Washington ? It isn’t just the thunder thighs that shrink after obesity surgery. Melting fat somehow thins bones, too.

Doctors don’t yet know how likely patients’ bones are to thin enough to break in the years after surgery. But one of the first attempts to tell suggests they might have twice the average person’s 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 may not be enough.

Here’s the irony: Obesity actually is considered protective against bone-weakening osteoporosis, possibly the only positive thing you’ll ever hear a doctor say about too much fat.

The big question is whether patients 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.