Endurance runners adapted to a low-carbohydrate diet can burn up to 1.54 g of fat per minute, which is at least 50% more than the highest previous estimate, researchers report.
A new study provides evidence that endurance athletes can perform at high levels without consuming carbohydrates during competitions, said Patrick Davitt, PhD, from Mercy College in Dobbs Ferry, New York.
"We're trying to put all the naysayers to rest," he told Medscape Medical News. Dr Davitt presented the finding here at the American College of Sports Medicine (ACSM) 62nd Annual Meeting.
In a joint position statement issued by the ACSM, the American Dietetic Association, and the Dietitians of Canada, it is recommended that athletes replace energy burned during sports with carbohydrates (Med Sci Sports Exerc. 2009;41:709-731).
However, a growing number of ultra-endurance athletes are living on low-carbohydrate diets in an effort to adapt their metabolism to draw more energy from fat, Dr Davitt reported. That way, they don't have to carry food with them and they don't have to stop to eat or drink carbohydrates during a competition, he said.
In addition, they can avoid the gastrointestinal upset that some people experience when they eat during intense exercise. "Blood is being shunted away from the gut," he explained.
To see how low-carbohydrate diets are affecting endurance athletes, Dr Davitt and his colleagues recruited 20 experienced ultra-endurance runners from all over the United States. All athletes had completed a 50-mile race, all were men, average age was 33.5 years, and an average body mass index was 22.6 kg/m².
Ten of the athletes habitually ate high-carbohydrate diets that were 28% fat, 15% protein, and 58% carbohydrate, and 10 ate low-carbohydrate diets that were 71% fat, 19% protein, and 11% carbohydrate. All had been on these diets for at least 6 months.
As the athletes ran on treadmills, the researchers measured consumption of oxygen and production of carbon dioxide. From these measurements, they calculated the maximal aerobic capacity of each athlete and the amount of fat and carbohydrates burned.
There were no significant differences in the aerobic capacity between the two groups. However, on average, the high-carbohydrate group burned less fat per minute than the low-carbohydrate group (0.67 vs 1.54 g; P < .0001).
People on low-carbohydrate diets don't have the energy for a surge. That's a real problem. You just can't get into higher gear.
"That's really a profound finding because it indicates they are able to run at a higher intensity for a longer time using mostly fat," said Dr Davitt.
Previous studies indicated that the maximum possible rate of fat oxidation was less than 1 g/min, he explained. But these studies did not assess ultra-endurance athletes who had adapted to low-carbohydrate diets.In addition, this study showed that the low-carbohydrate group reached maximal fat oxidation at a lower intensity of exercise. This indicates that these runners could rely on their store of fat more quickly than those in the high-carbohydrate group.
Table: Measures of Oxidation
VariableLow-Carbohydrate DietHigh-Carbohydrate DietP Value
Maximal aerobic capacity, mL/kg per min64.364.7.85
Maximal carbohydrate oxidation, g/min5.657.83.002
% maximal aerobic capacity at maximal fat oxidation70.2554.89<.0001Adapting to a low-carbohydrate diet might allow endurance athletes to avoid the gastrointestinal symptoms caused by eating during exercise, said Nancy Clark, RD, who is a sports nutritionist in Boston.
However, fats burn more slowly than carbohydrates, which could be a drawback for competitive athletes who need to put on a burst of speed, she explained.
"People on low-carbohydrate diets don't have the energy for a surge," she said. "That's a real problem. You just can't get into higher gear."
The study was supported by Quest Nutrition and the Robert C. and Veronica Atkins Foundation. Dr Davitt and Ms Clark have disclosed no relevant financial relationships.
American College of Sports Medicine (ACSM) 62nd Annual Meeting: Abstract 1799. May 28, 2015.
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