Lead researcher Michel Lucas, a research fellow in the Department of Nutrition at the Harvard School of Public Health, and colleagues found the risk of suicide for adults who drank two to four cups of caffeinated coffee per day was about half that of those who drank decaffeinated coffee or very little or no coffee.
Caffeine not only stimulates the central nervous system but may act as a mild anti-depressant by boosting production of certain neurotransmitters in the brain, including serotonin, dopamine and noradrenaline, Lucas said.
The authors did not recommend depressed adults increase caffeine consumption because most individuals adjust their caffeine intake to an optimal level for them and an increase could result in unpleasant side effects, Lucas said.
"Overall, our results suggest there is little further benefit for consumption above two to three cups/day or 400 milligrams of caffeine/day," the authors wrote in their study findings, published in the World Journal of Biological Psychiatry.
The researchers based their findings a review of data from three large U.S. studies involving the 43,599 men enrolled in the Health Professionals Follow-up Study 1988-2008, 73,820 women in the Nurses' Health Study 1992-2008 and 91,005 women in the Nurses' Health Study II 1993-2007.
Caffeine, coffee and decaffeinated coffee intake was assessed every four years by questionnaires.