Low-carbohydrate diets continue to be quite popular, especially among clients trying to lose weight by exercising. But can these diets actually backfire by causing frustration and early fatigue during an exercise session?

Researchers reporting in the October 2007 issue of the Journal of the American Dietetic Association set out to determine whether low-carb diets might result in early fatigue or affect the desire to exercise. The degree of carbohydrate restriction is an important consideration for people who work out, because reduced tissue glycogen stores are known to cause fatigue during sustained exercise.

Subjects in the “ketogenic” group of this study ate a diet very low in carbs (5% of energy) and high in fat (65% of energy), with the balance (30% of energy) coming from protein. The control group ate a conventional nonketogenic low-carb diet (40% carb, 30% fat and 30% protein). The researchers compared fatigue levels and ratings of perceived exertion (RPE) during exercise in untrained, overweight adults who adhered to the ketogenic diet versus control subjects who followed the nonketogenic diet. All meals and snacks were provided to study participants, and energy intake was strictly controlled to provide approximately 70% of that needed for weight maintenance.

At study’s end, both diet plans were equally effective at inducing weight loss and reducing fat mass. However, blood ketone levels were a whopping 3.6-fold higher in participants who adhered to the ketogenic diet. Interestingly, the higher blood ketone levels were also associated with elevated RPE scores and increased feelings of fatigue while exercising.

This was the first study to relate “tiredness” during exercise to ketosis, according to the
researchers. The results led them to conclude that “these data support the contention that
diets with extreme carbohydrate restriction can impact the desire to be physically active. Although low-carb, high-protein diets are effective for weight loss, they may thwart attempts to combine diet modifications with increased physical activity.”

The researchers went on to suggest that health professionals advise clients who want to lose weight to eat “moderate amounts of carbohydrate (35%-40% of energy) from vegetables, fruit and whole grains and ample protein (up to one-third of energy).”