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

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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.

Can Pelvic Widening Explain Away the Extra Pounds?

Aside from providing a new excuse for a widening girth, what does the finding mean? Even Dahners was hard-pressed to come up with anything important to the practice of treating patients with bone problems. "Does it make us fix broken bones differently now? Does it make us put in our special joints differently?" The answer was a resounding no.

"Basically, I just thought this [widening] was interesting to know, but I really don't know where it's going," said Dr. Elton Strauss, an associate professor of orthopedic surgery and geriatrics at the Mount Sinai School of Medicine in New York. "I think this is something that maybe is going to come to light 15 years from now, as they get autopsy specimens or other studies that look inside the bone to give some kind of idea of what the issues are. They really can't come up with a reason as to why the pelvis widens."

Dr. Clifford B. Jones, a clinical professor of orthopedics at Michigan State University, said the widening could come from muscle stress. "You've got muscle attachments to the pelvis, and they all want to pull. That could relatively widen your pelvis due to the fact that your pelvis brim area is going to get wider and wider from these muscles pulling off and stressing." Or maybe it's estrogen. "Men have female hormones, too, to a lesser degree, and that could be causing it," said Jones. "Or it can just be more maturation that you have over the years."

Despite the ambiguity about its cause and implications, a spreading pelvis does not offer a Get Out of Jail Free pass for packing on pounds. "We can say their bones are getting wider, and that's the reason they look wider," Dahners said. "We can't say you're not getting fat, because you might be getting fat, too."

As an explanation for the obesity epidemic, "this [widening] phenomenon explains exactly none of it," said Dr. David Katz, director of the Prevention Research Center at Yale University School of Medicine. "Whatever accounts for these skeletal changes, there is certainly no reason to think it … has occurred only in the last few decades. Presumably, skeletons have always behaved this way, yet epidemic obesity, at ever younger ages, is a new phenomenon. An old cause cannot be used to explain a new effect."

And that's little consolation for most of us.