The suicide risk in adults who drink two to four cups of caffeinated coffee per day is about half that of those who drink decaffeinated coffee or very little or no coffee, according to a new study at the Harvard School of Public Health.
Suicide risk in this study refers to those people who actually completed suicide; the researchers did not measure actual suicide risk as commonly referred to by other researchers (which would include those who attempted suicide, but failed).
“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, Ph.D., research fellow in the Department of Nutrition at HSPH.
Not only does caffeine stimulate the central nervous system, but it 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 depression risk among coffee drinkers found in past epidemiological studies, the researchers reported.
For the study, scientists examined data on 43,599 men enrolled in the Health Professionals Follow-Up Study (HPFS) from 1988 to 2008; 73,820 women in the Nurses’ Health Study (NHS) from 1992 to 2008, and 91,005 women in the Nurses’ Health Study II (NHSII) from 1993 to 2007.
Caffeine, coffee, and decaffeinated coffee intake were reported every four years through questionnaires.
Caffeine consumption was calculated from coffee and other sources, including tea, caffeinated soft drinks, and chocolate. Coffee was the major caffeine source, however, at 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. The researchers did not assess those people who had suicidal ideation, a suicide plan, or those who attempted suicide but did not complete it.
The U.S. Centers for Disease Control and Prevention estimate that for every completed suicide, another 25 people have attempted suicide but failed.
In spite of the findings, the researchers are not recommending that depressed adults increase their caffeine consumption, because most people adjust their caffeine intake to a level that feels right 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,” said the authors.
The researchers did not notice any significant 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, probably due to the small number of suicide cases in these categories.
However, in a prior HSPH coffee-depression study, scientists observed a maximal effect among those who drank four or more cups per day. Finnish researchers actually found a higher risk of suicide among people drinking eight or nine cups per day. Few participants in the two HSPH studies drank that much coffee, so the studies did not address the impact of six or more cups of coffee per day.
The study is published in The World Journal of Biological Psychiatry.
Source: Harvard University