Fasted training: how long and how intense?

Joëlle Flück 8. October 2020

Many endurance athletes have often wondered when fasted training makes sense and for how long. The most important facts.

Presented by Swiss Sports Nutrition Society

In general, you need to take a closer look at the sport or respective goal that you have set yourself. If you are preparing for a short, intensive competition (e.g. a 3 km or 5 km run), the carbohydrate availability has top priority. Intensive training units, such as interval training (e.g. 5 x 1000 m), thus need to be done under optimal conditions as far as the intake of carbohydrates before the unit is concerned.

If, on the other hand, the focus is on optimising the body composition (e.g. fat reduction) or even preparing for a long-term endurance event such as an Ironman, it is quite reasonable and necessary to complete some training units or entire training phases with low carbohydrate availability, so that the body can resort to optimised fat burning. Such units can either be done in the morning before breakfast or during the day without any additional carbohydrate intake.

A general answer is therefore not possible. What’s clear, however, is that «fasted training» is not done at maximum intensity, but ideally at a moderate or even low intensity. This can ensure maximum performance during intense loads and prevent the body from being unnecessarily burdened by fasted training.

The time gap and diet have a huge influence

In the case of a fasted unit, a distinction must also be made as to whether carbohydrates were consumed since the last training unit the day before and thus whether the carbohydrate reserves in the muscles (glycogen stores) have been replenished or not. If carbohydrates were consumed after the last unit, the carbohydrate reserves of the liver in particular are emptied during the fasted unit. This leads to a higher proportion of free fatty acids in the blood circulation. Fat burning is also stimulated, and the proportion of carbohydrates burned decreases. The lipid metabolism can thus be trained with the help of a fasted unit.

Fasted training also has disadvantages

During a fasted training unit, the body is exposed to higher stress potential. If no carbohydrates are consumed during a longer endurance unit (>2 h), an increase in the stress hormones (e.g. cortisol) can be observed. The immune system is weakened, and the body can become more susceptible to an infection. In addition to the high-intensity units, which are done with the usual carbohydrate availability, an additional stimulus such as fasted training can represent a higher stress stimulus for the body and impair recovery.

Furthermore, if fasted training or training units without a carbohydrate intake are done frequently, this can lead to gastrointestinal problems later in the competition due to the intake of carbohydrates. It is therefore worthwhile frequently integrating your competition nutrition into your training so as not to experience any nasty surprises.


For runners preparing for competitions in the range of 5 km to a half-marathon, doing fasted units only makes sense to a limited extent. Ideally, they should do the intensive training units such as intervals with optimal carbohydrate availability. In some cases, endurance units can be completed in a fasted state, but at a relaxed to moderate intensity and for a duration of 30 to 90 minutes. For marathon runners or ultra-endurance athletes (including Ironman, a bike marathon, etc.), it can definitely make sense to do longer units in a fasted state. However, you should make sure that the carbohydrate reserves are adequately stocked up again after the unit and that sufficient time is planned for recovery.