Their discovery reverses thinking about the effect of fat on bone health.
Previously it was believed that excess body fat actually protected against the brittle bone disease osteoporosis. But the new findings show that fatty tissue wrapped around internal organs can lead to thinning bones in women.
Scientists from Massachusetts General Hospital in Boston measured levels of total, abdominal, and under-skin fat, as well as deep visceral fat surrounding organs in the body, in 50 obese pre-menopausal women.
Women with more visceral belly fat were found to have increased levels of bone marrow fat, and also weaker bones.
However, there was no significant correlation between either under-skin fat or total fat and bone mineral density.
"We know that obesity is a major public health problem," said lead researcher Dr Miriam Bredella.
"Now we know that abdominal obesity needs to be included as a risk factor for osteoporosis and bone loss.
"Our results showed that having a lot of belly fat is more detrimental to bone health than having more superficial fat or fat around the hips.
"It is important for the public to be aware that excess belly fat is a risk factor for bone loss, as well as heart disease and diabetes."
The findings were presented today at the annual meeting of the Radiological Society of North America in Chicago.
Dr Bredella's team is now looking to see if belly fat also affects osteoporosis in men.