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NEW YORK, Nov 24 (Reuters) diet and nutrition may play a key role in helping people fight depression, Australian researchers report.

A number of nutrients, including polyunsaturated fatty acids, St John's Wort and several B vitamins, have the potential to influence mood by increasing the absorption of chemical messengers in the brain, Dr Dianne Volker of the University of Sydney in Chippendale and Jade Ng of Goodman Fielder Commercian in North Ryde, New South Wales note in the journal Nutrition and dietetics.

There is a wealth of epidemiological, experimental and circumstantial evidence to suggest that fish and the oils they contain, in particular omega-3 polyunsaturated fatty acid, are protective against depression, Volker and Ng write. They point out that the balance between omega-3 and omega-6 may also be important, given that the latter can prevent the body from absorbing the former.

Another candidate for dietary prevention of depression is the amino acid tryptophan, which is found in foods, including turkey, and is responsible for the drowsiness people feel after eating a hearty Thanksgiving dinner. The body converts tryptophan to the neurotransmitter serotonin, suggesting the amino acid may have modest effects on mood.

But studies investigating whether the B vitamin folate, vitamins B6 and B12, and S-adenosylmethionine (SAMe) play a role in depression have had conflicting results, the researchers write.

And while European studies have found that St John's Wort has antidepressant effects, US clinical trials have shown the opposite, which some think may be due to the herb's interaction with other medications.

Volker and Ng conclude: ''The role of balanced nutrition in mental health should be recognized,'' thus allowing for the use of nutrition and relevant nutrients in the maintenance of good mental health.