Americans' belt size continues to inch up, and women's waistlines are widening faster than men's, according to new government research.
The average waist size ballooned more than an inch -- from 37.6 inches to 38.8 inches -- between 1999 and 2012, U.S. Centers for Disease Control and Prevention researchers found.
Based on their waist circumference, 54 percent of Americans were abdominally obese in 2012, up from 46 percent 13 years earlier.
"Waists are still expanding in the U.S., and particularly so among women," said study researcher Dr. Earl Ford, a medical officer at the CDC.
While men's waists increased less than an inch -- about 0.8 of an inch on average -- women's midriffs grew about twice that, or 1.5 inches, Ford said.
Waistlines larger than 35 inches for women and more than 40 inches for men are considered abdominal obesity, a risk factor for heart disease and diabetes.
Waist circumference is a simple tool that reflects the amount of total body fat and intra-abdominal body fat. Like body mass index (BMI), which is a calculation based on height to weight, it is used to predict heart disease risk.
For the study, published Sept. 17 in the Journal of the American Medical Association,
Ford and his colleagues used data from the National Health and Nutrition Examination Survey (NHANES) on nearly 33,000 men and women ages 20 and older.
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The finding that waist sizes are still increasing was surprising, Ford said, since the prevalence of obesity seems to have reached a plateau recently. Even so, more than one-third of U.S. adults are obese, according to the CDC.
No single reason for the bulking up stood out. But the researchers speculated that sleep disruption, certain medications and everyday chemicals known to be endocrine disruptors may possibly play a role.
Why women's waist sizes are enlarging more than men's isn't clear, Ford said.
Connie Diekman, director of university nutrition at Washington University in St. Louis, said the study "seems to reflect a few things which we know contribute to body fat gain."
"As baby boomers age, the natural decrease in muscle mass and slowing metabolism leads to more body fat. In addition, given the size of the baby boomer generation, these aging changes will impact statistics," said Diekman, who wasn't involved in the study.
Diekman agreed that insufficient sleep may be partly to blame. Inadequate activity is probably a factor, too, as it leads to body fat gain, she said.
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Diekman reminds her clients that getting at least five or six hours of sleep nightly may help regulate hunger and help prevent more weight gain. Also, current guidelines call for at least 150 minutes of physical activity a week, "a figure few Americans achieve," she said.
Weight loss would be the main strategy to reduce waist size, Ford said.
Children and teens in the United States may be faring better, according to another study published in July 2014. For that report, University of Minnesota researchers used the same NHANES data and found the proportion of children ages 2 to 18 classified as obese based on waist size held steady at nearly 18 percent from 2003 through 2012.