Coffee is part of daily life for millions of people, and scientists have spent years studying whether the drink affects long-term health.
Earlier research has linked regular coffee drinking with lower rates of type 2 diabetes and some forms of heart disease. The reasons for these links are still not fully understood.
New research from Finland offers more clues. Scientists at the University of Oulu found that people who regularly drank more coffee tended to have less body fat, more muscle and several blood markers connected with healthier metabolism.
They also found differences in sex hormones, and these patterns were not exactly the same in men and women.
The researchers studied 2,264 people from the Northern Finland Birth Cohort 1966. Everyone included in this analysis was 46 years old, which helped reduce the effect of age differences when the researchers compared participants.
The team looked at usual coffee intake alongside body composition, blood chemicals linked with metabolism and several hormone measurements.
Finland is a useful place for this kind of research because coffee drinking is extremely common there. Average consumption has been estimated at about 11.8 kilograms of coffee per person each year. This gave researchers a population with a wide range of regular coffee habits to examine.
One of the clearest findings involved body composition. People who drank more coffee tended to have less total body fat and less visceral fat, which is the fat stored deep inside the abdomen around internal organs. At the same time, they tended to have more skeletal muscle.
This was interesting because people with different levels of coffee intake did not show major differences in body mass index, or BMI.
BMI is calculated from height and weight, but it cannot tell whether a person’s weight comes mainly from fat or muscle. The study suggests that looking only at BMI may miss some differences associated with coffee drinking.
Higher coffee intake was also linked with lower levels of branched-chain amino acids in the blood. These are natural building blocks of protein and are needed by the body. However, persistently high blood levels have been linked in previous research with insulin resistance and a greater chance of developing type 2 diabetes.
The hormone results were especially noticeable in men. Men who drank more coffee generally showed a healthier pattern involving glucose and insulin, and they had higher total and bioavailable testosterone. They also had higher levels of sex hormone-binding globulin, usually shortened to SHBG, a protein that carries sex hormones through the bloodstream.
At the same time, some measures of freely available male hormones were slightly lower among heavier coffee drinkers. In women, the hormone pattern was simpler. Greater coffee intake was mainly associated with higher SHBG and lower measures of freely available androgens, hormones that include testosterone.
Lead author Luca Verroest, a doctoral researcher at the University of Oulu, said the hormone pattern remained noticeable even after the researchers considered BMI and lifestyle factors.
The differences between men and women may offer a new direction for understanding how coffee is connected with metabolism. Hormones influence many processes, including muscle growth, fat storage and the way the body handles sugar.
Coffee contains hundreds of substances, so caffeine may not be the only important ingredient. It also contains plant compounds such as polyphenols as well as other chemicals created during roasting.
Researchers now want to learn whether coffee itself produces the changes seen in this study and, if it does, which ingredients are responsible.
The study has an important limitation: it was observational. Researchers measured coffee habits and biological differences at the same time, so they cannot prove that coffee caused people to have less fat, more muscle or different hormone levels.
People who drink different amounts of coffee may also differ in diet, sleep, activity, smoking or other habits that are difficult to completely separate from coffee consumption.
Another strength and limitation is that all participants were the same age and came from a Finnish birth cohort. This makes comparisons within the group cleaner, but it also means the results may not apply equally to younger people, older adults or populations with very different diets and coffee habits.
Intervention trials, in which researchers deliberately change coffee intake and measure what happens afterward, would provide stronger evidence.
Overall, the findings are intriguing because several results point in the same direction: higher coffee intake was associated with lower body fat, more muscle and blood markers that may indicate better metabolic health.
The hormone findings also suggest a possible biological pathway that deserves closer study. Still, the research should not be read as proof that drinking more coffee will make a person leaner or protect against diabetes.
The next step is to test these ideas more directly. The researchers are already investigating possible mechanisms in animal models, with the longer-term goal of human studies. Until those studies are completed, the findings are best viewed as evidence of an interesting relationship rather than a reason to dramatically increase coffee intake.
The study, titled “Associations of habitual coffee intake with testosterone and cardiometabolic markers: the Northern Finland Birth Cohort 1966 study,” was published in the European Journal of Nutrition.
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Source: University of Oulu.
