Running on an empty stomach – around 30 minutes at endurance pace – does not, in theory, pose any health risks. This is particularly true for runners who are relatively fit and used to training. An organism with enough reserves can tolerate moderate-intensity sessions without any prior caloric intake.
Remember: during the night, only hepatic glycogen is burned. The fuel stored in your muscles remains largely intact. However, training without having eaten beforehand mobilises the lipid channel earlier. Many runners therefore choose this type of session for weight loss (they sometimes speak of the desire to "dry out") and to increase their level of performance when sugar reserves are largely used up.
ADVANTAGES, BUT ALSO SOME DISADVANTAGES...
Regular training without having eaten beforehand improves the body's capacity for mobilising energy. A study published in 2008 proved that athletes regularly training in endurance sports burn fats more easily – and, at the same time, preserve more of their glycogen reserves – than sedentary or untrained subjects. They also rebuild their stores more quickly and naturally after exercise.
But be careful! Running on an empty stomach repeatedly and (sometimes) for too long can cause significant signs of fatigue, and even lead to headaches or digestive trouble. It is important to be reasonable in the frequency and length of this type of work, and no matter what, to make sure you listen to your body.
PRECAUTIONS TO TAKE
- Training without having eaten beforehand is not recommended for novice runners or those with no experience in endurance sports. Check with your physician to ensure that you do not have undiagnosed diabetes, which could lead to brutal hypoglycaemia.
- Eat a dinner rich in complex carbohydrates each evening before training on an empty stomach in order to improve your glycogen reserves.
- Be cautious and follow a true progression in the length of your morning outings. It is important to check how your body tolerates them and to take care when increasing their duration.
- Avoid leaving for an adventure across the countryside; instead, choose loop routes that can be shortened if you are not feeling well (dizziness, hunger, etc.).
- You should target your endurance pace (no more than 70% of your maximum heart rate) when training on an empty stomach. Avoid interval training.
- Unless training for a marathon – and only for experienced athletes – training on an empty stomach should not exceed 50 minutes. It is important to hydrate regularly during your workout.
- Eat immediately after the end of your workout.
"Low glycogen training", a training technique involving particularly intense efforts (intervals) by bodies low in carbohydrates, is practised more and more by Kenyan athletes. The goal is to become used to running in the same state as you would be during the last ten kilometres of a marathon – without a significant loss of performance.