CrossFit fasting

Training on an empty stomach is a subject that generates many doubts: will I have enough energy to train well? Is there a danger of catabolizing muscle? Is it easier to lose fat?

The ideal strategy will depend on each person's starting point and objectives.

We will answer all these questions with the most recent studies on fasting and training.

How we store and obtain energy

The purpose of consuming food before training is mainly for energy. Let's see how we obtain, store and use it.

Glycogen (already stored glucose) is our energy both in times of intense exercise and for the normal functioning of cells that do not metabolize fat. 

fasting crossfit

These are essentially those of our brain, although it can obtain its energy from ketone bodies, produced in a state of ketosis (low-carbohydrate diet).

Glycogen stores are concentrated in:

  • livershares its reserves when needed, although it only stores ¼ part of them.
  • musclesThey alone use their reserves, approximately ¾ of the total, and do not return to the bloodstream. Although they can produce lactate that the liver can transform into glucose.

If we train 8 hours after eating, the hepatic glucose reserves will be low, but the muscles will have enough stored energy. 

Benefits of training on an empty stomach

Fasting training has become popular because of all the reported benefits:

  • Improves fat oxidation and insulin sensitivity.
  • Helps metabolic flexibility. Your body wants to conserve energy so it makes efficient use of glycogen, develops new mitochondria and increases its storage capacity.

Risk of muscle catabolization

Surely no one puts that much effort into gaining lean mass only to lose it. The good news is that there is more fear or misinformation than real risk.

It must be understood that the destruction or catabolism of protein occurs with training. For muscle regeneration there must be an intake of new amino acids.

It is important to remember that this goes beyond the protein shake. What is really crucial is to provide the nutrients our body needs throughout the day and not so much the time when we do it. 

A shake by itself does not work magic, but rather the whole context of training and feeding.

Does it burn more fat? 

Training on an empty stomach favors the use of fat as fuel as we have seen, but the total impact is not very relevant. 

In addition, in individuals not adapted to fasting training, this can lead to a reduction in intensity and, therefore, in fat burning. 

So for real purposes CrossFit training on an empty stomach has little impact on fat loss.. But the post-workout meal is if your main goal is to reduce fat. 

Exercise increases growth hormone, which helps mobilize fat.

To extend its effect as long as possible, it is recommended that you skip the post-workout mealThe moment you ingest food, insulin is elevated and this growth and fat burning hormone is inhibited.

Does fasting CrossFit affect performance?

Training on an empty stomach negatively affects sports performance if:

  • Glycogen stores are very low due to fasting for more than 16 hours or very low carbohydrate diets.
  • the individual is not adapted to training on an empty stomach.

Once this process of adaptation to fasting is overcome, the burning of fat as a substrate to maintain physical activity improves and is not possible when training with high glycogen levels.

This point in particular is of great interest, not only for aesthetic reasons, but also because a crosfitter that is more efficient in its energy expenditure will be better able to withstand workloads.

Recommended strategy

As in most cases, a middle ground is usually best. 

It is interesting to train the body with low reserves to take advantage of the benefits mentioned above, but to compete with high reserves to be able to perform at maximum intensity.

On the other hand, doing CrossFit if you have eaten food in the last 2 hours it may be indigestible.. It depends on the rhythm of digestion of each person, but to avoid ending up vomiting due to a digestion cut, it is recommended to leave at least 2 hours (personally I need 4).


If your goal is fat loss: you can try to train on an empty stomach and after a process of adaptation you will be able to slightly increase fat burning. It is recommended not to ingest post-workout to take full advantage of the growth hormone raised by training.

If your goal is muscle gain: Training on an empty stomach is still a good option to help your body become more metabolically flexible, especially after you have managed to adapt to training on an empty stomach.

Post-workout food is important to avoid catabolizing, but even more important is the food you feed your body throughout the day.