In this large prospective study of African-American women, intakes of tea, coffee, and caffeine were not associated with overall risk of breast cancer. There were no significant associations within categories of menopausal status or hormone receptor status.
Epidemiologic studies of tea intake in relation to breast cancer risk have been inconsistent. A meta-analysis reported an inverse association between black tea and breast cancer incidence among eight case-control studies, but five cohort studies of predominantly postmenopausal women suggested a modest increase in breast cancer risk, with a summary OR of 1.15 (95% CI 1.02–1.31) for highest vs. lowest intake.8
A more recent analysis from the Swedish Mammography Cohort observed a significant positive association between tea and breast cancer (IRR=1.22, 95% CI 1.05–1.42, for ≥2 cups/day compared with none), and the association was stronger for ER+/PR+ breast cancer.9
No association between tea and risk of breast cancer was observed in the Nurses’ Health Study11
or in the Women’s Health Study.10
Although there was no association with breast cancer overall or with ER+/PR+ breast cancer in our study, there was a nonsignificant positive association of heavy tea consumption with postmenopausal breast cancer (IRR=1.44, 95% CI 0.89–2.34, for ≥4 cups/day compared with none). Relatively few women frequently drank tea, and power was therefore limited for detecting a modest association.
The mechanism by which tea could affect breast cancer risk is unclear. A study of postmenopausal Chinese women in Singapore reported lower plasma estrone and estradiol levels in green tea drinkers but higher levels in black tea drinkers than in non-tea drinkers. 22
An in vitro study demonstrated that tea catechins exhibited antiestrogenic effects at high concentrations but estrogenic activity at lower concentrations.23
Green tea contains much higher concentrations of catechins than black tea,24
and these data support findings from previous studies that observed a protective effect of green tea, but not black tea, on breast cancer risk.8, 25
Tea intake was positively associated with prolactin levels in a study among postmenopausal women in the Rotterdam Study,26
and the authors speculated that estrogenic effects of black tea may account for some of the protective association between tea intake and risk of coronary heart disease. However, no association between tea intake and circulating sex hormones was observed among postmenopausal women in the Nurses’ Health Study.5
Epidemiologic studies of the relation between coffee and caffeine intake and breast cancer risk have also been inconsistent. A meta-analysis that included nine cohort studies reported no overall association between coffee consumption and risk of breast cancer.12
In the Nurses’ Health Study, among postmenopausal women, there was a significant inverse association with caffeine intake (IRR=0.88, 95% CI 0.79–0.97, for the highest vs. lowest quintile) and a borderline significant inverse association with coffee (IRR=0.89, 95% CI 0.79–1.02, for ≥4 cups/day relative to none).11
Among postmenopausal women in our study, we observed a similar magnitude of association for coffee that was not significant (IRR=0.85, 95% CI 0.55–1.32, for ≥4 cups/day relative to none), but there was no association between caffeine intake and breast cancer. Half of the women in our cohort rarely drank coffee, and their levels of caffeine consumption are lower than that reported in studies comprised mostly of white women,10, 11
among whom coffee and caffeine intake would tend to be more highly correlated than among African-American women.
Studies have shown that intakes of coffee and caffeine were positively associated with sex-hormone binding globulin in postmenopausal women,5, 7
which could explain one mechanism for an inverse association between coffee intake and risk of breast cancer.27
Studies examining the association between coffee, caffeine, and circulating estrogens among premenopausal women have been conflicting. A study of older premenopausal women reported that caffeine was inversely associated with free estradiol,4
and the Nurses’ Health Study reported lower luteal levels of total and free estradiol with increasing coffee consumption.5
However, one study observed that increasing coffee intake was associated with higher early follicular phase estradiol.6
A study of premenopausal, nulliparous women reported that coffee intake was positively associated with the ratio of 2-hydroxyestrone (2-OHE1) to 16α-hydroxyestrone (16α-OHE1) plasma levels.28
Evidence suggests that 2-OHE1 has lower estrogenic activity than 16α-OHE1, and that a high 2-OHE1/16α-OHE1 ratio may be associated with a reduced risk of breast cancer.29–31
However, a study in a multiethnic population of premenopausal women observed higher urinary levels of both 2-OHE1 and 16α-OHE1 with increasing caffeine consumption.32
Coffee has also been shown to be an important source of lignans in Western populations,33
and there is increasing evidence that high lignan intake is associated with a reduced risk of breast cancer.34
Strengths of our study include its large size, prospective design, high rate and length of follow-up, information on breast cancer risk factors and other potential confounders, and focus on African-American women. Misclassification of long-term tea and coffee intake would likely be random and would have attenuated true associations. It is possible that unknown lifestyle factors may have influenced our findings, but we were able to control for several established breast cancer risk factors, which did not appreciably affect our results.
In conclusion, our results suggest that tea, coffee, and caffeine consumption is not associated with risk of breast cancer among African-American women.