The levels of caffeine in your blood could affect the amount of body fat you carry, a factor that in turn could determine your risk of developing type 2 diabetes and cardiovascular diseases.

Those are the findings of a 2023 study that used genetic markers to establish a more definitive link between caffeine levels, BMI, and type 2 diabetes risk.

The research team, from the Karolinska Institute in Sweden, the University of Bristol in the UK, and Imperial College London in the UK, said calorie-free caffeinated drinks could be explored as a potential means of helping reduce body fat levels.

"Genetically predicted higher plasma caffeine concentrations were associated with lower BMI and whole body fat mass," the researchers wrote in their paper, published in March 2023.

"Furthermore, genetically predicted higher plasma caffeine concentrations were associated with a lower risk of type 2 diabetes. Approximately half of the effect of caffeine on type 2 diabetes liability was estimated to be mediated through BMI reduction."

The study involved data from just under 10,000 people collected from existing genetic databases, focusing on variations in or near specific genes known to be associated with the speed at which caffeine is broken down.

In general, those with variations affecting the genes – namely CYP1A2 and a gene that regulates it, called AHR – tend to break caffeine down more slowly, allowing it to remain in the blood longer. Yet they also tend to drink less caffeine in general.

An approach called Mendelian randomization was used to determine likely causal relationships between the presence of the variations, illnesses like diabetes, body mass, and lifestyle factors.

While there was a significant link between caffeine levels, BMI, and type 2 diabetes risk, no relationship emerged between the amount of caffeine in the blood and cardiovascular diseases including atrial fibrillation, heart failure, and stroke.

Previous studies have linked a moderate and relative increase in caffeine consumption to better heart health and a lower BMI, and the new research adds more detail to what we already know about the effects that coffee has on the body.

It's important to also keep in mind the effects of caffeine on the body aren't all positive, which means care must be taken when weighing up the benefits of drinking it – but this latest study is an important step in assessing how much caffeine is ideal.

"Small, short term trials have shown that caffeine intake results in weight and fat mass reduction, but the long term effects of caffeine intake is unknown," the researchers explained.

The team thinks the association shown here could be down to the way caffeine increases thermogenesis (heat production) and fat oxidation (turning fat into energy) in the body, which both play an important role in overall metabolism.

However, more research will be needed to confirm cause and effect. While this study involved a large sample, Mendelian randomization isn't infallible, and it's still possible that other factors are at play that weren't accounted for in this study.

"Considering the extensive intake of caffeine worldwide, even its small metabolic effects could have important health implications," the researchers wrote.

The research was published in BMJ Medicine.

An earlier version of this article was published in March 2023.