Fasted Exercise in Triathlon Training: Is it Right for me?



It’s common for athletes to be told exercising in the overnight-fasted state (i.e. before eating breakfast) is a good thing, yet others say it is something that should be avoided. This post will help you understand this contrast and decide if and/or when it might be right for you.

Will fasted exercise burn more fat?

One of the main reasons people perform fasted-state exercise is to burn more fat. It is true that you will generally burn more fat (and less carbohydrate) when you are exercising in the fasted state compared to when you exercise after breakfast. However…

  1. This doesn’t necessarily mean that you will lose more body fat over time.
  2. This doesn’t necessarily mean that you will be better at burning fat during a race (when you’re not fasting).
  3. If you consume protein (without carbs) for breakfast before your workout, i.e. protein shake, you will burn the same amount of fat during exercise as if you were fasting.

Will I get more out of training when fasted?

Many people think that exercising in the fasted state will give your body a “better” workout than if you’ve eaten. Taking a closer look at the matter, it appears like fasted training might add some benefit when someone is usually eating too many calories or eating unrealistically large meals before exercise (i.e. 150 g carbohydrate). But under more typical circumstances the research is still unclear as to whether you’ll get additional benefits out of your workout, particularly for trained athletes.

Does training fasted affect my performance?

Yes and no. The best evidence we have (from a meta-analysis of 46 different studies) suggests it may depend on the length of exercise. If your workout is less than 1 hour in duration, it probably doesn’t matter for performance if you eat or not. For example, research in trained athletes shows no difference in interval training capacity during a 1-hour training session in the fasted or fed state. But as your workout duration extends you will probably perform better if you have eaten something beforehand.

Will having breakfast keep my blood sugar more stable?

Your blood sugar may be less stable after a meal. However, that’s not necessarily a bad thing. Your blood sugar will be higher after a meal, and then gradually come down during exercise.

In the fasted state your blood sugar will stay quite stable, but depending on the exercise intensity and duration, may eventually start to drop down a bit below baseline values. In the big picture, if your workout is around an hour or so, you probably don’t need to worry about it.

Convenience and personal preferences of fasted training

Some people do fasted training simply out of convenience (it’s easier to just wake up and train), preference (just don’t feel like eating), and/or gut comfort (don’t like training with a full stomach). These are all valid reasons to train on an empty stomach.

Will I break down my muscle by training fasted?

No. Muscle glycogen (our carbohydrate ‘gas tank’ in our muscle) does not get used overnight. This means whatever is in the tank when you go to bed will still be there for your workout in the morning, and this is what gets used during the workout. Just don’t forget to eat after exercise so your body can recover and build muscle!

Will I get too hungry from training fasted?

Research in trained cyclists performing a 1-hour training session found no difference in hunger between fed and fasted states. Similar to performance capacity and blood sugar, these differences would likely appear as your workout duration extends beyond an hour or so. It is also possible to start out in the fasted state, and then consume a sports drink, gel, or bar in the middle of the workout when you need a boost.

Can I create too big of a calorie deficit by training fasted?

Yes, and this is something athletes often don’t think about but maybe one of the most important reasons for the hard-training athlete to avoid fasted training.

It is easy to assume that daily calorie balance matters most, yet it has become increasingly clear that the distribution of calories within the day matters a lot too.

Imagine you have a bucket that represents your energy balance. Throughout the day your bucket is continually getting filled up (as you eat) and emptied out (as you burn calories). We don’t want to empty the bucket out too much by creating such a huge deficit, and we don’t want to overflow the bucket by eating a huge surplus.

In the morning you are already in a small energy deficit because you haven’t eaten since the previous evening. Imagine you do an early morning workout before breakfast, and burn 600 calories (you could easily burn 400-1200 kcal during a 1 h workout). Even if you eat a large breakfast, you could still be in a calorie deficit until lunchtime.

You may wonder if this calorie deficit matters, as long as you end up in a calorie balance at the end of the day. It does. Imagine an extreme example, where someone does a big training session in the morning, and eats all of their daily calories at dinner. Most people would (correctly) assume that type of eating pattern is not ideal. But it is surprisingly common for endurance athletes to eat a quick breakfast before rushing off to work, then try and have a “healthy” lunch, and find themselves making up most of their calories later in the day. This too, is far from ideal, as large within-day energy deficits have been associated with hormonal dysfunction and even a reduced metabolic rate in both male and female endurance athletes! While deficits are normal, we want to avoid spending too much time in too large of a deficit.

If this resonates with you but you want to get some of the potential benefits of fasted training like increased fat oxidation, you can try having a low-carbohydrate snack before training like a scoop of protein powder or a low-carbohydrate bar, which provides calories while still allowing high levels of fat oxidation.

Summary: How should I apply these insights on fasted training?

First, think about how long your workout is going to be. If your workout is longer than ~90 minutes, most people will probably be well-served by eating something before exercise. Then, decide what to eat based on how hungry you are. If you’re hungry but want the additional fat-burning benefit of a fasted workout, remember you can have a low-carbohydrate breakfast and still burn the same amount of fat as if you were fasted. The intensity of the workout may also play a role in your decision, particularly if the workout is longer than an hour. Finally, observe how you feel, and how you perform in both fasted and fed states. You might find you’re not as hungry as you think during a fasted workout, or you might find that it just doesn’t work for you!

Stay tuned