A recently published study found a correlation between higher rates of coffee drinking in women and decreased risk of depression. Naturally, that finding made headlines. But blogger Scicurious has a really nice analysis of the paper that picked up a significant flaw in the way the data is being interpreted. There was a correlation between drinking more coffee and a lowered risk of depression. But that wasn't the only correlation the researchers found—just the only correlation they made a big deal of in their conclusions.
On her blog, Scicurious lists the other correlations and explains why it's hard to draw any solid conclusion from this data set:
1) Smoking. The interaction between depression risk, smoking, and coffee consumption was "marginally" significant (p=0.06), but they dismiss it as being due to chance because it was "unexpected". Um. Wait. Nicotine is a STIMULANT. It is known to have antidepressant like effects in animal models (though the withdrawal is no fun). This is not unexpected.
2) Drinking: heavy coffee drinkers drink more. But note that they don't say that drinking coffee puts you at risk for drinking alcohol.
3) Obesity: heavy coffee drinkers are, on average, thinner, but not more physically active. They do not conclude that coffee drinking prevents obesity.
4) Church going: heavy coffee drinkers are less likely to go to church. Less likely to go to church, less likely to develop depression…heck, forget depression, maybe coffee prevents religion now! Now THAT would be a heck of a finding.
Here's the thing. I do believe that high coffee consumption correlates with decreased risk of depression. But a lot of other things do as well. I am not convinced that the high coffee consumption wasn't part of a lifestyle that correlated with decreased risk of depression, maybe they have stronger support networks or less incidence of depression in the family. It could be many other things.