As if eating badly and being overweight were not already harmful enough, research suggests that consuming too much of several kinds of fat can damage memory and intellect.

With about a third of the U.S. population either overweight or obese, the results could have broad significance for the national IQ.

"We are in the midst of an obesity epidemic in the U.S.," said Dr. Barry Levin, of the VA Medical Center in East Orange, N.J.

"These studies show that diets high in fat are a risk factor for not only heart disease, hypertension, and diabetes, but for cognitive decline as well."

All of the studies, which were announced at the annual conference of the Society for Neuroscience, involved animals. However, scientists agreed that the results almost certainly applied to humans as well.

"After I did this study, I didn't eat french fries anymore," said neuroscientist Lotta Granholm, who found that trans fats -- the sort found in many fast foods -- impaired memory and learning in middle-age rats.

Many recent studies have linked trans fat to heart disease and cancer, but Granholm's research is the first to connect it to problems with learning and memory.

"It's an important study. There's a real impact on the brain," said University of South Florida-Tampa neuroscientist Paula Bickford, an expert on the how food affects cognition.

Last year, Granholm fed one group of rats a diet that contained 10% hydrogenated coconut oil, a common trans fat. She gave another group the same diet, but replaced the coconut oil with soybean oil, which is not a trans fat.

After six weeks, the animals were tested in a series of mazes. The coconut oil group made far more errors, especially on the tests that required more mental energy.

"The trans fats made memory significantly worse," said Granholm, who is director of the Center on Aging at the Medical University of South Carolina in Charleston.

The quantity of trans fat fed to the rats was equivalent to amounts consumed by many Americans. "It was not more than what people would eat," said Granholm, who also noted that the rats in her study were not overweight. This indicates that the cognitive problems were not related to obesity, but to consumption of trans fat.

"You don't have to be overweight to be affected by this," she said.

CHANGE IN EATING
Granholm has taken her study to heart: After she got her results, she and her husband went through their refrigerator and threw out almost all the food that contained trans fats.

Over the past two decades, trans fat has become ubiquitous in this country. It is used in many crackers, cookies, doughnuts, cakes and breads, as well as many fast foods, which are often fried in partially hydrogenated vegetable oil, a common trans fat.

The fats, which are made by adding hydrogen to vegetable oil under high pressure, appeal to manufacturers and restaurants because they extend shelflife and remain solid at room temperature, as saturated fats do.

In response to recent research, and efforts by consumer groups, the U.S. Food and Drug Administration earlier this year decided that starting in 2006, products that contain trans fats must say so on the label.

Granholm said trans fat seems to produce its effect on the brain by destroying proteins that help neurons send and receive signals. In animals that ate coconut oil, these molecules, known as microtubule-associated proteins, were much less common.

Granholm suspects that trans fat increases inflammation in the brain, which damages the proteins.

FOLLOW-UP STUDIES
It is not clear whether the damage is reversible. Granholm plans to do a follow-up study to see whether the impairment decreases once the trans fat diet is discontinued.

She is also planning to look at mice that are fed a diet high in both trans fats and blueberry extract. Previous research has shown that blueberries can improve brain function, and Granholm thinks the extract might counteract the trans fat effects.

Trans fat is not the only offender. A study by researchers at the National Institute on Aging (NIA) in Baltimore found that a diet high in saturated fat -- the kind that comes from beef, butter and milk -- impairs memory in mice.

NIA neuroscientists Mark Mattson and Veerendra Kumar Madala Halagappa gave mice a diet high in trans fat and saturated fat and then ran them through a series of maze tests. The high-fat group did worse than mice fed a normal diet.

Halagappa and Mattson think these fats increase brain levels of a fat called sphingomyelin. This substance somehow reduces the amount of brain-derived neurotrophic factor (BDNF), a protein that protects and nurtures nerve cells, and fosters learning.

Their study did offer a bit of good news for those with a sweet tooth: Mice on a high-sugar diet had no cognitive difficulties.

"That surprised us," said Halagappa, who had expected the regimen to harm memory.

LEARNING, MEMORY
Triglycerides -- fat that has long been known to raise the risk of heart disease and stroke -- also seem to hinder learning. Researchers at the VA Medical Center in St. Louis fed mice a high-fat, high-calorie diet. The fatter group performed much worse on a series of learning and memory tests.

The scientists suspected that the problems were caused by an increase in triglycerides. To test this, they gave the mice gemfibrozil, a lipid-lowering drug that is particularly good at lowering triglycerides. The animals' triglyceride levels dropped, and the memory troubles disappeared.

"The same mechanism is likely important in people," said geriatrician John Morley, one of the researchers who did the study. "We're assuming (triglycerides) are doing the same thing in humans."

He noted that the animals' triglyceride levels were not enormously high -- the rodents had the equivalent of mild to moderately high human levels.

Researchers agreed that replicating the animal results in humans could be difficult. Morley pointed out that ethical concerns would probably keep researchers from subjecting humans to the same kinds of high-fat diets fed to the rats.

However, Bickford, the University of South Florida-Tampa researcher, said that fat's potential effects on cognition were too serious to ignore: "The bottom line is, what is the impact on humans?"