Your bathroom scale can tell you how much you weigh but it cannot tell you how much of that weight comes from muscle, body fat, or even where that fat is stored.
That distinction is at the centre of a new Finnish study on coffee drinkers. Researchers looking at 2,264 adults find that people who drink more coffee tend to have a different body composition from those who drink little or none, even though their BMI looks remarkably similar.
So what exactly did they find? And just how much of a difference could coffee really make?
The difference was hiding behind similar BMI
BMI is based on your height and weight. It's useful, but it doesn't tell you what that weight is made of. That's where the Finnish study comes in. The researchers observed that people who drank the most coffee had an average body-fat percentage of 27.14%, compared with 29.67% among people who didn't drink coffee. They also had more skeletal muscle. The high-coffee group averaged 32.46 kilograms of skeletal muscle, compared with 30.88 kilograms among non-drinkers. There was a difference in visceral fat too, although it was smaller. The high-coffee group averaged 102.52 cm², compared with 108.50 cm² among people who didn't drink coffee.
The groups had similar average BMI, waist circumference and waist-to-hip ratios. So the interesting part isn't simply that one group weighed less. It's that similar-looking numbers on the scale and tape measure could hide differences in what's underneath. But before you go and reach for another cup of coffee, there's something important to know about the people in the study.
Most people in the study drank coffee
The researchers used information from the Northern Finland Birth Cohort 1966, looking at people when they were 46 years old. They divided coffee drinkers into four groups: no coffee, 1–2 cups a day, 3–4 cups, and at least 5 cups a day. Most of the 2,264 people in the study drank coffee. Only 70 people said they didn't drink it. The group drinking at least five cups a day had the lowest average body-fat percentage and the highest average skeletal muscle mass.
That sounds interesting, but there's a catch. A "cup" wasn't the same thing as a measured amount of caffeine. The researchers didn't have enough information to work out exactly how much caffeine each person consumed. They also didn't have detailed information about things added to the coffee, such as milk or sugar. So this isn't a case of researchers discovering that five cups of coffee a day produces a certain amount of fat loss. It's simply a pattern they found when they compared the groups. And that leaves a bigger question: could something else be behind it?
Body composition wasn't the only difference researchers found
The body composition results weren't the only difference. People who drank more coffee also tended to have lower levels of three amino acids called branched-chain amino acids, or BCAAs: leucine, isoleucine and valine. These amino acids come from protein-rich foods and play a role in maintaining muscle.
The researchers also found differences in several hormones. In men, higher coffee intake was linked with higher total testosterone and SHBG, while free testosterone and the free androgen index moved in the opposite direction.
So the blood results add more clues, but they don't explain why the coffee groups differed in body composition. They also don't show that coffee caused any of these changes. And there's an even bigger reason to be careful about what we take from the study.
The study can't prove coffee caused it
The researchers looked at people's coffee habits and body composition at the same point in time. They didn't give one group coffee and another group no coffee and then watch what happened. That means they can say the two things were associated, but they can't say coffee caused the difference. People who drink a lot of coffee may also differ from non-drinkers in other ways.
They took several factors into account, including physical activity, smoking, alcohol use, education and BMI, but they couldn't rule out every possible difference between the groups. There's also a limitation in the way fat and muscle were measured. The researchers used bioelectrical impedance, which estimates body composition rather than measuring it directly.
So the numbers are useful for spotting a pattern. They aren't proof that drinking more coffee will change your body.
So, how much can coffee really make a difference?
That's still the unanswered question. The Finnish study found that people who drank more coffee tended to have less body fat, slightly less visceral fat and more skeletal muscle, even though their average BMI and waist measurements were similar. But it doesn't show that coffee caused those differences.
That means there's no basis here for saying that drinking five cups a day will burn belly fat or build muscle. What the study does show is something a little more interesting: the number on your bathroom scale doesn't always tell the whole story.
And when it comes to coffee, researchers now have another reason to look more closely at what might be happening underneath that number. For now, your daily coffee can remain just that — your daily coffee. The bigger question is whether coffee itself is responsible for the difference researchers spotted. That will take more research to answer.