With a fat-based diet, it’s possible to enhance one’s fat metabolism and thus increase endurance. Endurance training notoriously changes cellular metabolism, notably by improving the body’s ability to use fat for energy. As a result, the need for muscle glycogen during exercise decreases, preserving it for later stages of performance, which has advantageous effects on performance. The enhanced fat metabolism results from factors such as increased mitochondrial volume and enzymatic activity during submaximal (60-85% VO2max) exercise.

Performance declines with a fat-based diet

While glycogen stores are limited and suffice for only about 90 minutes of performance, there’s virtually an unlimited amount of fat stored in adipose tissue and, to some extent, in muscle intracellular fat stores. Even athletes with very low body fat percentages have enough fat in their bodies for several hours, even a day, of performance. Additionally, in well-conditioned athletes, the amount of energy derived from intramuscular fat stores can rival that from muscle glycogen, according to Maastohiihto.com.

One might think that enhancing fat oxidation would improve endurance performance and be advantageous in competitions. However, this isn’t necessarily the case, as cellular-level changes observed in the transport and oxidation of fats into energy haven’t always translated into improved performance. Sometimes, performance decline is even observed.

As many may know, the introduction of fats limits performance. If fats are the primary energy source, performance must be reduced because fat oxidation is slower than that of carbohydrates. Lower training intensity may not be ideal, especially when the intention is to train hard. Training with low carbohydrate stores often leads to elevated stress hormone levels and decreased immunity, potentially causing fatigue and overtraining. Especially for competitions, it’s crucial to start well-fueled to avoid performance faltering right from the start.

Nutritional periodization – ample fats for 5-14 days

Short-term fat-based diets affect fat metabolism similarly to endurance training. “Fat adaptation” refers to a situation where an endurance athlete follows a high-fat, low-carbohydrate diet for 5-14 days (70% energy from fats, 15% from carbohydrates). During this time, regular training continues, including both low- and high-intensity sessions. Fat adaptation can be done independently or combined with a few days of carbohydrate loading before a competition (1-3 days, 15% fats, 70% carbohydrates).

Compared to a carbohydrate-focused diet, the mentioned combination diet (fat adaptation -> carbohydrate loading) enhances the body’s ability to use fats for energy and reduces glycogen breakdown during subsequent performance. Fat adaptation differs from the responses induced by endurance training, as the ability to oxidize fats increases without changes in mitochondrial function. Other mechanisms, partly still unknown, are behind these changes.

Fat adaptation followed by a few days of carbohydrate loading allows sustained enhanced fat oxidation while replenishing glycogen stores before a competition. In theory, at the start of the competition, the body would be better able to burn fats for energy, postponing the need for carbohydrates to later stages, such as in a marathon. Full glycogen stores would suffice for a long time.

Endurance athletes need carbohydrates

The ability of muscles to use fats for energy increases due to a fat-based diet, but individual differences are significant: some benefit more than others. A fat-based diet has a detrimental effect on the activity of enzymes crucial for carbohydrate metabolism, likely affecting high-intensity performance negatively. If carbohydrates aren’t available in sufficient quantities, exercise intensity decreases, resulting in a smaller training response and slower progress. Of course, the body can adapt to high-intensity exercise under a low-carbohydrate diet, but this usually takes years of work and adaptation.

Although short-term carbohydrate loading after a fat period maintains the changes in fat metabolism reasonably well, the inhibitory effect of fats on carbohydrate metabolism persists even after carbohydrate loading, reducing the effectiveness of high-intensity performance. This is likely bad news for those competing in shorter endurance races, especially when performance is close to maximal oxygen consumption (90-100% VO2max), as is often the case in track races.

Endurance athletes need carbohydrates not only for energy but also for many other functions. Immunity and overall well-being suffer under a low-carbohydrate diet. The risk of overuse injuries and infections increases when carbohydrate intake is inadequate, which can be considered a stressful situation. Of course, there are individual differences, and a low-carbohydrate diet may work very well for some. This diet may also be a good weight management tool for those prone to weight gain with carbohydrate intake. Thus, a fat-based diet may have a positive effect on athletes who need to lose weight before the competition season.

A diet rich in fats and low in carbohydrates may not be suitable for all athletes in the long run. Based on research, fat adaptation can be recommended for experienced athletes, usually implemented periodically. A fat-based diet often yields positive results in ultra-endurance events and long performances, where enhanced fat metabolism is crucial for success, and a functioning fat metabolism helps avoid “hitting the wall” during performance. However, carbohydrates are still the primary fuel for high-intensity performance, so even during fat adaptation, it’s good to include high-intensity exercises and consume carbohydrates, allowing the body to use them efficiently.

As with any nutritional strategy, it’s advisable to test fat adaptation and a low-carbohydrate diet well in advance of important competitions, and the method’s effects on one’s body can be tested during the training season.

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