Literature Review Links Coffee & Bladder Cancer

This article was originally published on RealClearScience.

The most interesting man in the world has nothing on coffee, which is the most interesting beverage in the world. Coffee continues to be the subject of countless studies, some more serious than others. Thanks both to science and intrepid entrepreneurs, for instance, we have learned the chemistry of perfect coffee, thebest time of day to partake, and why drip coffee is likelier to spill than a latte. You may think that coffee and pooping have nothing in common, but you would be wrong. Again, thanks to science, we know why it is worthwhile to pluck beans out of elephant dung, why coffee makes you poop, and why coffee should go in your mouth, not your butt. If all of that isn’t enough, you can now take coffee classes at some universities.

Of course, coffee has also been the subject of intense epidemiological investigation. Overall, while the literature seems to indicate that coffee is more beneficial than harmful, it does have its downsides. And researchers from China have now highlighted one of those disadvantages.

Every year, there are around 330,000 new cases of bladder cancer worldwide, and 130,000 die from the disease. According to the National Cancer Institute, nearly 75,000 Americans acquire bladder cancer annually, making the U.S. incidence rate 20.5 per 100,000 people. Because the incidence rate for all cancers in the U.S. is 460.4 per 100,000, that means roughly 4% of new cancers in the U.S. are cancers of the bladder.

The cause of bladder cancer is not well understood, but some studies have fingered coffee as a risk factor. In order to clarify this, the Chinese research team conducted a meta-analysis of the literature. They started with 1,788 articles and, using various criteria, whittled the list down to 40 studies. They found that, on average, coffee drinkers were 33% more likely to develop bladder cancer than those who refrained from drinking coffee. (See figure.)

The figure shows the odds ratios (OR) for each study. (The odds ratio measures the association between a particular exposure, in this case coffee, and a particular outcome, in this case bladder cancer. If OR = 1, the exposure is not linked to the outcome; if OR < 1, the exposure may help prevent the outcome; if OR > 1, the exposure may help cause the outcome.) For coffee and bladder cancer, the researchers found that the OR = 1.33, meaning that coffee raises the risk of bladder cancer by 33%. (*See footnote.*)

Importantly, the authors also showed a dose-response relationship: The more coffee a person drinks, the more likely he is to develop bladder cancer. (See figure.)

Graphs A and B depict OR plotted against daily cups of coffee. (Chart A represents data from case-control studies, while Chart B represents data from cohort studies.) Graph A shows that the OR increases by 0.05 with each additional daily cup of coffee; Graph B shows that the OR increases 0.03 for each additional daily cup of coffee.

Translated, that means each additional daily cup of coffee you drink raises your risk of bladder cancer by 3% to 5%. If, for instance, you drink three daily cups of coffee, your risk of developing bladder cancer will be 9% to 15% higher than for those people who do not drink coffee.

Should that prevent you from drinking coffee? Nope. Like mentioned earlier, coffee has several health benefits. But, if you are a heavy coffee drinker, you might want to take it down a notch. However, most of you should not worry and should continue to enjoy your daily coffee (or two) with reckless abandon… or in my case, an iced, triple-shot, vanilla latte.

[Note: The epidemiology sticklers out there will know that this interpretation is not exactly correct. Technically, odds ratios do not allow for a direct comparison of risks. However, because bladder cancer is infrequent, the odds ratio is a good approximation of relative risk. For a deeper discussion on how odds ratios can be misleading, see this BMJ paper (PDF). Epidemiology can be quite tricky, which is why science/health journalists need to treat such studies with care.]

Source: Weixiang Wu, Yeqing Tong, Qiang Zhao, Guangxia Yu, Xiaoyun Wei & Qing Lu. “Coffee consumption and bladder cancer: a meta-analysis of observational studies.” Scientific Reports 5, Article number: 9051. Published: 12-Mar-2015. doi:10.1038/srep09051