In recent years, fats have become a trendy topic. Discussions around low-carb, high-fat (LCHF) diets and paleo eating highlight a common theme: a reduction in carbohydrate intake while favoring fats.

But how does this apply to endurance sports, where carbohydrates have traditionally been the primary nutrient?

A fat diet can potentially enhance one’s fat metabolism, thereby increasing endurance. Endurance training is known to induce changes in cellular metabolism, notably improving the body’s ability to utilize fats for energy. As a result, the demand for muscle glycogen during exercise decreases, allowing it to be conserved for later stages of performance, which can have beneficial effects on overall performance. The enhancement of fat metabolism is attributed to factors such as increased mitochondrial volume and enzymatic activity during submaximal (60-85% of VO2max) efforts.

Performance decreases with a fat diet

While glycogen stores are limited and sufficient for only about 90 minutes of exertion, the amount of fat stored in adipose tissue is virtually limitless, with additional reserves in muscle tissue. An athlete with a very low body fat percentage still has enough fat for several hours, even a day, of performance. Moreover, well-conditioned athletes can derive energy from intramuscular fat stores that can match glycogen levels.

It might be assumed that enhancing fat oxidation would improve endurance capabilities and be advantageous in competitions. However, this is not necessarily the case, as cellular-level changes observed in the transport of fats into muscle cell mitochondria and their oxidation into energy do not always translate into improved performance. In some cases, performance has even been found to decline.

As many are aware, the utilization of fats is limited by performance intensity. If fats are the primary energy source, performance intensity must be reduced, as fat oxidation is slower than that of carbohydrates. Lower intensity during training may not be ideal, especially when the goal is to train at high intensity. This could lead to suboptimal training responses.

Additionally, athletes may experience fatigue and risk overtraining, as training with low carbohydrate stores often results in elevated stress hormone levels and decreased immunity. It is crucial to start competitions well-fueled to avoid performance issues right from the start.

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Nutritional periodization – High fat for 5-14 days

Short-term fat diets have been shown to affect fat metabolism similarly to endurance training. “Fat adaptation” refers to a situation where an endurance athlete follows a high-fat, low-carbohydrate diet (70% energy from fats, 15% from carbohydrates) for 5-14 days. During this time, normal training continues, both low and high intensity, as usual. Fat adaptation can be conducted as is or combined with a carbohydrate loading phase (1-3 days, 15% fats, 70% carbohydrates) in preparation for competition.

Compared to a carbohydrate-focused diet, the aforementioned combined diet (fat adaptation -> carbohydrate loading) enhances the body’s ability to utilize fats for energy and reduces glycogen breakdown during upcoming performances. Fat adaptation differs from the responses induced by endurance training in that the ability to oxidize fats increases without changes in mitochondrial function. Other, partially unexplained mechanisms are thus behind these changes.

Fat adaptation followed by a few days of carbohydrate loading allows for the enhanced fat oxidation to persist after the fat diet while replenishing glycogen stores before competition. Theoretically, at the start of a competition, the body would then be able to burn fats more efficiently for energy, delaying the need for carbohydrates until later stages, such as during a marathon. When carbohydrates are finally needed, full glycogen stores would suffice for a long way.

Endurance athletes need carbohydrates

The muscles’ ability to utilize fats for energy increases as a result of a fat diet, but individual differences are significant: some benefit more from the method than others. A fat diet can negatively impact the activity of enzymes crucial for carbohydrate metabolism, likely having detrimental effects on high-intensity performance. Training intensity decreases if carbohydrates are insufficient, leading to a smaller training response and slower development. While the body can be trained for high-intensity workouts even under a low-carbohydrate diet, this typically requires years of effort and adaptation.

Although a short carbohydrate loading after a fat phase preserves the changes in fat metabolism quite well, the inhibitory effect on carbohydrate metabolism persists even after carbohydrate loading, which diminishes high-intensity performance. This is likely bad news for those competing in shorter endurance events, especially if performance intensity is close to maximal oxygen uptake (90-100% of VO2max), as is the case in many race distances.

Endurance athletes need carbohydrates not only for energy but also for various other functions. Immunity and overall body function are under strain during a low-carbohydrate diet. The risk of injuries and infections increases under stress, which can include insufficient carbohydrate intake. Of course, there are individual differences, and for some, a low-carbohydrate diet may work very well. This diet can also be a good weight management strategy for those prone to excessive weight gain from carbohydrate consumption. Thus, a fat diet may have a positive impact on athletes who need to lose weight before the competition season.

A diet high in fats and low in carbohydrates may not be suitable for all athletes in the long term. Based on research, fat adaptation is recommended for experienced athletes, and even then, usually implemented periodically. A fat diet often yields positive results for ultra-distance and long performances, where enhanced fat metabolism is crucial for success, helping to avoid the so-called “bonking” during performance. However, carbohydrates remain the most important fuel for high-intensity efforts, so even during fat adaptation, it is beneficial to perform high-intensity workouts while consuming carbohydrates, allowing the body to learn to utilize them effectively.

As with other nutritional strategies, it is advisable to test fat adaptation and low-carbohydrate diets well in advance of important competitions, experimenting with the method’s effects on one’s body during the training season.

This article was previously published and updated today.

Sources:
Yeo et al. 2011. Fat adaptation in well-trained athletes: effects on cell metabolism. Appl Physiol Nutr Metab 36, 12-22.