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updated 11/30/2010 1:07:21 PM ET 2010-11-30T18:07:21

Body fat, long thought to protect against osteoporosis, may instead promote the disease, according to a new study. Specifically, high levels of belly fat may hurt bone health and increase women's risk for developing the bone-weakening disease, the researchers said.

The results showed visceral fat — the fat located between organs in the abdominal cavity — was associated with reduced bone-mineral density in obese women. In contrast, fat under the skin in other places in the body, called subcutaneous fat, did not demonstrate such a link.

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This finding was "shocking," said study researcher Dr. Miriam Bredella, a radiologist at Massachusetts General Hospital in Boston, "because obese women at 30 should have totally normal bones."

A low density of minerals in the bones is known to increase the risk for osteoporosis, a condition characterized by weak bones that are prone to fracture.

"There is more bad news for obesity," Bredella said. "The one good thing about obesity was that it protects against bone loss, but it actually does not always."

The new study is being presented today (Nov. 30) at the annual meeting of the Radiological Society of North America in Chicago.

"Bad" fat vs. "good" fat
Previous studies that have looked at the effect of obesity on bone health have not distinguished between the two types of body fat — visceral and subcutaneous. And underweight women are known to have an increased risk for osteoporosis, so "it has been assumed that the heavier you are, the better your bones are," Bredella said.

Bredella and her colleagues examined the bone-mineral density and body mass index (BMI) of 50 obese women whose average age was 30.

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The women underwent a computed tomography (CT) scan to measure their bone loss, and magnetic resonance (MR) spectroscopy was used to evaluate the amount of fat in the marrow of their bones. Both scans focused on the lumbar section of the spine, a region of the lower back that is thought to be a good indicator of bone-mineral density and bone marrow fat in other parts of the body. The researchers also measured the women's amount of belly fat.

Belly fat and osteoporosis
In general, the more visceral fat the women had, the lower their bone-mineral density was. While none of the women had osteoporosis, the bone-mineral density for some was below normal, meaning they had a condition known as osteopenia that may lead to osteoporosis.

And obese women with more visceral fat also had more fat in their bone marrow, Bredella said. This suggests fat in the bones makes them weaker, she said.

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The researchers aren’t sure why visceral fat in particular appears has this effect. But it could be that this fat secretes certain hormones or other substances that harm bones, Bredella said.

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The results add to a growing body of work that suggests visceral fat is worse for your body than subcutaneous fat. Previous studies have linked belly fat with an increased risk of Type 2 diabetes and heart disease.

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Follow MyHealthNewsDaily staff writer Rachael Rettner on Twitter @Rachael_MHND.

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