FIT for LIFE Knowledge Base Nutrition

What does nutrition science say about fasting training?

News – or Myth

At the end of the 1970s at the latest, researchers began to study the metabolism during training after short-term fasting from a few hours to a day. Already the results of the early studies showed an increased fat burn and savings in muscle glycogen during such training. But does this also lead to a better (endurance) athletic performance?

In the Internet there are many contributions to fasting training in endurance sports. However the information is often contradictory, and sometimes you can even find technically correct and extremely questionable comments in the same article. There is some confusion.

What and why?

As fasting training or training on an empty stomach, most people think of training in the morning before breakfast. However, the point of fasting training is not that the training takes place in the morning or specifically before breakfast, but that you don’t gain energy for several hours before the training. So a fasting training can also take place in the early evening, when you have had breakfast but are fasting until the training.

In fasting training, the last meal was eight to twelve hours ago. The metabolism returns in this short period of time to its basic mode with increased energy supply from fats. The goal of a fasted training is it to start with the fat metabolism already stimulated state and to hold or even strengthen this state in the training.

Fat burning is increased

In various on-line articles it is maintained, the increased fat burn is pure theory and no scientific studies would have occupied this. This is nonsense. There are over 50 well controlled studies on fasted training and in eleven studies on trained men and women also the energy supply during mostly continuous load with 60 to 70% VO2 max was examined.(1),(2) The load duration amounted to on the average 75 minutes and varied between 30 and 120 minutes. In the fasting state and in comparison to an appropriate load approximately two to three hours after a carbohydrate-rich meal, fat burning was increased by three grams (1). The effect was higher in men than in women and with better training condition the effect became smaller.

Improved overall performance?

A three gram higher fat burning in a 75-minute workout looks modest. But this increased fat burning is the mirror of various metabolic adjustments that can, among other things, make training more effective(2). Accordingly, fasting training should be considered as acute modulators of metabolism with potential medium to long-term effects. Whether regular fasting training will improve endurance performance at a later stage cannot be assessed at present. The corresponding, well-controlled studies are lacking. What is clear, however, is that the absolute performance during an exercise in a fasting state of one hour or longer and compared to a corresponding exercise after carbohydrate intake is worse. (2) However, for workouts lasting less than one hour, no negative influence of the fasting training on the immediate and effective performance is seen.

Also good for your health?

Fasted workouts influence muscles, fat tissue, and energy balance in a way that suggests a long-term positive effect on health(3) They have the potential to improve not only endurance performance but also health. But even this is for the time being only an assumption. Studies on the effect of regular fasting training on health are still missing. Practical tips: Too frequent sobriety training increases the risk of fatigue and overtraining. They should therefore be carried out as an alternative form of training and by no means as the main form of training. There are no studies on the ideal frequency, duration and intensity. General recommendations should be taken with caution and tested individually. Training once a week for 60 to 90 minutes at moderate intensity about 10-12 hours after the last meal seems to be a good choice for endurance athletes. For everyone else, the following applies: In fasting training, things do not always go as well as usual. Those who do not have any experience with this, start with shorter distances and/or pack a bar, gel or sports drink for safety in case of emergency.

1. Vieira AF et al. Br. J. Nutr., 2016; 116: 1153–64
2. Aird TP et al. Scand. J. Med. Sci. Sports, 2018; 28: 1476–93
3. Wallis GA, Gonzalez JT. Proc.Nutr.Soc., 2019; 78: 110–7

Author: Dr. Paolo Colombani, Nutrition expert Paolo Colombani puts a current topic for sport nutrition under the magnifying glass. After nearly 20 years at the ETH Zurich Paolo Colombani works as Chief Science Officer at Vegisan AG


Text from FITforLIFE– This Blog article was made available to us from the Swiss Magazine FIT for LIFE. If you want to regularly read informative articles in the field of running and endurance sports, then click here.

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