There aren't many areas where men and women benefit equally but coffee has always been about bringing people together. Do you think Newton would have done his great work without coffee? No, he would have starved long before Principia. The man ate every meal in a coffee house.

A new review in The World Journal of Biological Psychiatry will get heads nodding among coffee acolytes for finding that drinking lots of coffee daily reduces the risk of suicide in men and women by about 50%. The authors reviewed data from three 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, they say. 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 analysis, the scholars examined data on 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. Caffeine consumption was calculated from coffee and non-coffee sources, including tea, caffeinated soft drinks, and chocolate. However, coffee was the major caffeine source – 80% for NHS, 71% for NHS II, and 79% 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 2-3 cups/day or 400 mg of caffeine/day,” the authors write.

The researchers didn’t observe any major difference in risk between those who drank two to three cups of coffee per day and four or more cups/day, most likely due to the small number of suicide cases in these categories. However, in a previous 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 Harvard School of Public Health studies drank such large amounts of coffee so the impact of six or more cups/day of coffee was not addressed in these two studies.

“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 Harvard School of Public Health.

Citation: Michel Lucas, Eilis J. O’Reilly, An Pan, Fariba Mirzaei, Walter C. Willett, Olivia I. Okereke  & Alberto Ascherio, 'Coffee, caffeine, and risk of completed suicide: Results from three prospective cohorts of American adults', The World Journal of Biological Psychiatry doi:10.3109/15622975.2013.795243