Is a high-fat diet safe for long-distance runners?

"I've heard about long distance runners eating a low-carb, high-fat diet for sustained energy. Is this safe and is it effective?

It is well known that carbohydrate is a substrate that provides an immediate source of energy for high intensity exercise, thus for many years a moderate to high-carb diet has been recommended by sports dietitians to support the energy needs of runners. Due to the limited supply of available carbohydrate in the body, dietary carbohydrate is consumed in a manner that allows muscle and liver carbohydrate stores to be maximized, providing about 1-2 hours of energy during a run. After 1-2 hours, carbohydrate replacement must occur. The need for carbohydrate replacement during exercise is driven by the bodies reliance on carbohydrate as the primary energy substrate during exercise. To maintain sustained energy the runner has traditionally been told to consume 50% or more of their calories from carbohydrate and to consume carbohydrates during exercise. In recent years the scientific viewpoint has been challenged by research that appreciates fat oxidation during exercise and metabolic adaptation that occurs when runners eat a high fat, low-carb diet.

A challenge to the conventional moderate- to high-carbohydrate diet is that this dietary approach is not ideal for all runners. Runners who have insulin resistance, chronic inflammation or gastrointestinal dysfunction will do better with a lower carbohydrate diet and may struggle with running performance when utilizing a higher carbohydrate approach. These runners may have blood glucose functions from higher carbohydrate intake and/or gastrointestinal discomfort that impair enjoyment and running performance and so those who are less tolerant to the higher carbohydrate approach are a great fit to learn more about the potential of a high-fat, low-carb diet. Research shows at least equal performance if not improved running performance when runners use a high-fat, low-carb diet compared to a higher carbohydrate diet. There have only been a handful of studies conducted on a low-carb, high-fat sport nutrition diet plan, and so it is not being routinely recommended, however, for those that are carbohydrate intolerant, it is an excellent option to experiment with.

If carbohydrate is the preferred energy substrate, how could a low carbohydrate dietary approach support long-distance running? The answer is found in the details of "preferred energy substrate". Carbohydrates are the preferred energy source for high intensity exercise, such as sprinting. However, during lower intensity exercise, fat is the preferred substrate. By definition long distance running is a lower intensity exercise because the pace is sustained over long periods of time. When utilizing fat as the substrate to fuel running, the concept of maximizing carbohydrate stores diminishes in importance, as does the importance of a higher carb diet. Interestingly, runners that use a low carbohydrate dietary approach will develop the ability to use even less carbohydrates during exercise, then carbohydrate used as a substrate during exercise is spared and the need for dietary carbohydrate immediately before, during and immediately after exercise is lower. Another benefit to the low-carb approach is lower circulating insulin levels. In response to dietary carbohydrate intake the pancreas produces insulin; higher circulating insulin levels pushes the body towards carbohydrate as a fuel and make it metabolically more difficult to tap into fat for energy. When carbohydrate intake goes down, insulin follows, leading to an improved use of fat as the energy source during exercise.

If you are taking part in long-distance running, especially ultra-distances, and especially if you are insulin resistant, have type 2 diabetes, gastrointestinal problems or chronic inflammation, a high-fat, low-carb diet may be suitable. If you are a sprinter this may not be the best fit for you, however, if you don't tolerate carbohydrate well, the improved metabolic function you experience from reducing carbohydrate intake may provide benefit that leads to improvement in sport performance.

Work with a qualified sport dietitian that can assist you with developing an appropriate diet plan. Be sure to find a practitioner open to considering all of your options in creating a sport nutrition plan for you.

Hana A. Feeney, MS, RD is an open-minded, progressive dietitian that blends evidence based nutritional science with intuitive eating and cutting-edge functional medicine. Hana specializes in sport nutrition, digestive health, fertility, hormonal health and eating disorders. Visit to explore, read, cook and reach out! Contact Hana directly by phone at 520-429-3418 or via email at

Please remember that the information contained in this article is provided for informational purposes only. It is general information that may not apply to you as an individual, and is not a substitute for personalized nutrition advice or healthcare. Never disregard medical or nutritional advice or delay seeking medical care because of something you have read or accessed through this article.



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