Training on an empty stomach
Within endurance sports, it is common for many athletes to perform training sessions on an empty stomach.

The main objective at the center of this method is the optimization of sports performance, by adapting the body to use more fat as a source of energy.
In reality, from a physiological point of view this is not the most effective solution ...
1) Physiological adaptations
During training on an empty stomach, the effort will be carried out with reduced hepatic glycogen reserves (stock of sugars in the liver), but not in the muscles.
According to the work of Dr. Fabrice Kuhn (author of the book Ultra Performance), it is the fact of having low muscle glycogen reserves during exercise, which will allow the physiological adaptations of the body to optimize sports performance in endurance.
We will then talk about low glycogen training
An example with "Sleep Low":
A high intensity session at the end of the day with optimal glycogen reserves
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A dinner without carbohydrate recharge
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A short, low-intensity session on an empty stomach the next morning
This type of method must be carried out punctually and introduced gradually within a structured training plan.
In a logic of progression, and in order to be fully efficient during intensity sessions, it is necessary to have a sufficient reserve of glycogen, in particular thanks to a periodization of carbohydrates according to the training schedule.
2) psychological adaptations
Simple training on an empty stomach, on the other hand, can have positive mental effects.
Indeed, performing this type of session allows us to overcome this psychological barrier of lack of food by getting used to the sensation, but also that of the fear of energy failure.
This will increase our self-confidence, proving to us our ability to adapt and make an effort without taking an energy intake beforehand.
CONCLUSION
Training on an empty stomach should not be seen as the most effective solution for optimizing the use of fat as a source of energy.
It is a leverage effect on certain points, which can be integrated into a structured planning over short and not very intense sessions.