A study published last year in the Archives of Internal Medicine found that women who drink two to four caffeinated cups of coffee a day are significantly less likely to develop depression – an interesting statistic, given that women are twice as likely to encounter depression as men. What’s more, the likelihood to be at a lower level of risk for depression increases as more coffee is consumed – suggesting that the caffeine content in coffee may have some effect on brain chemistry. (Caffeine has also been linked to lower risk levels for Parkinson’s disease, diabetes and suicide).
So, if you drink coffee, you’re likely to be happier, right?
Well, not quite. While the findings of this study are interesting and the methodology is fairly sound (the data comes from a long-running, longitudinal Nurses’ Health Study with around 50,000 women in which the average age of participants in the most recent draw was 63), the association that’s been found is based on correlation, not causation. The finding is that as coffee consumption increases, depression levels decrease, and vice versa. But correlations often occur in data sets in a spurious manner (where two unrelated variables vary together), and that means that there needs to be a stronger theoretical basis to draw a strong conclusion.
It’s important to keep in mind, after all, that other variables could be at play here. For example, coffee could be a beverage preferred by those who are not depressed because of the influence of some other group of variables. The depressed state could cause coffee to taste too bitter to some individuals, causing them to drink less of it. Depressed people could have a biologically-driven aversion to stimulants and favor depressants (such as alcohol) or carbohydrates instead. And so on down the line.
One theory put forth in the WebMD article about this study is that caffeine is thought to elevate levels of dopamine and serotonin in the brain, two of the chemicals the brain uses to make a person feel happier and less stressed. This may lead to lower levels of depression. The only way this hypothesis can be confirmed, however, is for an experimental study to be conducted focused specifically on establishing this causation.
One way this could be conducted would be a study that directly studies the effects of increasing caffeine on brain chemistry related to depression. This would likely require some sort of neuroscientific research (such as the use of an fMRI) and would be limited to understanding the direct impact of caffeine on brain scans.
Another could determine the direct effects of increasing levels of caffeine on a group of women who are depressed, a group of women not prone to depression or elation, a group of women who are elated, and a control group of randomly-selected women. In each group, there would be three subgroups: those who consume no caffeine, those who consume 1-3 servings of caffeine daily, and those who consume 4 or more servings of caffeine daily. The data would make a strong argument for whether or not the hypothesis should be accepted.
Proper experimental design is a lot more difficult than simply searching for correlations, but it helps to ensure that when you hear a claim like “Coffee can help curb depression!” that the basis is derived from observed cause and effect and not just some connections in a data set.