Do Low-Carb, High-Fat Diets Sap Performance?

Uwe is a 45-year-old runner from Germany. Like many other runners, he recently noticed a lot of online chatter about low-carb, high-fat (LCHF) diets for endurance athletes. Intrigued, he decided to give it a try. He completely removed bread, pasta, rice and potatoes from his diet. Within three months he had lost 17 pounds.

There was only one problem. Uwe's marathon training completely tanked. On long runs he found himself unaccustomedly exhausted after 2 hours and 15 minutes, which was troubling because his expected marathon time was 3:30. Desperate, Uwe contacted me through my website. "From day to day I feel more tired and sluggish," he told me. "What should I do?" Naturally, I told him to get reacquainted with bread, pasta, rice and potatoes as quickly as possible.

Uwe's story is typical of runners and other endurance athletes who make the unwise decision to leap aboard the runaway LCFH diet bandwagon. They do so with high hopes of boosting their fitness and setting PRs but soon discover that the diet saps their endurance and their speed, slows their recovery between workouts, and leaves them tired all the time. Most LCHF dieters do lose weight, but there's nothing intrinsically good about that. You could lose weight by skipping lunch every day and that wouldn't boost your running performance either.

More: The New Rules of Marathon Nutrition

The Ideology Behind LCHF
The core issue with the LCHF diet is that, like so many diets, it's based on ideology rather than reality. The ideological foundation of the LCHF is the notion that carbohydrate is bad. It's so bad that everyone should eat as little of it as possible. And not only that, but endurance athletes should try to avoid letting their muscles burn carbs (in the form of muscle glycogen and blood glucose) when running, relying on fat to fuel their muscles instead.

The specific goal of increasing the fat-burning capacity of the muscles is not without merit. The supply of fat fuel in the body is vastly greater than the supply of carbohydrate fuel. Carbs are a much more efficient fuel, however, and the body prefers to use them at higher-exercise intensities. But through training and dietary manipulation, runners can teach their muscles to burn more fat at moderately high intensities. As a result, carbohydrate is spared, which is critical, because when the body's carbohydrate stores run low, runners hit the wall.

More: How Many Carbs Are Enough?

The normal training process increases the muscles' fat-burning capacity significantly. An LCHF diet increases it even further. But whereas training boosts performance along with fat-burning capacity, a LCHF diet sabotages performance. Numerous studies have demonstrated that maintaining a LCHF diet is a terrible idea for runners and other endurance athletes. The trouble with ideology, though, is that it's impervious to facts. As evidence against the wisdom of LCHF diets piles up, advocates of these diets blithely continue to advocate them, and athletes like Uwe are harmed.

More: How Much Healthy Fat Should You Eat?

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