The rat experiments used a well-established animal model of depression, according to the researchers from Harvard-affiliated McLean Hospital in Belmont, Mass.

The rats were placed in a tank of water, where they had no choice but to swim. After a while, the rats realized swimming was futile, so they simply began to float, a sign of surrender to depression. Given an antidepressant drug, however, they started swimming again, the researchers said.

But combined doses of omega-3 fatty acids and uridine were as effective as three different antidepressants in prompting the rats to start swimming again, said study author William Carlezon, director of McLean's Behavioral Genetics Laboratory.

"We had given these two components [omega-3 fatty acids and uridine] separately," Carlezon said. "As it became clear that each treatment in its own way was having an effect, we came up with the idea of giving them together to see if there would be a synergistic effect, because they act on the same system."

The drugs and the dietary components used in the study probably act on mitochondria in brain cells, he said. "Mitochondria produce energy for brain cells," Carlezon explained. "Imagine what happens if your brain does not have enough energy. Basically, we were giving the brain more fuel on which to run."

The findings appear in the Feb. 15 issue of Biological Psychiatry.

Omega-3 fatty acids are well-known ingredients in many fish, and are most abundant in oily species such as salmon and tuna. Cardiologists recommend a diet rich in oily fish because omega-3 fatty acids are good for the circulatory system. And what's good for the heart is also good for the brain, said Dr. Bruce Cohen, president and psychiatrist-in-chief at McLean Hospital.

"If you study people around the world and take people of similar background, the group eating more fish has a lower rate of heart disease and depressive illnesses," Cohen said.

Omega-3 fatty acids are best obtained by eating fish rather than in dietary supplements, he said. "In fish, they are fresh and in the form you need," Cohen said.

Uridine is a different matter. It's not found in high levels in any particular food, Carlezon said. It is an important element in mother's milk, and baby formula is enriched with uridine because it is essential for early nerve growth, he said.

There are no uridine supplements now on the market, but there might be a case for them, Carlezon said. More studies are needed to see whether uridine in the diet affects mental capacity and learning, he added.

There are growing indications that mitochondria are involved in psychiatric conditions other than depression, Carlezon said. McLean researchers have found major alterations in the genes for mitochondria in people with bipolar disorder, a condition that includes cycles of depression.