And if the coffee is decaffeinated, the reduction in risk rises to over 30 per cent - a result that suggests the benefits of coffee for this population group are not due to caffeine.
The scientists of the new study say that several large prospective studies have reported a beneficial link between coffee intake and the risk of type-2 diabetes, but conflicting reports exist in the literature as to the potential benefits, if any, of caffeine for the development of diabetes.
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.
The new study, published in the journal Archives of Internal Medicine(Vol. 166, pp. 1311-1316), analysed the diet of 28,812 post-menopausal women using 126-item food frequency questionnaires. A 16-page questionnaire was also completed that measured risk factors for diabetes, including BMI, age, waist-hip ratio, alcohol consumption and smoking habits.
Coffee intake was divided into five groups based on the number of cups drunk per day: zero; less than one; one to three; four to five; or six or more. About half the women drank between one to three cups per day, 5,554 drank four or five cups per day, and 2,875 drank more than six cups a day.
The average consumption of coffee in the US is reported to be 3.2 cups per day.
During the 11 years of follow-up, 1,418 cases of diabetes were recorded. After accounting for the diabetes risk factors from the questionnaires, the researchers calculated that women who drank more than six cups of coffee per day had a reduction in the risk of developing diabetes of 22 per cent, compared to the women who drank no coffee.
When the data was analysed with respect to decaffeinated coffee, the reduction in the risk of diabetes was 33 per cent, compared to 21 per cent for caffeinated coffee.
This result indicates that caffeine is not responsible for the apparent benefits, leading the researchers to examine other nutrients found in the beverage.
"Magnesium, for which coffee is a good source, could explain some of the inverse association between coffee intake and risk of type-2 diabetes mellitus through known beneficial effects on carbohydrate metabolism," suggested lead author Mark Pereira from the University of Minneapolis.
However, when the numbers were crunched to account for magnesium, no change in the results was observed.
Coffee also contains a range of polyphenols that are potent antioxidants, but the researchers could not account for these compounds adequately to be able to elucidate a link.
The US Nurses Health Study and Health Professionals Follow-up study reported different results, but Pereira said that such differences could be due to the differences between the population groups.
"Perhaps high caffeine intake carries more detrimental effects in older adults than in middle aged adults, or perhaps the association between caffeine and diabetes in the present study was confounded by some unmeasured or poorly measured factor," wrote Pareira.
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. The researchers argue however that the results are more likely to be underestimates of the true strength of the link.
"Although the first line of prevention for diabetes is exercise and diet, in light of the popularity of coffee consumption and high rates of type-2 diabetes mellitus in older adults these findings may carry health significance," concluded the researchers.
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.