In this large population-based prospective cohort study, we found no association between green tea consumption and the risk of lung cancer among this Japanese population, who consume green tea much more frequently than people in Western countries. There was no indication that green tea reduces lung cancer risk among past smokers, current smokers, or passive smokers.
It has been hypothesised that the consumption of green tea may decrease the risk of lung cancer risk, specifically in some case–control studies (Ohno et al, 1995
; Mendilaharsu et al, 1998
; Zhong et al, 2001
; Hu et al, 2002
). However, other case–control studies found either no association (Le Marchand et al, 2000
; Bonner et al, 2005
) or a positive association (Tewes et al, 1990
). The wide range and crude categorisation of tea consumption, different study populations, choice of controls, inadequate control for confounding, and inevitable recall bias might have obscured possible relationships. Our findings are in general agreement with those of the prospective cohort study of atomic bomb survivors in Japan (Nagano et al, 2001
) and postmenopausal women in America (Zheng et al, 1996
), although tea consumption was differently categorised.
Our study had several strengths. Our individuals were recruited from the general population. The information on green tea consumption and other health-related lifestyle factors was obtained before the individuals developed lung cancer, thus avoiding recall bias. The questionnaire used had been tested and a reasonably high level of validity and reproducibility was found. Our sample size was appropriate given the high incidence in the study region, and the period of follow-up was reasonably long.
Our study also had some limitations. First, the information on green tea consumption and other lifestyle factors was collected only once. Some individuals might have changed their frequency of tea consumption during the follow-up so some misclassification was inevitable. Second, we excluded 6355 individuals who had not answered the question on green tea consumption and 420 individuals who had reported extreme daily energy intake, among whom 52 lung cancers were diagnosed. The characteristics of individuals without tea consumption details similar to those with mean age (61.8 vs
59.9), percentage of men (51.3 vs
47.3), and percentage of current smokers (29.4 vs
28.4). There was no difference in lung cancer incidence between the individuals included (n
440) and those excluded from the analysis (n
=6775), and the HR was 1.03 (95% CI: 0.77–1.37; P=
0.85). Thus, our results would not be substantially biased by exclusion of the individuals without tea data. The third concern is the 11.9% (4928 individuals) of participants lost to follow-up, but their characteristics were similar to those who were fully followed-up (n
512) with respect to: mean age (59.6 vs
58.0), percentage of men (47.3 vs
39.6), % of current smokers (30.6 vs
28.9), and percentage of individuals consuming more than 5 cups of green tea per day (30.8 vs
26.1). Thus, our results would not be remarkably biased by loss to follow-up.
This large population-based prospective cohort study in Japan finds no evidence that the consumption of green tea reduces lung cancer incidence.