type 2 diabetes
Potential mechanisms
The one thing still lacking with these associations, between consumption of regular coffee and decaffeinated coffee or tea and a lower risk of developing type 2 diabetes, is a plausible mechanism.
A role for caffeine?
Since coffee and tea are the main sources of caffeine in the diet in most countries, it is difficult to directly separate an effect of caffeine from either coffee or tea. However, since decaffeinated coffee is reported to have a similar size association as regular coffee, it is unlikely that caffeine plays a role in the negative association for development of type 2 diabetes.
Paradoxically, acute caffeine intake leads to glucose intolerance and insulin insensitivity while chronic caffeine has only small effects on glucose metabolism12.
An acute study tested for the effects of caffeine in women with or without gestational diabetes. Caffeine did not affect glucose and insulin levels in the group without gestational diabetes. However caffeine impaired insulin sensitivity in women with gestational diabetes13.
Other constituents
Other coffee constituents, in particular antioxidants like chlorogenic acid and trigonelline reduce early glucose and insulin levels at 15 minutes in oral glucose tolerance tests (OGTT). Decaffeinated instant coffee shows no effect in this test14.
These observations are in good accordance with the data of a French study reporting the strongest association for coffee consumed at lunchtime9.
Coffee could also partly inhibit postprandial hyperglycemia and hence prevent the occurrence of type 2 diabetes15. According to a recent cross-sectional multi-ethnic study on 954 non-diabetic adults, the effect of regular coffee is positively related to insulin sensitivity while decaffeinated coffee rather improves pancreatic beta-cells function16.
Effect on subclinical inflammation – a new hypothesis
A Finnish study tested the effects of a progressively increasing coffee consumption (1st month abstinence, 2nd month 4 cups/day, 3rd month 8 cups/day) in obese volunteers in a medium term intervention trial. No effects in the OGTT were seen. Coffee consumption appeared to have beneficial effects on some markers of subclinical inflammation, considered to be risk factors for type 2 diabetes17.
Because of its high content in antioxidant compounds18, coffee could contribute to the total antioxidant capacity of the diet that is necessary to reduce oxidative stress that in turn leads to favourable conditions for the development of type 2 diabetes19. This hypothesis has not yet been verified.
Other areas under investigation
Among the other mechanisms studied, a Dutch study did not find that either magnesium, potassium, caffeine or blood pressure were involved in the coffee or tea association with type 2 diabetes8.
One Japanese study reported a link with psychological factors, perceived mental stress in men and Type-A behaviour in women20.
Finally, a US case-control study found a correlation between coffee consumption and plasma levels of sex-hormone-binding-globulin that directly modulates intracellular signalling of sex hormones that, themselves, play a critical role in type 2 diabetes development. This association was not found for decaffeinated coffee and tea21.
