The average sports nutritionist usually recommends that bodybuilders and others engaged in high-intensity anaerobic exercise emphasize carbohydrates in their diets, with 65 to 70 percent of calories being the suggested intake. That recommendation is based on the fact that dietary carbs provide the raw material for muscle glycogen, which is stored in the liver and muscle tissues and is the primary fuel for anaerobic exercise.

Even so, studies conducted over the years show that a high carb intake may not be as important for anaerobic exercise as some people believe. A 2003 study, for example, found that consuming a 65 percent carb diet offered no advantage over a 40 percent carb diet in promoting gains in strength and lean body mass. Since glycogen is the preferred fuel for high-intensity exercise, however, it seems rational that eating more carbs before training would promote more efficient workouts due to more available energy.

A new study compared high and low carb intakes in 11 recreational weightlifters. The subjects started by depleting the muscle glycogen stores in their legs through cycling, then followed either a high-carb (7.66 grams per kilogram of body-weight) or low-carb (0.37 grams per kilogram) diet for 48 hours. The subjects then did a weight-training routine consisting of five sets to failure of each of the following exercises: squats, leg presses and leg extensions.

The researchers took blood samples both before and after the exercise to determine blood glucose and lactate levels. Neither group displayed any significant differences in exercise performance, although those in the high-carb group had higher blood glucose levels after the workout. Blood lactate levels were similar in both groups. That's significant because it's believed that higher blood lactate levels occur with a lower carb intake.

The researchers suggest that the low-carb group maintained exercise performance due to a stable blood glucose level. The glucose may have been released during exercise by breakdown of preexisting glycogen stores in the men's livers. Since the study took place over a 48-hour period, it's possible that continuing a low-carb diet for a longer time may deplete liver glycogen stores. Once that happens, some muscle breakdown may occur during extended exercise, unless the exerciser consumes other fuel sources, such as fat.

The trick to avoiding such muscle breakdown may be to intersperse higher carb days with lower carb days, a precompetition practice of many bodybuilders. That helps to restore depleted liver glycogen levels, thus providing a source of fuel (glucose degraded from liver glycogen) during workouts. The controlled carb intake would depress insulin levels for a few days each week, which in turn would maximize bodyfat mobilization.