Low-carb/high-fat diets like the keto diet are a popular dietary approach for weight loss and health improvement. This interest has leaked over into the elite athlete community.
Especially since some early studies have renewed intrigue around potential benefits of very low-carb, high-fat diets (PMID: 31713719).
Endurance exercise is prolonged activity sustained at a moderate-to-vigorous intensity. To fuel this activity, we depend on our stored carbohydrate, in the form of glycogen. Our glycogen stores are depleted after 80 – 120 mins (depending on the intensity), and we start to rely primarily on our fatty acid stores as our main form of energy.
This has sparked the question, if we improve our ability to oxidize (burn) these fatty acids more efficiently, can we improve our performance in these stages of a race?
Also, if we can increase our ability to use these fatty acids earlier in performance, can this spare some of the glycogen for later use?
The study done by Cao et al., (2021) is the only known systematic review and meta-analysis examining these questions. They examined randomized controlled trials and reported:
- No changes in V02 max between keto and normal
- Among 3 studies (48 participants) that studied time-to-exhaustion, there was no difference between keto and high-carb
- Among 8 studies (126 participants) that studied max heart rate, there was no difference between keto and high-carb
- Among 8 studies (103 participants) that studied respiratory exchange ratio (RER), it was significantly lower in the keto groups.
This finding simply shows that participants oxidized more fatty acids when exercising, which was to be expected.
Long-term studies may show different findings but, as of now, there is no convincing evidence to show that it can be beneficial over high carb intake for endurance athletes.
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