Muscle mass, not fat mass, predicts vertebral fracture risk: Vietnam osteoporosis study

The association between body weight and vertebral fracture is established, but whether muscle mass or fat mass drives this relationship remains unclear. In Vietnamese adults, our study demonstrates that muscle mass, rather than fat mass, predicts vertebral fracture risk. Both low and excessive muscle mass significantly increase this risk, indicating a U-shaped relationship.

The purpose is to examine the relationship between various body composition measures and vertebral fracture in a Vietnamese population.

This cross-sectional study included 1,372 Vietnamese adults aged ≥ 50 years from the Vietnam Osteoporosis Study. Body composition was measured using dual-energy X-ray absorptiometry to derive lean body mass index (LBMI), appendicular skeletal muscle mass index (ASMI), body fat mass index (BFMI), and percentage-based indices. Vertebral fractures were identified via thoracolumbar radiographs using Genant's semiquantitative method. Adjusted logistic regression models and restricted cubic splines were used to assess associations and explore nonlinear relationships.

Vertebral fractures were diagnosed in 173 participants (12.6%). Compared with those without fractures, affected participants were older (63.4 ± 9.6 vs 58.8 ± 7.3 years; p < 0.001) and more often male (38.7% vs 29.4%; p = 0.017). Both LBMI and ASMI showed U-shaped associations with fracture risk, with higher odds at the extremes; for ASMI, the lowest and highest quartiles had increased odds of fracture (OR 1.72; 95% CI 1.02-2.95 and OR 2.59; 95% CI 1.36-5.02, respectively). In contrast, BFMI was not significantly associated with fracture risk (p = 0.365). Higher trunk lean percentage was consistently related to fracture (ORs 1.82-2.54 across non-reference quartiles), whereas total body and leg lean percentages were associated with increased risk only in the highest quartile (OR 2.18; 95% CI 1.07-4.45 and OR 2.06; 95% CI 1.03-4.12).

In Vietnamese adults, extremes of muscle mass (low and high), rather than fat mass, are associated with higher vertebral fracture risk. This emphasizes incorporating muscle mass evaluation into fracture risk assessment beyond reliance on BMI alone.