How many carbohydrates do you need to fuel a high-intensity bodybuilding workout? This question has triggered a heated debate. High-carb proponents often base their position on studies that examined endurance athletes, who tend to deplete muscle-glycogen stores and thus require higher carb intake. Low-carb advocates point to recent studies that indicate that the body might adapt to a higher-fat, lower-carb intake by up-regulating fat-burning enzymes in muscle. In addition, fat byproducts, such as ketones, can be a direct energy source for muscle. Another valid point is that there is no physiological requirement for carbs, since they can be synthesized from protein and, to a lesser extent, from the glycerol portion of triglycerides (fat).
A recent study from Texas Christian University analyzed the effects of low-carb vs. high-carb diets consumed by 11 male weight trainers. First, the men depleted their leg-muscle glycogen stores through cycling. Then, for two days, they followed either a highcarb or low-carb diet. Next, the men were put through a workout consisting of five sets each of squats, leg presses and leg extensions, reps done to failure, with three minutes rest between sets.
No differences in either muscle strength or endurance were observed in either the high-or low-carb groups. However, those who consumed the low-carb diet showed an increased reliance on circulating blood glucose relative to those on the high-carb plan.
What does this mean? Circulating blood glucose is controlled by levels of liver glycogen. The glycogen stored in muscle, in contrast, can be used only by the muscle it's stored in, due to lack of a certain enzyme in muscle that permits systemic release of glucose. However, byproducts of muscle-glycogen metabolism, such as lactate, can be released into the blood, then converted into glucose in the liver through a process called the Cori cycle.
As long as liver-glycogen stores are intact, limiting carbs before a bodybuilding workout will not adversely affect workout intensity. Taking carbs right after a workout might provide an anticatabolic effect through limited cortisol release.