Feeling depressed? Ask not what your parents did or didn’t do when you were a child. Ask yourself what you had for dinner last night, and the night before, and the night before that.

For half a dozen years now, the evidence has been growing that omega-3 fatty acids, the kind found in fatty fish like salmon, sardines, and tuna, can help prevent and treat depression.

Rich in EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), these are among the “good” oils that have long been known to reduce the risk of heart attacks and strokes. They are also the oils that, in recent decades, in tandem with rising depression rates, Americans have not been getting enough of. Omega-3s combat auto-immune diseases like rheumatoid arthritis, reduce cardiac arrhythmias, and are crucial to the development of the spinal cord, brain, and retina in infants and to healthy brain functioning in adults as well.

The case for linking low omega-3 levels to depression is strong, though not yet a slamdunk. But there is little risk – and significant benefit – to following the American Heart Association recommendation to eat fish at least twice a week and, if you already have heart disease, taking at least 1 gram a day of supplements containing EPA and DHA.

The latest evidence for the role of omega-3 fatty acids and depression came last month, when researchers from McLean Hospital in Belmont, Mass., reported that omega-3 fatty acids, plus uridine, another substance found commonly in meat and other protein-rich foods, prevented depression in rats just as well as antidepressant drugs. The effect of uridine was immediate, said Bill Carlezon, director of the behavioral genetics lab at McLean. It took 30 days for omega-3 to kick in. But combining the two made omega-3 effective three times faster.

“There is something to this story,” said Dr. Andrew Leuchter, vice chair of psychiatry at the Neuropsychiatric Institute at UCLA. “I have seen enough patients who treat themselves with omega-3 fatty acids to think these substances may have, at least in some individuals, potent effects on mood.”

No one knows exactly why omega-3s might protect against depression, but theories abound. One is that depression may, in part, be an inflammatory problem, which omega-3s can damp down, said Dr. Andrew Stoll, director of psychopharmacology at McLean. Another is that the oils keep cell membranes more fluid, making it easier for receptors to respond to neurotransmitters like serotonin, which is often deficient in depression. Another is that omega-3s may boost levels of serotonin.

Whatever the underlying mechanism, “the epidemiological evidence is huge,” Dr. Stoll said, that omega-3s can protect against depression.

Overall, major depression is 60 times more prevalent in countries where little fish is eaten, said Dr. Joseph R. Hibbeln, senior clinical investigator at the National Institute on Alcohol Abuse and Alcoholism.

In 1998, Dr. Hibbeln and others showed that high fish-eating countries like Japan and Taiwan have very low rates of depression, while low fish-eating countries like Germany and the United States have high rates. “When you compare rates of depression across populations, there is a consistent finding of strikingly lower rates of major depression, bipolar disorder, seasonal affective disorder, and postpartum depression in countries where people eat more seafood,” he said. Small clinical trials also support the association – though not uniformly.

A 2002 double-blind, placebo-controlled study of 20 people in Israel, for instance, showed that adding EPA to standard antidepressants significantly decreased depression after three weeks. A 2002 Scottish study of 60 people came to similar conclusions when patients added a 1-gram daily supplement of EPA to their standard treatment. (Interestingly, higher doses did not work as well.) A 2002 Taiwanese study of 28 people using both EPA and DHA also found significant improvement in depression scores, compared to placebo.

But not all studies support this. A New Zealand team studied 77 mildly depressed people and randomly assigned them to add 8 grams a day of fish oil or a placebo (olive oil) to standard antidepressant therapy. Mood improved in both groups.

A Finnish study that asked nearly 30,000 men to recall their fish oil consumption over the years also found no link between omega-3 fatty acids and depression or suicide, although studies based on recall are notoriously inaccurate. And a double-blind, placebo controlled study of 35 people at Baylor College of Medicine in Houston found no improvement in depression when they gave patients only DHA, not EPA.

In the plus column, bipolar disorder, also known as manic depression, seems to be helped by omega-3 fatty acids. A study comparing 10 countries showed that higher fish consumption correlated with lower rates of bipolar disorder. A 1999 study by Dr. Stoll of McLean showed that giving fish oil supplements to people with bipolar disorder reduced episodes of depression and mania.

Omega-3 fatty acids also appear to reduce hostility and homicide. Several studies have found that low intake of fish and omega-3s correlates with higher rates of hostility, which is often associated with depression in males. A 36-country study showed lower rates of homicide in countries where people ate more fish.

And postpartum depression also appears linked to omega-3 levels. A study of women in 23 countries showed that women who ate less seafood and had lower rates of DHA in their breast milk were more likely to suffer postpartum depression. Pilot studies by Dr. Marlene Freeman, director of the Women’s Mental Health Program at the University of Arizona, suggest taking DHA and EPA can reduce postpartum depression by 50%.

(Many pregnant women and nursing mothers, added Dr. Freeman, have been frightened about eating any kind of fish because of government warnings of particularly high mercury levels in king mackerel, shark, swordfish, and tilefish. The Center for Science in the Public Interest, a nutrition advocacy group, says that sardines and salmon contain little mercury; women of reproductive age should probably limit their consumption of canned tuna to one can of white or two cans of light a week.)

Even borderline personality disorder, characterized by volatile interpersonal relationships and impulsivity, seems to respond to omega-3 treatment. Yet another McLean study of 30 patients found that EPA, without any other medication, improved symptoms of borderline personality.

That’s more than enough evidence for me. Given the long-standing overall health benefits of omega-3 fatty acids and the newly emerging psychiatric benefits, the conclusion is a no-brainer. Eat fish two times a week. And if you hate fish (as I do), take at least 1 gram a day of a supplement with EPA and DHA.

Ms. Foreman’s column appears every other week. Past columns are available on www.myhealthsense.com.