Those Extra Pounds and Inches Are Not Just Fat -- Your Bones Are to Blame Too

Height for most people caps at about age 20, but the hips continue to expand.

ByABC News
June 6, 2011, 2:38 PM

June 7, 2011 — -- Can't zip your jeans, cinch a belt or pull those boy shorts over your hips anymore? Take heart. It doesn't necessarily mean you're getting fatter.

You can now blame your bones.

Most people don't grow any taller after the age of 20, but a recent study published in the Journal of Orthopaedic Research found evidence that the pelvis -- the hip bones -- continues to widen in both men and women up to about age 80, long after skeletal growth is supposed to have stopped.

While it's hardly news that people find themselves to be wider at 40 and 60 than they were at 20, the extra inches were assumed to come from an increase in body fat, said Dr. Laurence E. Dahners, a professor of orthopedics at the University of North Carolina School of Medicine, and lead author of the study.

"You think you're getting fat, but the evidence is your bones are getting wider," Dahners said.

Dahners and his researchers stumbled on the "widening" phenomenon while studying the X-rays of patients in an unrelated bursitis study. "It seemed that the older you got, the wider your pelvis was, which made no sense," Dahners said. "It goes against everything we hear, that young people are bigger and stronger than the older generation."

They decided to investigate further, reviewing the CT scans from 246 randomly selected patients, including about 20 males and 20 females in each 10-year age group from 20-29 and 70-79. They measured the distance between the hip joints, the width of the hip joints, the width of the pelvic inlet (the birth canal opening that sits in the middle of the pelvis) and the height and width of the L4 vertebra, to make sure their patients didn't skew larger and taller to begin with.

"We thought maybe we were having some kind of fluke. Perhaps we just happened to recruit a bunch of old big people and young, small people," Dahners said.

They found that all the widths increased as the patients got older.

The width of the pelvis in the oldest patients was, on average, almost an inch larger than in the youngest patients, which can lead to a 3-inch increase in waist size from age 20 to age 79, and a weight gain of about a pound a year. "Say you wore 30-inch waist pants when you were 20, by the time you were 80, it should be 33-inch pants, even if you didn't gain any fat," Dahners said.

"We found older people really are wider, almost 8 to 9 percent wider," Dahners said. "An almost 10 percent increase in your circumference if you consider yourself to be a cylinder would be enough to explain a big part of a pound a year gain over the age of 20."

The finding can offer comfort -- and insight -- to the weight-afflicted.

"I used to blame the dryer for my tight jeans," said Ruth, 45, who works in advertising. "My friends would say, no, jeans don't shrink that much. It's not the dryer. It must be you. Maybe it is, but now I can say it's my bones."

Paula, 39, a writer, said, "I figured the widening of my hips was water retention, or that I'd put on weight. If it's the bones, now I feel better."

Neither woman wanted her last name used.

And the relief they feel is only natural. "We would like to blame weight gain on big bones, not that we're actually fat," Dahners said.