But how does this trend sit with endurance sports, where carbohydrates have traditionally been the key nutrient? Can a hard-training endurance athlete perform without a daily dose of pasta? This article dives into the pros and cons of a high-fat diet—specifically from an endurance athlete’s perspective.

Boosting Fat Metabolism—and Endurance

A high-fat diet can enhance fat metabolism and thus increase endurance. It’s well known that endurance training induces metabolic changes in cells, including improvements in the body’s ability to use fat as fuel. As a result, the need for muscle glycogen during exercise decreases, conserving it for later stages of the effort—an advantage in performance terms. Enhanced fat metabolism results partly from increased mitochondrial volume and enzymatic activity during submaximal effort (60–85% of VO₂max).

High-Fat Diet Lowers Performance Intensity

While glycogen stores are limited (enough for about 90 minutes of exercise), fat stores are virtually unlimited—stored both in adipose tissue and to some extent within the muscles themselves. Even athletes with very low body fat still have enough fat to fuel several hours—or even a full day—of activity. Well-trained athletes can even derive as much energy from intramuscular fat as they do from muscle glycogen.

It would seem logical that enhancing fat oxidation would improve endurance and be beneficial in races. However, this is not always the case. In studies, cellular-level changes in fat transport into mitochondria and fat oxidation have not always translated into better performance. In fact, some have even shown performance declines.

As many already know, the main limitation to using fats as fuel is the intensity of exercise. When fats are the primary energy source, performance intensity must be lowered because fat oxidation is slower than carbohydrate metabolism. Lower intensity in training isn’t ideal, especially when the goal is high-intensity work. The resulting training response may be suboptimal.

Furthermore, athletes may become fatigued and risk overtraining, as training with low glycogen often increases stress hormone levels and suppresses immunity. It’s especially important to start competitions well-fueled to avoid an early crash.

Nutritional Periodization – High Fat for 5–14 Days

Short-term high-fat diets affect fat metabolism in similar ways as endurance training. “Fat adaptation” refers to following a high-fat, low-carb diet (70% of energy from fat, 15% from carbs) for 5–14 days while continuing both low- and high-intensity training. This adaptation phase can be followed by a 1–3 day carbohydrate-loading period (15% fat, 70% carbs) before a race.

Compared to a standard high-carb diet, this combo (fat adaptation -> carb loading) enhances the body’s ability to use fat and reduces glycogen breakdown during performance. Unlike adaptations from endurance training, fat adaptation increases fat oxidation capacity without altering mitochondrial function—meaning other mechanisms, still not fully understood, are at play.

Fat adaptation followed immediately by carbohydrate loading allows for sustained fat oxidation while replenishing glycogen before competition. In theory, this enables better fat burning early in a race, conserving carbohydrates for later stages—e.g., in a marathon. When carbs are needed, full glycogen stores can carry the athlete further.

Endurance Athletes Still Need Carbs

Muscle capacity to use fat increases after fat adaptation, but individual responses vary—some benefit more than others. However, fat adaptation suppresses enzymes critical for carbohydrate metabolism, potentially impairing high-intensity performance. Training quality suffers when carbs are lacking, leading to a reduced training effect and slower progress. While it’s possible to train the body to tolerate high-intensity work on a low-carb diet, this typically takes years of gradual adaptation.

Even though a short carb-loading period post-fat-adaptation maintains many of the fat metabolism benefits, the suppressive effects on carb metabolism persist, compromising high-intensity performance. This is bad news for athletes competing in shorter endurance events—especially those requiring efforts close to maximum oxygen uptake (90–100% VO₂max), such as track races.

Carbohydrates are essential not just for energy, but for many bodily functions. Immunity and overall health can suffer during a low-carb phase. The risk of overuse injuries and infections rises when the body is stressed—including under low-carb intake. That said, individual variation exists, and some athletes thrive on low-carb diets. Such a diet may also help with weight control, particularly for athletes prone to gaining weight when consuming more carbohydrates. In these cases, a high-fat diet may aid athletes needing to lose weight before competition season.

In the long run, a high-fat, low-carb diet is not necessarily suitable for all athletes. Based on current research, fat adaptation is best reserved for experienced athletes and should generally be applied periodically. It often works well for ultramarathons and long-duration events where improved fat metabolism is key to success, helping avoid the dreaded “bonk.” However, for high-intensity efforts, carbohydrates remain the primary fuel. Even during fat adaptation, it’s smart to include high-intensity training sessions with carbs to ensure the body maintains its ability to use them efficiently.

As with all dietary strategies, fat adaptation and low-carb diets should be tested well in advance of key competitions. Training season can be a good time to experiment and observe how your own body responds.

This article was originally published last spring and has been updated today. Read more about training and nutrition at ProXCskiing.com.

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