Low-Carbohydrate Diets Harm Athletic Performance


    A study of elite race walkers showed that a low-carbohydrate, high-fat diet slowed their race times and training (J of Physiology, February 14, 2017).

    Thirty world-class race walkers participated in an intense high speed training program and took in the same total calories. They were divided into three groups:
    • High carbohydrate
    • Alternating days of high and low-carbohydrate intake
    • Low-carbohydrate, high fat (LCHF) diet

    They trained intensely for three weeks. Results showed that:
    • The high carbohydrate and the alternate-day high-carbohydrate groups improved their race times. The LCHF group did not improve.
    • The LCHF group burned a greater percentage of fat during intense exercise.
    • The LCHF group required more oxygen to exercise intensely at 80 percent of their maximum speed for two hours. The LCHF diet forced the athletes to burn more fat and less carbohydrates so they required more oxygen, and therefore had to slow down.

    Why LCHF Diets Have Incorrectly Been Promoted to Improve Athletic Performance
    Your muscles burn primarily fat and carbohydrates for energy. You have enough fat stored in your body to exercise for many days. However, you can store only 1600-2000 calories worth of sugar (carbohydrate) in your muscles and liver, and will start to run out of your meager supply of sugar after 70 minutes of intense exercise.

    Some people believe that if you restrict carbohydrates, you will teach your muscles to burn more fat and preserve their small store of sugar (Metabolism, 2016;65(3):100-10). That is correct, but burning more fat and less sugar for energy slows you down (Nutrients, 2014;6.7:2493-508). The limiting factor to how fast you can move over distance is the time it takes to move oxygen into muscles. Since sugar requires about 20 percent less oxygen than fat does to fuel your muscles, you need to slow down and have less power when your muscles burn fat rather than sugar.

    The faster you move, the greater percentage of sugar that your muscles use for energy, so restricting carbohydrates (your source of sugar) and forcing your muscles to burn more fat slows you down in both training and racing. The key to improving your speed over distance is to move very fast in training. Anything that makes you train at a slower pace limits your improvement, and of course, anything that slows you down in a race can make you a loser.

    Low-Carbohydrate Diets Can Slow You Down
    In another study, competitive bicycle racers ate a high-fat or high-carbohydrate diet for six days followed by a high-carbohydrate diet for one day and then completed time trials on their bikes. Then they ate the opposite diet for six days followed by a high carbohydrate diet for one day and repeated their time trial. The diets did not affect their times or power output for 100 kilometers (62 miles), but the high-fat diet slowed their ability to sprint for 0.6 miles (J Appl Physiol, 2006, 100(1):194-202).

    Many other studies show that it doesn’t make any difference what a trained endurance athlete eats on the week before competition because the muscles of trained athletes store the most sugar as glycogen when they reduce training for several days, regardless of what they eat, provided that they do not restrict all carbohydrates. Any sprint that takes less than 50 seconds is not affected by diet, because you can work up to 50 seconds anaerobically, without requiring additional oxygen. However, a low-carbohydrate diet before extended sprinting hurts performance. Restricting carbohydrates before a sprint taking more than 50 seconds increases oxygen needs which slows you down. See my article, Eat to Compete, on what you should eat and drink before and during a competition.

    Checked 3/29/21