The National Heart, Lung and Blood Institute makes it easy to tell if you're overweight or obese: Divide your weight by your height to find your Body Mass Index (BMI). If your BMI is greater than 30, the U.S. government considers you obese.
BMI is one of the most widely used calculations for determining if you're at a healthy weight. But although it's simple, it isn't necessarily accurate. It has no way of differentiating between muscle mass and fat, for one, meaning that super-fit athletes with bulging quads and biceps still get classified as "obese." It also doesn't take into account that people naturally have different builds — some are born stockier, others slimmer.
And let's not forget: BMI wasn't even invented to determine how much fat we have on our bodies.
A recent paper published in the journal PLOS One proposes an alternative calculation to BMI. This new indicator, called Surface-based Body Shape Index (SBSI), takes into account body shape and body size, and could be a more accurate predictor of mortality risk.
SBSI is determined using four measurements: body surface area (BSA), waist circumference (WC), height (H) and vertical trunk circumference (VTC). What's vertical trunk circumference? Imagine a circle extending from your shoulder, through your groin, and up your back to your shoulder again.
Here's what the new calculation looks like — a much more complex equation than simply dividing your weight by your height:
The researchers say SBSI provides a better picture of a person's health because it specifically accounts for abdominal fat, measured by waist circumference and vertical trunk circumference. Honing in on abdominal fat — as opposed to measuring a person's overall mass — is a more accurate way to predict a person's cardiovascular risk, they say.
To analyze how well their new model predicts mortality risk, the researchers applied it to data on more than 11,000 subjects from the National Health and Human Nutrition Examination Surveys from 1999 to 2004.
"Applying SBSI initially gives reasonable performance when compared with existing body shape measures," the researchers found, noting that it performed better, in some categories, than BMI, and on par with A Body Shape Index (ABSI), another recently-developed calculation for determining mortality risk based on body shape and size.
This BMI replacement still has problems. As Real Clear Science notes, popularizing BMI alternatives such as SBSI and ABSI is no easy feat. When it comes to health research, "simplicity trumps accuracy," they write:
"Unfortunately, ABSI seems to have mostly fizzled out, earning only 83 mentions in PubMed, the international database for biomedical science, since its inception. Over the same period, BMI has been mentioned more than 34,000 times! The new indicator, SBSI, is probably doomed to the same fate. Thus far, it seems that simplicity trumps accuracy in health research, at least as far as BMI is concerned."
But there's also another issue: that we shouldn't use any one number to paint a picture of our overall health. People of all shapes and sizes can be physically fit — in fact, according to a recent study, your mortality risk is lower if you're fat and fit than if you're skinny and unfit. Additionally, BMI, ABSI and SBSI say nothing about a person's mental and emotional health. These tools can be useful for developing nationwide statistics, but perhaps shouldn't be used to evaluate an individual's health.
Another limitation of SBSI, the researchers note, is that it could be affected by myriad other things that potentially define our health, such as smoking, pregnancy, socio-economic status and ancestry.