Following the research on the health effects of caffeine is dizzying. Positive in some cases, negative in others – it’s hard to know whether that morning cup of joe is a health elixir or slow-acting poison.
In one of the latest major studies on caffeine’s effects, researchers from the Harvard School of Public Health found a correlation between drinking 2-4 cups of caffeinated coffee each day and lower suicide risk among adults.
The study, published in The World Journal of Biological Psychiatry, was a meta-review of three extensive U.S. health studies that included a total of 43,599 men and 164,825 women. Consumption of caffeine (from tea, soda and chocolate), coffee and decaffeinated coffee was evaluated among study participants every four years via questionnaire. Across all three studies, coffee accounted for the majority of caffeine consumed at 71% of the total.
Causes of death were tracked during the study period by reviewing death certificates; 277 deaths were the result of suicide.
The analysis showed that the risk of suicide among adults drinking 2-4 cups of coffee a day (the equivalent of about 400 mg of caffeine) was 50% less than the risk for adults who drank decaffeinated coffee or one cup or less of caffeinated coffee. Drinking more than 4 cups of coffee didn’t drop the suicide risk lower.
As with all correlative results, it’s worth noting that this analysis does not show causation between drinking coffee and lower suicide risk, and there’s nothing in this study to suggest that suddenly bumping up your caffeine intake will curb depression. It's also worth noting that the three studies examined in this one were cohort studies, meaning they tracked multiple health risk factors across large groups over the course of several years, and it's notoriously difficult to control for variables with this study design. Cohort study results aren't very popular with statisticians.
Having said that, the neurochemistry behind the finding makes sense. As discussed here, caffeine acts as an expert mimic of a chemical called adenosine in the brain and other parts of the body. Adenosine is a sort of checks-and-balances chemical produced by neurons as they fire throughout the day; the more adenosine is produced, the more the nervous system ratchets down activity, until we eventually fall asleep and reboot the process.
By mimicking adenosine, caffeine blocks receptors in the nervous system from receiving the signals to decrease energy expenditure. When that happens, levels of the brain’s homegrown neuro-stimulants—dopamine and glutamate—increase, and we experience the brain stimulating effects associated with drinking a big cup of java.
Seen this way, coffee may act as a mild antidepressant -- at least to an extent. Previous research has found similar correlations reinforcing the possibility that coffee--the most frequently ingested psychoactive substance in the world--can help alleviate depression.
All of this research, however, should be taken with an enormous caveat that the findings are anything but conclusive. And given the drawbacks of cohort studies, it's possible that the latest study results are a mirage that wouldn't hold true outside of this particular correlative fishbowl.