Since the 1960s, carb-loading has been the cornerstone of sports nutrition for endurance athletes. Research published in 1967 by a group of Swedish physiologists — the first to research muscle biopsy in the field of athletic performance — revealed that high carbohydrate intake increased levels of glycogen in the muscles, allowing endurance athletes to run farther and faster. The study was seen as one of the first major breakthroughs in sports science, and running performance came to be viewed as a function of how much glycogen an athlete could store before a race.

What is carb loading?

Carb-loading diets typically recommend increasing carbohydrate consumption during the week before a high-endurance activity, while tapering activity and cutting back on foods higher in fat. The combination of eating more carbs and scaling back on training — including resting completely the day before the event — appears to increase muscle glycogen stores, providing runners with the additional fuel needed to complete a half marathon or full marathon.

The case for a high-fat diet

More recent research, however, throws the idea of carb-loading into question. Some researchers believe that the optimum fuel for endurance sports is actually fat. The body naturally contains greater reserves of fat, and a sustained high-fat diet can force the body into a state of ketosis, where it uses fat stores for energy. Given the body’s natural supply of fat reserves, this can provide endurance athletes with the energy they need to cover a full marathon or even greater distances.

So which diet is better for endurance athletes? It depends. Experts in sports nutrition are beginning to understand that there is no one-size-fits-all diet for athletes. Several factors — including age and genetics — determine how an individual processes and stores fuel from foods, meaning that two athletes with the same goals and training regimen could have completely different nutritional requirements.

How the body uses carbohydrates

Carbohydrates are broken down into sugar, which enters the bloodstream. As blood sugar levels rise, the pancreas releases insulin, a hormone that allows cells to absorb glucose and use it for energy. Over time, if the pancreas secretes continually high levels of insulin to process all the sugars in a high-carb diet, the body can become resistant to it. But the amount of carbohydrates required to develop insulin resistance varies from person to person. People who metabolize carbohydrates quickly can benefit from a high-carb diet, while individuals with certain genetic variants may perform better on a low-carb, high-fat diet.

How should athletes determine their ideal diet type?

Determining the right diet type for your body is often a matter of trial and error. Pay attention to what works for you and don’t worry too much about what your friend or running partner is eating. If you want to try to become “fat adapted” — making the shift to burning fat for fuel — be aware that it takes time, so if you’re used to carb-loading, don’t try to switch to a high-fat diet right before a race. Genetic testing can also reveal how your body processes different foods and whether exercise will improve your body’s response to insulin. Request a free consultation with a Vivaliti DNA Health Coach to learn more about the benefits of genetic testing and how eating the right diet for your individual genotype can help you reach your fitness goals.