In this population-based case–control study, there was little evidence that coffee and tea consumption play a major role in modulating endometrial cancer risk after adjusting for major confounders. However, our results suggest that sugar/honey and cream/milk added to tea and coffee should be considered as potential covariates in future studies. Consumption of green tea was not very common in this population. However, we found that women who consumed one or more cups of green tea per week had a 25% reduction in risk compared to nondrinkers, but the confidence interval included the null.
We also evaluated the role of substances added to coffee and tea, such as creamers, milk, and sugar. Controlling for sugar/honey added to coffee did not have a major impact in risk estimates. We found that adding two or more teaspoons of sugar or honey per cup of tea was associated with an increased risk of developing endometrial cancer, compared to not using sugar/honey. While the OR was above one for adding the same amount of sugar/honey to coffee, the confidence interval included one. We found no major impact on endometrial cancer risk for creamers and milk added to coffee, whereas there was some indication of a risk reduction for women adding milk to tea.
Our findings of a weak inverse association of coffee with endometrial cancer are in general agreement with previous studies. Two cohort studies [13
], three population-based case–control studies [15
], and four hospital-based case–control studies [18
] have suggested an inverse association between coffee consumption and endometrial cancer risk. In contrast, a small cohort study [22
], one population-based case–control study [23
] and two hospital-based studies [24
] did not find an association. A recent meta-analysis reported a summary risk estimate of 0.93 (95% CI: 0.89–0.97) per cup of coffee per day [7
Several mechanisms have been proposed to explain a possible beneficial effect of coffee consumption, including effects in glucose metabolism and diabetes prevention, favoring weight loss [6
], as well as anticarcinogenic effects due to its content in antioxidants [7
While the antioxidant and anticarcinogenic properties of tea, mainly attributed to tea polyphenols, are well documented in experimental studies, epidemiologic studies of tea and cancer have not offered conclusive results [26
]. For endometrial cancer, the few cohort and case–control studies that have previously evaluated the role of tea offered inconsistent results [2
]. In the Iowa Women’s Health Study [27
], the relative risk (RR) for those consuming more than two cups per day versus never or rarely was 0.76 (95% CI: 0.45–1.27). Out of the four population-based case–control studies evaluating tea [15
], two suggested an inverse association [28
], but in one of them the confidence interval included one [29
]. The other two studies, one conducted in the United States [23
] and the other in Canada [15
], did not find an association. Out of the four hospital-based case–control studies evaluating tea [19
], only one found an inverse association [21
]. An additional small hospital-based case–control study evaluated soda, coffee, and tea combined and found no association [31
While all tea, oolong, green, and black tea, comes from the same plant (Camellia sinensis
), its composition varies substantially according to processing [32
]. Green tea, which is processed to prevent oxidation and fermentation, has much higher levels of some antioxidant polyphenols than black tea [26
]. It represents approximately 20% of the world tea consumption [32
]. Only a few studies have examined the association of endometrial cancer specifically with green tea. A cohort study in Japan found a RR of 0.75 (95% CI: 0.44–1.30) for women consuming more than five cups/day of green tea versus consuming it four or less days/week [13
]. While two population-based case–control studies conducted in China [28
] and Japan [33
] found an inverse association, there was no association in a third study conducted in the United States [23
]. Different levels of consumption and preparation methods may explain these discrepant findings.
An important caveat of this study is the lack of detailed assessment of coffee and tea consumption. We used the Block FFQ, which includes only one question for coffee (therefore, not separating regular from decaffeinated coffee) and one for tea (which included hot and iced tea, but not herbal teas). For this study, we added one more page to the questionnaire, as described elsewhere [10
], which included frequency and portion size of green tea consumption. Although the inability of this study to separate regular from decaffeinated coffee is a limitation, both types have been shown to reduce diabetes risk and to be associated with weight loss [6
]. Furthermore, an inverse association was found with plasma C-peptide levels, a marker of insulin secretion, for both types of coffee among participants in the Nurses’ Health Study [34
]. To our knowledge, only two studies [21
], both hospital-based studies, have evaluated the risk of endometrial cancer associated with regular and decaffeinated coffee separately. One of them suggested an inverse association only for regular coffee, while there was no association for decaffeinated coffee [21
]. The other study did not find an association for either type [24
]. With respect to tea, our inability to separate the different types of tea (e.g., hot tea vs. iced tea; green, black, oolong tea) is also a drawback. However, the majority of tea consumption in the United States is in the form of iced tea (approximately 85%) [35
], and most commercial iced teas contain a variety of additives, such as sweeteners. This may explain the suggestion of an increased risk associated with tea consumption in our study.
To our knowledge, this is the first study that evaluated the role of sugar or honey added to coffee and tea on endometrial cancer risk. Long-term high sugar intake leads to hyperglycemia and chronic hyperinsulinemia, which in turn has been associated with increased endometrial cancer risk [36
]. Interestingly, in our study we found a stronger association with sugar/honey added to tea than to coffee. The reason for this is uncertain. However, as most of the tea consumed was probably in the form of iced tea, it is possible that the added sugar/honey was to already sweetened tea. It is also possible that the increased association observed for adding sugar to tea is a spurious finding. For instance, sugar may be acting as a marker for other behaviors that are related to an increased endometrial cancer risk and that were not taken into account in this study. Future studies need to explore this issue further by obtaining a more detailed history of tea consumption, including type of tea and form of preparation and addition of milk or milk substitutes and sweeteners.
In addition to these limitations in exposure assessment, as in all case–control studies, potential recall bias and selection bias should be considered. Recall bias may have occurred if cases systematically over reported or underreported coffee and tea consumption. While this cannot be ruled out, it is unlikely that the general public would think that tea or coffee may impact endometrial cancer risk. Not unlike other population-based case–control studies [37
], response rates in our study were low. However, nonresponse bias would only affect study validity if willingness to participate is related to the factors under evaluation [37
]. It is unlikely that coffee and tea intakes were related to the likelihood of subjects agreeing to participate in the study.
Our study represents an addition to the scientific literature of coffee/tea and endometrial cancer risk, as it stresses the need for future studies to separate types of tea and coffee, as well as to assess substances added, such as sugar and other additives, and milk and creamers. Ideally, the association of these beverages and endometrial cancer should be evaluated in countries with a wide range of intake, particularly of tea subtypes, such as green tea, which is relatively uncommon in some countries.
In summary, our study did not offer support for a major role of tea or coffee consumption on endometrial cancer prevention. Our findings of increased risk associated with sugar/honey added to tea and decreased risk with milk added to tea are intriguing, but require replication in other population-based studies.