Going on a keto diet for just 5 days makes elite cyclists slower in time trials, adding about 3 more minutes to their ride time.

What we've found so far suggests that a 5-day ketogenic diet may impair cycling time-trial performance in trained endurance athletes. Our analysis of the available evidence shows a consistent direction in the findings. We reviewed 60.0 assertions from the research, and all of them support the idea that short-term keto dieting reduces performance in elite cyclists. Specifically, we found that when trained endurance athletes follow a ketogenic diet for just five days, they tend to be slower in time trials, with one analysis indicating an increase of about 3 minutes in their ride time [1]. No studies in our review refuted this effect. This means that even a brief shift to a high-fat, low-carb diet may interfere with how well elite cyclists can perform in timed endurance events. While the exact reasons aren’t detailed in the evidence we’ve reviewed, the performance drop could be linked to reduced carbohydrate availability, which muscles rely on during high-intensity efforts. Our current analysis shows the evidence leans toward a negative impact on performance, but we also recognize that this is based on a limited scope of assertions. We don’t yet know how individual differences—like how well someone adapts to ketosis—might influence these results. Also, we can’t say how longer-term keto adaptation might change the outcome, since the data we analyzed only covers five days. As always, our understanding is based on what we’ve reviewed so far and will continue to improve as more evidence is added. Practical takeaway: If you're a trained cyclist aiming for your best time-trial performance, cutting carbs for just five days might not be the best move—your clock time could pay the price.