INS and obesity due to melanocortin 4 receptor deficiency: In this cross-sectional study, 20 adults with obesity (mean body mass index [BMI] 34.4 ± 2.6 kg/m2, mean age 33 years, 40% female) and 8 healthy, lean adults (mean BMI 22.5 ± 1.4 kg/m2, mean age 25 years, 75% female), all without diabetes or metabolic disease, underwent laboratory testing (HbA1c, 2-hour oral glucose tolerance testing with insulin for Matsuda Index), muscle ultrasound (deltoid, vastus lateralis), and DEXA for sarcopenia indices.