Americans' Waistlines Are Expanding, And That's Not Good Fat

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If your belt needs to be let out a notch, you're not alone. The average American waistline is growing even though obesity rates haven't grown. And excess abdominal fat increases the risk of heart disease, diabetes and stroke.

The collective American waistline grew by more than an inch from 1999-2000 to 2011-2012, according to a study published Tuesday in JAMA, the Journal of the American Medical Association.

The study results come at a time when the percentage of Americans who are overweight or obese has stabilized. In short, people haven't been getting fatter, but their waistlines are still increasing.

"We're a little bit puzzled for explanations," Dr. Earl Ford, a medical epidemiologist at the Centers for Disease Control and Prevention and lead author of the study, tells Shots. The two measures are closely related: While body mass index or BMI measures fat overall, waist circumference helps measure fat distribution.

Stress, hormonal imbalances, environmental pollutants, poor sleep or medications that help pack on abdominal weight are possible causes, health and nutrition researchers speculate. And older adults typically lose muscle as they age, while fat continues to increase.

Ford and his team used data from the National Health and Nutrition Examination Survey (NHANES), which polled 32,816 adult men and women who were not pregnant. The mean waist circumference of Americans increased from 37.6 inches in 1999-2000 to 38.8 inches in 2011-2012. Men, women, non-Hispanic whites, non-Hispanic blacks and Mexican-Americans showed significant gains.

The prevalence of abdominal obesity (a waist circumference of greater than 40.2 inches or size 44 in men and 34.6 inches or size 12/14 in women) increased from 46.4 percent of the sample in 1999-2000 to 54.2 percent in 2011-2012.

Generally, as waist circumference increases, the amount of belly fat increases, too. But that's not true for everyone. African-Americans tend to have somewhat less abdominal fat for the same waist size than do whites and Hispanics.

Regardless, "people should not only watch their weight, but also their waistline," Frank Hu, a professor of nutrition and epidemiology at the Harvard School of Public Health, tells Shots via email.

Ford believes it's important to continue looking at abdominal obesity to judge how American health is doing.

"I can't really say exactly why BMI has gained favor in our national statistics," he says. "It may have to do that height and weight have been more consistently measured and that waist circumference has been included on some [statistics] but not as many as height and weight have been."