Researchers from the Harvard School of Public Health reviewed data from three previous US studies and found the risk of suicide amongst adults who drank several cups of caffeinated coffee on a daily basis was about half that of that compared to those who drank decaffeinated coffee, very little coffee or no coffee at all.
The study, of 200,000 men and women, examined data which outlined their caffeine consumption both coffee and non-coffee sources - including tea, caffeinated soft drinks, and chocolate.
However for the majority, coffee was the main caffeine source and over the average 6 and-a-half year assessment period, there were just 277 deaths from suicide.
'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.
Caffeine works by stimulating the central nervous system but may act as a mild antidepressant by increasing production of certain 'feel good' neurotransmitters in the brain, including serotonin, dopamine, and noradrenaline.
This could explain why previous epidemiological studies have found a lower risk of depression among coffee drinkers in past, the researchers reported.
In spite of the findings, the authors do not recommend that depressed adults self-medicate by increasing their caffeine consumption as 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 a day or 400 mg of caffeine a day,' the authors wrote.
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 a day, most likely due to the small number of suicide cases in these categories.
However, in a previous HSPH coffee-depression study, the investigators observed a maximal effect among those who drank four or more cups per day.
In fact one previous 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 impact of six or more cups a day of coffee was not addressed in these two studies.