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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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