By Unni Greene

Endurance and high-performance athletes traditionally avoid low-carb diets due to the notion that the body needs carbohydrates for fuel. Instead, many athletes carb load before a big event to increase their storage of glucose and glycogen calories to be burned as fuel. Carb loading involves depleting the storage of glucose and glycogen about seven days in advance of a competition or performance and then “loading,” with up to 70 percent of calories coming from carbohydrates, three to four days before the event.

New research has shown that a low-carb, ketogenic diet can help an athlete maintain energy and, in some instances, improve performance when compared to carb loading. To achieve this success, an athlete must be able to switch from carb burning to fat burning for fuel. This is called ketosis. A state of ketosis generally can be achieved by consuming no more than 50 grams of fibrous carbohydrates per day. This estimate varies by individual and should be established for each athlete. 

An athlete must follow a low-carb diet for a prolonged period of time, usually a week or more, before ketosis is achieved. The liver creates ketone bodies that are used to convert fat into energy as ketosis is achieved. To find your personal carb limit, it’s important to actually measure your ketones through urine, breath or blood. This will give you a fair assessment of achieving ketosis, instead of simply counting carbohydrate grams consumed. It’s important to note that carbs should come from vegetables, not from non-fibrous carbs. 

A low-carb, high-fat diet for performance athletes has gained a lot of interest in the ultra-endurance world, where athletes are exercising constantly for several hours. By limiting carbs, the body learns to use fat for fuel instead of relying on glucose and glycogen for energy. The problem with relying on glucose and glycogen is that they eventually will run out and you will hit the famous “wall,” whereby performance suffers. Your body can store only about 2,000 calories as glycogen. By switching to a low-carb diet you train the body to use adipose tissue (fat) for fuel. Even the leanest athlete carries around at least 20,000 calories of body fat. If you are burning fat and sparing carbohydrates, you do not hit the “wall” because you do not run out of fuel. This is one of the most commonly perceived benefits of a low-carb diet for athletes. 

Ultra-endurance athletes who have switched to low-carb, high-fat diets are now winning races and, in some cases, setting new course records. They’re also experiencing other benefits, such as speedier recovery rates, improved metabolic health and a leaner body composition.

Unni Greene is the owner of SoMi Fitness and a Certified Nutrition specialist. For more information, visit


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