In light of the findings the potential role of calorie free caffeinated drinks for lowering the risks of obesity and type 2 diabetes is probably now worth exploring. Previously published research indicates that drinking 3-5 daily cups of coffee, a rich source of caffeine, is associated with a lower risk of type 2 diabetes and cardiovascular disease, note the researchers. An average cup of coffee contains around 70–150 mg caffeine. 

Researchers used Mendelian randomisation to find out what effect higher blood caffeine levels have on body fat and the long term risks of type 2 diabetes and major cardiovascular diseases—coronary artery disease, stroke, heart failure, and irregular heart rhythm (atrial fibrillation). They looked at the role of two common genetic variants of the CYP1A2 and AHR genes in nearly 10,000 people of predominantly European ancestry, who were taking part in 6 long term studies. The CYP1A2 and AHR genes are associated with the speed of caffeine metabolism in the body. 

People who carry genetic variants associated with slower caffeine metabolism drink, on average, less coffee, yet have higher levels of caffeine in their blood than people who metabolise it quickly to reach or retain the levels required for its stimulant effects. The results of the analysis showed that higher genetically predicted blood caffeine levels were associated with lower weight (BMI) and body fat. Higher genetically predicted blood caffeine levels were also associated with a lower risk of type 2 diabetes.  

Read the full University of Bristol news item

'Appraisal of the causal effect of plasma caffeine on adiposity, type 2 diabetes, and cardiovascular disease: two sample mendelian randomisation study' by Susanna C Larsson, Benjamin Woolf and Dipender Gill in BMJ Medicine [open access]