Many bodybuilders tend to complain of muddled thinking when they begin low-carbohydrate diets. This isn't surprising, since
the brain is a "glucose hog," extracting 10% of the glucose circulating in blood. Few people realize that the brain also stores
glycogen, albeit in amounts too small to have any meaningful effect.
Past medical studies do suggest that a carb-restricted diet has several interesting effects on the brain. For example, some studies describe subjects experiencing a "positive mood state" akin to relaxation. And because of the brain's glucose dependence, reduced carb intake may decrease brain activity.
In the case of epilepsy, this offers advantages. Epilepsy is characterized by a random firing of brain neurons, leading to fits and seizures. But studies show that under a reduced carb intake, the brain activity of epileptics slows down, leading to a distinct lowering of the severity of symptoms. The mechanism behind this remains unclear, however.
Since the brain appears to depend on glucose as a primary energy source, shouldn't bodybuilders on diets containing almost no carbohydrates pass out? Chalk up the fact that they don't to the wisdom of nature. The body has evolved alternative energy sources to deal with the brain's sensitivity to hypoglycemia (low blood glucose). Think of a car: Although it runs best on premium gasoline, it will also run on a lower- octane fuel, although the engine may protest with knocks and sputters.
The brain works in a similar manner. Sure, it prefers glucose, but if the optimum fuel is in short supply, it can adapt by exploiting at least two other fuel sources: ketones and lactate. Ketones are intermediate byproducts of fat oxidation, and the body produces greater amounts of them during low carb diets. In time, the body can adapt to using ketones as a substitute fuel albeit a less-efficient one - for glucose.
This idea is illustrated in a recent study published in the International Journal of Obesity (19:811-816, 1995). Researchers observed the cognitive effects of a low-carb diet on 21 overweight women over 28 days of dieting. The subjects took various psychological tests designed to measure brain performance.
The study indicated that most mental tasks weren't adversely affected by the low-carb diet, with one exception: mental flexibility. Significantly, this characteristic was measured by the most-demanding tests. Impairment occurred during the first week of the diet. In addition, some of the women appeared to be more affected than others, suggesting individual tolerances to the diet.
Some women, when faced with a task that required quick thinking, exhibited a slight loss of ability during the initial week of low-carb dieting. This wasn't entirely unexpected considering the lag that occurs while the brain shifts from glucose dependence to an alternate fuel source.
Most dietitians do not suggest forgoing carbs completely on a long-term basis. Low-carb diets often include "off" days, during which greater amounts of carbohydrates are consumed to replenish depleted body-glycogen stores. Consuming carbs on those days will quickly mitigate any brain drain caused by low glucose.