The study, which appears in the February 2009 edition of the journal Appetite, showed that dieters performed less well on cognitive tests when carbohydrates were restricted than if they reduced calories.
The researchers said that this is because glucose, the brain’s primary fuel, is not stored, but produced by the body when it breaks down carbohydrates. They are then converted to glucose, and the resulting energy is used immediately by nerve cells.
The researchers suggested that restricting carbohydrates would restrict the amount of energy available to the brain, thereby adversely affecting cognitive ability.
Professor of psychology and corresponding author Holly A. Taylor said: “The popular low-carb, no-carb diets have the strongest potential for negative impact on thinking and cognition.”
Low-carb vs. low-calorie
The study involved 19 women who selected either a low-carbohydrate (LC) diet, similar to the Atkins diet, or a macro-nutrient reduced-calorie diet as recommended by the American Dietetic Association (ADA). Nine chose the LC diet, which restricted carbohydrates to under 20g a day (the recommended daily intake is a minimum of 130g), while 10 chose the ADA diet.
The researchers allowed the women to self-select their diets, in order to ensure the best possible compliance.
Their cognitive skills, including long and short-term memory, spatial memory and visual attention were tested 72 hours before starting the diets, and 48 hours and one week after starting. They were then tested in the second and third weeks, after carbohydrates had been reintroduced.
“Although the study had a modest sample size, the results showed a clear difference in cognitive performance as a function of diet,” said Taylor.
The LC dieters had slower reaction times in all tests and showed a gradual decrease in memory performance. However, they did better than the ADA group in attention tests, concurrent with previous studies in the area which have shown that LC diets can improve attention span in the short-term.
The researchers wrote: “Taken together the results suggest that weight-loss diet regimens differentially impact cognitive behaviour.”
The LC dieters’ cognitive impairment improved on reintroducing carbohydrates to the diet.
“Although this study only tracked dieting participants for three weeks, the data suggest that diets can affect more than just weight,” said Taylor. “The brain needs glucose for energy and diets low in carbohydrates can be detrimental to learning, memory and thinking.”
The study was conducted partly in response to the popularity of low-carbohydrate diets which make promises about ‘dieting without hunger’ and ‘drop a jean size in two weeks’.
The authors suggest: “Another common phrase may be an important reminder to these prospective dieters — youare what you eat.”
The researchers also noted that subjective hunger ratings did not vary between the two groups and nor did average weight loss – at 2kg.
Source: AppetiteFebruary 2009, Volume 52, Issue 1, Pages 96-103 "Low-carbohydrate weight-loss diets. Effects on cognition and mood"Authors: Kristen E. D’Anci, Kara L. Watts, Robin B. Kanarek, Holly A. Taylor