In a report published online on October 14, 2009 in the American Journal of Clinical Nutrition, researchers from Tohoku University in Japan report that older people who frequently drink green tea have a significantly lower incidence depressive symptoms.

The study included 1,058 men and women aged 70 and older who resided in Sendai, Japan. Depression was evaluated using Geriatric Depression Scale scores, and dietary questionnaire responses were analyzed for the type and frequency of tea consumption. Blood samples were analyzed for C-reactive protein, a marker of inflammation.

Twenty-seven percent of the men and 29 percent of the women in the study were classified as experiencing mild and severe depressive symptoms, and 15 percent of the men and 24 percent of the women were categorized as having severe symptoms. The adjusted risk of mild and severe depression was 44 percent lower for those who reported drinking four or more cups green tea per day. For severe depression, determined by a high Geriatric Depression Scale score or antidepressant use, consuming four or more cups green tea per day was associated with a 52 percent lower adjusted risk.

The scientists originally hypothesized that tea’s protective effect against depression was due in part to its anti-inflammatory effect, however, the study failed to find an association between green tea intake and C-reactive protein levels. The other mechanism they proposed, that of an anti-stress response due to tea’s theanine content, could explain the study’s findings. Theanine, an amino acid found in green tea, is able to penetrate the blood-brain barrier and may increase brain serotonin and dopamine, which could help improve mood and reaction to stress. “Thus, these data prove a useful hypothesis that higher consumption of green tea is related to a lower prevalence of depressive symptoms, possibly because it leads to a decrease in the stress response,” the authors write. “A further study is required to clarify whether green tea or theanine have a beneficial effect on the prevention and treatment of depressive symptoms.”