Advice on whether or not to exercise on an empty stomach can be contradictory. Nutrition research scientist Tim Crowe weighs in.
To exercise fasted or fed? It’s a controversial topic with strong views coming from both camps.
Exercising in a fasted state is frequently endorsed as essential for those promoting weight loss, to ensure the most ‘fat burning’ possible. Then there is the opposing view that you want to be well-fuelled before exercise so you can power the length and duration of your exercise. But reality is more complicated than both arguments.
Exercise for weight management
We exercise for all sorts of reasons. General health and fitness, training for competitive sport, or to help shed a few kilos are all common reasons. If your aim is to lose weight, then are you better off exercising on an empty stomach?
A common stance on exercise for weight loss is driven by the belief that breaking a sweat on an empty stomach uses up your body’s fat stores better than grinding your gears and going nowhere because you’re metabolising the meal you just ate before exercise. So what’s really going on?
Fat burning 101
Standard advice claims that you need to exercise at a low to moderate level of intensity to burn fat.
While it is true that the body burns the greatest percentage of fat at lower intensities of aerobic exercise, at higher intensities you burn far more kilojoules – and more fat kilojoules – overall.
When time is limited to exercise, then it makes sense to work as hard as you safely can to get the most health, performance and weight loss gains from your workout.
Fed or fasted?
So if the intensity and duration of exercise matters most for long-term weight balance, what is it about being fed or fasted that makes a difference when you exercise?
Cardio exercise increases fat oxidation and it seems that people who do this in a fasted state oxidise more fat than glucose. But this doesn’t translate into increased body fat loss overall when compared with exercising fed. Why should this be the case?
If two people have the same meals to eat in a day, and one delays their first meal until after they exercise while the other eats their first meal before exercise, what happens?
The key is that short-term transient spikes in fat oxidation are pretty meaningless. It’s the long-term balance over days and weeks that will dictate whether you lose or gain body fat. So if you exercise fasted and experience a small spike in fat burning, it is offset by the fact that you’re just shifting what you would have eaten before exercise to later on, at which time you’ll be in a state of energy excess.
The person who exercises fed may experience less fat oxidation during exercise, but the energy deficit will be delayed as they won’t be eating as much later on.
What the science says
A 2017 systematic review and meta-analysis looked at the effect of overnight fasting before exercise compared with eating just before exercise, and how this affected weight loss and body composition. And while only five studies could be included in the review, the conclusion was that performing exercise in a fasted state did not influence weight loss or changes in lean and fat mass compared to exercising in a fed state.
The findings from this review support the notion that weight loss and fat loss from exercise are more likely to be enhanced through creating a meaningful caloric deficit over time, rather than exercising in fasted or fed states.
Exercise performance
Interestingly, a 2018 systematic review and meta-analysis looking at fed versus fasted cardio found no difference in exercise performance when workouts lasted less than an hour. But when workouts went for more than an hour, eating before the workout improved performance. As far as blanket population advice goes, there is no strong reason to recommend someone exercise fasted or fed when it comes to performance or weight loss goals, when we’re only talking about an hour or so of exercise. Go with what your body tells you – some prefer to work out on an empty stomach, others need to eat first, as they feel their blood sugar falling and experience a lack of energy.
Take your pick, unless going hard and long
If you’re planning a serious endurance session for more than an hour or your goal is to perform or hit maximal effort, you may want to consider eating before you train. For exercise under an hour in duration, or a low-intensity training session, eating before will make little difference, unless you just feel better for it.
Intermittent fasting for athletes
On the surface, some of the principles of intermittent fasting seem to go against the core sports nutrition concepts of refuelling post-exercise and spreading food consumption (especially protein) over the day. If you’re an athlete training for an event or competition, the research is interesting.
A 2024 systematic review on intermittent fasting and athletic performance concluded that intermittent fasting provides body composition benefits without reducing physical performance, maintenance of lean mass, and improvements in maximum power. But everyone’s mileage is different here and your own performance would be the best guide.
If you’ve consistently trained in a fasted state, the metabolic and muscle adaptations you’ll acquire may boost performance when you then compete in a fed state. A fasted workout trains you to perform under difficult physiological conditions of low fuel availability and that can come in handy later. You probably wouldn’t want to enter a race or powerlifting competition in
a fasted state, but the fasted workouts you did in the months leading up to the competition make you more likely to perform better in a fed state.
The bottom line
Fasted training isn’t superior to fed training if your goal is weight loss. If you enjoy training fasted or your schedule only permits training early in the morning, then train fasted. There’s very little difference between cardio in the fed or fasted state with regards to fat loss, muscle preservation, daily caloric intake or metabolic rate. What really matters, then, is you. Some people feel lighter and more energised when they do cardio on an empty stomach, while others feel light-headed and sluggish. Fed or fasted state? Go for whichever makes you feel better.
Article sources and references
- Aird et al. 2018. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scand J Med Sci Sports. 28(5):1476-1493.
- Conde-Pipó et al. 2024. Intermittent Fasting: Does It Affect Sports Performance? A Systematic Review. Nutrients. 16(1):168.
- Hackett D, Hagstrom AD. 2017. Effect of Overnight Fasted Exercise on Weight Loss and Body Composition: A Systematic Review and Meta-Analysis. J Funct Morphol Kinesiol. 2(4):43.
- Mysportscience. 2015. 6 ways to train low.https://www.mysportscience.com
- Wiley Online Library. Study on effects of exercise (exact title and authors not provided).https://www.onlinelibrary.wiley.com
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