Editorial NexoScience
You and a friend order the same coffee, drink it at the same time, and somehow her heart barely notices while yours is racing an hour later. Caffeine guidelines are usually built around body weight: a moderate dose sits around 5 to 6 milligrams per kilogram, an amount often used in supplements, energy drinks, and even some clinical protocols. But a new study suggests that formula misses something important.
Body fat, not total weight, changed how much caffeine ended up in the blood. Researchers at Poznan University of Medical Sciences gave 38 women a single 6 milligram per kilogram dose of caffeine in capsule form, then measured its concentration in their plasma an hour later. Fourteen of the women had 30% body fat or less; the other 24 had more than 30%. Both groups received the exact same relative dose, adjusted for their own body weight, and both had abstained from any caffeine for 48 hours beforehand.
The results split sharply. Women in the higher body fat group had a median caffeine concentration of 10.64 micrograms per milliliter, more than four times the 2.32 micrograms per milliliter measured in the leaner group. The difference was statistically significant (p < 0.001). Paraxanthine, the main compound the body converts caffeine into, was also markedly higher in the higher fat group (1.73 versus 0.85 micrograms per milliliter, p = 0.007). Theobromine, a milder byproduct, showed no meaningful difference between the two groups.
Fat mass explained more of the variation than the scale ever could. When the researchers modeled what actually predicted caffeine levels, group membership, obese versus non-obese, accounted for the largest share, explaining close to 57% of the variance on its own. Fat mass alone correlated strongly with caffeine concentration, a Spearman correlation of 0.689, while age and body mass index played a much smaller role once fat mass was accounted for.
None of this proves caffeine is unsafe for people with higher body fat, and it does not mean the effect described here would hold at the lower, more typical doses most people actually drink in a cup of coffee. This was a single 6 mg/kg dose measured 60 minutes after ingestion, and the authors note their sample size, while enough to detect the differences reported, is still modest. What the data does show is a plausible biological mechanism. Adipose tissue may act as a slower draining reservoir for caffeine, and liver enzymes that break the compound down may work less efficiently in people with more body fat, both of which could leave more caffeine circulating in the blood for longer.
The authors argue caffeine dosing built purely on total body weight may need rethinking. Guidelines used in sports nutrition, and increasingly in weight loss supplements that lean on caffeine as a stimulant, tend to treat a kilogram of fat and a kilogram of muscle as interchangeable. This study is among the first to test that assumption directly by measuring body composition rather than just weight, and it found the two are not the same when it comes to how much caffeine actually shows up in the bloodstream.
The study, “Body fat percentage is a key factor in elevated plasma levels of caffeine and its metabolite in women,” was conducted by Przemyslaw Domaszewski, Mariusz Konieczny, Pawel Pakosz, Jakub Matuska, Anna Poliwoda, Elzbieta Skorupska and Manel Santafe, and published in PeerJ in 2025. DOI: 10.7717/peerj.19480.
