Unraveling the link between resting metabolic rate and phase angle in facioscapulohumeral dystrophy: a comparative and associative analysis

Background: Resting metabolic rate (RMR) and phase angle (PhA) are muscle-related measures whose relationship has been analyzed in several cohorts. However, such parameters and their association are underexplored in facioscapulohumeral dystrophy (FSHD). Methods: Thirty-nine genetically confirmed and clinically categorized FSHD patients (13 females, 26 males; mean age 37.9 years) and 26 matched healthy controls (HC) were recruited. RMR and PhA were assessed using indirect calorimetry and bioimpedance analysis, respectively. Physical activity level (PAL), as potential confounder, was assessed using the International Physical Activity Questionnaire in HC and a subgroup of 18 patients. Comparative and associative analyses were conducted. For associative analyses, PAL and age were included as covariates, while clinical category [grouping patients into Group 1 (category A, classical phenotype, n = 28) and Group 2 (categories B-D, non-classical phenotypes, n = 11)] and sex were used as stratification factors. Results: Patients showed significantly lower PhA and PAL than HC, but similar RMR. The association was strong in HC, weaker but still significant in the overall FSHD group, and non-significant in the PAL subgroup. As a covariate, PAL weakened the association only in HC, while age weakened the association only in patients. Stratification by sex resulted in loss of significance in both groups, while category stratification showed significant association only in Group 2. Conclusions: While the reduced PhA in FSHD aligns with literature, an RMR comparable to HC contradicts previous findings, suggesting the need for further investigation. Overall, PhA does not emerge as a parameter strongly associated with RMR in patients with FSHD.