Low-carb, high-fat diets negatively affect performance
Posted November 17th, 2006 at 12:00 PM by Martin Kennedy
Section: Nutrition, Healthy Eating, Race Prep & Recov
A recent study from South Africa, published in the Journal of Applied Physiology, shows that eating a low-carbohydrate diet slows extended sprint performance of cyclists. 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.
***Note: We encourage EVERYONE to see a doctor before altering their diet, taking a supplement and/or performing athletic, fitness or other strenuous physical activity. It is your responsibility to evaluate the accuracy, completeness and usefulness of any information, instruction, opinion or advice contained in the content. Please also see our complete disclaimer.***
This post is written by Dr. Gabe Mirkin, M.D. and was originally published on his blog “Fitness and Health E-Zine”.
Dr. Mirkin is board certified in Sports Medicine and has practiced for over 40 years. He has completed more than 40 marathons and was a talk show host of a nationally-syndicated radio program for about 25 years. For more articles by Dr. Mirkin, please check out: www.DrMirkin.com
Please also be advised that Dr. Mirkin’s opinions and the references cited are for information only, and are not intended to diagnose or prescribe. For your specific diagnosis and treatment, consult your doctor or health care provider.
***Note: We encourage EVERYONE to see a doctor before altering their diet, taking a supplement and/or performing athletic, fitness or other strenuous physical activity. It is your responsibility to evaluate the accuracy, completeness and usefulness of any information, instruction, opinion or advice contained in the content. Please also see our complete disclaimer.***
Tags: carbohydrates, cycling, diet, dr gabe mirkin, endurance athletes, glycogen, high fat, low carb, performance, sprint
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The Final Sprint
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I have a Burn The Fat Feed The Muscle blog. It pretty much covers fat burning.
Come and check it out if you get time ;)
December 10th, 2006 at 9:48 amYou have some very good information on this blog. I would very much like your permission to recommend my readers to visit this site. I have a blog that encourages weight loss by changing your eating habits and this site would help alot.
January 13th, 2007 at 4:25 pmCora Mclaughlin-Fuller
I can only say that 6 days is not far enough to make your body change from carb-burning to fat-burning. For me it took a mounth but it was definately worth it. No I can go on for ever cause I never run out of energy. I’ve also read about a man who tried a low carb diet as a long distance biker. On carbs he run out of energy after 70 kilometers but on fat it lasted 130 Km.
January 29th, 2007 at 2:27 pmIt is well known that a ketogenic (fat burning) diet will “negatively effect performance” in sprinting. If you need to sprint optimally, you need glucose. Also, most people will take up to 3 weeks to adapt to changing their diet to low carb. This is well-known and well-reported in numerous studies and books. If you want to try to bash a low-carb diet, make sure you put your study subjects on it for only a week or 2, just like this study did! A better-designed study regarding high-fat diets and cyclists was done by Stephen D Phinney — http://www.nutritionandmetabolism.com/content/1/1/2
April 9th, 2008 at 12:42 pm