Is there an association between coffee consumption and incidence of cancer? The answer seems to depend on whom you ask, the study group involved, how much and what type of coffee the study participants drank and a host of other factors. Many research studies have found no link or only a weak link between the two, but recently a new study that showed a stronger association between coffee consumption and a lower risk of prostate cancer was published in the Journal of the National Cancer Institute (1). Why should we believe this new study when so many other studies have been unable to show a strong link?
…because this recent prostate cancer study is one of the biggest studies ever done on this topic to date and, due in part to the large sample size, the authors of this paper discovered something new and interesting about coffee consumption and prostate cancer.
[For you female readers who are thinking “OK, but I don’t have to worry about prostate cancer” and are tempted to quit reading at this point, skip ahead to learn how coffee consumption was recently linked to a reduced incidence of a certain type of breast cancer.]
As part of their prostate cancer study, the Harvard researchers collected data from 47,911 men in biennial questionnaires about their age, smoking habits, diet, health problems, physical activity, family medical history and consumption of alcohol and coffee. During the 20-year study, 5,035 men were diagnosed with prostate cancer, of whom 642 had a lethal form of the cancer, defined as fatal or metastatic cancer. Armed with this wealth of data, the authors performed a multitude of statistical calculations to examine a possible association between coffee consumption and incidence of prostate cancer, adjusting for potentially confounding factors such as smoking, race and diet. These adjustments were important because participants who drank the most coffee were more likely to engage in other behaviors that could possibly affect the incidence of cancer, such as smoking.
What the researchers found was a weak inverse association between coffee consumption and prostate cancer: Men who consumed >6 cups of coffee each day had an 18% lower risk of prostate cancer than men who did not consume coffee. The 18% drop, while statistically significant, isn’t that exciting. The real discovery came when they split prostate cancer patients into subcategories of nonadvanced, advanced and lethal cancer. They uncovered a much stronger link between coffee consumption and the risk of advanced and lethal prostate cancers. Men who consumed >6 cups of coffee per day were less than half as likely to get lethal forms of prostate cancer than men who refrained from a daily cup of joe. The fact that coffee drinkers were more likely to engage in behaviors that are thought to increase prostate cancer risks lends more support to the protective effect of coffee: These risky behaviors should increase, not decrease, the incidence of prostate cancer. Coffee was able to overcome any increased risk due to these other behaviors and then some.
The authors considered reverse causation as a possible explanation for their findings: Men with undiagnosed prostate cancer experience urinary symptoms that might cause them to limit their coffee intake. However, the authors examined the frequency of urinary symptoms in coffee drinkers and abstainers and found no statistically significant differences.
The protective effect of coffee could not be traced to caffeine, since there was no difference in prostate cancer rates between drinkers of caffeinated and decaffeinated coffee. Plus, the effect did not extend to men who ingested similar amounts of caffeine in other forms. That left the other components of coffee. Coffee is a complex mixture of compounds found in raw coffee beans and compounds formed during the roasting process. Components in coffee are known to improve glucose metabolism, have anti-inflammatory and antioxidant effects and influence sex hormone levels. All of these can play a role in prostate cancer progression.
The beneficial effects of these compounds seems to extend beyond the prostate. In a recent Breast Cancer Research paper, Jingmei Li and colleagues present the results of their study about coffee consumption and the incidence of breast cancer. They recruited 5,929 postmenopausal Swedish women to complete questionnaires about their diet, reproductive history, family medical history, education level and other factors, including coffee consumption. Of these women, 2,818 had been diagnosed with breast cancer, while 3,111 had not and served as controls. Daily coffee consumption was categorized as 0–1 cups of coffee, 1–3 cups, 3–5 cups and >5 cups. Women who drank only 1 cup of coffee per day were classified as nondrinkers due to the small number of women who completely abstained from coffee.
After taking into account the influences of potentially confounding factors such as smoking, hormone-replacement therapy, level of physical activity and alcohol consumption, the authors found no statistical difference in the overall rate of breast cancer in coffee drinkers versus abstainers. However, when they split the breast cancer cases into estrogen receptor (ER)-positive cancer, which tends to be slower growing and has a better chance of responding to hormone-suppression therapy, and ER-negative cancer, they found that women who drank more than 5 cups of coffee per day had a 57% lower risk of developing ER-negative breast cancer than women who drank no more than 1 cup of coffee per day. While this risk reduction was statistically significant in the Swedish study group, this reduction was not significant when a larger study group of 3,464 breast cancer patients in Germany (the Mamma Carcinoma Risk Factor Investigation, or MARIE, study) was examined.
The authors hypothesize that the risk reduction was not observed in the MARIE study due to differences in the brewing method, bean type, caffeine content or some other factor related to coffee drinking. The authors did not differentiate between caffeinated and decaffeinated coffee in their questionnaire, so the authors were unable to determine the role of caffeine, although they were able to cite previous studies that showed both positive and negative correlations between caffeine consumption and breast cancer risk. The authors noted that most coffee consumed in Sweden is brewed by the boiling method, while filtered coffee tends to be more prevalent in Germany. Perhaps a difference in brewing methods explains the difference? Chemical differences between boiled and filtered coffee exist and could explain the fact that in another study (3), boiled coffee, but not filtered coffee, reduced the risk of breast cancer.
Again, the authors believed that the benefits of coffee consumption are due to noncaffeine components of coffee, such as polyphenols, phytoestrogens, minerals and others, and that these components could differentially affect breast cancer of different ER subtypes. For example, plasma levels of the phytoestrogen enterolactone are higher in coffee drinkers, and this is associated with a decrease in ER-negative breast cancer risk. However, another phytoestrogen present in coffee, trigonelline, seems to activate the estrogen receptor via an estrogen-independent mechanism and stimulates the growth of ER-positive cancer cells. This differential effect on cancer subtypes (aggravates one, yet attenuates the other) could explain why the link between coffee consumption and overall risk of breast cancer seems so weak.
So, everyone should run out to nearest coffee house and order the largest vat of coffee possible, right? Well, don’t reach for your wallet just yet. Even though these newer studies showed a reduced cancer risk with increasing coffee consumption, there is still enough uncertainty that it would be premature to recommend massive volumes of coffee to stave off cancer. For the time being, continue to drink coffee for the usual reasons—the smell, the flavor or the jolt that only coffee can provide to get you moving in the morning—but don’t expect it to reduce your risk of cancer.
References
1. Wilson, K.M. et al. (2011). Coffee consumption and prostate cancer risk and progression in the Health Professionals Follow-up Study. J. Natl. Cancer Inst. 103 , 876–84. PMID: 21586702
2. Li, J. et al. (2011). Coffee consumption modifies risk of estrogen-receptor negative breast cancer. Breast Cancer Res. 13. PMID: 21569535
3. Nilsson, L.M. et al. (2010) Consumption of filtered and boiled coffee and the risk of incident cancer: A prospective cohort study. Cancer Causes Control 21, 1533–44.
Terri Sundquist
