Published: Apr 12, 2013
By Nancy Walsh , Staff Writer, MedPage Today

Action Points

• Older women with low abdominal fat stores were at substantially higher risk for fractures, particularly of the vertebrae.
• Note that fracture risk among both women and men was significantly greater with older age, previous fracture, history of falls, and lower bone mineral density.

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Older women with low abdominal fat stores were at substantially higher risk for fractures, particularly of the vertebrae, an Australian prospective study showed.

Each 1-kg decrease in abdominal fat mass among older women was associated with a 50% increase in risk of any fracture after adjusting for factors that included age, bone mineral density (BMD), and physical activity (HR 1.50, 95% CI 1.10 to 2.05), according to Tuan V. Nguyen, MD, of the University of New South Wales in Darlinghurst, and colleagues.

The highest risk was for vertebral fractures (HR 1.96, 95% CI 1.22 to 3.13), the researchers reported online in the Journal of Clinical Endocrinology & Metabolism.

“Although the association between fat mass and BMD has been extensively investigated, there have been few data on the relationship between fat mass and fracture,” they noted.

To examine this, and particularly to see if the risk of fracture was independent of BMD, Nguyen’s team analyzed fracture incidence among 766 women and 360 men enrolled in a population-based study of osteoporosis.

Participants underwent whole body scanning with dual energy X-ray absorptiometry, and lean tissue mass, fat mass, and BMD were calculated.

Mean age was 71. During a mean follow-up period of almost 5 years, 14% of the women and 5% of the men had fractures not caused by major trauma.

Fracture risk among both women and men was significantly greater with older age, previous fracture, history of falls, and lower BMD.

For women, risk also was elevated in those with less whole body fat mass and physical activity.

A total of 49 fractures in women involved the vertebrae and 58 were at other sites. For men, the corresponding numbers were 12 and seven.

In an unadjusted analysis, greater abdominal fat mass was associated with higher BMD, with linear correlations being seen at the femoral neck for both women (r = 0.29) and men (r= 0.14).

Similar correlations were found for the lumbar spine, again in both women (r = 0.24) and men (r = 0.14), with P values of 0.01 for all.

After adjustment for factors including age, smoking, and prior fracture, the association between abdominal fat mass and BMD remained significant at the lumbar spine and femoral neck (P<0.001) in women, though not for men.

The researchers also compared risks according with tertile of abdominal fat mass, and found that women in the highest tertile had significantly lower risk than those in the bottom tertile for any fracture (HR 0.49, 95% CI 0.30 to 0.80) and vertebral fracture (HR 0.41, 95% CI 0.20 to 0.85).

Among the possible reasons why body fat might be protective against fracture are its “padding” effect in falls, mechanical loading on bone, which can upregulate formation of new bone, and various metabolic and hormonal effects of fat on bone.

“This protective effect, together with the increase in obesity prevalence in recent years, might partly explain the reduction in fracture incidence which has been observed in many parts of the world,” the researchers commented.

However, the same abdominal fat that is associated with lower risk of fracture also has been linked with diabetes and heart disease, they noted.

“Although the findings have extended our understanding of the relationship between fat mass and osteoporosis, further research is necessary in order to elucidate underlying causes of the relationship,” they concluded.

Limitations of the study included its observational design and the small number of fractures in men, leaving uncertainty as to their risk.