NEW YORK (Reuters Health) - Low-carb diets may make people's short-term memory a little foggy, but they could improve people's ability to focus and pay attention, new research hints.

The key to keeping one's smarts while dieting seems to be not to cut out carbs completely, Dr. Holly A. Taylor of Tufts University in Medford, Massachusetts, the lead researcher on the study, told Reuters Health. "Low carbohydrate is OK; no carbohydrate is not," she said.

"Low-carb diets," Taylor added, "in the initial time period when they're actually no-carb diets, have the greatest potential to impair cognitive function because the brain uses glucose (sugar) as its primary fuel. The body breaks carbohydrates down into smaller components, including glucose, which the brain gets from the bloodstream, Taylor explained. So once carbohydrate stores are gone, the brain starts to starve.

Hypoglycemia, or low blood sugar, is known to worsen brain function, while low-carbohydrate diets that force the body to use body fat for fuel, a process known as ketosis, have long been used to control seizures, which "suggests that they can profoundly influence brain functioning," Taylor and her team note in the journal Appetite.

To investigate how low-carb diets might impact thinking and mood, they had 19 women choose either a low-calorie, balanced diet recommended by the American Dietetic Association (ADA), or a low-carb diet in which they cut out carbohydrates completely for a week and then gradually reintroduced them to their diets.

The study participants completed several tests of mood and cognitive function 72 hours before they began the diets and 48 hours, one week, two week and three weeks after starting the diet.

The nine women who chose the low-carb diet fared worse on tests of their memory during the first week of the diet, when no carbohydrates were allowed, than the 10 women on the ADA diet. Once they started eating carbs again, the memory differences between the two groups disappeared.

"Even with a very small amount of carbohydrate, performance returned to normal," Taylor said. She pointed out that the diet allowed them to add just 5 to 8 grams of carbs a day, while the daily recommended intake of carbohydrates for people who aren't trying to lose weight is 130 grams.

After the first week, the low-carb group performed better on a test of sustained attention than the ADA group, and also reported feeling less confused.

Past research has shown that people do better on tests requiring attention and vigilance after a high-protein meal compared to a high-carb meal, and also feel less fatigued, the researchers note.

The current findings, Taylor said, show that "there's more to weight reducing diets than just losing weight."

SOURCE: Appetite, February 2009.

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