THE BLOG
12/14/2011 12:16 pm ET Updated Feb 13, 2012

Does Coffee Reduce Cancer Risk?

As I type this post, I am drinking my second coffee of the day. I had my first cup within an hour of waking up; I brewed some beans at home before heading out to study. Now, it's noon, and I'm drinking a skim latte at The Wormhole.

If I continue sipping at this pace, I could reduce my risk of developing endometrial cancer later in life. According to a Harvard University study, "Women who consumed four or more cups of coffee had 25 percent lower risk of endometrial cancer than those who consumed less than one cup per day."

I admit I was skeptical when I first saw headlines about this study. I began wondering how the data was collected. Were the coffee drinkers self-reporting their intake? Was the study longitudinal, and if so, for how many years were the subjects followed? What other variables need to be assessed? Do coffee drinkers belong to a certain socioeconomic demographic? Do coffee drinkers engage in other activities that reduce their risk of developing cancer? What if the coffee is decaf? What is a proposed mechanism for the inverse relationship between coffee and cancer?

(What can I say? My undergraduate professors taught me to be wary of any headline that claims causation.)

Here's what I found out.

First of all, the study is longitudinal. The data comes from the Nurses' Health Study, which began 26 years ago and follows some 67,000 women. In this population, there were 672 documented cases of endometrial cancer.

Indeed, the subjects self-reported their coffee intake. There are several potential limitations of self-reporting. For example, I might tell a researcher that I drink two cups of coffee each day. But in reality, I drink three cups on some days, one cup on other days, and solely tea on other days. But if I had to tell you a number, that number would be two. Maybe it's more like 2.5? No, on average, it's two. I think.

Another potential limitation of self-reporting is the lack of standardization. That is, my cup of coffee may be bigger than your cup. Or maybe I brew my coffee stronger than you brew yours. My cup of coffee could be the equivalent of someone else's three cups, or vice versa.

The authors of the paper acknowledge these limitations and hope that future studies can address them.

One thing I found interesting is that decaffeinated coffee offers virtually the same protective benefits as regular coffee. Tea, however, does not provide any protection against endometrial cancer. This, of course, implies that caffeine probably isn't involved in the mechanism. This finding also offers one reason for drinking decaf coffee. (To me, decaf coffee and non-alcoholic beers elicit one question: What's the point? But that's neither here nor there.)

If not caffeine, what component of coffee might help reduce cancer risk? One possibility, proposed by the authors of the study, is the anti-oxidative properties of coffee. This suggestion seemed strange to me, given tea -- especially green tea -- is packed with antioxidants. But the authors found no correlation between tea intake and cancer, which suggests that antioxidants may not all be created equal.

One possible mechanism involved in the inverse relationship between coffee and cancer could involve estrogen. A risk factor for some forms of endometrial cancer is excessive estrogen exposure. According to a past studies cited in the paper, coffee may help remove excess estrogen from the bloodstream.

The authors of the study note that exercise and weight management offer the best protection against endometrial cancer. This is because obesity is a major risk factor for endometrial cancer and many other diseases. In other words, the addition of cream and sugar to your coffee may negate the potential benefits of the bean itself. And pastries, though delicious accompaniments to lattes, are best eaten only on rare occasion.

This study, like most scientific studies, establishes a correlation. It does not prove causation. Without a definite mechanism -- and without a controlled, randomized study -- it is impossible to prove cause-and-effect.

For now, the inverse relationship between coffee intake and endometrial cancer should be taken with a grain of salt. Or perhaps with a grain of sugar. Or, actually, hold the sugar.