Keto helps burn fat for fuel — but can hurt athlete race times

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A new study shows keto athletes burn more fat — but get slower — when compared to those who eat a high-carb diet.

This leads us to inquire whether cutting out carbs actually hurts or helps competitive endurance athletes. And, perhaps, more importantly, what does it mean for us “mere mortals” who aren’t sports professionals?

This is a question that has been batted around for years, especially as we see examples of low-carb athletes like Zach Bitter breaking endurance world records or cycling phenom Chris Froome winning the Tour de France while reportedly eating a ketogenic diet.

Even while we have evidence of athletes who have excelled while restricting carbs, there is some scientific evidence that seems to suggest that keto diets are harmful — not helpful — for competitive athletic performance.

Louise Burke, an Australian sports dietitian, published a study in The Journal of Physiology back in 2016, which showed how eating a keto diet dramatically improved race walkers’ ability to burn fat for fuel.

Burke’s study fits well with another recent study, which shows how muscle adapts much better to burning fat when on a low-carb diet. This study also shows how muscles can actually maintain this fat-burning ability — even after shifting back to a higher-carb diet.

However, Burke and her colleagues also found that this increase in fat oxidation actually hindered the body’s ability to efficiently utilize energy. As a result, race times eventually began to suffer for those who were restricting carbs when compared to those who ate a higher-carb diet.

Burke’s study was met with significant pushback about the trial design. Criticisms of Burke’s process include having too short of an adaptation period and not using a meaningful performance measure, among other issues.

In response to these criticisms, Burke and her colleagues performed a replication study, which addressed some, although not all, of the concerns. This was published in PLOS ONE, a peer-reviewed open access scientific journal.

The results of the second trial matched the first. The race walkers who ate a low-carb keto diet improved their fat oxidation but decreased their 10k and 20k race times.

The high-carb athletes improved by 134 seconds, and the low-carb athletes slowed by 61 seconds. And so, the authors concluded that very low-carbohydrate diets are harmful for endurance athletic performance.

On the surface, this seems very reliable. Based on this study, if you are an endurance race walker considering a keto diet, it may not be the best choice for your performance.

But, what about those who are chronically in ketosis? Drs. Stephen Phinney and Jeff Volek have mentioned that it may take up to six months for an athlete to fully adapt to keto performance.

However, it’s important to note their conclusions appear to be based more on clinical experience than scientific studies.

Since we don’t have strong evidence that chronic ketosis provides a competitive advantage, some, like Alex Hutchinson, an award-winning journalist, claims that we should default to the position that a keto diet is harmful to performance. The burden of proof, he says, is on those who claim otherwise.

From a scientific perspective, Hutchinson has a great point. But, here is my question: Does this matter for most of us? Is this evidence having to do with competitive race times worth applying to the life of the everyday person striving for better health?

If you are an elite athlete competing professionally, then 61 seconds could be considered an eternity. In this case, it would be wise to make sure you’re following the latest science so that you can maximize your competitive edge.

But, if you are someone who’s interested in optimizing your health, burning fat for fuel, and experiencing the benefits of a low-carb lifestyle, then it’s likely the case that you can afford that 61-second loss.

For me, it’s worth it. Which category are you in?

Thanks for reading,
Bret Scher, MD FACC


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