Should I train fasted? As with all nutrition questions, there is no simple answer, there is a lot of nuance, there are individual factors that need to be taken into account, and there is a lot of misinformation out there.
With that said, we are going to look from a purely scientific, performance driven, competitive CrossFit standpoint.
With this in mind, my first answer would be- not during the competition season.
At this point in the season, the focus is on high intensity training, getting fit to compete. It is important to undertake high intensity sessions with normal or high carbohydrate availability, to make sure that intensity is achieved, and to ensure the body’s ability to effectively utilise carbs is maintained.
Training fasted, or in a low carb state may be important for endurance athletes, as by training our fat metabolism we can make this process more efficient. As crossfitters however, the longest competition workout we do is going to be about an hour, which, assuming we have reasonably full glycogen stores on competition day, is not long enough to require a switch to fat metabolism. Therefore there is little point in training the fat energy system to be efficient, and instead we should be concentrating on training the ability to use carbs.
During the off-season however, the story may be a little different.
Training at prolonged low intensity in a fasted state has been shown to upregulate many of the signalling molecules that are associated with mitochondrial biogenesis, meaning we make more mitochondria. Mitochondria are the engine room of the cell, the place where oxygen is converted into energy in the form of ATP. By increasing the number of mitochondria, we increase our ability to use oxygen and produce energy. We become more efficient.
This is where the concept of periodised nutrition comes in. We still need to do some training at high intensity during the off season, and obviously strength training, neither of which are best done when fasted. What we could do then would be to have a high carb dinner the night before and high carb breakfast on the morning of a high intensity or strength training session. Following the session, when glycogen stores are low we have a low carb meal, then ‘sleep low’ and carry out our prolonged low intensity (zone 2) training in a fasted state the following morning. We then have a high carb meal following this session to replenish our glycogen stores, and can start the cycle again. The diagram below illustrates this nicely.
By carb cycling in this way we are able to reap the benefits of adaptation, without increasing muscle protein breakdown, compromising immune function or being unable to reach high intensity when needed, all of which can happen if training is carried out in a chronically carb-deplete state.
If cardiovascular fitness is the main fitness modality that you need to improve during the off-season, then periodising nutrition could just be of use to you.
References
https://www.science.org/doi/10.1126/science.aau2093
https://pubmed.ncbi.nlm.nih.gov/26112242/
https://pubmed.ncbi.nlm.nih.gov/26741119/
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