For decades, carbohydrates have been at the centre of sports nutrition guidelines to help maximise physical performance and maintain muscle glycogen levels. But interest in an alternative approach, that being very low-carbohydrate ketogenic diets, has been growing for some time. With the dual aim of reducing body fat and enhancing the use of fat as a primary fuel source during exercise, there are plenty of glowing social media testimonials attesting to the benefits of this way of eating for athletes. But the research evidence for some years has been painting a very different, and often, opposing picture. And that’s what I explore in today’s podcast episode when I look at the findings of a recent systematic review on the topic of how the keto diet performs on exercise performance and body composition in athletes and trained individuals.
Keto is the theme of today’s podcast, so let’s start by talking about what defines a keto diet. Here, you are talking about limiting carbohydrate intake to less than 50 grams per day. So that means for a normal diet that isn’t low in calories, carbohydrates will contribute to less than 10 percent of the total energy of the diet. The remaining macronutrients come from fat – up to 80 percent of energy - and then of course protein.
The idea behind going so low with carbohydrates is to increase ketosis. Ketones are what our body can use as a fuel source when carbohydrate is in short supply. We make ketones from fat. You have ketones in your blood right now, but when you are in ketosis from going on a ketogenic diet, you have more of them.
It is promoted ad nauseam by those advocating keto diets that being in ketosis means you’re burning more fat. Just no – you’re being hoodwinked. Let me explain why. Yes, you’re burning more fat, BUT get the image out of your head that it is all coming from your body fat stores. It isn’t. The extra fat burning seen on a keto diet is majorly augmented by…..all that extra fat you are eating in place of carbohydrates. In the end, it is the calorie deficit caused by following a restrictive diet that results in weight loss, not any magical body fat-burning ability of ketosis.
But could keto have a greater advantage over carbohydrate fuelling when it comes to performance? Certainly, keto diets saw a surge of popularity some years back with promises of enhanced athletic performance and with plenty of champions on social media singing its praises. Now firstly, in defence of keto diets, if a keto diet helps a person to lose weight and that then improves their power-to-weight ratio, then sports performance benefits could follow. But any diet can do that for you so this only applies if keto is a diet that connects with a person more than other approaches.
Most of the claims about keto were more to do with sparing glycogen stores and being able to better use fat as a fuel source in sport. However, when it comes to energy production, even though fat is much more energy dense than carbohydrates on a per gram basis, you need more oxygen to gain a fixed amount of energy from fat compared to carbohydrates. So keto is fine for lower-intensity steady state exercise, but for peak power output, this could be a detriment. And this is by no means academic speculation as the research base looking at this has been building for some years.
Now we have a new meta-analysis that has compared the effects of a ketogenic diet against carbohydrate-rich diets and their effects on physical performance and body composition in adult athletes and trained individuals.
The review included 18 controlled trials that investigated the effects of a ketogenic diet on physical performance and body composition in trained participants. A ketogenic diet was considered as having less than 50 grams of carbohydrates per day or less than 10 percent of energy coming from carbohydrates. The comparison control diets had a minimum of 40 percent of their energy coming from carbohydrates. The studies ranged from 4 days to 12 weeks – which is plenty of time to see fat adaptation on a keto diet.
The outcomes looked at were endurance exercise performance (measured as average power output), 1-repetition maximum performance as a measure of strength and finally body composition changes. And I’ll link to the study in the show notes https://pubmed.ncbi.nlm.nih.gov/35757868
So, what did the review find? When compared to a standard diet that contained at least 40 percent of the energy coming from carbohydrates, the ketogenic diet was found wanting. There was a drop in endurance exercise performance measured in a time trial by 3.3 percent. And this performance impairment was greater in running or race walking than in cycling, and also greater in shorter time-trial durations. As well, the higher the VO2max of the participant (VO2max is a measure of fitness), the greater the impairment in performance when following a ketogenic diet.
When it came to pure strength power output using the 1-repetition maximum test, again the ketogenic diet was inferior with a drop in strength of almost 6 percent. This drop in strength was partly explained by a lower fat-free mass in participants on a keto diet of 0.8 kilograms. Muscle is a key component of fat-free mass. No athlete ever wants to lose muscle mass
But if it was only about what the scales read, then yes, keto was superior for weight loss and saw an additional 2.4 kilograms in body mass decline. And the longer a ketogenic diet was followed, the higher the body mass loss. But this needs to be tempered with the very well-known fact that a ketogenic diet causes a greater loss of water and glycogen. And the day will come when a person eats carbohydrates again and that weight will just go back on. So it isn’t all really ‘weight loss’ in the sense of what most people want – that being fat loss.
Interestingly, across the studies, the researchers saw a likely sex effect with greater impairment when on a ketogenic diet seen in female athletes than in males.
It is hard to find many positives in this review to give any endorsement to ketogenic diets in athletes. About the best you can say is that if you’re interested in more rapid weight loss to get the shredded pics for your Insta account then keto is for you, but for a serious athlete, keto diets should be treated as ‘approach with caution’ and if used, only for periodic parts of a training cycle to help with fat adaptation benefits.
For long endurance steady-state activity however lower-carbohydrate diets may be beneficial, and why such diets are popular in the ultra-endurance world, but you will struggle to find much of a groundswell of support for them in most sports, especially on competition day.
So, there can be a place for keto as part of diet cycling in training, especially for endurance athletes, and maybe even for weight loss, but you won’t find many podium athletes who got there from doing keto day in and day out.
So, let’s wrap all this up. For endurance and power athletes, keto diets can help with weight and fat loss. Which for these athletes could improve their power-to-weight ratio. But the price paid for that is on muscle mass and endurance performance. So it isn’t too much of a surprise that you are probably hearing less about this diet in the sporting world these days. And while you certainly can see evidence of metabolic changes in favour of fat adaptation when following keto, to paraphrase the doyen of sports nutrition, Professor Louise Burke, they don’t give out Olympic gold medals for having optimised metabolic enzymes.
So that’s it for today’s show. You can find the show notes either in the app you’re listening to this podcast on if it supports it, or else head over to my webpage www.thinkingnutrition.com.au and click on the podcast section to find this episode to read the show notes.
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I’m Tim Crowe and you’ve been listening to Thinking Nutrition.