Carb-restricted ketogenic diets have become wildly popular in the last five years. This trendy low-carb, high-fat diet has gained a cult following among those wanting to lose weight, increase energy and improve blood sugar control.
Keto is proving to be much more than just a quick way to lose weight, though 
. Research suggests that ketogenic eating may also be beneficial for treating chronic diseases, including diabetes, cancer, epilepsy, Alzheimer’s, and Parkinson's disease, although much of this research is still in its infancy [2
More and more endurance athletes — including Tour de France cyclists like Chris Froome and Romain Bardet — are also adopting carbohydrate-restricted ketogenic diets in hopes it will boost their performance. Carbs have long been favored for fueling exercise, but some believe a keto-adapted athlete might be able to perform better because their bodies are primed to pull energy from a variety of sources, including free fatty acids, ketone bodies, and fat- and protein-derived glucose, in addition to muscle and liver carbohydrate (glucose) stores 
It sounds good in theory, but does going keto really improve cycling performance? Before we dive into the research let’s first take a look at what a ketogenic diet is and how your metabolism changes when you trade carbs for fat.
What is the ketogenic diet?
A ketogenic diet is a high-fat, moderate protein, low carbohydrate diet comprising about 70-80% calories from fat, 20-25% calories from protein, and a scant 10-40 grams of carbohydrates per day 
. If we’re talking about food, this might look like an egg and veggie omelet for breakfast, chicken avocado lettuce wraps for lunch, a bowl of zucchini noodle bolognese for dinner, and some nuts or olives for a snack. Doable right? Hold your horses. Keto is one of the most restrictive eating approaches out there and requires a lot of careful planning to keep intake to less than 40g/day.
What happens to your body when you eat keto?
By drastically cutting carbohydrate intake and simultaneously increasing fats, your body becomes incredibly efficient at burning fat, largely because carbs are in short supply. Over time, the body kicks into a metabolic state called ketosis, where fat replaces glucose as the body’s primary fuel source. Unlike Atkins® and other low-carb diets that put a lot of emphasis on protein, protein is restricted on the ketogenic diet. This is because, if consumed in high amounts, protein converts to glucose which can interfere with ketosis .
Once you start a ketogenic diet, it can take several weeks for your body to switch from using carbohydrates as its main energy source to using fats. The initial adaptation period often referred to as “keto flu”, can have some unpleasant side effects, including headaches, fatigue, nausea, increased hunger, sugar cravings, muscle cramps, poor concentration, and diminished energy. Athletes appear to have a longer adaptation period likely due to the heightened stress and energy demands placed on their bodies. Without long-term adaptation to the ketogenic diet (which can take several months), athletes might experience some additional, not-so-pleasant side effects including low blood sugar, significantly impaired performance, and slower recovery .
Once these adaptation symptoms subside, many keto-converts report a decrease in cravings and increased energy and clarity. Keto-adapted athletes often report improved energy for both training and competition, improved inflammatory-related symptoms, reduced muscle soreness, and GI distress 
Does eating keto help (or hurt) cycling performance?
A body that is primed to utilize energy from a variety of sources sounds like it might have some promising performance benefits for cyclists, but can ketones and fatty acids really supersede glucose during exercise?
At this time, limited research shows no clear performance benefit to athletes following a ketogenic diet — endurance or otherwise [7
]. The only exception to this has been in vigorous, shorter-duration exercise in recreational athletes when diet-related weight loss was likely a contributing factor 
The keto diet has two big downsides for athletes: limited glucose availability and increased oxygen demands.
Despite increased fat burning, endurance capacity in keto-adapted athletes still appears to be limited by glucose availability 
. A keto-adapted cyclist will simply have less glucose available to fuel a long ride than a carbohydrate-fed competitor, and research to date suggests that this impairs performing at your very best, even if carbohydrate is temporarily restored pre-ride [9, 10]
Another reason why keto might not offer bigger performance benefits is that ATP (energy) production from fatty acids requires more oxygen than glucose does [9, 10]
. These greater oxygen demands decrease efficiency during exercise and thus limit the potential performance benefits for athletes following a strict ketogenic diet.
It is worth mentioning that, while a ketogenic diet likely won’t improve
cycling performance, it might not hurt it either if you’re working below maximum effort. A multi-study review found keto-adapted endurance athletes (including cyclists) were able to maintain moderate-to-vigorous intensity endurance exercise performance while following a keto diet 
. So if you’re considering going keto for its other health benefits, it’s possible to continue exercising at moderate-vigorous intensity once you’re fully adapted, however, keto is probably not the best approach for peak cycling performance. As for how long that adaptation period should be, the research suggests you should give it months at least 
One more thing to keep in mind is that the keto diet is highly restrictive and while it may not completely tank your cycling performance, you may fall short on other nutrients such as fiber, calcium, magnesium, selenium, vitamin C and B-vitamins. And over time, these nutrient deficits may leave you feeling pretty lousy.
If you’re a trained cyclist, relying on fat as your primary fuel source likely won’t help you break any personal or world records. That’s not to say you can’t still perform well on a keto diet — just keep in mind the length of your adaptation period is key. If you’re committed to keto and have a longer training ride or race on the agenda, having some carbs handy may temporarily kick you out of ketosis but will help you keep up with your carbohydrate-fed competitors.
Disclaimer: The text, images, videos, and other media on this page are provided for informational purposes only and are not intended to treat, diagnose or replace personalized medical care.