Daily coffee drinkers looked fine on fasting glucose and HbA1c. A two-hour sugar test in 905 Polish adults told a different story, and their genes made no difference

Gene tests that promise to tell you how your body handles coffee or carbs are easy to buy. A cross-sectional study published in Nutrients on 27 October asked a plainer question first: in ordinary adults, do blood sugar, body fat and cholesterol track everyday habits, a couple of well-known diabetes genes, or both?

The volunteers, aged 18 to 79, were seen at the Medical University of Białystok in north-east Poland as part of a registered project. Of the 905 who filled in the habit questionnaires, 612 said they never drank coffee, 182 drank it weekly and 111 daily. Snacking was counted as eating or drinking something with calories between main meals; two or more such occasions a day made someone a frequent snacker, and about two thirds were. Everyone without diagnosed diabetes drank 75 g of glucose and had blood taken at 0, 30, 60 and 120 minutes. The team also read four gene variants: one in BDNF, a gene involved in appetite, and three in TCF7L2, a gene region with some of the strongest known links to type 2 diabetes risk.

The clearest signal came from coffee, and it showed up only after the glucose drink. Compared with people who never drank coffee, daily drinkers had higher glucose and insulin responses over the two hours, and their two-hour glucose was about 15 mg/dL higher, after allowing for age, sex, calories eaten and inactivity. Their fasting glucose and HbA1c, the two tests most people get at a check-up, did not differ. In unadjusted averages, the gap opened at 30 and 60 minutes. It held when body mass index or snacking was added to the model, and when 14 people whose results met diabetes thresholds on the day were left out.

Frequent snackers carried more body fat: about 1.3 points more BMI, 2.6 kg more fat mass and 1.8 percentage points more body fat after adjustment. They also ate around 170 kcal a day more in their three-day food records. Coffee and snacking were not linked to each other at all; roughly two thirds of never, weekly and daily coffee drinkers alike were frequent snackers. Weekly coffee drinkers, oddly, had higher HDL cholesterol than non-drinkers, while daily drinkers did not.

The genes added little. One BDNF genotype came with lower odds of type 2 diabetes, but the pattern did not rise or fall with the number of copies, and the authors call it a lead for replication. The TCF7L2 variants were not clearly linked to diabetes here. Most telling, the team ran 184 tests of whether a gene changed how coffee or snacking related to an outcome. Eleven came up at the usual p < 0.05 threshold, about the nine you would expect by chance, and none survived correction for multiple testing. Their conclusion: these results do not support gene-based advice on coffee or snacking.

Now the brakes. This is a snapshot, so it cannot say coffee raised anyone's glucose. Daily drinkers had twice the rate of diagnosed diabetes (22.5% against 10.6%), a hint that coffee habits travel with other health and lifestyle differences. The questionnaire did not ask about cup size, brewing method, sugar, milk or creamer, or what the snacks were, and smoking and sleep were not accounted for. None of the main results passed the study's own correction for multiple testing either (the coffee and glucose finding came closest). And large prospective studies, which the authors cite, keep finding that regular coffee drinkers are less likely to develop type 2 diabetes over time. The authors read their result as a reason for researchers to use the two-hour test, not as a reason to drop coffee.