Drinking several cups of coffee daily appears to reduce the risk of suicide in men and women by about 50 percent, according to a new study by researchers at the Harvard School of Public Health (HSPH). The study was published online July 2 in The World Journal of Biological Psychiatry.

“Unlike previous investigations, we were able to assess association of consumption of caffeinated and non-caffeinated beverages, and we identify caffeine as the most likely candidate of any putative protective effect of coffee,” said lead researcher Michel Lucas, research fellow in the Department of Nutrition at HSPH.

The authors reviewed data from three large U.S. studies and found that 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 antidepressant by boosting production of certain neurotransmitters in the brain, including serotonin, dopamine, and noradrenaline. This could explain the lower risk of depression among coffee drinkers that had been found in past epidemiological studies, the researchers reported.

In the new study, researchers examined data on 43,599 men enrolled in the Health Professionals Follow-Up Study (HPFS) (1988–2008), 73,820 women in the Nurses’ Health Study (NHS) (1992–2008), and 91,005 women in the Nurses’ Health Study II (NHSII) (1993–2007). Caffeine, coffee, and decaffeinated coffee intake was assessed every four years by questionnaires. Caffeine consumption was calculated from coffee and other sources, including tea, caffeinated soft drinks, and chocolate. However, coffee was the major caffeine source — 80 percent for NHS, 71 percent for NHS II, and 79 percent for HPFS. Among the participants in the three studies, there were 277 deaths from suicide.

In spite of the findings, the authors do not recommend that 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. “Overall, our results suggest that there is little further benefit for consumption above two to three cups/day or 400 mg of caffeine/day,” the authors wrote.

The researchers did not observe any major difference in risk between those who drank two to three cups of coffee per day and those who had four or more cups a day, most likely due to the small number of suicide cases in these categories. However, in a previous HSPH coffee-depression study published in JAMA Internal Medicine, the investigators observed a maximal effect among those who drank four or more cups per day. One large Finnish study showed a higher risk of suicide among people drinking eight or nine cups per day. Few participants in the two HSPH studies drank such large amounts of coffee, so the studies did not address the impact of six or more cups of coffee per day.

Other HSPH researchers participating in the study included senior author Alberto Ascherio, professor of epidemiology and nutrition; Walter Willett, chair, Department of Nutrition and Fredrick John Stare Professor of Epidemiology and Nutrition; and research associates Eilis O’Reilly and An Pan. Pan now works at the National University of Singapore.