For the study, researchers conducted advanced three-dimensional bone scans on 115 women ages 18 and 19 with normal (less than 32 percent) and high (greater than 32 percent) body fat.
After taking into account the differences in muscle mass surrounding the bone, they found that the bones of participants with high body fat were 8 to 9 percent weaker than those of normal body fat participants.
"Obesity is an epidemic in this country, and I think this study is critical because it highlights another potential negative health effect that people haven't considered," said study co-author Richard D. Lewis, professor of foods and nutrition at the UGA College of Family and Consumer Sciences.
In the study, researchers used a three-dimensional imaging technique that measures both the amount of mineral in the bone and its shape and geometry. The study found that, surprisingly, normal- and high body-fat young adult females have comparable bone strength in a direct comparison that does not account for muscle mass.
"The fact that the two groups had similar bone strength measures is remarkable in itself, because you would expect it to be higher in the heavier person," Lewis said.
Doctoral candidate Norman Pollock, lead author of the study, said that muscle exerts force on bones, which stimulates bone growth. Overweight people tend to have more muscle surrounding their bones than their leaner counterparts, leading most researchers to assume that being overweight is good for bone health.
"When we corrected for the amount of muscle, we found that the obese person is not making as much bone as they should be for the amount of muscle that they have. People haven't observed that in the past because they weren't using the three-dimensional scan," Pollock said.
Lewis said the exact mechanisms by which excess fat hinders bone strength are unclear, but studies of obese rats show that they produce more fat cells in the bone marrow and fewer bone cells. Since fat and bone cells originate from the same precursor, it may be that fat cell production is favoured over bone cell production in obese people.
Researchers studied women, who were 18 and 19, an age at which the bones have stopped growing but before age-related degeneration begins.
Lewis said future studies using three-dimensional bone imaging should follow kids with normal and high levels of body fat through time to see how their skeletons grow. Other researchers have documented increased fractures in overweight kids, suggesting that childhood obesity may be particularly detrimental to bone health.
"When you're young you have the capacity to change the shape of your bones, but when you get older you don't have that capacity. And because of that, childhood obesity could have a significant, long lasting negative impact on the skeleton," Lewis said.
The new study is published in the November issue of the American Journal of Clinical Nutrition.