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It's a question students have long pondered: What foods boost performance on difficult learning tasks and tests? Some swear by protein-rich eggs and sausage, but according to recent brain research, potatoes, bread and other low-fat carbohydrates are a better bet.

The key ingredient is glucose, which boosts people's cognitive performance, according to psychologists Paul Gold, PhD, and Donna Korol, PhD, of Binghamton University, and Carol Manning, PhD, of the University of Virginia. In research described in a 1998 article in the American Journal of Clinical Nutrition (Vol. 67, p. 764S­771S), they found that it particularly improves people's adeptness at tasks involving memory and attention.

Researchers have long known that glucose fuels the brain, and have assumed that the brain always has a ready supply. But in rat studies, Gold and his graduate student, Ewan McNay, discovered that learning tasks quickly deplete the brain's glucose reserves. What's more, they found that replenishing that supply with injections of glucose helped rats learn more.

To see if the same is true in people, Gold and Korol gave elderly people and college students tests of short-term memory, attention and motor function. Before taking the tests, participants drank lemonade sweetened with either glucose or saccharin. The glucose showed strong effects: Elderly people who drank it recalled almost twice as much from a narrative prose passage as those who drank saccharin.

The researchers found a similar pattern in college students, but only when the students tackled a harder prose passage than the elderly group.

"To reveal glucose effects, the task must be difficult enough for a particular population," explains Gold. "In rats, the more of a cognitive challenge a task is, the more brain glucose is depleted."

So, what's the best vehicle for a healthy dose of glucose? Definitely not a chocolate bar, says Gold, because fat stalls the energizing effects of glucose. He also advises against eating large amounts of candy and refined sugar, "which would in any case be a horrible diet," he says.

There are other reasons not to use sugar to boost cognition. The effective dose range is rather narrow, with too much glucose impairing, rather than enhancing, cognitive functions. How much is best? That varies with people's metabolism and with the amount of glucose in their brains when they ingest it. Ultimately, Gold looks to further research to explore which meal combinations produce the right doses of glucose for optimal brain power. Knowing how it's best delivered could be particularly useful for school children, he says.

"It could help us come up with a better school meal plan and better integration of academic activities with meals," says Gold.

--B. MURRAY