Scientists (along with basically anyone with a rudimentary understanding of health) have long been perplexed by the so-called obesity paradox, which seemed to suggest that obese people with chronic illnesses lived longer than those with a healthy weight. While the absolute rule of “more body mass equals longer life” has since been debunked, a new study shows that there may be a bit of truth to the idea that more mass yields a longer life span. But it’s important what kind of mass it is.

Nearly 20 years after the paradox first become mainstream, a study published in PLoS ONE last week suggests that the seemingly contradictory claims spawned from our overreliance on body mass index (BMI) when determining body weight and health classifications. BMI values — which are determined using your weight and height — fails to take into account a number of important health factors, like muscle mass or specific body fat data.

The researchers found that subjects’ proportion of lean muscle mass to body fat was the key bit of data needed to contradict the infamous obesity paradox. The study followed 11,687 individuals for an average of around 9 years, and seemed to show that “participants with low muscle mass had higher body fat percentage (%TBF), an increased likelihood of diabetes, and higher adjusted mortality than other participants.” Stated more plainly: mo’ muscle, mo’ life.

The implications of this study go far beyond establishing a more nuanced relationship between size and health. The researchers’ findings on the importance of muscle mass add to a pile of evidence that we rely too much on BMI as a baseline measurement, and the concept of health is much more complex than raw size and mass data; body composition matters.