BioEd Online

When the star of the movie Super Size Me ate only McDonald's for a month, his physical health went down the tube. Now researchers have warned that such diets could hit mental abilities too.

Although this idea has been suggested before, a slew of animal studies, unveiled on 25 October, all conclude that learning and memory suffer when fat intake rises. Rats and mice raised on the rodent equivalent of junk food struggle to learn their way around a maze and take longer to recall the solution to problems they have already solved, researchers said at the annual meeting of the Society for Neuroscience, held in San Diego.

In one experiment, rats were asked to remember the position of platforms in a pool of water; the animals are motivated to do so because they dislike swimming. Two groups took the test: controls and a set that had munched on a high-fat and high-cholesterol diet for eight weeks. "Those on the high-fat diet made many more mistakes," says Ann-Charlotte Granholm of the Medical University of South Carolina in Charleston.

Another study challenged mice to learn how to navigate a maze without running over areas that gave them mild electric shocks. When John Morley and colleagues at Saint Louis University in Missouri tested the mice a week after they had learned the task, those raised on a high-fat diet took significantly longer to remember how to avoid the shocks.

Trans-fat trouble

Morley's group speculates that triglyceride, a cholesterol-like substance that is found at high levels in rats on a high-fat diet, could be causing at least some of the cognitive damage. Morley says that when rats take a drug that cuts triglyceride levels but that does not cut weight, their performance on the memory tasks improves.

Memory problems have also been documented in diabetics suffering from high levels of triglyceride, but both Morley and Granholm say little work has been done on the cognitive impact of such diets in humans.

"We believe, on the basis of these data, that high-fat diets are bad for cognition," says Barry Levin, a physician at the New Jersey Medical School in East Orange. "But we need a note of caution: we work on rats and this may not extrapolate to humans."

Granholm says, however, that there is enough evidence for people to avoid certain kinds of foods, such as those containing trans-fatty acids (trans-fats), which are known to raise levels of triglyceride and cholesterol.

Trans-fats are found in everything from cereals to margarine and bread, and manufacturers prefer them to other fats because they prevent food going off. "Decreasing the shelf life of the foods you eat might increase your own shelf life," notes Levin.