Caffeine impairs glucose metabolism in type 2 diabetes.

C affeine is a widely used drug despite evidence that it has deleterious consequences for health, including diabetes (1). In 1967, a study reported that drinking two cups of instant coffee significantly impaired glucose tolerance in a small group of men with “maturity-onset diabetes” (2). Recent studies showed that caffeine acutely decreased insulin sensitivity in young, nondiabetic adults (3–5). This study tested how oral caffeine affects carbohydrate metabolism in patients with type 2 diabetes, for whom decreases in insulin sensitivity might result in exaggerated hyperglycemic responses to glucose and other carbohydrates, which would aggravate the glycemic dysregulation found in the disease. We tested the effects of caffeine on fasting glucose and insulin levels and on glucose and insulin responses to a mixed-meal tolerance test (MMTT).