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New research shows obesity rates may not be leveling off

Tim Sloan/AFP

In recent years, the overall rate of obesity in the US has plateaued. But new data published in JAMA shows that Americans' waistlines may be expanding anyway.

Researchers looked at waist circumference measurements taken from over 32,000 adults in 1999 and 2012. During that period, participants' waists grew nearly a whole pants size, from 37.6 inches to 38.8 inches.

Some groups gained an even more significant amount of abdominal girth. White women, aged 40 to 49, experienced a 2.6-inch expansion; the waists of black men, aged 30 to 39, got padded with 3.2 extra inches; Mexican-American men, aged 20 to 29, added 3.4 inches to their frames; Mexican-American women over the age of 70 packed on 4.4 inches; and black women between the ages of 30 to 39 increased their waists by 4.6 inches. (Abdominal obesity was defined as a waist circumference greater than 40 inches in men and 35 inches in women.)

That racial minorities are experiencing greater gains maps on to the fact that they're also disproportionately struggling with obesity compared to white people in the US.

What does this new research mean?

There's a debate in the research community about whether waist circumference or BMI is the better measure of overweight or obesity. The most common measure of obesity is still body mass index, which is calculated by dividing weight in kilograms by the square of height in meters (or using this online calculator). Even though BMI can characterize very muscular people (and movie stars) as obese, researchers keep using it because it's less error-prone than taking a tape measure to someone's waist and potentially measuring the wrong part of their belly.

Still, some argue waist circumference would give a truer measure of obesity. And proponents of the metric, such as Dr. Earl Ford of the Centers for Disease Control and Prevention and an author on the study, noted that the new research could mean obesity rates aren't actually plateauing at all.

"The data about obesity in the US, and many countries, has relied mostly on body mass index to characterize obesity," Ford said. According to BMI measures, obesity has leveled off. But Ford and others have been looking at waist circumference, a different way to characterize obesity, and they've found another story.

Particularly concerning, he added, is that women's waist circumferences seemed to increase for every two-year period the researchers looked at. "There seems to be a divergence between men and women," he said. But again, the researchers aren't sure why.

Even if the waistline hypothesis turns out to be a less accurate reflection of the state of our bodies than BMI, Ford said that obesity "is still a serious problem in the US and people need to pay attention to diet and physical activity."

So how do you know if your waist is an unhealthy size?

The size of your waistline is a key health indicator and larger bellies are linked to an increased risk cardiovascular disease, Type 2 diabetes, colorectal cancer, and premature death.

To figure out whether you're at a healthy size, you just need a measuring tape. According to the Mayo Clinic, you want to measure around your bare stomach, just above your hipbone, while relaxing your belly. For women, a waist measure of 35 inches or more is concerning, and for men, it's 40 inches.

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