Decoupling Muscle Mass and Quality: Insights from Adult Body Mass Index Trajectories via Growth Mixture Modeling

Sarcopenia is characterized by a decline in muscle mass and quality and has emerged as a life-threatening aging-related syndrome. However, the association between adult weight trajectories and muscle health remains unclear.

Using data from 1,007 adults aged ≥50 years from the United States. National Health and Nutrition Examination Survey (NHANES) 2011-2014 cycles, a cross-sectional analysis of body mass index (BMI) trajectories across four adult life stages was conducted using growth mixture modeling. Dual-energy X-ray absorptiometry (DXA)-derived appendicular skeletal muscle mass index (ASMI) and muscle quality indices, including the appendicular muscle quality index (MQI.app) and total muscle quality index (MQI.total), calculated as grip strength per unit of muscle mass, were used to assess muscle health. Multivariate regression analysis was applied to evaluate the associations between BMI trajectories and muscle outcomes.

Three BMI trajectories were identified: stable normal (38.5%), rapid increase (16.5%), and stable high (45.0%). The ASMI was greater, whereas the MQI was markedly lower in the rapid increase group than in the stable normal group, with odds ratios (ORs) of 11.38 (95% CI: 6.18-20.98) for the extremely low MQI.app category and 10.07 (95% CI: 5.85-17.11) for the extremely low MQI.total category. The stable high group also showed increased odds of lower MQI categories.

BMI trajectory was heterogeneously associated with muscle phenotype. Static indicators such as BMI and the ASMI alone may not fully capture functional risk. These findings highlight the need for further research on muscle quality and long-term weight dynamics, and for prospective studies before any clinical screening or intervention implications can be considered.