Is Dieting a Brain Drain?

By: Rachael Moeller Gorman  |  May/June 2009
Some weight-loss programs may impact memory.
Watch: Top Memory-Boosting Foods
Losing weight can have lots of benefits: you look better, feel better and slash your risk of developing heart disease, diabetes and a host of other problems. But picking the wrong diet may muddle your memory, say researchers.
In a new study from Tufts University, 19 overweight women followed either a “low-carbohydrate” or a “low-calorie” diet, based on American Dietetic Association guidelines, for three weeks. After the first week, people in the low-carbohydrate group, who were told to completely eliminate carbohydrates from their diets, did worse on tests of working memory (i.e., why did I walk into this room?) and visuospatial memory (remembering locations on a map) than people in the low-calorie group.
“The brain’s primary fuel is glucose,” says Holly Taylor, Ph.D., cognitive psychologist at Tufts and co-lead investigator on the study. Eating carbohydrate-rich foods—grains, fruits, vegetables—is by far the most efficient way of keeping the brain’s glucose supply on “full.” But the body can only store one to two days’ worth of glucose, and when these stores are gone, glucose levels in the blood (also known as “blood sugar”) drop. Fats and proteins can be backup fuel sources, but they don’t provide the glucose needed to sustain peak brain power.
Luckily for the study subjects, “memory performance ­returned to normal when we re­introduced carbohydrates in the second week of the study,” said Taylor. A piece of fruit or one-­quarter of a slice of bread seemed to be enough.
Even though Taylor’s study suggests that tiny amounts of carbs are enough to preserve memory, a few other studies suggest that popular low-carb diets—such as Atkins, which allows more carbohydrates than Taylor’s study does in the second week—may compromise mental abilities. One study, published in The American Journal of Clinical Nutrition in 2007, showed that people on a higher-carbohydrate diet processed information more quickly than those on a low-carbohydrate diet.
Fortunately, all diets don’t dampen brain power. A balanced, low-calorie diet may, in fact, boost it. A study in the Proceedings of the National Academy of Sciences earlier this year showed that restricting total calories by up to 30 percent for three months actually increased verbal memory scores (i.e., how many words were remembered 30 minutes after seeing them) in a group of elderly people. Researchers suspect that losing weight improved the body’s ability to use glucose, and that this allowed the brain to work more smoothly.
Bottom Line: Be cautious about striking carbohydrates from your diet: you may lose more than just a few pounds. The Recommended Dietary Allowance for carbohydrates is 130 grams per day, a number based on the amount of glucose the brain needs to function optimally. What does 130 grams look like? A cup of oatmeal, an apple, two slices of whole-wheat bread and ¾ cup of cooked pasta.