We often hear that practicing physical activity on an empty stomach would burn more fat and accelerate weight loss. An attractive idea, especially for those who train in the morning. But is it really effective, and above all without risk? In short, should you stick to it? Answers from a doctor.
The principle of sport on an empty stomach: burn more fat, really?
Exercising on an empty stomach means working out after 10 to 12 hours without eating, usually first thing in the morning. In this context, glycogen stores – the fast fuel for muscles – are low. The body is then forced to use more fat to produce energy.
“When glycogen stores are low, the body will actually turn more quickly to lipids”explains Dr. Kierzek. On paper, this seems ideal for losing fat. However, this increased use of fat during exercise does not necessarily translate into greater overall weight loss in the long term.
What really happens in the body
Deprived of hepatic and muscular glycogen, the body primarily mobilizes fats, a process that is slower and more costly in terms of oxygen. Exercising on an empty stomach also increases the stress hormone cortisol, as well as oxidative stress. Effects which, in the long term, can disrupt metabolic balance.
“Studies show that there is no significant difference in body composition between sport on an empty stomach and sport practiced after eating”reminds the doctor. In other words, burning more fat during the session does not guarantee losing more weight in the end.
Limited and very targeted benefits
For trained athletes, sport on an empty stomach can have some advantages. It may promote certain metabolic adaptations, including a better ability to use fat as an energy source. Some also describe a feeling of lightness linked to the absence of digestion.
“This may be suitable for short, moderate sessions, such as a 15 to 20 minute jog, leisurely cycling or yoga”specifies the doctor. Under these well-regulated conditions, the negative impact remains limited.
But also risks not to be underestimated
On the other hand, for beginners or during more intense efforts, the risks quickly take over. Hypoglycemia, dizziness, malaise, loss of balance: even if the body has mechanisms to regulate blood sugar, they are not always enough.
“At high intensity or over long sessions, we observe a clear drop in performance, rapid exhaustion and an increased risk of injury”warns Dr. Kierzek. In the long term, repeating this type of training can also lead to chronic fatigue, loss of muscle mass and digestive problems after exercise.
Should we then ban sport on an empty stomach?
Not necessarily. “It is not completely useless for targeted fat loss in experienced athletes”recognizes Dr. Kierzek. But this practice remains controversial and should be used sparingly. For the majority of people, alternating nourished training with, perhaps, a few light sessions on an empty stomach seems to be a more balanced and safer strategy.
The golden rule remains the same: hydrate well, listen to your sensations and stop immediately if you feel unwell. When it comes to sport and weight loss, regularity and adaptation to one’s own level remain much more effective than promising shortcuts.