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 an essential criterion 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 the simple logic of both camps gets muddied when you consider the complexity of physiology and psychology of us as human beings. In this blog post, I’ll look at the fasted versus fed exercise debate.
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. I’ll dive straight into that last point first. If your aim is to lose weight, then are you better off exercising on an empty stomach?
It is a seductive idea about weight loss: you want to ensure all that sweat is using up your body’s fat stores to fuel its endeavours rather than grinding your gears and going nowhere because you’re metabolising the meal you just ate before exercise. But what is really going on?
Fat burning 101
Let’s take a step back first and talk about ‘fat burning’ in general. Well-meaning advice claims that you need to exercise at a low-to-moderate level of intensity to burn fat.
Now yes, it is true that the body burns the greatest percentage of fat at lower intensities of aerobic exercise. But at higher intensities, you burn far more kilojoules – and more fat kilojoules – overall.
If you were only concerned about burning the greatest percentage of kilojoules from fat, then sleeping is where it is at, but the total energy cost of this is tiny. Low-intensity workouts do promote weight and fat loss; you just have to do them for a longer time.
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 it.
Fed or fasted?
So if we take it that it is the intensity and duration that matter most for long-term weight balance, what about the follow-up question of whether being fed or fasted makes a difference when you exercise?
Doing cardio exercise does increase 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 all things being equal when comparing it to a similar situation when doing the exercise fed. Why should this be the case?
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 ‘delayed gratification’ that you’re just shifting what you would have eaten before exercise to later on. Meaning you would be in a state of energy excess at that time.
Now, for the person who exercises fed, sure, they may experience less fat oxidation during exercise, but the energy deficit will be delayed as they won’t be eating as much later on.
Again, remember I said here that all things being equal in that our fed and fasted exercise scenarios both involved the person eating the same amount of food over the course of the day.
That’s your theory done, so let’s delve into what the actual science and clinical research studies have to say.
Clinical trials have the answer
A 2017 systematic review and meta-analysis looked at the effect of overnight fasting before exercise compared to 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.
What about exercise performance when fasted though, is it impaired? 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 the situation was different when workouts went for more than an hour and this is where eating before the workout improved performance.
I want to stress that the findings from these reviews represent the average of many individuals. There are always lots of inter-individual differences. But 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. I’m a morning runner and really struggle to run with food in my stomach – I just feel sluggish for it so whether it’s psychological or physiological, I just feel better running on an empty stomach. Others need to eat before exercise as they feel their blood sugar falling and experience a lack of energy.
So, whether you train fasted or fed comes down to personal preference and scheduling. If you feel you need to eat before exercise and it sits well with you, then eat. Do you prefer or enjoy training fasted? Then train fasted.
As a disclaimer though, 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 taking on some food before you train. For exercise under an hour in duration or if it is a low-intensity training session, eating before will make little difference unless you just feel better in your mind for it.
The case for intermittent fasting
Another extension on the question of exercising in a fasted state is how it applies to intermittent fasting. On the surface, some of the principles of intermittent fasting do seem to go against the core sports nutrition concepts of refuelling post-exercise and spreading food consumption (especially protein) over the day.
But in fact, there could be some benefits from ‘training low’. These are times when carbohydrate especially is limited to promote favourable aspects of metabolic fat adaptation. Periods of ‘training low’ are then offset by times of ‘training high’ for high-quality sessions and race days where peak performance is needed and where you would revert back to standard feeding regimens. So, intermittent fasting could fit within this type of protocol if it was periodised.
A systematic review published in 2024 on this very topic of intermittent fasting and athletic performance concluded that intermittent fasting provides benefits in terms of body composition 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. This is separate though to using intermittent fasting for weight loss to help improve the power-to-weight ratio in a sport where a performance benefit would be likely, but intermittent fasting is just one in a long line of ways to achieve weight loss.
If you’ve consistently trained in a fasted state, the metabolic and muscle adaptations you’ll acquire will boost performance when you compete in a fed state. And that’s everything, isn’t it? A fasted workout trains you to perform under difficult physiological conditions of low fuel availability and that comes in handy. 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.
What it all means
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.