In the current study, coffee consumption at baseline of 12 or more cups of coffee per day was independently associated with a significant 67% reduction in incidence of diabetes over the following 7.6 years compared to those who were not drinking coffee.
We found an association between high coffee consumption and decreased diabetes risk in a population with high prevalence and incidence of type 2 diabetes. Although such an association was not found in Pima Indians[23
], differences in the categories of coffee consumption (the highest category was 3 cups of coffee or more per day), adjusted lifestyle risk factor (the report from Pima Indian only adjusted for age, sex, and BMI), geographic location of the tribe, and tribal customs related to coffee drinking may contribute to the difference of the study results. By only including the people with normal glucose tolerance, a bias from coffee consumption behavior changes caused by diagnosis of preclinical diabetes has been avoided [5
]. Our study adds to the existing literature concerning the association between coffee consumption and the risk of type 2 diabetes[4
Although our study was not designed to assess by which biological mechanisms coffee was involved in reducing diabetes risk, there are several possible mechanisms. First, coffee contains compounds that are antioxidants[25
], which may be related to the risk of diabetes because the oxidative stress may be a pathogenic mechanism linking insulin resistance with dysfunction of both beta cells and endothelium [27
]. Second, phenol chlorogenic acid and its degradation products (quinides) in coffee contribute to inhibition of glucose absorption in the intestine [28
], reducing hepatic glucose output through inhibition of glucose-6-phosphatase[30
]. Third, caffeine may also contribute to the inverse association between coffee and diabetes through increasing of basal energy expenditure[31
], stimulating fat oxidation and mobilization of glycogen in muscle [32
], and weight loss[33
]. Caffeine has also been related to acute decrease of insulin sensitivity [34
], although prolonged use of caffeine may result in tolerance for such acute effect [35
]. Finally, other components in coffee such as magnesium have been associated with a lower risk for type 2 diabetes [37
]. We did not collect other dietary information at the baseline exam. When Exam 2 dietary data were analyzed to explore relations of dietary intake according to coffee consumption of baseline exam, we found that participants who drank 12 or more cups of coffee per day had the highest magnesium intake among all groups. Energy intake, polyunsaturated-to-saturated fat intake ratio, and fiber intake were not significantly different among coffee groups. Further studies on the biological mechanisms relating consumption are needed. Several characteristics of coffee drinkers, such as BMI, physical activity, and smoking are known to be associated with diabetes incidence; these, however, were included as covariates in all analyses.
The strength of this study is the longitudinal design, the systematic diagnosis of diabetes by using blood glucose values measured following a glucose-tolerance test instead of self-reported diabetes, and the availability of information on a wide range of potential confounders.
The limitations of this study include the lack of availability of dietary data on the participants. Given that an independent, inverse association of diabetes risk and coffee consumption was observed only for the highest consumption category (≥ 12 cups/glasses per day), it may be that high coffee consumption is a marker for other factors such as energy intake, meal patterns, and food choices not measured here that are related to diabetes risk. This study was conducted in a single ethnic group; although this population has been shown to be representative of many populations in the US with high levels of obesity and diabetes, analyses in other populations must be done. In addition, analyses were limited to individuals with normal glucose tolerance.
In summary, the consumption of 12 or more cups of coffee per day was associated with a significant, 67% reduction in the incidence of diabetes compared to those who did not drink coffee. Further studies are needed to determine whether this association is causal or a result of unmeasured confounders. The potential effects of high level of coffee consumption on risk of type 2 diabetes may be important but should be considered in light of putative health effects of coffee, such as potential detrimental effects on blood pressure and sleep quality [39