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When Furious 7 set box-office records on its opening weekend, there was one story lurking in the shadows of the celebrity-industrial complex that no one wanted to talk about: The star, Vin Diesel, is overweight.

And costar Dwayne Johnson? Dude’s obese. Johnson has a body-mass index, or BMI, of 34.3. Which means, according to U.S. government standards, the Rock is obese. And Diesel, at 27.1, is overweight.*

If Johnson and Diesel aren’t the first guys you think of when you ponder America’s obesity problem, you begin to understand the problem with using BMI, a ratio of weight to height, as a tool to judge an individual’s fitness or health risks.

And yet, BMI appears in the news with disturbing regularity. The French National Assembly recently passed a ban on hiring models with a BMI below 18. (A model who’s 5-foot-8 would fall just below the cutoff at 118 pounds, but she’d be catwalk-legal at 119.)

And in Belton, Missouri, a parent cried foul when her 7-year-old daughter came home from school with a note saying the child was overweight, based on her BMI.

Every article you’ve read about the obesity epidemic relies on BMI to tell us who’s too fat, and who’s just right.

BMI is indeed a terrible way to assess whether you’re lean or fat. But not for the reason you think.

* I don’t actually know what either guy currently weighs. I got their BMI from this site, which allows you to calculate your own BMI, and compare it to random celebrities. My BMI is similar to those of Matt Damon, Russell Crowe, and Leo DiCaprio. Alas, it doesn’t let me compare my deadlift to theirs.

Belgian waffle

What we now call body-mass index dates back to 1832, when a Belgian mathematician named Adolphe Quetelet observed that human weight “increases as a square of the height,” except during infancy and the adolescent growth spurt.

Quetelet didn’t set out to study obesity; his goal was to standardize the use of statistics in social science.

The focus on obesity and its complications for human health began with the insurance industry in the early 20th century. Specifically, it started with the company we now know as MetLife, sponsor of all those Peanuts specials many of us watched as kids.

They published their first set of weight-for-height tables in 1942, and then updated it in 1959. The U.S. government started using BMI in 1980 to establish cutoffs for what was variously described as “ideal,” “desirable,” “suggested,” or “acceptable” weight.

Originally, both the insurance industry and the government recognized something we all understand by virtue of basic biology and common sense: Humans come in two genders, several body types, and countless ethnicities. Each has a slightly different distribution of height and weight.

If you happen to reach middle age, you also understand that weight naturally increases over the years. It takes a lot of effort just to keep the gain to a minimum, and few of us have the time, energy, or genes to maintain the same weight through life.

(But if you want to shed extra weight and have more energy, check out the Lose Your Spare Tire exercise and diet program—a cutting-edge training plan that can help you lose 30, 40, even 50 pounds.)

Unfortunately, in 2000 the U.S. Centers for Disease Control settled on a single definition of healthy weight for every adult, regardless of gender, age, body type, or ethnicity.

If your BMI is below 18.5 or above 24.9, your doctor will probably tell you you’re under- or overweight. If it’s 30 or above, you’re obese. Extreme obesity begins at a BMI of 40. Thus, a guy who’s 5-foot-10 is underweight at 130 pounds, overweight at 175, obese at 210, and extremely obese north of 275.

The first and most obvious problem is the one we opened with: People who are large and solid—like countless athletes, actors, and Men’s Health readers—are lumped in with people who are soft and sedentary.

BMI can’t distinguish between fat and lean tissue. Nor can it distinguish between different types of fat.

We know the visceral fat, the type that accumulates around your organs, puts your health in danger, whereas the fat women hold in their hips and thighs is linked to a lower risk of chronic health problems.

The less obvious problem is that weight isn’t a very good predictor of mortality—who lives and who dies. As Katherine Flegal of the National Center for Health Statistics explains in this summary, there’s a “small but consistent” advantage to being slightly overweight.

Those few extra pounds have been linked to better outcomes among those with heart, lung, and kidney diseases, and to higher survival rates following surgery and car crashes. Among the elderly, the lighter you are, the more likely you are to have a hip fracture, or to die from any cause.

When Flegal and her coauthors crunched the numbers for an earlier study, they found that all-cause-mortality risk is 6 percent lower for those classified as overweight, compared to those in the “normal” range.

Nobody can say why this is, but we can take a reasonable guess.