In the United States, the recent increase in prevalence of type 2 diabetes primarily is caused by diet and lifestyle factors, including overweight, physical inactivity, smoking, and a diet with a high glycemic index. Because caffeine adversely affects insulin sensitivity and glucose metabolism, coffee drinking has been considered to be associated with diabetes. However, caffeine can increase thermogenesis and energy expenditure, so the impact of coffee and other caffeine-containing drinks is unclear. Salazar-Martinez and associates reviewed data from two large, prospective cohort studies to evaluate the effect of coffee and decaffeinated drinks in relation to diabetes.

Participants in the Health Professionals Follow-up Study and the Nurses’ Health Study who did not have diabetes at baseline and who had adequately completed food intake questionnaires were included in the study.

More than 5,400 new diagnoses of diabetes were noted during the 12- to 18-year follow-up periods. In both cohorts, coffee intake was positively associated with smoking, alcohol intake, and fat ingestion, and inversely associated with physical activity, glycemic load, and tea consumption. After adjustment for these and other risk factors, coffee consumption was found to be associated with a statistically significant decrease in type 2 diabetes risk in both cohorts. The inverse relationship between decaffeinated coffee and type 2 diabetes was more modest. Tea consumption was not associated with diabetes risk in either cohort.

The authors conclude that caffeine intake (as nondecaffeinated coffee) is independently associated with decreased risk of diabetes in men and women. This association becomes stronger with increased coffee consumption. Because the cause of this relationship is unclear, more studies are needed before recommendations are made about coffee ingestion for prevention of new-onset diabetes.