Application of the COSIAM method for muscle protein turnover and skeletal muscle mass in young females: A comparison of methods for body composition assessment

Evaluating rates of myofibrillar protein synthesis (MyoPS) and breakdown (MyoPB) is essential for understanding muscle protein turnover, which underpins changes in skeletal muscle mass (SMM). For the first time, we applied the Combined Oral Stable Isotope Assessment of Muscle (COSIAM) method in a group of young, healthy females. This protocol allows simultaneous measurement of MyoPS, MyoPB, and SMM using deuterium oxide, D 3 -3-methylhistidine, and methyl-D 3 -creatine (D3-Cr), respectively. Additionally, we evaluated the agreement between D3-Cr-derived SMM and several SMM proxies, to determine the bias of each method relative to D3-Cr-derived SMM. Using the three-day COSIAM protocol, we assessed MyoPS, MyoPB and SMM in a cohort of twenty-one healthy, females (age: 22 ± 2 years, BMI: 24.7 ± 3.5 kg/m 2 ). Bioelectrical impedance (BIA), DXA and BodPod were used to estimate SMM, lean mass (LM), and fat-free mass (FFM), respectively. BIA, DXA, and ultrasound (US) were also used to assess appendicular lean mass (ALM), while US was additionally used to measure vastus lateralis cross-sectional area. Rates of MyoPS (1.97 ± 0.29 %/d) and MyoPB (rate constant [k] of 0.045) aligned with previous literature. DXA-derived LM, BIA-derived SMM, and BodPod-derived FFM overestimated D3-Cr-derived SMM by 20.1 ± 3.6 kg, 4.0 ± 2.2 kg, and 22.8 ± 4.1 kg, respectively. Conversely, DXA, BIA, and US-derived ALM uniformly underestimated D3-Cr-derived SMM by approximately 2.50 kg. Our findings suggest that the COSIAM method is a viable approach for assessing muscle protein turnover in young, healthy females, and provides reference values for future research. Additionally, this study reveals the degree to which commonly used proxy measures of SMM, overestimate or underestimate D3-Cr-derived SMM. These insights support the application of COSIAM in future female-focused research and advance our understanding of biases in body composition assessments for young females.