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Women with obesity at lower risk for fracture
Women with obesity are less likely to experience a major osteoporotic or hip fracture than women of normal weight, despite a decrease in femoral strength as BMI increases, according to research in The Journal of Clinical Endocrinology & Metabolism.
“The association of higher BMI with lower [major osteoporotic fracture] risk was mainly explained by [bone mineral density] in both men and women,” Jian Shen, MD, PhD, of the bone and mineral unit at Oregon Health and Science University in Portland, and colleagues wrote. “However, the protective effects of higher BMI on hip fracture were stronger and persisted in obese women even after BMD adjustment.”
Shen and colleagues analyzed data from 51,313 women (mean age, 65.9 years; 26% with obesity) and 4,689 men (mean age, 68.7 years; 23% with obesity) aged at least 50 years participating in the Manitoba Bone Density Program in Canada. All participants underwent DXA scans of the hip; researchers derived hip structural parameters for 41,999 women and 4,085 men. Using information from DXA scans, researchers measured cross-sectional moment of inertia (CSMI), cross-sectional area and femoral strength. Height and weight were obtained at time of DXA scan; participants were classified as underweight (BMI < 18.5 kg/m²), normal weight (BMI 18.5-25 kg/m²), overweight (BMI 25-30 kg/m²), obese 1 (BMI 30-35 kg/m²) or obese 2 (BMI > 35 kg/m²). Researchers used hospital records to determine major osteoporotic fracture incidents.
During a mean follow-up of 6.2 years in women and 4.7 years in men, 7.3% of women and 5.9% of men experienced major osteoporotic fractures; 2% of women and 1.6% of men experienced hip fractures. After adjustment for BMD, women with overweight and obesity had a 21% to 33% lower risk for hip fracture vs. normal-weight women. Researchers also observed a lower risk for hip fracture in men with overweight and obesity, but the association did not persist after adjustment for BMD.
In both men and women with a BMI less than 30 kg/m², increasing BMI was associated with progressive increases in femoral neck BMD (r = 0.32 for women and men), cross-sectional area (r = 0.29 for women and men) and CSMI (r = 0.18 for women; r = 0.2 for men), whereas increasing BMI was associated with decreases in femoral strength in women (r = –0.41) and men (r = –0.28).
“Femoral strength index progressively declined as BMI increased in both men and women, suggesting that increments in strength with increased BMI might not be as effective in countering the higher forces involved in a fall in the obese,” the researchers wrote. “Despite those structural and biomechanical disadvantages, obesity was associated with lower fracture risk.”
The researchers did not observe any significant interactions between sex and BMI categories with fracture risk. – by Regina Schaffer
Disclosure: The researchers report no relevant financial disclosures.
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Fracture risk and adiposity have a U-shaped relationship. Individuals who are underweight or obese (BMI of 30 kg/m² or greater) have a greater risk for fracture than individuals with a normal BMI. However, several studies have shown that overweight individuals, up to a BMI of 30 kg/m², have a reduced fracture risk. This study by Shen and colleagues is particularly robust, including 51,313 women and 4689 men. The study included incident fractures in hip, clinical vertebral distal forearm and proximal humerus. The effect of increasing adiposity on protection against fracture was particularly strong in the hip, but only partially explained by the increasing bone density. The higher BMI was not significantly associated with fracture risk in men.
The study also looked at femoral strength index, an estimate of the hip to withstand the impact of trauma, derived from the bone mineral density. The femoral strength index decreased with increasing adiposity and yet the occurrence of fractures in women decreased up to a BMI of 30 kg/m². This suggests that factors other than the bone density are important.
A study by Ishii and colleagues (J Bone Miner Res. 2014;29:2561-2570.) concluded that the protective effect of obesity on hip fracture risk was due to the effect of "soft tissue padding" absorbing impact forces. The authors in this study also acknowledged the importance of increased bone density in these overweight individuals (due to skeletal loading from the adiposity) and also the greater negative effect of impact forces, which occur when the overweight and obese fall.
Gonnelli and colleagues (Clin Cases Miner Bone Metab. 2014;11:9-15.) suggest, as have others, that being overweight may result in increasing bone density because of several factors, such as increased production of estrogen in the fat, as well as increased levels of insulin, adiponectin and leptin, all of which may promote increased bone density. In obesity (BMI of 30 kg/m² or above), inflammatory cytokines from fat (adipokines) may promote increased bone turnover. The authors acknowledge that there are divergent opinions about the importance of these hormones and cytokines in overweight and obese individuals and the impact upon bone health.
This paper another step in understanding the complex relationship between adiposity and fracture risk.