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Relation
Lafranconi A, Micek A, De Paoli P, et al. Coffee consumption reduces the risk of postmenopausal breast cancer: a dose-response meta-analysis of prospective cohort studies.Nutrients. 2018;10(2). pii:E112.
Objective
Summary of findings from prospective cohort studies on the association between coffee consumption and breast cancer risk.
Draft
A dose-response meta-analysis of prospective studies assessing associations between doses of coffee consumption (including decaffeinated) and breast cancer risk. All studies included data on dose-response relationships, highest vs. lowest consumption levels or subgroup analyses.
Target parameters
Coffee consumption was calculated against the relative risk (RR) of a primary breast cancer diagnosis from 0 to 7 cups of coffee per day for the categories of BMI, hormone receptor status, and menopausal status.
Key insights
Dose-response analysis of 13 prospective studies (over 1 million participants in total) showed no significant association between coffee consumption and breast cancer risk in the nonlinear model. However, an inverse relationship was observed when the analysis was restricted to postmenopausal women. Consuming 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 analyzes showed consistent results for all potential confounders examined. These results support the hypothesis that coffee consumption is associated with a reduced risk of postmenopausal breast cancer. The association of reduced risk in postmenopausal women was independent of whether the coffee was caffeinated or decaffeinated.
Practice implications
The relationship between coffee and breast cancer has been difficult to determine. 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 reduced risk of a number of diseases, including cardiovascular disease and cardiovascular mortality, Parkinson's disease, type 2 diabetes, and breast, colon, endometrial, and prostate cancers.1
Prospective observational cohort studies suggest that moderate to high coffee consumption is associated with a lower risk of all-cause, cardiovascular, and cancer mortality compared to lower consumption.2
The association of reduced risk in postmenopausal women was independent of whether the coffee was caffeinated or decaffeinated.
Coffee consumption has been hypothesized to influence the risk of female cancers, including breast, endometrial and ovarian cancers, particularly in postmenopausal women. A recent analysis of the Nurses' Health Study II grouped low coffee consumption into a dietary pattern that also included low intake of green leafy and cruciferous vegetables.3Previous attempts at meta-analysis of data looking at coffee consumption and cancer risk in women have not produced clear results, particularly not finding definitive dose-response associations.4In a 2013 meta-analysis, Li et al. 16 cohort and 10 case-control studies and found only a borderline association between the highest and lowest coffee consumption. However, they found a significant inverse association between coffee consumption and cancer risk in estrogen receptor-negative women and also BRCA1-positive women.5Their results suggest that attention should be focused on the effects 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 recent analysis by Lafranconi et al. has done just that, focusing on the effects of coffee specifically on postmenopausal women. They analyzed results from 13 prospective studies for a study population totaling more than 1 million women. While they found no significant association between coffee consumption and overall breast cancer risk, an important finding in itself, they found a significant inverse relationship in postmenopausal women, i.e. h. a decrease in the risk of breast cancer. Consuming 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 didn't matter whether the woman drank caffeinated or decaffeinated coffee. Therefore, no credit for protecting against breast cancer can be attributed to caffeine; other compounds in coffee must be responsible. It could be the many antioxidants in coffee. It could be the effect of coffee on liver function or on metabolic syndrome. The exact role coffee plays and the mechanisms by which it might alter breast cancer risk in this subgroup is unclear. We await future studies that could shed light on all of this.
It is now becoming increasingly clear that coffee does not increase the risk of breast cancer and may provide protection against breast cancer in certain subgroups. These data relate to the diagnosis of primary cancer and likely tell us little about the effects of coffee on the risk of recurrence.
There is another fundamental flaw in this data that no amount of statistical manipulation can correct: participants were not randomly selected as to who would drink coffee and who would not. These “free-living people” decided for themselves whether and how much coffee they drank. It may well be that these self-selected groups are divided over an as yet undefined quality that distorts the results. The obvious possibility is what we see is a difference in liver function or digestive function. A percentage of people intentionally 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 showed protective benefits, or at least seemed so in this data. However, many of us will think along these lines and not rush to encourage every postmenopausal woman to drink 4 cups of coffee a day. However, I will not discourage the habit for women who do this.