Coffee consumption is strongly associated with a reduced risk for type 2 diabetes, regardless of whether the coffee is caffeinated or not, according to a new meta-analysis of 28 prospective studies, published in the February issue of Diabetes Care.

"Compared with no coffee consumption…6 cups/day of coffee was associated with a 33% lower risk of type 2 diabetes," the authors write, adding, "Caffeinated coffee and decaffeinated coffee consumption were both associated with a lower risk of type 2 diabetes."

Drinking coffee has been well-established in previous meta-analyses as being associated with a reduced risk for type 2 diabetes; however, the new review was needed to account for more recent trials evaluating the benefits of caffeinated vs decaffeinated coffee, the authors explain.

"We found that a 1-cup/day increment of regular coffee was associated with a 9% reduction in diabetes, and 1 cup/day of decaf was associated with 6% reduction in diabetes, but the difference in risk reduction between the 2 types of coffee was not statistically different," said senior author Frank B. Hu, MD, PhD, a professor of nutrition and epidemiology with the Harvard School of Public Health in Boston, Massachusetts.

"This study provides strong evidence that regular consumption of coffee is beneficial for prevention of diabetes," Dr. Hu told Medscape Medical News. "For individuals who already drink coffee, they may enjoy this and other potential health benefits, and for those who are sensitive to the effects of caffeine, decaf may confer similar benefits."

Robust Findings

The 28 studies in the analysis included 1,109,272 participants, and all had the outcome of risk for type 2 diabetes; there were 45,335 cases of diabetes, with follow-up ranging from 10 months to 20 years.

The relative risk for type 2 diabetes with coffee consumption, compared with no or rare consumption, ranged from 0.92 for 1 cup per day, 0.85 for 2 cups, 0.79 for 3 cups, 0.75 for 4 cups, 0.71 for 5 cups, and 0.67 for 6 cups.

Meanwhile, the relative risk for diabetes associated with 1 cup of caffeinated coffee per day (compared to no or rare coffee consumption) was 0.91 compared with 0.94 for 1 cup of decaffeinated coffee per day (P = .17).

The findings were consistent for men as well as women, and while coffee-brewing methods were not assessed in the studies, the inclusion of diverse populations likely covered a wide array of methods, the authors note.

"Most coffee is likely to be filtered coffee, and the results from studies conducted in various populations, including US, European, and Asian, were similar, indicating consistency of the results despite potentially different preparation and processing methods," they observe.

And while none of the studies assessed levels of sugar and dairy added to coffee, the amounts are "likely to be small compared with other food sources," they add.

While this meta-analysis does provide "strong evidence" that higher consumption of coffee is associated with a significantly lower risk for diabetes, "longer-term randomized controlled trials are needed to establish causality and to elucidate the underlying mechanisms," they conclude.

Coffee Just a Small Part of the Diabetes Equation

One possible explanation for the reduced risk for diabetes with coffee consumption could be the role of chlorogenic acid, a phenolic compound and a major component of coffee, Dr. Hu said.

"Chlorogenic acid has been shown to improve insulin sensitivity and reduce blood sugar absorption," he told Medscape Medical News.

"Compounds in coffee also include antioxidant polyphenols, minerals such as magnesium and chromium, [and] vitamins; however, it is impossible to tease out the effects of individual compounds, because they don't exist in isolation in coffee and they may have synergistic effects."

He stressed also that coffee consumption remains a small piece of the picture.

"Coffee is only one of many dietary and lifestyle factors that can contribute to diabetes prevention. Clearly, maintaining a healthy weight through diet and exercise is the most important way to reduce risk of diabetes. For those who drink coffee regularly, they should enjoy it, but they still need to watch their weight and be physically active."

The research received funding from the National Institutes of Health. The authors have reported no relevant financial relationships.

Diabetes Care . 2013;37:569-586. Abstract