Researchers from the Harvard School of Public Health (HSPH) reviewed data from three large U.S. studies, analyzing data from tens of thousands of people regarding consumption of caffeine, coffee and decaffeinated coffee every four years through food-frequency questionnaires, while simultaneously, physicians reviewed and analyzed death from suicides.
The researchers assessed "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 NHS II (1993-2007)."
The amount of coffee consumed was assessed from both coffee and non-coffee sources like chocolate, tea and caffeinated soft drinks, though researchers add that coffee was the main source of caffeine in the study, accounting for a minimum of 71 percent in all three studies reviewed.
Suicide was the cause of death for 277 people over the course of the study, though researchers made a surprising discovery: the risk of suicide for adults who drank between 2-4 cups of coffee each day was 50 percent lower than adults who drank decaf, very little or no coffee.
According to the researchers, there were no major differences in the risk of suicide between those who drank 2-3 cups of coffee per day and those who drank 4 or more, though they did note that this may be due to smaller rates of suicides overall in this category.
So how does coffee affect the nervous system and the brain, and why might it help in the prevention of suicide?
Researchers explained that coffee and caffeine stimulate the central nervous system, acting as a mild anti-depressant by boosting the brain's production of neurotransmitters including noradrenaline, dopamine, and serotonin, chemicals all associated with depression. This, they add, could help explain why the results of past studies have linked coffee to lowered risks of depression.
Researchers concluded by advising a coffee intake of 2-4 cups a day for healthy adults, though experts advise that too much caffeine can result in nasty side effects like insomnia, nervousness, restlessness, muscle tremors and elevated heartbeat.
However, researchers also noted a previous study from HSPH linking caffeine and depression, in which participants had heightened depression symptoms from drinking 4 or more cups of coffee a day.