Rectus femoris thickness, SARC-F positivity and obesity in older adults with type 2 diabetes: associations with body composition and mortality

Muscle ultrasound is an accessible method for assessing muscle morphology, but interpretation of absolute muscle thickness may be challenging in older adults with obesity. To evaluate rectus femoris thickness (RFT) in relation to SARC-F-defined sarcopenia risk, obesity, body composition, and mortality in older adults with type 2 diabetes (T2D). Ninety older adults with T2D recruited between May 2022 and June 2024 underwent SARC-F screening, muscle ultrasound, and bioimpedance analysis. SARC-F positivity was defined as a score ≥ 4 and obesity as BMI ≥ 30 kg/m². All-cause mortality was assessed until June 30, 2025, using individual time-to-event follow-up. Sixty participants (66.7%) were SARC-F-positive, 32 (35.6%) had obesity, and 21 participants (23.3%) died during follow-up. Mortality was higher among SARC-F-positive than SARC-F-negative participants (20/60 [33.3%] vs. 1/30 [3.3%]). Time-to-event analyses suggested differences in survival according to SARC-F status; however, sparse events in the SARC-F-negative group and violation of the proportional-hazards assumption precluded robust estimation of an adjusted constant hazard ratio. These mortality findings should be considered exploratory because of sparse events and potential residual confounding. RFT showed significant positive correlations with the BIA-derived Sarcopenia Risk Index ( r = 0.36; p = 0.001), phase angle ( r = 0.23; p = 0.044), and right-leg lean mass ( r = 0.30; p = 0.012). RFT differed across the four SARC-F/obesity phenotypes (Kruskal–Wallis p = 0.013): among participants without obesity, mean RFT was lower in SARC-F-positive than SARC-F-negative participants (0.640 vs. 0.703 cm), whereas among participants with obesity, mean RFT was higher in SARC-F-positive than SARC-F-negative participants (0.840 vs. 0.759 cm). However, the adjusted RFT-by-obesity interaction was not statistically significant (ratio of odds ratios 1.44 per 0.1-cm increase in RFT; 95% CI 0.91–2.27; p = 0.121). RFT showed poor discrimination of SARC-F positivity in participants without obesity (AUC = 0.611) and no meaningful discrimination in the expected direction in those with obesity (AUC = 0.408). SARC-F positivity identified participants with higher observed mortality, although the limited number and distribution of events precluded conclusions regarding independent prognostic value. Absolute RFT alone showed limited discrimination of SARC-F-defined sarcopenia risk. Although RFT differed across SARC-F/obesity phenotypes, formal interaction testing did not demonstrate statistically significant effect modification by obesity, although clinically relevant interaction cannot be excluded. Muscle ultrasound may provide complementary information on regional muscle morphology but should be interpreted alongside functional and body-composition assessment.