Skeletal muscle mass can be estimated by creatine (methyl-d3) dilution and is correlated with fat-free mass in active young males

Abstract

Background: Assessing whole-body skeletal muscle mass (SMM) and fat-free mass (FFM) is essential for the adequate nutritional management and training evaluation of athletes and trained individuals. This study aimed to determine the relationship between SMM assessed using the creatine (methyl-d₃) dilution (D₃-creatine) method and SMM estimated by whole-body magnetic resonance imaging (MRI) in healthy young men undergoing exercise training. Additionally, we examined the association between FFM measured using the four-component (4C) method (FFM_4C) and the total body protein value estimated using 4C (TBpro_4C).

Methods and results: We analyzed the data of 29 males (mean age, 19.9 ± 1.8 years) who exercised regularly. SMM measurements were obtained using the D₃-creatine method (SMM_D3-creatine) and MRI (SMM_MRI). The SMM_D3-creatine adjusted to 4.3 g/SMM kg was significantly higher than SMM_MRI (p < 0.01). The fit of the creatine pool size compared with SMM_MRI was 5.0 g/SMM_MRI kg. SMM_MRI was significantly correlated with both SMM_D3-creatine adjusted to 4.3 g/kg and 5.1 g/kg. TBpro_4C was significantly lower than SMM_MRI (p < 0.01). Contrastingly, FFM_4C was significantly higher than SMM_MRI (p < 0.01).

Conclusions: SMM_D3-creatine adjusted to 4.3 g/SMM kg-a previously reported value-may differ for athletes and active young males. We believe that a value of 5.0-5.1 g/SMM kg better estimates the total muscle mass in this population. Traditional FFM estimation highly correlates with SMM_MRI in well-trained young males, and the relationships appear strong enough for total body protein or SMM to be estimated through the FFM value.