WASHINGTON - It isn't just thunder thighs that shrink after obesity surgery. Bones somehow get thinner, too.

Doctors don't yet know how likely patients' bones are to break in the years after stomach surgery. But one of the first attempts to tell suggests patients might have twice the average risk, and be even more likely to break a hand or foot.

The Mayo Clinic's finding is surprising, and further research is under way to see if the link is real. But with bariatric surgery booming and even teenagers in their key bone-building years increasingly trying it, specialists say uncovering long-term side effects and how to counter them takes on new urgency.

Simply popping today's doses of calcium supplements might not be enough.

"These procedures are now being sold as a panacea," Shonni Joy Silverberg of Columbia University told last week's annual meeting of the Endocrine Society in Washington. "It is of heightened importance to find the answers to these questions."

Here's the irony: Obesity actually is considered protective against bone-weakening osteoporosis.

"They're starting better than most of us," cautions study coauthor Jackie Clowes, a Mayo bone-metabolism expert. So the big question is whether people who get the surgery really end up with worse bones, or just go through a transition period as their bones adjust to their new body size.

About 15 million Americans are classified as extremely obese: 100 pounds or more overweight. Dieting often isn't enough to fend off diabetes and other problems.

Surgery is fast becoming the preferred option - from the stomach stapling called gastric bypass to less invasive stomach banding. Patients tend to lose 15 percent to 25 percent of their original weight, and diabetes dramatically improves.

More than 1.2 million U.S. patients have undergone the surgery in the last decade, 220,000 in the last year alone, according to the American Society for Metabolic and Bariatric Surgery.

There are little data yet on how patients fare years later. But doctors have long noted that radical weight loss can speed bone turnover until the breakdown of old bone outpaces formation of new bone.

Silverberg cites recent studies showing that one year after gastric bypass, adults' hip density drops as much as 10 percent, raising concern about a common fracture site of old age. (Stomach banding causes less thinning because it does not alter nutrient absorption as much.)

To see if such changes translate into fractures, the Mayo team is comparing the medical records of nearly 300 adults who have had bariatric surgery with similarly aged Minnesotans who have not.

A quarter of the 142 surgery patients studied so far experienced at least one fracture in the following years, Mayo's Elizabeth Haglind told the endocrinology meeting. Six years after surgery, the group had twice the average risk.

But in a puzzling finding, the surgery recipients had three times the risk of hand and foot fractures than their Minnesota neighbors.

Those fractures are not usually connected to osteoporosis. Did the once-obese start exercising and simply fall down more? Clowes doubts it.

"I was shocked" at the numbers, says Scott Shikora, president of the bariatric surgeons group, who says he has not seen a significant fracture problem in his practice.

Surgeons routinely tell patients to take some extra calcium and Vitamin D. Shikora estimates about half follow that advice, and other research suggests higher doses may be needed anyway.

A key next step will be to compare the patients who have fractures with others of the same weight to see if their bone mass just had to adjust - or if something about surgery alters the complex mix of hormones and other factors that keep bones strong.

For now, Clowes' advice for those who have had surgery: Don't skip checkups, where doctors monitor bone health. And treat nutrient deficiencies aggressively.