During the session: a little more fat
This is the part that does hold true. A meta-analysis of 27 studies compared the same aerobic exercise fasted and after eating. When fasted, about 3 g more fat was used for energy during the session 1.
Two important nuances:
- Intensity is what counts. The difference appeared below 70% of maximum aerobic capacity. Above that, it disappeared 1.
- It is grams, not kilos. About 3 g of fat per session is a small figure compared with what a calorie deficit shifts in a week.
4 weeks, same deficit
Body fat: fasted or after eating
After a few weeks: the same result
What matters is whether you lose more fat over time, and here the studies agree.
In the most cited trial, 20 young women followed the same calorie-deficit diet for 4 weeks and did 1 hour of moderate cardio, 3 days a week. Half trained fasted and had a shake afterwards; the other half had the same shake beforehand 2.
Both groups lost weight and fat: from 16.5 to 15.4 kg of fat when fasted and from 15.7 to 15.0 kg after eating. The difference between groups was not significant. The authors concluded that the change is similar whether or not you train fasted 2.
A later meta-analysis pooled 5 studies with 96 people, lasting 4 to 6 weeks. There were no differences in weight or fat between exercising fasted or fed. Its conclusion: losing weight and fat depends on maintaining a calorie deficit, not on when you eat 3.
Why it does not show on the scales
Because the fat you do not use during the session you use at another time of day, provided that by the end of the day you have spent more than you have eaten. A review on the subject confirms that, in studies lasting 4-6 weeks on a maintenance or calorie-deficit diet, there were no differences in weight, fat or lean mass 4.
The same authors believe studies of 12 weeks or more would be needed to detect small differences 4. For now, the practical answer is clear: do your cardio whenever it suits you best.
What about hunger?
Here fasted cardio has a point in its favour. In a study with 12 young, active men, an hour on the treadmill before breakfast did not make them eat more at lunch. They ate the same as when they trained after eating, and their energy balance that day was more negative, mainly at the expense of fat 4.
If training fasted helps you keep what you eat under control, that is a good reason to do it. If it leaves you ravenous for the rest of the day, that is a good reason not to.
If you like it fasted
Fasted cardio, done well
If you like training fasted
- Drink water before you start.
- Keep the intensity moderate: that is where the difference in fat used shows 1.
- Save intervals and long sessions for the days when you have eaten 5.
- Include protein in the meal afterwards, to get your recovery off to a good start. If you are wondering about amino acids before training without breakfast, we cover it in BCAAs before fasted training.
When it is worth eating first
There is one case where eating first does change something: performance. A meta-analysis of 46 studies found that eating beforehand improves performance in prolonged aerobic exercise. In short sessions, no difference was seen 5.
| Your session | What suits you | Why |
|---|---|---|
| Easy or moderate cardio, first thing in the morning | Whichever you prefer | After a few weeks, fat loss is the same |
| Long session: a long run, a bike ride | Eat first | It improves performance in prolonged aerobic exercise |
| Intervals or very intense cardio | Eat first | Above 70% of your maximum capacity, the advantage in fat used disappears |
Our own summary based on Vieira et al., 2016; Hackett and Hagstrom, 2017; and Aird et al., 2018.
And if you train fasted, remember that the breakfast afterwards counts towards your daily total, like any other meal.
What does work for losing fat
- A moderate calorie deficit. If you do not know how much to eat, start with calorie deficit: how much to eat.
- Plenty of protein and strength training, so that what you lose is fat and not muscle.
- The cardio you can keep up, at a time you can keep up.
If you want the data from each study, you will find them in fasted cardio: what the studies say.
Your L-carnitine before cardio
L-carnitine is the classic companion to cardio during a fat-loss phase. In a meta-analysis of 9 trials with 911 people, those who took it lost 1.33 kg more weight than the control group 6.
If you are pregnant, take medication or have diabetes, check with your doctor first.
Frequently asked questions
Is fasted cardio better for losing fat?
There is no difference after a few weeks. In 5 studies lasting 4-6 weeks, doing aerobic exercise fasted or after eating gave the same weight and fat loss. What decides it is your calorie deficit.
Do you use more fat when you exercise fasted?
During the session, yes: in a meta-analysis of 27 studies, about 3 g more fat was used for energy when fasted. The effect is seen at moderate intensities and disappears above 70% of maximum capacity.
How much fasted cardio should you do?
In the most cited study, the participants did 1 hour at moderate intensity, 3 days a week, for 4 weeks. They lost the same fat as the group who had breakfast first, because both had the same deficit.
Can I take L-carnitine before fasted cardio?
Yes. If you take medication, have diabetes or are pregnant, check with your doctor first.
Sources
- Vieira AF, Costa RR, Macedo RCO, Coconcelli L, Kruel LFM. Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. Br J Nutr. 2016;116(7):1153-1164. View publication
- Schoenfeld BJ, Aragon AA, Wilborn CD, Krieger JW, Sonmez GT. Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr. 2014;11:54. View publication
- Hackett D, Hagstrom AD. Effect of overnight fasted exercise on weight loss and body composition: a systematic review and meta-analysis. J Funct Morphol Kinesiol. 2017;2(4):43. View publication
- Wallis GA, Gonzalez JT. Is exercise best served on an empty stomach? Proc Nutr Soc. 2019;78(1):110-117. View publication
- Aird TP, Davies RW, Carson BP. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: a systematic review and meta-analysis. Scand J Med Sci Sports. 2018;28(5):1476-1493. View publication
- Pooyandjoo M, Nouhi M, Shab-Bidar S, Djafarian K, Olyaeemanesh A. The effect of (L-)carnitine on weight loss in adults: a systematic review and meta-analysis of randomized controlled trials. Obes Rev. 2016;17(10):970-976. View publication
Information based on published studies. It does not replace the advice of a doctor or registered dietitian. The results describe what was measured in each study, not the properties of any specific product. A varied and balanced diet and a healthy lifestyle are important.
