Exercise on fasting to lose more weight, good or bad idea ?
Doing sport for weight loss is a good resolution. But if you don’t start to quickly lose weight, you won’t manage to stay motivated! So you’ve decided to find the most efficient and fast way to lose weight. Why don’t you try to do sport while fasting? Efficient or not?
Why would doing exercise while fasting be a better way to lose weight?
As we’ve explained in our article “Running for weight loss”, your organism uses two main sources of energy: carbohydrates and fats. Your goal in order to lose weight is to burn as much fat as possible. In the morning, your carbohydrate stores are small because they’ve been used by your body during the night. Exercising while fasting would thus force your organism to burn body fat and then enable you to lose more weight.
Second reason to do sport while fasting: force your body to get used to burning fat as a first choice energy source and be more efficient in burning fat. This is really interesting for endurance sports like marathons.
A study showed that on 20 men who did a 6 week endurance training program (one hour to one and a half hour of cycling four times per week), those who exercised in a fasted state had a better increase of their muscles’ capacity to burn fat.
Doing sport while fasting could also increase the body’s ability to utilize oxygen which is necessary to burn fat efficiently. In 2010, a study was done on 8 men and 6 women, untrained, who undertook four weeks of endurance sport in either an overnight fasted or acutely fed state. Those who fasted showed a higher resting muscle glycogen concentration and a greater increase in VO2 max, the maximal amount of oxygen an individual can utilize.
But it is not that simple!
Scientific studies do not agree about the efficiency of doing sport while fasting to lose weight.
In 2011, 8 healthy young men performed a moderate intensity training lesson for 36 minutes, in a fasted state or not. Results: If during the exercise, the amount of fat burnt is more important in a fasted state; 12 hours after the exercise, the amount of fat in total energy expenditure is more important for those who had breakfast and it is the same after 24 hours. So if during the exercise, being fasted enables you to burn more fat, the trend reverses during the rest of the day. All in all, it seems better not to be in a fasted state to do sport and lose weight.
In 2014, a study showed there was no significant difference of weight loss between people having done cardio exercises three times per week whether in a fasted state or not.
On overweight women, after 16 weeks of low-volume HIIT (high intensity interval training) in a fasted state or not, Canadian scientists showed there was no significant difference concerning the oxidative capacity of muscles.
But apart from these controversial results, some disadvantages are associated with doing sport whilst fasted:
- It could make you lose muscle. If your stores of carbohydrates are too low, the organism can use protein as a source of energy. But there are not many studies about the effects of doing sport while fasting on the muscles.
- When you do sport in a fasted state, you can’t do as hard a session as when you have eaten, which means you will have less of an afterburn effect (your body will burn less fat after the exercise than with a more intense session). For more information on the afterburn effect, have a read of our article “Running for weight loss”.
- If you do too much sport in a fasted state, your body could get used to storing more fat during the meals.
- Sport whilst fasting can make you feel more tired.
- Watch out for signs of hypoglycemia.
All in all, should we do sport while fasting to lose weight or not?
Let’s take note that these studies have been done on a very small number of people, with different metabolisms, in different conditions. Efficiency of sport while fasting for weight loss definitely depends on the metabolism and the lifestyle of every single individual. In any case, if you do sport while fasting, don’t forget to eat slow release carbs the night before, drink water during the exercise and listen to your body.
What do you think about doing sport while fasting?
 : Riché, D. (1995). Guide nutritionnel des sports d’endurance. Edition Vigot