Research published in Diabetologia (the journal of the European Association for the Study of Diabetes) showed that increasing coffee consumption by on average one and a half cups per day (approximately 360 ml) over a four-year period reduced the risk of type 2 diabetes by 11%. The research was led by Dr. Frank Hu and Dr. Shilpa Bhupathiraju, Department of Nutrition, Harvard School of Public Health, Harvard University, Boston, MA, and colleagues.

Coffee and tea consumption has been associated with a lower type 2 diabetes risk but little is known about how changes in coffee and tea consumption influence subsequent type 2 diabetes risk. The authors examined the associations between 4-year changes in coffee and tea consumption and risk of type 2 diabetes in the subsequent 4 years.

The authors used observational data from three large prospective, U.S.-based studies in their analysis: the Nurses' Health Study (NHS) (female nurses aged 30-55 years, 1986-2006), the NHS II (younger female nurses aged 25-42 years 1991-2007), and the Health Professionals Follow-up Study (HPFS) (male professionals 40-75 years, 1986-2006). Detailed information on diet, lifestyle, medical conditions, and other chronic diseases was collected every 2 to 4 years for more than 20 years.

The availability of these repeated measures and the long-duration of follow-up allowed the authors to evaluate 4-year changes in coffee and tea intake in relation to risk of type 2 diabetes in the following 4 years. They also examined whether the association with diabetes incidence differed between changes in caffeinated and decaffeinated coffee. Diet was assessed every 4 years using a validated food frequency questionnaire. Self-reported incident type 2 diabetes cases were validated by supplementary questionnaires. The final analysis included 48,464 women in NHS, 47,510 women in the NHS II and 27,759 men in HPFS.

The authors documented 7,269 incident type 2 diabetes cases, and found that participants who increased their coffee consumption by more than 1 cup/day (median change=1.69 cups/day) over a 4-year period had a 11% lower risk of type 2 diabetes in the subsequent 4-years compared to those who made no changes in consumption. Participants who decreased their coffee intake by 1 cup a day or more (median change=-2 cups/day) had a 17% higher risk for type 2 diabetes. Changes in tea consumption were not associated with type 2 diabetes risk.

Those with highest coffee consumption and who maintained that consumption—referred to as "high-stable consumers" since they consumed 3 cups or more per day—had the lowest risk of type 2 diabetes, 37% lower than the "low-stable consumers" who consumed 1 cup or less per day.

The authors said the higher risk of type 2 diabetes associated with decreasing coffee intake may represent a true change in risk, or may potentially be due to reverse causation whereby those with medical conditions associated with risk for type 2 diabetes (such as high blood pressure, elevated cholesterol, cardiovascular disease, cancer) may reduce their coffee consumption after diagnosis. However, even when cases of cardiovascular disease or cancer were excluded during follow-up, the results were very similar.

While baseline decaffeinated coffee consumption was associated with a lower type 2 diabetes risk, the changes in decaffeinated coffee consumption did not change this risk. Regarding tea consumption, the authors said: "We found no evidence of an association between 4-year increases in tea consumption and subsequent risk of type 2 diabetes. This finding may have potentially been due to the relatively low number of participants who made significant changes to their tea consumption over a 4-year period thereby limiting statistical power to detect true associations. The overall low levels of tea consumption in this group may also be responsible for these findings."

The authors continued: "In these 3 large prospective cohorts with more than 1.6 million person-years of follow-up, we observed that increasing coffee, but not tea, intake over a 4-year period was associated with a lower type 2 diabetes risk in the next 4 years. Decreasing coffee intake was associated with a higher type 2 diabetes risk. These changes in risk were observed for caffeinated, but not decaffeinated coffee, and were independent of initial coffee consumption and 4-year changes in other dietary and lifestyle factors."

"Changes in coffee consumption habits appear to affect diabetes risk in a relatively short amount of time. Our findings confirm those of prospective studies that higher coffee consumption is associated with a lower type 2 diabetes risk and provide novel evidence that changes in coffee consumption habits are related to diabetes risk."