If you want to spark controversy on a group ride, ask your fellow cyclists if they ride fasted or not. But if you want to spark an even more spirited discussion, bring up keto.
The keto diet for endurance athletes focuses on cutting carbs and increasing fat intake. It breaks down into some interesting macros: 75 percent of the diet is fats (think olive oil, butter and avocado), 20 percent of the calories are protein (meat, eggs or cheese), and only 5 percent of the calories come from carbohydrates. Eating this way puts your body in a state called ketosis.
Our body’s primary source of energy comes from glucose, derived from carbohydrates. Since the keto diet limits carbohydrates, it forces your body to burn fat as fuel instead.
Excess carbs are stored in the body as glycogen. When these stores get full, the body converts them to fat. If the body’s glucose stores are low, it will use fat for energy instead. This metabolic state is called ketosis. In ketosis, the body breaks fat down into molecules called ketones, which the body uses as fuel.
Eating low carb doesn’t automatically put you into ketosis. A low carb diet is a broader term for any diet that reduces carb intake, whereas keto specifically aims for ketosis.
The science of keto and endurance performance
Current findings are mixed on ketogenic diet cycling performance. Ketosis helps the body by burning ketones for fuel, providing a steady source of energy for endurance events without relying heavily on glycogen stores. It can also be a useful strategy for weight loss.
However, dietary ketosis causes an athlete to need more oxygen to maintain the same pace. When it comes to high-intensity exercise, interval training or sprints, the athlete on a keto diet may be at a disadvantage.
Training typically aims to increase the power or pace an athlete can sustain at a given percentage of VO2 Max. Since an athlete in ketosis requires more oxygen, they operate at a higher percentage of VO2 Max without a corresponding performance gain.
Overall, the science suggests a mixture of both traditional fuelling and the keto diet for endurance athletes. Keto may cause favourable adaptations during the training season, when a cyclist can focus on high volume and moderate intensity training. This can increase fat metabolism, reduce body mass, and decrease post-exercise muscle soreness. Because the diet reduces high-intensity capacity, however, returning to a regular-carb diet for racing season may be more practical.
Potential benefits for cyclists and triathletes
One of the key keto-diet benefits for cyclists is improved fat utilisation. Since even lean athletes have significant fat stores, the body will have plenty of steady-state energy for very long rides.
You won’t experience the peaks and troughs of glycogen depletion, and you won’t need to constantly eat carbs to replenish energy on the bike.
Some riders say keto helps them avoid bonking.
Another benefit of the keto diet is reduced gastrointestinal distress. Endurance athletes often struggle with stomach issues due to overloading on sugar, gels and sports drinks. By not needing to constantly top up glycogen stores, many of these GI issues can be avoided.
Keto diet: Benefits and risks in endurance sports
A common drawback of the keto diet is reduced aerobic capacity. While it helps maintain a steady pace in lower zones, high-intensity intervals require more oxygen to hit the same targets, making them feel harder.
Another challenge is the adaptation period. The initial phase can cause the “keto flu,” with fatigue, aches and flu-like symptoms as the body adapts to fewer carbs.
Lastly, restricting carbs also restricts nutrient-dense foods such as fruits, whole grains and legumes. This can create deficiencies in calcium, vitamin D, phosphorus, and magnesium. Longer-term, risks include kidney stones and reduced bone density.
Keto vs. traditional endurance nutrition
Traditional nutrition for cyclists and other endurance athletes relies heavily on carbohydrate fuelling – carb loading for races and consuming 90-120 grams of carbs per hour while cycling. Carbs provide fast energy but require constant replenishment and may cause GI upset.
A periodised carbohydrate approach restricts carbs during specific training phases and increases them when higher intensities are required. Strategies include:
Cyclical ketogenic diet (CKD): Strict keto for 5-6 days, followed by 1-2 days of carbs.
Targeted ketogenic diet (TKD): Using carbs around workouts for faster recovery, otherwise remaining in ketosis.
Train low hybrid strategy: Adjusting carb intake based on workout intensity for metabolic flexibility.
Exogenous ketones: Supplements that mimic some benefits of ketosis without strict dietary adherence.
How to start keto as an endurance athlete
If you’re curious about starting keto, consult a doctor first. Choose a period when you have several weeks to adapt – don’t attempt it just before racing.
Carbs: Keep below 50 grams per day. Fat: 65-80 percent of daily calories. Protein: 0.6-1.0 grams per pound of lean body mass.
Monitor electrolytes, as increased urination may cause dehydration and loss of magnesium, potassium, and sodium. Add electrolyte-rich foods such as avocados, leafy greens, or supplements.
Training on keto with ROUVY
During adaptation, focus on low-stress activities like yoga, walking, or easy recovery rides on the ROUVY indoor cycling app. Once past the keto-flu phase, return to Zone 2 rides where you can still hold a conversation. Gradually increase workout intensity based on how your body responds.
When keto works best
The keto diet works particularly well during base training season, supporting long, slow rides without constant refuelling. It may also suit ultra-distance events or athletes pursuing specific metabolic goals, like improving fat burning while still using carbs for high-intensity racing.
When keto may not be ideal
This form of low-carb endurance training can impair high-intensity performance, making it unsuitable for sprints or back-to-back race days requiring rapid glycogen replacement. Athletes training or racing at high intensities will likely struggle to recover quickly on keto.
Final thoughts: is keto right for you?
The keto diet might suit you if:
- You want to lose weight and do well with strict guidelines.
- You prefer high-fat, high-protein foods.
- You can manage 1-2 weeks of keto flu.
- Your training is steady and moderate, not highly intense.
- You have no underlying health conditions that make keto unsafe.
The keto diet might not suit you if:
- You have kidney issues, or other medical conditions.
- You are pregnant or breastfeeding.
- You have disordered eating or are underweight.
- You don’t want to restrict carbs.
- You rely heavily on high-intensity training and VO2 Max capacity.
8 Tips: Smart ways to experiment and track your journey
Before committing fully to a ketogenic approach, it helps to monitor your progress carefully and make small, informed adjustments along the way. Here are some tips:
Start a journal and set a baseline for 3-7 days to track food, weight, sleep, energy, symptoms and workouts.
Choose your approach: Full keto or a hybrid, for at least 8-12 weeks.
Set food targets: 50 net grams of carbs or less per day, approximately 2.2 grams of protein per kilogram of bodyweight (or 1 gram per pound), 65-80 percent calories from fat.
Meal-prep weekly, being sure to check labels for hidden sugars.
Prioritise fluids and electrolytes in the first weeks.
Train easy during adaptation, then increase intensity gradually.
Reassess if progress plateaus.
Ask yourself weekly: What worked? What didn’t? How was energy, mood and performance? Adjust accordingly.
Further reading
- Levels, “A low-carb diet may boost exercise performance and health | Dr. Tim Noakes & Josh Clemente”
- Journal of Physiology, “Ketogenic Low-Carbohydrate, High-Fat Diet: The Future of Elite Endurance Sport?”
- Nutrients, “The Effect of a Ketogenic Low-Carbohydrate, High-Fat Diet on Aerobic Capacity and Exercise Performance in Endurance Athletes”
- Frontiers in Physiology, “Low carbohydrate high fat ketogenic diets on the exercise crossover point and glucose homeostasis”
