"Coffee drinking was associated with a reduced risk of type 2 diabetes in both men and women, and this association was observed regardless of the levels of physical activity, BMI and alcohol consumption," wrote lead author Gang Hu in the current issue of the International Journal of Obesity
The research adds to previous large prospective studies have reported a beneficial link between coffee intake and the risk of type-2 diabetes, but whether these apparent benefits are related to the caffeine content is controversial.
However, such doses are considerably higher than the average worldwide daily coffee consumption of one and a half cups, while the US average is more than three and a half cups.
The new study analysed the diet of 11 197 women and 10 188 men (age range 35-74) with no history of coronary heart disease, stroke, or diabetes at the start of the study. Dietary consumption of coffee, tea, alcohol and other food consumption was assessed using a self-administered food frequency questionnaire, and information about physical activity, smoking habits, socio-economic factors, height, weight and blood pressure was also collected.
The researchers, from the National Public Health Institute in Helsinki and including collaborators from the University of Helsinki, the Institute of Biomedicine, and the South Ostrobothnia Central Hospital, documented 964 incident cases of type-2 diabetes, during a follow-up of about 13 years.
Classed as an epidemic by the World Health Organisation, at least 171m people worldwide suffer from diabetes, a figure likely to more than double to 366m by 2030. The American Diabetes Association estimates that at least 90 per cent of the 17m Americans diagnosed with diabetes have type 2.
Hu and colleagues report that, after adjusting for a range of possible confounders like age, systolic blood pressure, smoking, BMI, fruit, vegetable, alcohol, bread, sausage and tea consumption, drinking between three and six cups a day was associated with a reduction in the risk of developing type-2 diabetes of 23 per cent in men and 29 per cent in women.
Drinking seven or more cups a day was associated with a reduction in the risk of developing type-2 diabetes of 34 per cent in men and 48 per cent in women.
"Among obese and inactive people, coffee drinking of seven cups or more daily reduced the risk of type 2 diabetes to half," wrote Hu.
The researchers did not undertake any kind of mechanistic study, but state that association between diabetes and coffee appears to be complex. Indeed, this study and previous studies have reported that coffee's magnesium content could improve insulin sensitivity. Additionally, the range of polyphenols may explain some of the inverse association between coffee intake and risk of type-2 diabetes mellitus.
Some reports have stated that caffeine could also increase insulin sensitivity, but this relationship is controversial. A study published in June in the journal Archives of Internal Medicine(Vol. 166, pp. 1311-1316) reported that drinking six or more cups of coffee every day could reduce the risk of developing type-2 diabetes by more than 20 per cent, but if the coffee is decaffeinated, the reduction in risk rises to over 30 per cent - a result that suggested the benefits of coffee for this population group are not due to caffeine.
There are several limitations with this study, most notably that the data was obtained by observational self-reporting, which is dependent on the recall and accuracy of the individual participants.
Significant further research needs to be done, but the association between coffee intake and range of diseases continues to generate interest inside and outside of the scientific community. Previous research has suggested that increased coffee intake may have beneficial effects in a range of diseases, from liver cancer to heart disease.
Source: International Journal of Obesity Volume 30, pp. 1742-1749. doi:10.1038/sj.ijo.0803341 "Joint association of coffee consumption and other factors to the risk of type 2 diabetes: a prospective study in Finland" Authors: G Hu, P Jousilahti, M Peltonen, S Bidel and J Tuomilehto