A study of the elderly in Japan showed that those who drank four or more cups of green tea a day were considerably less likely to show symptoms of depression.

This effect was not seen in drinkers of coffee, black tea or oolong tea.

Prior studies have suggested that green tea drinking is linked to lower levels of psychological distress. This led the researchers to look at the association between green tea drinking and symptoms of depression in 1,058 relatively healthy elderly individuals. The 488 individuals who reported drinking four or more cups of green tea daily were 44% less likely to show symptoms of depression than those who drank one cup or less daily.

The apparent ability of green tea to ward off depression did not fade after adjustment for other factors, such as socioeconomic status and medical history.

Green tea drinking is much more common in Japan than it is in the United States.

Just because people who drink a lot of green tea are less likely to exhibit depression doesn't mean that green tea is the cause. That's why the researchers stop short of recommending green tea as a treatment to minimize depression in the elderly. The results do suggest that this possibility should be tested in clinical trials. Green tea is inexpensive, readily available and has been in use for centuries.

If green tea does have an anti−depressive effect, this may be due to the presence of the amino acid theanine. Some studies have shown theanine to reduce physical and mental stress and promote relaxation. So far, theanine has been found only in the tea plant and a single species of mushroom. Chromatographic analysis of tea has shown that green tea contains more theanine than black or oolong tea does.

In the study, 1,058 community−dwelling people aged 70 or older were tested for depressive symptoms by filling out the 30−item Geriatric Depressive Scale. Additionally, any subject taking antidepressants was considered to have depressive symptoms. Green tea consumption was determined from a questionnaire. The apparent ability of green tea to ward off depression did not fade after adjustment for other factors, such as socioeconomic status and medical history.

Depressive symptoms were fairly common in the study population, with 34.1% of the participants to be showing mild depressive symptoms and 20.2% showing severe depressive symptoms.

The study was performed by researchers at the Tohoku University Graduate School of Biomedical Engineering in Sendai, Japan. An article detailing the study was published in the December 2009 issue of the American Journal of Clinical Nutrition.