WASHINGTON (Reuters) - Omega-3 fatty acids, found in soy, fish and other oils, and known to provide a range of health benefits, may help protect against Alzheimer's disease, U.S. researchers reported on Wednesday.

Tests on mice showed that a diet high in one particular omega-3 fatty acid called DHA helped protect the brain against the memory loss and cell damage caused by Alzheimer's disease.

"We saw that a diet rich in DHA, or docosahexaenoic acid, dramatically reduces the impact of an Alzheimer's gene, said Greg Cole, a Professor of Neurology at the University of California Los Angeles School of Medicine.

In the journal Neuron, Cole and colleagues reported the results of studies with mice bred to have genetic mutations that cause brain lesions associated with Alzheimer's disease.

The investigators were looking for something else, but noticed the mice did not have the expected memory loss or brain damage. Notably, the synapses, the connections between brain cells, were not as damaged as would be expected.

"We discovered that the mice lived on a nutritious diet of soy and fish -- two ingredients chock-full of omega-3 fatty acids," said Sally Frautschy, who worked on the study.

"Because earlier studies suggest that omega-3 fatty acids may prevent Alzheimer's disease, we realized that the mice's diet could be countering the very thing we were trying to accomplish -- showing the progression of the Alzheimer's-related brain damage," she added in a statement.

The researchers removed fish and soy from the mouse diet and substituted safflower oil instead, which is low in omega-3 and rich in another fatty acid called omega-6, which does not include DHA. Some mice stayed on the original diet and others got the new, less-healthy diet.

"We found high amounts of synaptic damage in the brains of the Alzheimer's-diseased mice that ate the DHA-depleted diet," Frautschy said. "These changes closely resembled those we see in the brains of people with Alzheimer's disease."

Mutant mice on the DHA-rich diet did better on memory tests than the mice fed safflower oil, the researchers said.

"After adjusting for all possible variables, DHA was the only factor remaining that protected the mice against the synaptic damage and memory loss that should have resulted from their Alzheimer's genes," said Cole. "We concluded that the DHA-enriched diet was holding their genetic disease at bay."

People are already advised to eat omega-3 fatty acids to protect the heart.

DHA and a related fatty acid called AHA are also added to some infant formulas and milks to promote brain development. They are found naturally in human breast milk.

SOURCE: Neuron, September 2, 2004.