Athletes who adopt a high-protein, low-carbohydrate diet may hurt their performance, not enhance it, sports nutritionists say.
For years, such diets have attracted throngs of athletes, from high school to professional levels, who seek to build muscle and drop unwanted pounds.
Although conceding that the diet may well lead to weight loss, sports nutritionists warn that it may not be the kind of weight athletes want to lose.
Instead of taking off fat, athletes on the diet who train vigorously and compete regularly probably are shedding muscle mass.
"It's a trendy diet that is a big mistake for athletes," said Chris Rosenbloom, an associate professor of nutrition at Georgia State University. "They find out eventually that their body's run out of fuel too quickly."
The mainstream appeal of many low-carbohydrate diets has helped promote the popularity of similar eating programs among athletes, who are constantly seeking a competitive edge.
Leading health and fitness magazines reinforce the diets' appeal by bombarding athletes with ads trumpeting "carb-free" energy drinks, "zero-carb" energy bars and low-carb nutrition shakes.
But carbohydrates are essential to athletic performance because they are converted into glucose, which is needed to power muscles. Carbohydrates stimulate the production of insulin, which helps release amino acids into muscles so they can be built, maintained and repaired.
Carbohydrates also supply more energy for muscles -- and do it more efficiently -- than either proteins or fats, nutritionists say. Eating enough carbohydrates is crucial for almost any activity -- regardless of the fitness level of the athlete -- especially if the exerciser has low body fat and is unable to call upon fat reserves for energy.
Nutritionists also caution athletes against accepting dietary recommendations from personal trainers who don't have proper credentials or certifications.
"You want your diets based on research," said Rosenbloom, who has consulted with the Atlanta Hawks pro basketball team. "Not on what some guy tells you at the gym."
Although athletes differ, sports nutritionists say a good general guideline for a sports diet is to consume 5 to 7 grams of carbohydrates and 1.2 to 1.5 grams of protein for each kilogram of body weight. (In nonmetric terms, the equivalent is 0.16 to 0.22 ounces of carbohydrate and 0.04 to 0.05 ounces of protein for each 2.2 pounds of body weight.)
At minimum, athletes need to eat at least 500 total grams (about a pound) of carbs each day. "You don't want to eat Cap'n Crunch for breakfast," she added. "Oatmeal will work."