Led by Nina Paynter from the Johns Hopkins Bloomberg School of Public Health, the scientists looked at coffee and sweetened beverage consumption and the risk of type-2 diabetes among 12,204 nondiabetic, middle-aged men and women taking enrolled in the Atherosclerosis Risk in Communities (ARIC) Study.
While no significant link between sweetened beverage consumption and type-2 diabetes was observed, the researchers do report a significant protective association amongst coffee drinkers.
The research adds to previous large prospective studies that 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, the doses reported to offer a protective effect are 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 ARICS study, conducted between 1987 and 1999, assessed dietary intakes using a semi-quantitative food frequency questionnaire (FFQ) and classified coffee and sweetened beverage consumption according to the number of cups per day. Diabetes diagnosis was obtained by self-reporting of physician-diagnosed diabetes, diabetes treatment, and/or a fasting or non-fasting blood sugar test
Paynter and her co-workers report in the American Journal of Epidemiology that consumption of at least four cups of coffee was associated with a 23 per cent reduction in the risk of men developing type 2 diabetes mellitus, compared to men who "almost never" drank coffee. Similar intakes in women were associated with an 11 per cent reduction of type-2 diabetes in women, although this was not statistically significant, said Paynter.
Sweetened beverage consumption is reported to have no significant link to the incidence of type 2 diabetes mellitus.
"The lack of association between sweetened beverage consumption and type-2 diabetes risk is surprising," said the researchers. "Sweetened beverage consumption may be more important risk factor for younger or leaner people and should be studied further in that population."
"In conclusion, a higher consumption level of coffee was associated with a decreased risk of type 2 diabetes in middle-aged adults," concluded the researchers.
Being an epidemiological study, no investigation of a potentially protective underlying mechanism was performed. Previous studies have stated that association between diabetes and coffee appears to be complex., with some scientists advocating coffee's magnesium content for improve insulin sensitivity. Additionally, the range of polyphenols, particularly chlorogenic acid, 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.
"Although several observational prospective studies have yielded consistent findings and although biological explanations for decreased diabetes risk with increased coffee consumption have been postulated, further research, particularly experimental studies that can examine the long-term effect of coffee consumption on glucose metabolism, insulin sensitivity, and diabetes risk, is needed before recommendations can be made about coffee drinking with respect to the prevention of type-2 diabetes," said Paynter.
Source: American Journal of Epidemiology Volume 164, Issue 11, Pages 1075-1084; doi:10.1093/aje/kwj323 "Coffee and Sweetened Beverage Consumption and the Risk of Type 2 Diabetes Mellitus - The Atherosclerosis Risk in Communities Study" Authors: N.P. Paynter, H-C. Yeh, S. Voutilainen, M.I. Schmidt, G. Heiss, A.R. Folsom, F.L. Brancati and W.H.L. Kao