Decade-long study finds no clear link between daily coffee consumption and diabetes risk

For a decade, researchers in Italy tracked 1,158 adults to answer a question that has nagged at coffee drinkers for years: does a daily habit of several cups actually raise the risk of diabetes? The answer, based on data collected at the start and end of the study period, appears to be no—at least not in any measurable way. Those who drank at least two to three cups of coffee each day developed type 2 diabetes and impaired fasting glucose at roughly the same rate as those who drank none.

The scientific puzzle behind the question is real enough. Caffeine, in the short term, can weaken the body's ability to control blood sugar and handle glucose efficiently. That acute effect raised a legitimate concern: if caffeine does this acutely, might years of daily consumption wear down glucose metabolism and tip someone toward diabetes? The PAMELA study, which recruited participants aged 25 to 74 from Monza, Italy, was designed to test whether chronic coffee drinking showed any such long-term consequence.

The researchers divided their participants into two groups: non-consumers, who drank no coffee daily, and consumers, who drank at least two to three cups per day. The study did not examine intermediate levels of intake, nor did it assess tea, decaffeinated coffee, or other caffeinated beverages. At the baseline measurement, the two groups were comparable on the measures that mattered most—blood pressure, heart rate, fasting glucose levels, and the prevalence of impaired fasting glucose or diabetes. The coffee drinkers, however, were more likely to smoke and drink alcohol, and they carried higher body weight and BMI.

Over the following decade, both groups changed in similar ways. Weight increased, blood pressure rose, triglycerides climbed, and fasting glucose levels edged upward. Both groups also saw increases in the proportion of people with impaired fasting glucose—defined as a fasting glucose level between 110 and 126 mg/dL—and type 2 diabetes, defined as a fasting glucose level of 126 mg/dL or higher, or the use of antidiabetic medication. But the rate of new cases did not differ significantly between the coffee drinkers and the non-consumers. The pattern held even when researchers examined coffee consumed with or without added sugar.

The finding is reassuring for coffee drinkers, but the study carries important caveats. The non-consumer group was small—only 99 people compared to 1,059 coffee drinkers—which means the study had limited statistical power to detect a real difference if one existed. The researchers acknowledge that their estimates were imprecise, and they cannot definitively rule out a difference in risk. The study also relied on self-reported coffee intake, did not track the caffeine content of the coffee consumed, and did not capture changes in diet or energy intake over the decade. Participants who did not complete both the baseline and follow-up assessments were excluded, which could have introduced bias.

What the PAMELA study does suggest is that moderate daily coffee consumption, at least at the level of two to three cups, does not appear to accelerate the development of glucose disorders in this Italian population over a 10-year span. But the limitations mean the question is not fully settled. Larger studies with more balanced groups, detailed tracking of caffeine and coffee type, and diverse populations would be needed to confirm whether the apparent safety of habitual coffee drinking holds across different contexts and consumption patterns.