Loosen your belts: U.S. waist sizes keep expanding

If your pants feel tight, you are not alone. The size of the average American waist just keeps getting bigger, new research shows.

The waists of men, women, blacks, whites and Mexican Americans all grew significantly from 1999 to 2012, researchers from the federal Centers for Disease Control and Prevention reported Tuesday in the medical journal JAMA.

The average man's waist grew from 38.9 inches to 39.7 inches; the average woman's belly expanded even more, from 36.3 inches to 37.8 inches.

Those are just averages. The study shows that more American guts are official health hazards – in an "abdominal obesity" zone that starts at 40.2 inches for men and 34.6 inches for women. Bellies that big are linked with increased risks of diabetes, heart disease and premature death.

The new study shows 43% of men and 64% of women are in that zone. That's up from 37% of men and 55% of women in 1999.

All of this expansion, which the CDC is documenting for the first time since 2008, happened even as overall adult obesity appeared to level off.

"The increase in abdominal obesity in the U.S. population is very worrisome," says Frank Hu, professor of nutrition and epidemiology at the Harvard School of Public Health. He was not involved in the new research.

"The total amount of body fat is important, but the location of the fat is also important," he says. Abdominal fat is particularly dangerous, he says, even in people who are not otherwise obese.

Government researchers using tape measures recorded the waist sizes of 32,816 men and women age 20 and older for the new study. One limitation, they said, was that they did not collect data on specific Hispanic groups and had no data on Asian Americans before 2011.

It's not clear why waists are expanding faster than BMI (body mass index – the standard measure of overall obesity), the report says. Other researchers have suggested that everything from sleep deprivation to medication side effects may lead extra fat to collect around mid-sections, says the CDC's Earl Ford, lead author of the paper.

Hu says high-sugar diets and stress hormones also might be at work. He says weight training, in addition to diet, can help many people control their bulges.

Ford agrees: "People just have to pay a lot more attention to their lifestyles."

The new numbers come as no surprise to Ed Gribbin, president of Alvanon, a company that conducts body measurement research for clothing retailers.

He says Alvanon's research, based on hundreds of thousands of body scans, shows similar trends, though it finds women's waists a bit slimmer than the CDC does.

Gribbin says he tells his clients it's a fact of American life that waists get bigger, especially as people age.

He says anyone can see the result.

"Just go to the mall and watch people go by," he says. "There's some belly spilling over the waistband in almost everybody."