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Low carb diet v high carb diet and cycling performance

What is better for performance: a high fat diet or a high carbohydrate? This is a question that is discussed over and over. Let me state upfront that I believe that the answer is probably it depends and sometimes a diet should have a focus on carbohydrate and sometimes a focus on low carb. In this blog we will look at one study that will help our overall discussion on this topic and in future blogs we will gradually form a complete picture. A few years ago researchers in South Africa published a study that sheds some light on this question. Let's have a look at this experiment.

High fat diet plus carb loading

The study, published in the highly respected Journal of Applied Physiology compared the performance effects of a low carbohydrate (high fat diet) with a high carbohydrate diet. It is known that a low carb diet will result in low glycogen stores and this has been shown to reduce performance the investigators made sure that the 6 days on either of these diets was followed by a day of carbohydrate loading so that glycogen levels would always be high and this could not be the reason for potential decreases in performance. The hypothesis was that the 6 days on a low carb, high fat diet would result in adaptations that would allow the body to burn more fat. Consequently the athletes would be less dependent on this diet and they would not run out of carbohydrate so easily. Ultimately the high fat diet plus carb loading was hypothesized to improve performance. By making sure the high fat diet was followed by carbo-loading the researchers put the low carb-high fat diet in pole position…

The hypothesis was that 6 days on a low-carb, high-fat diet would result in adaptations that would allow the body to burn more fat

Unique study design

The study was unique in that the performance test was multifaceted. The investigators included a simulation of a 100-km time trial (mimicking a real life event) and during this time trial they also measured 1-km and 4-km all-out efforts.

The study had a cross-over design. This means that every athlete performed all tests twice: once with each diet. They ingested either a high-carb diet (68% energy from carbs) or a low carb, high fat diet (68% energy from fat) for 6 days, followed by 1 day of carbo-loading (8-10 g carbohydrate per kilogram body weight). On days 1 and 8 participants completed a 100-km time trial. During the time trials subjects ingested a 10% glucose polymer solution at regular intervals (200 ml every 20 min). On selected days during the diet-week measurements of fat metabolism were performed during exercise.

What was found?

As expected the low carb diet resulted in higher fat oxidation during exercise in the week leading up to the 100-km time trial and this effect still existed when the participants carbo-loaded. However, it was found that 100-km time trial performance was not different between the two trials. Important, however is that 1-km sprint power output was lower during the low-carb, high-fat diet. Also the 4-km sections were getting slower on the low carb, high-fat diet and pace was maintained much better on a high carbohydrate diet. Statistically, there were no differences in 4km time trail performance between the diets.

1-km sprint power output was lower during the low-carb, high-fat diet


A low-carb, high-fat diet does result in adaptations that favour fat metabolism (Or impair carbohydrate metabolism). This increased fat metabolism did not translate in a performance effect, even when carbo-loaded before the event. It was clear though that high intensity sprint performance was compromised. Of course, we should never draw conclusions based on a single study and it is important to look at the totality of evidence. However, there are no studies at present that would contradict these findings and the vast majority of studies would support it.

Take home message

So even when a low-carb, high-fat diet was placed in pole position to outperform a high-carb diet, there was no benefit, and in fact, on average, the average 100-km time trial was slightly slower (not statistically significant). Important to note is that high intensity sprint performance, highly important in cycling races and many other events, was worse with the low carb diet.

Pros and cons


- A well conducted and well controlled study

- A good simulation of real life conditions by including sprints and using a time trial approach


- Muscle glycogen was not measured, so we cannot be certain that muscle glycogen was fully restored by the 1 day carb-loading

- The study was single blind not double blind. In other words it was impossible to mask what diets the subjects were receiving and ideally the subjects would not know what diet they would receive. This is of course very difficult to do when diets are so extremely different as in this study.



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