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Low Carb Diet and Performance

A study from South Africa showed that eating a low-carbohydrate diet slows extended sprint performance of cyclists (Journal of Applied Physiology, January 2006). Competitive bicycle racers ate a high fat or high-carbohydrate diet for six days followed by a high-carbohydrate diet for one day and 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. Diets did not affect their times or power output for 100 kilometers (62 miles), but the high fat diet slowed their sprint performance over one kilometer (0.6 miles).

Muscles get their energy from sugar and fat stored in muscles or from the bloodstream. The limiting factor in how fast an endurance athlete can exercise is the time it takes to transport oxygen from the blood in the lungs to the muscles. Muscles require far more oxygen to burn fat than to burn sugar for energy. So when a muscle runs out of its stored sugar, called glycogen, it becomes less efficient, hurts, is difficult to co-ordinate and slows you down.

Many previous studies show that it doesn't make any difference what an trained endurance athlete eats on the week before competition because the muscles of trained athletes store the most glycogen when they reduce training for several days, regardless of what they eat. 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. This study shows that a high-fat diet before extended sprinting hurts performance. A high fat diet causes muscles to burn a higher percentage of fat. Using fat for energy requires more oxygen than carbohydrates do, and how fast you can sprint 0.6 miles on a bicycle is limited by how rapidly you can deliver oxygen to muscles. Restricting carbohydrates before a sprint taking more than 50 seconds increases oxygen needs which slows you down.
Also see Low Carb-High Fat Diets for Endurance?

Checked 12/19/15

May 11th, 2013
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About the Author: Gabe Mirkin, MD

Sports medicine doctor, fitness guru and long-time radio host Gabe Mirkin, M.D., brings you news and tips for your healthful lifestyle. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin is a graduate of Harvard University and Baylor University College of Medicine. He is board-certified in four specialties: Sports Medicine, Allergy and Immunology, Pediatrics and Pediatric Immunology. The Dr. Mirkin Show, his call-in show on fitness and health, was syndicated in more than 120 cities. Read More
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