A Low-Carb Diets for Athletes?

By Laird Harrison

Just about everyone I know has at least flirted with a low-carbohydrate diet — Atkins, Zone, Paleo, Southbeach — and lots swear by the weight-loss effects.

But a low-carb diet for athletes goes against the standard recommendations of sports nutritionists. In fact, the American College of Sports Medicine (ACSM) personal training text book recommends pancakes as a healthy alternative when traveling.

So I was fascinated to talk to some researchers at the ACSM Annual Meeting in May who have examined the effects of a low-carb diet for athletes.

The bottom line: low-carb diets work fine for endurance. The benefits for power are less clear.

One study looked at the effects on long-distance — and I mean really long-distance – runners. All 20 men in the study had completed a 50-mile race.

Ten had spent at least six months living on a low-carb diet. They got only 11% of their calories from carbs. They did that so that their bodies would shift to extracting more energy from fat.

That way they don’t have to carry food with them, or even stop to eat during the competition, the lead researcher, Patrick M. Davitt, of Mercy College told me. Some people get sick to their stomachs if they eat or even drink carbohydrates during intense exercise, he said. “Blood is being shunted away from the gut.”

The other ten men followed the standard recommendations about carbo-loading before a big race. Their diets supplied 58% of their calories from carbohydrates.

Equal Aerobic Capacity

When they tested these ultra endurance runners on a treadmill, Davitt and his colleagues found no difference in aerobic capacity between the two groups. That is, the low-carb men and the high-carb men used the same amount of oxygen when running at full speed, which is a standard measure of fitness.

But the researchers found that the high-carb group on average could burn up to .67 grams per minute of fat, while the low-carb group could burn up to 1.54 grams per minute.

“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 gram per minute, said Dr Davitt. But these studies did not include ultra-endurance athletes who had adapted to low-carb diets.

In addition, the study showed that the low-carb group reached maximal fat oxidation at lower intensity of exercise. This indicates these runners could rely on their store of fat more quickly than the high-carb group.

Low-Carb Diet for Athletes Results


Low-Carbohydrate Diet, n =10 High-Carbohydrate Diet, n = 10
Carbohydrate in Diet, % 11 58
Fat in Diet, % 71 28
Protein in Diet, % 19 15
VO2max, mL/kg/min 64.3 64.7
Maximal Carbohydrate Oxidation, g/min 5.65 7.83
Maximal Fat Oxidation, g/ 1.54 .67
% VO2max @ Maximal Fat Oxidation 70.25 54.89

But a low-carb diet seemed to work less well in another study on a low-carb diet for athletes presented at ACSM. That study compared 18 experienced CrossFit participants who typically ate moderately low-carb diets — about 40% of calories.

When half the group switched to higher-carb diets, they could do more repetitions of the CrossFit exercises. The difference was not statistically significant, but the researchers believe it’s worth following up.

And Boston-based sports nutritionist Nancy Clark says burning fat for exercise might provide enough energy over a long period, but not for a burst of exertion.

“People on low-carbohydrate diets don’t have the energy for a surge,” she told me. “That’s a real problem. You just can’t get into the higher gear.”

For more technical details on this study, read this article in Medscape.

Featured photo: Competitors in the Donadea 50-kilometer run. By Peter Mooney.

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2 thoughts on “A Low-Carb Diets for Athletes?”

  1. What Nancy Clark is saying only holds true for a person with a moderate low carb diet. If your are truly fat adapted (meaning over 70% energy from fat sources.) you can exert the same amount of energy (in my opinion even more energy) as if you where on a normal carbohydrate based diet.
    I am weightlifting (3xweek), (+ running (3-4xweek), + HIIT (2xweek) and when i am in between the zone: meaning having a cheat meal on weekends, carb-backloading, or intermittent fasting i notice a decrease in energy output when muscles are depleted from carbs. If my muscles are loaded with carbs, the power i can output is stable. But if i am fully fat adapted (after 10-14 days) i can exert more than on a carb based diet. The (small) downside of it is that it also takes more time to regenerate because the muscles can not recover as easily compared to using carbohydrates. The ketone bodies in your blood need to be high to exert more power than with carbs. The statement: “People on low-carbohydrate diets don’t have the energy for a surge. That’s a real problem. You just can’t get into the higher gear.” needs to be reconsidered.

    1. Great to hear your experience. I haven’t tried a really low carb diet myself, so I can’t offer my own. I think Nancy’s point is that fat can’t be converted to energy as quickly as carbs. I’d love to hear from others who know about the biochemistry.

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