Beyond BMI: Cellular Integrity, Low Muscle Mass and Their Association with Disease Duration in Adults with Type 1 Diabetes—A Cross-Sectional Study

Background: In adults with type 1 diabetes (T1D), routine anthropometric assessment may not detect deterioration in muscle quality and cellular integrity. Body mass index (BMI) remains useful as a first-line screening tool but cannot distinguish lean mass, fat mass and tissue quality. This study aims to describe bioimpedance-derived cellular integrity, apply the full diagnostic algorithm of the European Working Group on Sarcopenia in Older People (EWGSOP2), compare two bioimpedance muscle-mass equations, and examine the associations of disease duration and long-term glycemic exposure with muscle-related outcomes. Methods: This cross-sectional study included 193 adults with T1D recruited by consecutive sampling at the outpatient clinics of two hospitals. Participants underwent phase-sensitive 50 kHz bioelectrical impedance analysis (BIA), handgrip dynamometry and chair stand testing, and historical HbA1c and continuous glucose monitoring data were retrieved. Low muscle mass was estimated using the Janssen and Sergi equations with predefined sex-specific thresholds, and the complete EWGSOP2 algorithm was applied to distinguish low muscle mass from probable, confirmed and severe sarcopenia. Multivariable ordinary least squares models were adjusted for age and sex and fitted separately to preserve the largest complete-case sample for each outcome, with Benjamini–Hochberg correction for multiple testing. Results: Estimated low muscle mass differed markedly by equation, from 28.4% (Janssen) to 8.7% (Sergi); probable sarcopenia, defined by low handgrip strength, was present in 8.5%, whereas confirmed sarcopenia occurred in 2 of 167 participants (1.2%) and severe sarcopenia in none. The 16 participants identified by the Sergi equation were a strict subset of the 52 identified by the Janssen equation. Longer T1D duration was associated with a lower phase angle (β = −0.018 per year; 95% CI −0.028 to −0.009; and p < 0.001), independently of chronological age. Historical HbA1c was not associated with the appendicular skeletal muscle mass index (β = +0.092; p = 0.190) but was associated with lower handgrip strength (β = −1.67 kg per 1%; p = 0.013), a longer five-repetition chair stand time (β = +0.71 s per 1%; p = 0.004) and a lower muscle quality index (β = −0.064 per 1%; p = 0.025). Conclusions: In long-standing T1D, disease duration was associated with impaired cellular integrity and cumulative glycemic exposure with reduced muscle function and quality rather than with reduced muscle quantity. Estimated prevalence of low muscle mass varied more than threefold with the equation applied, while confirmed sarcopenia was rare. These cross-sectional findings suggest that BIA-derived muscle-mass equations should not be used in isolation and support anchoring morphofunctional assessment in measured muscle strength, complemented by phase angle.