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Carbohydrates,performance and weight loss
Is low the way to go or the way to bonk?


Arizona State University, School of Nutrition and Health Promotion
550 N. Third Street, Phoenix, AZ 85004, USA


Despite their popularity, meta-analyses demonstrate that low-carbohydrate diets are no more effective for weight loss than low-fat or balanced diets. Carbohydrates are essential for high-intensity exercise because the maximum rate of energy production is substantially higher for carbohydrates compared to fats. Low-carbohydrate diets reduce tolerance for very high-intensity exercise. Training under low-carbohydrate conditions may enhance skeletal muscle adaptations, but no study has shown that this type of training results in superior endurance performance.  The fact that world’s best marathoners consume relatively high carbohydrate diets is testament to the superiority of carbohydrates as a fuel for competitive endurance sports.


Low-carbohydrate diets have been prescribed for weight loss and, more recently, been advocated for athletes to enhance endurance performance (1).  Popularity of low-carbohydrate diets was high in the 1960s and early 1970s, after which interest in carbohydrate restriction waned. In late 1990s there was a resurgence of interest in low-carbohydrate dieting, and diets advocating restriction of carbohydrates have remained popular ever since.  Despite the popularity of low-carbohydrate diets over the years, only during the past 15 years or so has research on low-carbohydrate diets provided sufficient data to evaluate the efficacy of carbohydrate-restrictive diets for weight loss and health. 

In endurance sports, reaching a point in the race where an athlete can no longer maintain the desired pace is referred to as “bonking.”  The precipitous decline in performance associated with the bonk has been largely attributed to depletion of carbohydrate stores in skeletal muscle and liver, and possibly to reductions in blood glucose concentration.  Indeed, carbohydrate is the fuel of choice for endurance athletes, ...