Green tea and black tea consumption in relation to colorectal cancer risk: the Singapore Chinese Health Study

Abstract

2002

Experimental studies support both green and black tea as chemo-preventive agents for colorectal cancer development. Epidemiologic data are limited and inconclusive on the relationship of either type of tea with colorectal cancer development. The relationships between green tea and black tea consumption and colorectal cancer risk were examined within the Singapore Chinese Health Study, a prospective cohort study of diet and cancer involving 63,000 men and women enrolled between 1993 and 1998. Intake of green tea and black tea separately was assessed through in-person interviews using a validated food frequency questionnaire. Incident cancer cases and deaths among cohort members were identified through record linkage of the cohort database with databases from the nationwide Singapore Cancer Registry and the Singapore Registry of Births and Deaths. The proportional hazard regression method was used to examine the associations between intake of green and black tea separately and colorectal cancer risk with adjustment for potential confounders. After an average of 8 years of follow-up, 735 colorectal cancer cases (451 colon and 284 rectal cancer cases) were identified. With adjustment for potential confounders, subjects who drank green tea daily exhibited a statistically significant increase in risk (RR = 1.25, 95% confidence interval (CI) = 1.01-1.55) relative to nondrinkers of green tea. This risk increase was mainly confined to men (RR = 1.46, 95% CI = 1.12-1.91); the comparable RR in women was 0.92 (95% CI = 0.61-1.37). In men, dietary fat was a modifier of the green tea-colorectal cancer association (p for interaction=0.02). Relative to nondrinkers of green tea who consumed low (below population median value) dietary fat, those with high dietary fat intake regardless of green tea intake status and green tea drinkers with low dietary fat intake exhibited similar levels of increased risk that were all statistically significantly different from 1.0 (RR of 1.5-1.7). Irrespective of gender, intake of black tea was not associated with risk of colorectal cancer (RR = 0.93, 95% CI = 0.79-1.09) in this Asian population. We speculated that the presence of a threshold effect on colorectal cancer risk from dietary fat and green tea, which implicates a common etiologic pathway for the two exposures, and the absence of a comparable effect in women are compatible with the ‘bile acid’ hypothesis in colorectal carcinogenesis. There is laboratory evidence that both dietary fat and green tea are inducers of bile acids while estrogen decreases bile acid secretion. Thus, the inhibitory effect of estrogen on bile acid output may render the women ‘resistant’ to the deleterious effects of dietary fat and green tea via the bile acid pathway. Our novel observations require confirmation by other studies.