Increasing coffee consumption may reduce the risk of developing type 2 diabetes
Increasing coffee consumption by an average one-and-a-half cups per day (approximately 360mL) over a four-year period is associated with an 11 per cent reduction in the risk of developing type 2 diabetes, according to research from Harvard University.
The study, published in Diabetologia, was led by Dr Frank Hu and Dr Shilpa Bhupathiraju from the Department of Nutrition, Harvard School of Public Health, Harvard University, Boston.
The researchers used observational data from three large prospective US-based studies in their analysis: the Nurses' Health Study (NHS), which collected data from female nurses aged 30 to 55 years between 1986 and 2006; the NHS II, which collected data from younger female nurses aged 25 to 42 years between 1991 and 2007; and the Health Professionals Follow-up Study (HPFS), which collected data from male health professionals aged 40 to 75 years between 1986 and 2006. Detailed information on diet, lifestyle, medical conditions and other chronic diseases was collected every 2 to 4 years for over 20 years.
"In these three 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 four years," the authors said.
"Decreasing coffee intake was associated with a higher type 2 diabetes risk," the authors said. "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," the said.
Study method
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 of the Harvard University paper examined the associations between 4-year changes in coffee and tea consumption and risk of type 2 diabetes in the subsequent 4 years.
The availability of the repeated measures and the long-duration of follow-up allowed the researchers to evaluate 4-year changes in coffee and tea intake in relation to the 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 NHS II and 27,759 men in HPFS.
The researchers documented 7,269 incidents of type 2 diabetes cases and found that participants who increased their coffee consumption by more than 1 cup per day (median change was 1.69 cups per day) over a 4-year period had an 11 per cent 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 was 2 cups a day) had a 17 per cent higher risk for type 2 diabetes.
Decaffeinated coffee consumption
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.
Tea consumption not associated with type 2 diabetes risk
Changes in tea consumption were not associated with type 2 diabetes risk.
"We found no evidence of an association between 4-year increases in tea consumption and subsequent risk of type 2 diabetes," the authors wrote. "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 limited statistical power to detect true associations. The overall low levels of tea consumption in this group may also be responsible for these findings," they said.
The researchers said changes in coffee consumption habits appeared to affect diabetes risk in a relatively short amount of time.
High coffee consumption associated with much lower risk
Those with the 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. This group's risk was 37 per cent lower than the "low-stable consumers" who consumed 1 cup or less per day.
Causes unclear
The researchers said that 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 researchers said results were similar.
"Our findings confirm those of prospective studies that higher coffee consumption is associated with lower type 2 diabetes risk and provide novel evidence that changes in consumption habits are related to diabetes risk," the researchers said.
This study was funded by research grants P01 CA87969, P01 CA055075, R01 HL034594, and HL60712 from the US National Institutes of Health. The work of Dr Shilpa Bhupathiraju was supported by a postdoctoral fellowship grant from the American Heart Association (grant no. 13POST14370012).
Conflict of interest statement: Study author Dr Rob M van Dam (National University of Singapore and National University Health System, Singapore) received grant funding from Nestec Ltd for a randomized trial of the effects of coffee consumption on insulin sensitivity. Nestec Ltd is a broad food company that also sells coffee. The researchers said this is grant funding specific for that project with a contractual agreement that ensures that the company cannot influence the design of the study or decision to publish the results. They said the funding "does not in any way affect the current study". Other authors declared that there is no duality of interest associated with this manuscript.