High levels of caffeine in your blood may help lower your amount of body fat and even your risk of type 2 diabetes, according to new research.
However, the study does not recommend any particular amount of coffee a person should drink to stay healthy - and more is not necessarily better, as the fat or sugar contained in caffeinated drinks might offset the benefits.
Previous research has suggested that drinking three to five cups of coffee - containing an average of 70-150 mg of caffeine - each day was associated with a lower risk of type 2 diabetes and cardiovascular disease.
But those were observational studies, which makes it difficult to disentangle the actual effect of caffeine from that of other potentially influential factors.
The latest study, published in the open-access journal BMJ Medicine, instead used a technique known as Mendelian randomisation, which uses genetic evidence in support of a causal effect.
Exploring caffeine metabolism
The researchers examined the role of two common gene variants - which by definition are randomly allocated at birth - associated with the speed of caffeine metabolism.
People who carry genetic variants associated with slower caffeine metabolism drink less coffee on average but have higher levels of caffeine in their blood than those who metabolise it quickly.
The researchers used these variants to determine “genetically predicted” blood caffeine levels among nearly 10,000 people who took part in six long-term studies.
They found that higher blood caffeine levels were associated with both lower body mass index (BMI) and a reduced risk of type 2 diabetes.
"It's small, lifelong effects," Dipender Gill, a senior author of the study, told Euronews Next.
Caffeine is known to boost metabolism, increase fat burning and reduce appetite, with a daily intake of 100 mg estimated to increase energy expenditure by around 100 calories a day.
The researchers further used Mendelian randomisation to pinpoint the extent to which the effect of blood caffeine levels on type 2 diabetes risk might be driven by the concurrent weight loss - and found it accounted for nearly half of it (43 per cent).
What does this mean for coffee addicts?
If you feel you have to guzzle down way too many cups of coffee to function each day, this might mean that your caffeine metabolism is of the faster type - so the levels of caffeine in your blood wear off quickly and you need to refuel often to sustain them and feel the benefit.
However, this study doesn’t say whether having a fast or slow caffeine metabolism puts you at greater risk of obesity or diabetes, Gill cautioned.
"At an individual level, the rate at which we metabolise caffeine has such a small impact on our body weight that it in itself doesn't explain it. But the approach that we use helps infirm or support that there is an effect in large sample sizes," he explained.
In other words, the study shows there is genetic evidence that higher caffeine levels in the blood reduce body weight and diabetes risk, but there’s not much we can take away from that finding at an individual level, and we certainly shouldn’t be changing our coffee-drinking habits based solely on this study.
"Individuals should not alter their dietary preferences or lifestyle based on the findings from this study alone, because as well as having potential beneficial effects, caffeine can also have harmful effects if consumed excessively," Gill said.
The researchers are calling for more research into the benefits of calorie-free caffeinated drinks, to further isolate the effect of caffeine and dispel potential myths about coffee drinking.
"When people consume coffee, it's unclear whether it’s the effect of caffeine in the coffee or the other things that are in coffee. And there are a lot of other chemicals in coffee too, including water - actually, just staying hydrated is good for people," Gill explained.
One of the study’s limitations is that it looked at people of mostly European ancestry, and the researchers say that further clinical study is needed to validate and confirm their findings.
The gold standard would be a randomised clinical trial "looking at somehow therapeutically altering an individual's plasma caffeine levels and the effects that has on various metabolic parameters," said Gill.
"At the end of the day, it's a drug, right? So you could alter someone's plasma levels of that drug [caffeine] by any means that you might otherwise administer a drug - so, as a tablet, as an injection".