The body relies on carbohydrates as its primary source of energy, which is in the form of glucose. Without carbohydrates, the body adapts by converting fat into ketones in the liver, which are used as an alternative energy source.

This process is called ketosis, and for the body to adapt, fat intake needs to make up around 80% of the diet, while carbohydrates are restricted to less than 50g a day (equivalent to a couple of slices of bread).

• How to keep your carbohydrate intake in control

While training and competing in endurance sports, the body relies predominantly on carbohydrates as a source of fuel during long periods of sustained activity.

Glycogen is the form that carbohydrates takes when stored in muscle tissue (300g) and the liver (90g), which equate to around 1,600 calories of available energy at any one time.

During an endurance event, these stores are depleted, so athletes need to consume carbohydrates regularly during a race to maintain energy levels.

The body stores around 7,700 calories of energy in one kilogram of fat. This equates to a massive amount of energy, which is why the ketogenic diet has such an appeal to endurance athletes.

Accessing this energy source can reduce the need to refuel during exercise. Relying on fat may also reduce the risk of gut disturbances experienced by some athletes when they consume high amounts of carbohydrates. It’s also been suggested that the ketogenic diet reduces lactate accumulation after exercising, which may enhance recovery¹.

Can the ketogenic diet help improve body composition?

Claimed benefits of a ketogenic diet include improvements in body composition. An athlete’s power-to-weight ratio is considered a predictor of performance, especially for cyclists.

This reflects the amount of power an athlete can produce in relation to their body weight. In other words, the more power athletes have per kg of body weight, the faster they will be.

One approach to improving this ratio is losing weight, but this comes with the risk of losing muscle mass which is counterproductive.

Research published in the journal Metabolism studied a group of endurance athletes following the same training protocol over 12 weeks.

The group was divided into those following a ketogenic diet and those following a high carbohydrate diet. It was shown that those following the ketogenic diet experienced a 5.9kg weight loss over 12 weeks versus a 0.8kg loss in those following a high carbohydrate diet.

It was also shown that both groups maintained lean mass. Improvements in fat oxidation were also observed, and this is also one of the few studies to show improvements in specific performance outcomes².

Does the ketogenic diet help improve performance?

Many studies have shown how the ketogenic diet results in greater fat oxidation rates at higher percentages of VO2 max compared to a high carbohydrate diet.

• How to improve VO2 max

VO2 max is a primary measure of performance and reflects the maximum amount of oxygen an athlete can utilise during exercise.

Fat oxidation rates reach maximal levels at moderate-intensity exercise, corresponding to 59-64% of VO2 max in highly trained endurance athletes.

Research has shown that following the ketogenic diet increases fat oxidation at rates above the normal VO2 max. One study even showed improved fat oxidation in athletes exercising to 70% of their VO2max.

However, fat oxidation is not as accurate a measure of performance as improvements in VO2 max, race times and time trials.

Despite these studies, there’s not enough research to prove that the ketogenic diet increases performance. Evidence still clearly shows that a high carbohydrate diet is more beneficial in improving performance outcomes.

What are the downsides of following the ketogenic diet?

Despite its appeal, the ketogenic diet requires drastic dietary changes and the difficulty lies in maintaining this way of eating.

The diet relies on fat foods, including meat, poultry, cheese, eggs, cream, avocado, oils, nuts, and seeds. All carbohydrate foods are excluded, including grains, cereals, and starchy vegetables such as potatoes, as are legumes including beans, pulses, and lentils.

Vegetables are generally limited to salad vegetables and greens, while acceptable fruits are limited to berries in moderation.

Given the restrictions on this diet, athletes choosing to follow it may benefit from a daily multivitamin and mineral supplement, such as Elite All Blacks Gold Multivitamin.

Preparing keto meals also takes more planning and preparation, which may challenge some athletes. Some may also find the continuous intake of rich, fatty foods unpalatable.

It also doesn’t take much to knock you out of ketosis, such as over-eating protein, so followers must be very strict with their food intake.

The keto diet is a high-fat diet, not high protein, so followers need to be wary of overeating meat, poultry, and fish, as excessive protein in the absence of carbohydrates can be converted to glucose in the body.

As athletes adapt to ketosis, they may feel sluggish and tired, impacting training and performance. According to the research, this is a crucial consideration as becoming fully adapted takes 3-4 weeks⁴.

What about periodised nutrition as a low carbohydrate approach?

Periodised nutrition is an approach that may help athletes encourage adaptations that can help improve exercise performance. The practice of ‘training low’ manipulates carbohydrate intake to achieve these adaptations.

This is when you train with a low glycogen availability, but it’s not the same as ketosis because it’s a short-term depletion of carbohydrates done periodically.

Training low aims to drive adaptations to increase fat metabolism, which can spare glycogen stores for critical high-intensity race periods.

Eating this way is also thought to increase mitochondria in cells responsible for making energy in the body. AMP-activated protein kinase (AMPK) is a protein that has an essential role in muscle adaptation and is thought to be upregulated in the absence of glycogen.

Endurance training promotes many of the same adaptations in the body, but it seems that training low may help them.

Several methods have been proposed, which include:

• Twice-a-day training sessions that withhold carbohydrate intake between workouts
• Overnight fasting
• Prolonged training and restricting or postponing carbohydrate intake during sessions
• Delaying carbohydrate intake during recovery

This technique can be included in your training session and you could start by including it once a week. The best sessions to try this in are low to moderate intensity, so you have enough time to recover before heavier sessions.

Maintaining a higher carbohydrate intake during high-intensity training will help improve performance and adaptation.

The available research suggests that it is possible to maintain performance on a ketogenic diet, but the evidence for improved performance is lacking.

Adopting a periodised diet that involves ‘training low’ may be a better way for endurance athletes to reap the performance benefits of limiting carbohydrates in their diet during certain times in their training schedule.

We recommend to practice caution and refer to your GP when following any diet, and always seek nutritional advice from registered nutritionists and health professionals.  

Top image credit: Dovile Ramoskaite / Unsplash