In Chinese men and women living in Singapore we observed an inverse association between coffee intake and risk of type 2 diabetes mellitus, a suggestive inverse association between black tea intake and diabetes risk, and no association between green tea intake and diabetes risk. The inverse association for black tea with risk of diabetes became slightly stronger after adjustment for magnesium and caffeine intake. The findings for coffee are consistent with multiple other prospective cohort studies on this topic (3
), as well as a recent meta-analysis (8
). Of note, one study observed no association between coffee consumption and diabetes risk (39
). To our knowledge there are no long-term trials published on this topic.
Multiple components of coffee could explain or contribute to the association with decreased risk of type 2 diabetes. Magnesium is a component of coffee, and higher dietary intakes may be associated with lower risk of type 2 diabetes in large prospective cohort studies (14
), improved insulin sensitivity, and glucose control (40
). However, dietary magnesium was not associated with type 2 diabetes risk in our cohort; thus, it did not change the relative risks for coffee intake. Caffeine's possible role in type 2 diabetes risk is still unclear, and caffeine and coffee may be too highly correlated to take any mechanistic analysis approaches in this study. Indeed, there are mixed findings in the literature (5
), probably because many studies are not able to thoroughly assess decaffeinated coffee. An alternative explanation to our findings could be that coffee drinking may be an indicator of sensitivity to caffeine, a potential agent that decreases insulin sensitivity (7
), implying a physiologic susceptibility to the pathogenic processes leading to type 2 diabetes. Conversely, evidence from a recent meta-analysis and prospective cohort study found decaffeinated coffee to be inversely associated (8
) and more strongly inversely associated than caffeinated coffee with incident type 2 diabetes (38
). In addition, a clinical study found that glucose intolerance because of caffeine is blunted by coffee (47
), suggesting that other coffee components besides caffeine may be more important with respect to type 2 diabetes risk.
Our findings underscore the possibility that any causal mechanism involved is probably due to the many other minerals and phytochemicals or to the interaction of these components and the overall antioxidant capacity of coffee. However, we were not able to examine these possibilities. Hypotheses on specific bioactive components are driven by laboratory and clinical studies relating to chlorogenic acid, a phenolic compound in coffee (9
), quinides (12
), and the potential of polyphenolic compounds contributing to a lower iron status (50
). Moreover, another hypothesis is coffee's high antioxidant activity, and a high antioxidant contribution to the diet may reduce free radical generation (52
), thus protecting the pancreatic β cells from oxidative stress or potentially promoting insulin sensitivity in the peripheral tissues (17
Unlike most other studies on this topic, we specifically assessed green and black teas, whereas past studies have typically grouped tea types into one category and have reported null findings (5
). A retrospective cohort study in a Japanese population showed that a high intake of green tea, but not black tea, was associated with a decreased risk of type 2 diabetes (28
). However, there are potential methodologic concerns in that study with respect to population selection and follow-up because the final analysis included only 17 413 of the original 110 792 participants. Of note, that study did observe a much higher intake of green tea than in our study.
In our study we were not able to assess specific mechanisms beyond any contribution magnesium and caffeine may play in the tea-diabetes association. Although our results in the main model are only suggestive of an inverse association between black tea intake and risk of type 2 diabetes, if there is a true causal relation, it may be explained by a number of plausible mechanisms from laboratory and clinical investigations related to improved glucose metabolism (22
), antiinflammatory activity (54
), insulin-potentiating activity (55
), and the ability of tea extracts to induce malabsorption of carbohydrates in humans (30
). Those studies all considered tea from the plant Camellia sinensis
, as opposed to herbal teas that do not contain any leaves from the plant.
To our knowledge this is the first large prospective study addressing the topic of coffee, black tea, and green tea consumption and incident type 2 diabetes in an Asian population. Other strengths of this study include the high participant response rate, <1% lost to follow-up, data obtained through a detailed face-to-face interview that included a FFQ specific to this population, and validated diabetes case status with a high positive predictive value obtained through the validation study.
Limitations include potential misclassification of the exposures because of poor self-report, biases, and other errors; this would probably bias the results toward the null, assuming it is nondifferential in nature. Residual confounding as an explanation also needs to be considered, yet this appears unlikely to play an important role because of the observation that the relative risk for the coffee and diabetes association was strengthened (away from the null) when we adjusted for lifestyle and dietary factors. In addition, we have not estimated the negative predictive value of our diabetes definition, although missing cases in this manner would tend to bias the relative risks toward the null; this is a relatively benign threat to the validity of the study given the findings for coffee and diabetes risk. In addition, although we were able to study both black and green tea consumption, our results only apply to a smaller range of intake compared with coffee.
In conclusion, we observed a significant inverse association between coffee consumption and risk of incident type 2 diabetes mellitus in middle-aged Chinese men and women of Singapore. We also observed a suggestive inverse association between black tea consumption and incident type 2 diabetes in a population at high risk of developing type 2 diabetes. These findings are important because coffee and tea are 2 of the most commonly consumed beverages worldwide and other prospective studies on this topic have been restricted to essentially European-based populations, whereas Asians have among the world's highest rates of type 2 diabetes. Second, we were able to take advantage of our rich data set and the unique dietary patterns in Singapore to simultaneously examine the associations of coffee, black tea, and green tea in the same analysis. The associations we observed are noteworthy because they provide evidence that the coffee findings in other prospective cohort studies are not likely artifacts of the reported dietary patterns, nor are they likely to be explained by residual confounding. Further human studies, especially clinical trials, are needed to investigate the role of long-term coffee and tea consumption, and their innate bioactive compounds, in relation to the risk of type 2 diabetes mellitus. Indeed, diet plays an important role in the prevention of diabetes. Given the high consumption of coffee and tea worldwide and the growing type 2 diabetes epidemic, especially in Asia, these findings convey a potential high significance for public health. However, it is too early to recommend increasing coffee and tea consumption until there is more thorough data from clinical trials related to the topic, with respect to not only the possible benefits but possible side effects or harm as well.