One benefit of obesity, conventional wisdom has held, is that it protects women against osteoporosis and fractures.
But a study by researchers at Columbia University has found that abdominal fat is associated with poor bone quality and lower bone formation in premenopausal women.
“There’s been a real change in the way we think about bone and obesity,” says Adi Cohen, MD, MHS, Florence Irving Assistant Professor of Clinical Medicine and a member of the Metabolic Bone Diseases Program at Columbia University Medical Center.
“It is true that larger people tend to have stronger bones, but studies that separate fat mass from muscle mass show that fat is associated with lower, not higher, bone density and that fracture risk may be higher in obese individuals.”
The reason why obesity is associated with lower bone density is unclear, says Cohen, because previous studies looked at bone only with imaging or indirect markers of bone formation. Some of those studies suggested that fat releases inflammatory molecules that increase bone degradation. Others suggested that obesity reduces new bone formation.
Bone biopsy, which is the most direct way of looking at bone formation and degradation, was used in Cohen’s most recent study, published in the June issue of the Journal of Clinical Endocrinology and Metabolism.
“We found that women with the most abdominal fat had the worst bone structure.”
In the study, 40 premenopausal women with BMIs ranging between 20 and 40 kg/m2 underwent bone biopsy and a bone density scan that also measured abdominal fat.
“We found that women with the most abdominal fat had the worst bone structure,” Cohen says. Although all women had normal bone density scans, women with the most abdominal fat had bone-biopsy evidence of thinner, less stiff bone in the inner trabecular compartment and more porous bone in the outer cortical compartment.
The bone biopsies also showed that higher abdominal fat was associated with markedly lower bone formation.
“What we have here, for the first time, is tissue-level evidence supporting the new hypothesis that fat is bad for bone, at least at certain times of life,” Cohen says.
But She cautions that researchers still don’t know what effect this will have on women after menopause (when bone degradation accelerates) or if losing weight will benefit bone health.
“This is just the beginning of the research we need to conduct to answer those questions.”
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