Reviewed: Kuriyama S, Shimazu T, Ohmori K, et al. Green tea consumption and mortality due to cardiovascular disease, cancer, and all causes in Japan. The Ohsaki study. JAMA. 2006;296(10):1255-1265.
Tea ranks second to water as the most consumed beverage worldwide. Forms of tea include green, oolong, and black (depending on the level of processing), all of which originate from the leaves of the tea plant (Camellia sinensis L., Theaceae). Green tea polyphenols have been studied in vitro and in animals as a protectant against cardiovascular disease (CVD) and cancer. However, according to the authors of this large-scale epidemiological study, the effects of green tea consumption in humans remains unclear. They examined the association between green tea consumption and mortality due to all causes, to CVD, and to cancer within a large population-based cohort study in northeastern Japan.
In the Ohsaki National Health Insurance Cohort Study, the authors delivered a self-administered questionnaire, including items on dietary intake, between October and December 1994 to all national health insurance beneficiaries aged 40 to 79 years living in the catchment area of Ohsaki Public Health Center, Miyagi Prefecture, Japan.
The questionnaire included items about the frequency of consumption of 4 beverages (green tea, oolong tea, black tea, and coffee), and 36 items about food, as well as items regarding the consumption of alcohol and tobacco, personal and family history of disease, job status, level of education, body weight, height, amount of time participating in sports or exercise, and time spent walking every day. The frequency of green tea consumption was divided into 5 categories.
Of 54,996 eligible persons, 52,029 (95%) responded. Of those, 40,530 were included in the study analysis. Participants received follow up for as many as 11 years for all-cause mortality and for as many as 7 years for cause-specific mortality (i.e., CVD and cancer).
At baseline, participants who consumed green tea more often tended to be older and were more likely to be unemployed, to engage in sports or exercise, or to have a history of hypertension and diabetes mellitus, and were less likely to spend time walking. Men were more likely to have a history of a gastric ulcer condition and women to be obese. No apparent associations between smoking status or alcohol drinking and green tea consumption were noted.
A follow up was conducted for participants regarding migration and mortality. Participants who withdrew because of emigration could not be reached for subsequent information and were therefore withdrawn from the study. For those who died, the authors investigated cause of death by reviewing the death certificates filed at Ohsaki Public Health Center. Cause of death was coded according to the International Statistical Classification of Diseases and Related Health Problems, Tenth Revision (ICD-10).
During 11 years of follow-up (1995-2005), 4,209 participants died. The authors found that green tea consumption was inversely associated with mortality due to all causes and that the inverse association was more pronounced in women (P=0.03 for interaction with sex).The multivariate hazard ratios of mortality due to all causes associated with different frequencies of green tea consumptions are shown in Table 1 (above).
During 7 years of follow-up (1995-2001), the authors report that 892 participants died of CVD and 1,134 died of cancer. The inverse association with CVD mortality was stronger than that with all-cause mortality (with the strongest inverse ciation observed for stroke mortality). The association appeared to be more pronounced in participants who had never smoked. Again, the inverse association was stronger in women (P=0.004 for trend).
The multivariate hazard ratios of mortality due to CVD associated with different frequencies of green tea consumptions are shown in Table 2 (below).
The participants who consumed 5 or more cups per day of green tea had a risk of all-cause mortality that was 16% lower (during 11 years of follow-up) and a CVD mortality that was 26% lower (during 7 years of follow-up) than those who consumed less than 1 cup per day.
The authors report that the hazard ratios of cancer mortality were not significantly different from 1.00 in all green tea consumption categories compared with the lowest-consumption category. Thus, the authors conclude that “green tea consumption is associated with reduced mortality due to all causes and due to cardiovascular disease but not with reduced mortality due to cancer.”
— Shari Henson
Table 1. Green Tea Consumption and Multivariate Hazard Ratios for All Causes of Mortality
| Green Tea Consumption | Multivariate Hazard Ratio |
| Less than 1 cup/day | 1.00 (reference) |
| 1-2 cups/day | 0.96 (95% confidence interval, 0.87-1.05) |
| 3-4 cups/day | 0.90 (95% CI, 0.82-0.98) |
| 5 or more cups/day | 0.84 (95% CI, 0.77-0.92) |
Table 2. Green Tea Consumption and Multivariate Hazard Ratios for Cardiovascular Disease as Cause of Mortality
| Green Tea Consumption | Multivariate Hazard Ratio |
| Less than 1 cup/day | 1.00 (reference) |
| 1-2 cups/day | 0.87 (95% CI, 0.72-1.06) |
| 3-4 cups/day | 0.77 (95% CI, 0.63-0.93) |
| 5 or more cups/day | 0.74 (95% CI, 0.62-0.89) |