Lafranconi A, Micek A, De Paoli P, et al. Coffee intake decreases risk of postmenopausal breast cancer: a dose-response meta-analysis on prospective cohort studies. Nutrients. 2018;10(2). pii:E112.
To summarize evidence from prospective cohort studies regarding the association between coffee intake and breast cancer risk.
A dose-response meta-analysis of prospective trials that assessed associations between dosages of coffee consumption (decaffeinated included) and breast cancer risk. All studies contained data on dose-response, highest vs lowest consumption levels, or subgroup analysis.
Coffee consumption was calculated against relative risk (RR) of primary breast cancer diagnosis from 0 to 7 cups of coffee per day for categories of BMI, hormone receptor status, and menopausal status.
The dose-response analysis of 13 prospective studies (totaling over 1 million participants) showed no significant association between coffee consumption and breast cancer risk in the nonlinear model. However, an inverse relationship was seen when the analysis was restricted to postmenopausal women. Consumption of 4 cups of coffee per day was associated with a 10% reduction in postmenopausal cancer risk (RR: 0.90; 95% confidence interval [CI]: 0.82-0.99). Subgroup analyses showed consistent results for all potential confounding factors examined. These findings support the hypothesis that coffee consumption is associated with decreased risk of postmenopausal breast cancer. The association of decreased risk in postmenopausal women was independent of whether the coffee was caffeinated or decaffeinated.
The relationship between coffee and breast cancer has been difficult to pin down. In recent years coffee has been viewed as potentially beneficial to human health. An August 2017 literature review suggested that coffee may be associated with a decreased risk of a number of morbidities, including cardiovascular disease and cardiovascular mortality, Parkinson's disease, type 2 diabetes, and cancers of the breast, colon, endometrium, and prostate.1
Observational prospective cohort studies suggest that moderate-to-high coffee intake is associated with lower risk of all-cause, cardiovascular, and cancer mortality compared to lower consumption.2
The association of decreased risk in postmenopausal women was independent of whether the coffee was caffeinated or decaffeinated.
It has been hypothesized that coffee consumption affects risk of female cancers including breast, endometrial, and ovarian cancers, particularly in postmenopausal women. Recent analysis of the Nurses’ Health Study II grouped low coffee consumption in a dietary pattern that also included low intake of green leafy vegetables and cruciferous vegetables.3 Prior attempts at meta-analysis of data looking at coffee consumption and female cancer risk have failed to produce clear results, in particular not finding definitive dose-response associations.4 In a 2013 meta-analysis, Li et al examined 16 cohort and 10 case-control studies and found only a borderline association comparing highest vs lowest coffee consumption. However, they did find a significant inverse association between coffee consumption and cancer risk in estrogen receptor–negative women and also BRCA1-positive women.5 Their findings suggest attention should be focused on the effect of coffee on specific subgroups of women. (A review of the meta-analysis by Li et al appeared in a previous issue of Natural Medicine Journal.6)
This current analysis by Lafranconi et al did just that, focusing on the effects of coffee specifically in postmenopausal women. They analyzed results from 13 prospective studies, for a total study population of more than 1 million women. While they did not find a significant association between coffee consumption and overall breast cancer risk, which in itself is an important finding, they did find a significant inverse relationship, that is, a decrease in breast cancer risk, in postmenopausal women. Consumption of 4 cups of coffee per day was associated with a 10% reduction in postmenopausal cancer risk (RR: 0.90; 95% CI: 0.82-0.99).
In this study, it did not matter if the woman was drinking caffeinated or decaffeinated coffee. Thus, credit for the breast cancer protection cannot be given to caffeine; other compounds in coffee must be responsible. It could be the many antioxidants in coffee. It could be the effect coffee has on liver function or on metabolic syndrome. The exact role coffee plays and the mechanisms by which it might change breast cancer risk in this subgroup is unclear. We await future studies that may elucidate all of this.
In the meantime, it is becoming increasingly clear that coffee does not increase breast cancer risk and may offer protection against breast cancer in certain subgroups. These data are related to diagnosis of primary cancer and probably tell us little about the effects of coffee on risk of recurrence.
There is another fundamental flaw with these data that no amount of statistical manipulation will compensate for: the participants were not randomized as to who would drink coffee and who wouldn’t. These “free-living people” decided on their own whether or not to drink coffee and how much. It may well be that these self-selected groups divide themselves over a yet undefined quality that confounds the results. The obvious possibility is what we are seeing is a difference in liver function or digestive function. A percentage of people purposefully avoid coffee because it keeps them awake at night, likely a symptom of slow liver detoxification capacity.
The possible counterargument in this study is that decaffeinated coffee also exhibited protective benefits, or at least appeared to in these data. Still, many of us will be thinking along these lines and not rushing to encourage every postmenopausal woman to drink 4 cups of coffee a day. Yet, for those women who do, I am not going to discourage the habit.