Carb-loading before events such as marathons and triathlons have been a popularized and heavily utilized strategy for probably as long as these events have existed. And this is for good reason too. Many studies, beginning as early as the late ‘30s, were examining the phenomenon of how higher glycogen (usable form of glucose) stores in the muscle played a key component in exercise bouts lasting longer than 90 minutes [1]. Researchers then began to discover that higher carbohydrate intake, specifically timed around endurance competitions, improved common endurance parameters such as time to exhaustion (TTE) and VO2 max in cyclists [2]. It’s a tried-and-true method to improving your performance in these long-distance, long-duration endeavors.

Or is it?

As time has gone on and our understanding of sports performance has improved, has a more superior method of fueling before competition emerged?

Well, with the ongoing trend of the ketogenic (keto) diet over the past few years, it looks like carb-loading may have met its match. Let’s dive into both carbohydrate and fat fueling before endurance competitions and see which one reigns supreme.

Holding On To Our Commonly-Held Beliefs

Sports nutritionists have advocated high-carbohydrate diets for endurance athletes for quite some time now. These recommendations are made mostly because of studies that were released between the ‘70s and the early 2000s, indicating that low-carbohydrate diets impair endurance performance by increasing perceptions of fatigue in the athlete [3,4,5].

However, just like anything in science, there are very few things that have such black-and-white answers to them. One study measured the level of intensity of elite cyclists after following 6 days of a low-carbohydrate diet [6]. They found that levels of fat oxidation (the utilization of fat for energy) were increased significantly in the cyclists even after this short-term, low-carbohydrate diet. What this entails is that the athletes were able to more efficiently utilize fat for energy production, even in the absence of carbohydrates.

Another interesting finding by researchers is that long-term adherence to a low-carbohydrate diet (9-36 months) may be just as effective as a high-carbohydrate diet, while also providing various metabolic advantages to the athlete, such as reduced appetite, and a decrease of blood sugar [7]. It was found that when elite cyclists followed a low-carbohydrate, high-fat diet (keto), time to exhaustion was not affected as compared to a standard high-carb diet [8]. Surprisingly, even though the cyclists had lower levels of muscle glycogen after following the low-carb diet, their levels of muscle glycogen were similar to the high-carbohydrate group post-exercise. This indicates that a high-fat diet may be just as effective in providing fuel for the body during endurance activities are carbohydrates are.

Could Keto Possibly Be Even Better Than High-Carbs?

It turns out that those who are “fat-adapted” (those who have been following the keto diet for at least 6 months) may perform better than those on a standard high-carb diet. Because those who follow the keto diet have a higher fat oxidation rate, this causes something called a glycogen sparing effect [9]. What this basically means is that the keto-adapted athlete would be able to maintain a high standard of intensity, while having the ability to preserve their muscle glycogen for when they really need it, such as when they need to sprint to the finish line at the end of a race. However, this is only speculation, as the research on keto and athletes in general is quite limited.

The Problem With a High-Fat Diet

The keto diet has been around for decades, being utilized for primarily for children who suffer from seizures, caused by something called GLUT1 deficiency, which is when the body lacks the ability to metabolize glucose as it crosses the blood-brain-barrier.

However, the keto diet is still in its infancy in the athletic world. Most of the studies have not started to develop until the 90s and 2000s. While on the other hand, the traditional high-carbohydrate model has been studied ever since the ‘30s.

Because of this, there are still many aspects of the keto diet in conjunction with endurance performance that remain unexplored. This includes how being “fat-adapted” affects one’s central fatigue and their perception of fatigue during exercise, the optimal composition of the types of fatty acids to eat on the diet, such as saturated, polyunsaturated, and monounsaturated fats, among many other variables.

Lastly, most of the studies that actually are out there are focused on utilizing the diet as a means to control body composition over the long term. There aren’t many out there that utilize it in a shorter time frame, such as replacing carbs with fats in order to see how “fat-loading” would differ from a traditional “carb-load” before an endurance competition.

The Bottom Line

Fortunately, with keto being a popular trend for quite some time now, this creates a higher demand for research regarding its application in endurance activities.

But for now, you have to do what all true scientists do, and that is to use yourself as your own guinea pig and test it out on yourself. Before one event, use a carb-load, and before the next event, use a fat-load, and see how each one affects you.

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  1. Christensen EH, Hansen O. Arbeitsfahigkeit und Errichtung. Skandinavische Archlv fUr Physiologie 1939; 8: 160-71
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  3. Karlsson J, Saltin B.. Diet, muscle glycogen, and endurance performance. J Appl Physiol. 1971;31:203–206.
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  5. White AM, Johnston CS, Swan PD, Tjonn SL, Sears B.. Blood ketones are directly related to fatigue and perceived effort during exercise in overweight adults adhering to low-carbohydrate diets for weight loss: a pilot study. J Am Diet Assoc. 2007;107:1792–1796.
  6. Burke LM, Hawley JA.. Effects of short-term fat adaptation on metabolism and performance of prolonged exercise. Med Sci Sports Exerc. 2002;34:1492–1498
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