An incredibly popular health stat might not have any muscle behind it

Is body mass index (BMI) worth anything? According to a recent study, it might not be. The paper, which reviewed 72 studies on obesity and mortality comprising 2.5 million participants, found that individuals with high levels of stomach fat are at a higher risk for early death, regardless of their BMI, a measurement that tracks a person’s weight according to their height.

The paper also found that individuals with varying regional fat distribution — like larger hips and thighs, but with small waists — did not have higher risk levels than individuals with comparable body fat percentages but who had more stomach fat. These findings were published in September in the medical journal The BMJ.

The study, at first blush, sits somewhere between a celebration of body positivity and leg day. Its results shift blame away from traditional obesity signifiers, like total poundage and BMI, and toward a specific range and location of fat.

Fat, which is correlated in the study with heart disease, some cancers, kidney disease, and neurological disorders, is not necessarily bad in itself but is troubling when found at the waist: The study suggests that for every four inches a person’s waist grows, their likelihood of disease increases by about 11 percent. The study’s hip and thigh circumference measurement results may have been even more surprising: a nearly two-inch increase in thigh circumference corresponds to an 18 percent lower rate of mortality; a four-inch increase in waist circumference had a 10 percent lower risk rate.

The study, which distinguishes between stomach and leg size, notes that BMI “does not reflect regional body fat distribution,” casting that whole measurement into doubt. Its shortcomings are widely known among lifters, who are considered obese regardless of their body fat, and because of their muscles. It’s less a useful measurement than a category error, and the study builds on this acknowledgment, showing that so-called “skinny” people aren’t automatically healthier.

Testing for BMI — Measuring body fat percentage can be done in a number of ways. There are calipers, which you might remember from high school gym class; priced like bicycles, some are cheap and some are not. Eye tests — an expression of bodily literacy you can pick up if you look at enough photos of shirtless people — are free. (If you don’t live near a beach, there are (weird) websites with breakdowns.) Percentage numbers are helpful to a point, and reveal some truths: men, whether as big as The Rock or as cut as Pete Doherty, show abs under 10 percent, are “hard-bodied” under 15, and look obese at 25 and above. The range for women is more robust, but 15 percent and below is a super muscular build.

No test may be better than the Marines body fat test, effective and simple, involving only bodyweight and a tape measure. Male recruits take their circumference at their neck and abs, with hips added for women, and both input their weight; a number of online calculators spit out the ratio. It’s fairly precise, and contained by the tension most bodies are held by. A big neck and small waist means a big back (and likely big thighs); the inverse means less muscle and more fat. Thigh measurements would be an improvement; the results would probably correspond well to The BMJ study, or outpace the most expensive calipers.

Still, questions remain. While most bodies shake out a certain way — no one really is built like Barbie — it’s hard to tell how the specifics of the measurements taken in this study shake out. The authors acknowledge that BMI’s reliability gets criticized — which is why they moved towards measurements in their own research — but the ratios and circumferences referred to here still leave room for confusion.

Looking at the breakdown, it’s unclear how many inches are muscle, how much is fat, and how much is luck of the genetic draw. Big thighs could be a cyclist or a yogi’s, or someone with big thighs; the hip-to-thigh ratios in the study are not made as explicit as, say, an eye test would. (It’s likely the studies they were reviewing didn’t track their participants’ squat numbers.) And the study’s specificity on waist measurements looks, initially, to be a bit obvious. The four-inch belly circumference variance that’s cited is huge: the difference between a 31” and 35” waist for a 6" man can turn a model to a construction worker; for someone shorter it’s a complete body makeover. Of course a four-inch spread makes a difference! Still, it distinguishes between muscle and fat better than BMI.

The study dovetails too with some old bodybuilding wisdom about the danger of being “skinny fat.” The term, which refers to individuals with a low BMI who hold fat deposits in their stomachs, is a vague insult: people who look thin, and are worse off for it. The term is a break from the mass of dieting literature that focuses on making people lose weight and get skinny, through calorie reduction, shame, or worse. But skinny-fat people are thin because they don’t have muscle, they have plenty of fat. We know dieting is an unpleasant experience, done wrong, it doesn’t seem like a health goal either.

A study that correlates big legs to better health is a shot in the arm for body positivity, which has caught on outside of the fitness world but has lagged within. While the study advocates for flatter stomachs, the results seem nonetheless revelatory and welcoming, when taken in the context of a top-down “skinny is better” approach to fitness and dieting that seems to prevail.

And while there are more visible examples of body positivity now than there was, marking a distinct visual change, there’s something substantive about a scientific study joining the fray and breaking up the skinny rule. The truth might be that different body types — lifter bodies, bigger people, folks with thunder thighs — are healthier in the long term than plenty of skinny people. That could change the way we exercise.

We all might work out more if we’re not trying to lose weight.

Leg Day Observer is an exploratory look at fitness, the companion to GQ.com’s Snake America vintage column, and a home for all things Leg Day. Read past editions of Leg Day Observer for more thoughtful approaches to lifting and eating.