A new study from scientists in the US found that when women went on low or zero-carb diets they performed worse on thinking and memory tests compared to reducing calories without reducing carbohydrates. When they put carbs back into their diet, their thinking and memory skills went back to normal.
The study was the work of researchers from the psychology department of Tufts University in Medford, Massachusetts. It is published in the February 2009 journal Appetite and is already available to view online.
Dr Holly Taylor, professor of psychology at Tufts and corresponding author of the study, said the findings showed 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,” she added.
Taylor’s co-authors and research colleagues were Professor Robin Kanarek, former undergraduate Kara Watts and research associate Kristen D’Anci.
Our brain cells need glucose to work, but they have no way of storing it so they rely on a continuous supply via the bloodstream. The researchers had a hunch that reducing carbohydrate intake would reduce the body’s ability to keep the brain supplied with glucose and therefore affect cognition, since glucose comes from breaking down carbohydrates.
For the study, Taylor and colleagues recruited 19 women aged 22 to 55 and let them each choose to go on either a low carb or low calorie diet as recommended by the American Dietetic Association. Nine of them chose the low carb diet and the other 10 chose the low calorie diet.
Altogether the participants attended five assessment sessions. Session 1 was just before they started on their chosen diet, sessions 2 and 3 were during the first week of dieting (when the low-carb dieters eliminated carbohydrates), and sessions 4 and 5 were in weeks 2 and 3, after the low-carb dieters started eating carbohydrates again.
During the assessment sessions the dieters performed a range of tests that measured attention, short and long term memory, visual attention and spatial memory. They also answered questions about how hungry they felt and their mood.
The results showed that:
- Low carb dieters showed a gradual decrease on memory tasks compared with low-calorie dieters.
- Reaction time for the low-carb dieters was slower, and their visual-spatial memory was not as good as that of the low-calorie dieters.
- But low-carb dieters responded better than low-calorie dieters in the attention-vigilance tasks.
- This last result is consistent with previous studies that found people on high protein or high fat diets showed short term improvements in attention.
- Hunger levels did not vary between the two diet groups, and the only difference in mood was that the low-calorie dieters felt more confusion during the middle period of the study.
“Although the study had a modest sample size, the results showed a clear difference in cognitive performance as a function of diet.”
“The data suggest that after a week of severe carbohydrate restriction, memory performance, particularly on difficult tasks, is impaired,” she added.
Taylor also explained that:
“Although this study only tracked dieting participants for three weeks, the data suggest that diets can affect more than just weight.”
“The brain needs glucose for energy and diets low in carbohydrates can be detrimental to learning, memory, and thinking,” she concluded.