The average American's waistline has expanded over the past decade, with about one inch (2.5 centimeters) of belly fat adding among the general public from 1999 to 2012, researchers observed on Tuesday.
The findings in The Journal of the American Medical Association (JAMA) are the first to show how the nation's obesity epidemic has translated to added paunch in recent years, and the study said the trend toward larger waistlines was "significant."
About one third of American adults -- 34.9 percent or 78.6 million people -- are considered obese, according to the US Centers for Disease Control and Prevention, a condition that raises the risk of heart disease, stroke, cancer and diabetes.
Those figures are based on body mass index, a ratio calculated from weight and height. Recent research has shown no change in obesity rates according to BMI between 2003-2004 and 2011-2012.
But when CDC researcher Earl Ford and colleagues examined data about waist circumference from the same national health registries, dating from 1999-2012, they found that there was a definite trend toward bigger bellies in the United States.
"Our analyses using data from the same surveys indicate that the prevalence of abdominal obesity is still increasing," said the report.
"The reasons for increases in waist circumference in excess of what would be expected from changes in BMI remain speculative, but several factors, including sleep deprivation, endocrine disruptors, and certain medications, have been proposed as potential explanations."
For the purposes of the study, abdominal obesity was defined as a waist circumference greater than 40.2 inches (102 centimeters) in men and greater than 34.6 inches (88 centimeters) in women.
From a pool of nearly 33,000 men and nonpregnant women ages 20 years or older, "the overall age-adjusted average waist circumference increased progressively and significantly, from 37.6 inches in 1999-2000 to 38.8 inches in 2011-2012," said the study.
Men gained on average 0.8 inches and women added 1.5 inches, it said.
"The overall age-adjusted prevalence of abdominal obesity increased significantly from 46.4 percent in 1999-2000 to 54.2 percent in 2011-2012," the study added.
Men and women saw "significant increases," with male abdominal obesity increasing from 37 to 44 percent, and women going from 55 to 65 percent.
The gains were highest among African Americans and Mexican Americans, the researchers found.
"Our results support the routine measurement of waist circumference in clinical care consistent with current recommendations as a key step in initiating the prevention, control, and management of obesity among patients," said the study.