Low Carb Dieting and Impact on Cognitive Ability

Holly A. Taylor, professor of psychology at Tufts, has recently published a new study that highlights some serious concern about the way low-carb diets affect cognition. When dieters eliminate carbohydrates from their meals, they performed more poorly on memory-based tasks than when they reduce calories, but maintain the carbs. When carbohydrates were reintroduced, cognition skills returned to normal.

"This study demonstrates that the food you eat can have an immediate impact on cognitive behavior. The popular low-carb, no-carb diets have the strongest potential for negative impact on thinking and cognition," says Dr. Taylor.

She collaborated with Professor Robin Kanarek, former undergraduate Kara Watts and research associate Kristen D'Anci in the study entitled "Low-carbohydrate weight-loss diets: Effects on Cognition and Mood," which appears in the February 2009 edition of the journal Appetite.

The study says that while the brain uses glucose as its primary fuel, it has no way of storing it. Instead, the body breaks down carbohydrates into glucose, which is carried to the brain through the blood stream and used immediately by nerve cells for energy. Reduced carbohydrate intake should thus reduce the brain’s source of energy. Therefore, this study hypothesizes that diets low in carbs would affect cognitive skills.

Study participants included 19 women ages 22 to 55 who were allowed to select the diet plan they preferred -- either a low-carbohydrate diet or a low-calorie, macronutrient balanced diet recommended by the American Dietetic Association. Nine women chose a low-carbohydrate diet and 10 selected the low-calorie diet.

"Although the study had a modest sample size, the results showed a clear difference in cognitive performance as a function of diet," says Taylor. Five testing sessions were performed by these patients that assessed cognitive skills such as attention, long-term and short-term memory, visual attention, and spatial memory. The first session was held before participants began their diets, the next two sessions occurred during the first week of the diet, which corresponded to the week when low-carb dieters eliminated carbohydrates. The final two sessions occurred in week two and week three of the diets, after carbohydrates had been reintroduced for those on the low-carb diet.

The data suggest that after a week of severe carbohydrate restriction, memory performance, particularly on difficult tasks, is impaired. Low-carb dieters showed a gradual decrease on the memory-related tasks compared with the low-calorie dieters. Reaction time for those on the low-carb diet was slower and their visuospatial memory was not as good as those on the low-calorie diet. However, low-carb dieters actually responded better than low-calorie dieters during the attention vigilance task. Researchers note that past studies have shown that diets high in protein or fat can improve a person's attention in the short-term, which is consistent with the results in this study.

Dave