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.
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