For many years when I was trying to lose weight. This was a frequent question I’d google: ‘Is fasted exercise better for weight loss?”
I would read articles stating the body needed to be fasted to burn the fat stores during exercise. It didn’t help with bodybuilding forums and Netflix documentaries preaching the benefits of fasted exercise, with very little practical evidence.
The premise being if you have eaten, your body will store some of that energy as glycogen (carbohydrate). Therefore if you exercise your body will use glycogen for energy instead of fat. However, if your body is depleted with glycogen then you would just burn fat.
Then there was the case of the fat burning zone. Where you had to exercise for 20 minutes before your body would burn any fat.
Like a lot of fitness myths, there is a small element of truth. So it’s only fair I credit that truth and explain it. Then I’ll demonstrate how it fits into the bigger picture.
Overall for the majority of people fasted exercise will not be superior. But there might be some nuance, which I think is only fair to explain.
Fat burning when fasted
The thought is, when the body has no carbs, it will burn its fat stores for energy. The thing is, this is true. The bodies preferred source of energy is carbs. But when carbs aren’t available, fat is usually abundant.
Therefore when you train fasted, your body will use its fat stores, to provide energy.
The problem is however, just because you burnt more fat during your exercise session, does not mean you burn more body-fat during the day.
Fat loss is governed by the number of calories burned throughout the whole day. So, if you trained fasted in the morning and burned 2500 calories per day. But let’s say you ate 3000 calories that day. Yeah, you would have burned more fat in that session. But because you ate 500 calories extra during that day, you will still gain weight. Your body has taken in more energy than it has burned, therefore the extra energy will be stored as fat, regardless of training fasted or not.
On the contrary, if you ate breakfast and trained an hour later for example. Your body would burn glycogen carbohydrate for fuel. This means body fat will likely not be burned during your session. But let’s say you burned 3000 calories throughout the whole day. But you ate 2500 calories that day. Despite not burning fat in that session. You have still undereaten during the day, so to make up for it, your body will have to dip into its fat stores regardless of when you ate to make up for that deficit.
Therefore, whether you eat before or after your session is irrelevant to whether you burn body fat or not. Focus on the number of calories burned, throughout the whole day. Fat loss is determined by what happens in the whole day, not the 1 hour of exercise in isolation.
One study was done with 20 young women. 10 women were assigned to a fasted before training group. The other 10 ate before training. Both groups had 3, 1-hour cardio sessions per week. Both groups had to adhere to a calorie deficit. Both groups lost a significant amount of weight. They worked out the weight loss and body composition was no different for either group. (1)
These findings indicate that body composition changes associated with aerobic exercise in conjunction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training.Schoenfeld et al, 2014
I will say it was a small sample and a short time frame. But this study showed what it showed.
In 2017 a meta analysis was done, analysing the best studies and this is what they found:
Every study showed there were so significant changes in weight between the fed and fasted groups. All that mattered was that the participants in each study created a calorie deficit, which was controlled by diet manipulations. (2)
These findings support the notion that weight loss and fat loss from exercise is more likely to be enhanced through creating a meaningful caloric deficit over a period of time, rather than exercising in fasted or fed statesHacket & Hagstrom, 2017
As usual, the evidence suggests what we don’t want to hear. Eat fewer calories than you burn. There’s no hacks to it, it just is what it is.
Our review of a small number of studies does not support the use of fasted exercise for weight lossHacket & Hagstrom, 2017
and positive changes in body composition. Futhermore, our findings also suggest there is no detrimental
effect on body mass and body composition with utilizing this practice
On the contrary, just because fasted exercises doesnt impriove fat loss, doesnt mean it’s a bad thing to do.
Should I quit fasted exercise?
This leads people wondering, whether they shouldn’t bother with fasted exercise altogether then. This is a double edged sword. There is some evidence to suggest that training an hour or so, with a little carbohdrate in the system, allows for a better performance.
In 2018, a meta analysis reviewing 46 studies showed, that training after eating, did elicit better performance in participants over a longer duration. (3)
On the other hand, you have people like me, who genuinely prefer to train fasted. There’s a couple reasons why.
- I just personally feel better when I don’t have any food in my system. Not saying this is right, this is just my personal preference. If I train after eating something small, I genuinely don’t feel any difference myself.
- I like to train first thing in the morning. Exercising first thing in the morning gives me a psychological win for the day. It makes all my food choices and everything else I do good. If I was to train fed, I’d have to wait a little while before training. But I like to get it done right away.
For other people, fasted exercise is part of their morning routine and part of their habit loop. A great book called ‘Power of habit” by Charles Duhigg talks about creating good habits. He talks about the habit loop, which goes cue, habit and reward. For some people eating after their workout is their reward. It’s what they associate working out with. So their cue may be putting on the workout gear. The habit is working out. Finally, the reward may be a chocolate peanut butter banana protein shake afterwards, which they associate with working out.
What I’m saying is for some people fasted training may work really well and fit your schedule perfectly. Whereas for others, training fasted may make you feel worse and lethargic and you may feel more fueled with some carbs a couple of hours before.
So when it comes to either fasted exercise or fed exercise, do what you prefer. Just a couple of points if you choose either:
- If fed, give yourself at least an hour after eating before exercising
- If fasted, make sure to consume some protein within at least 2 hours after training
When it comes to fasted exercise or fed exercise, it really comes down to personal preference. When it comes to fat loss, what matters, is sustaining a calorie deficit over a long period of time. That way the body will burn fat to sustain the energy it is producing. Worrying about whether to do fasted exercise for fat loss is like worrying about whether to pay the bills before you get paid or after. Neither way is going to save you money.
It doesn’t matter what fuel your body uses for exercise. What matters is the size of the calorie deficit created throughout the whole day.
If you train fasted make sure to consume some protein at least a couple of hours after. If you train fed, make sure to wait at least an hour for your food to digest.
Josh is a Registered Associate Nutritionist, with the Association for Nutrition (AFN). He completed his degree in Nutrition at the University of Roehampton in 2021. He passed with a First Class with Honours.
Josh is also a tutor for up-and-coming Personal Trainers, where he teaches a Level 4 Advanced Nutrition course. This is for Personal Trainers looking to upskill their nutrition knowledge. This is done at Norfolk Health & Fitness.