Critical appraisal of D₃-creatine dilution method for skeletal muscle mass measurement

The D₃-creatine dilution method is gaining rapid momentum as a tool for quantifying skeletal muscle mass (SMM), with emerging discourse positioning it as a reference standard and influencing peer-review and funding decisions. A critical appraisal of the method's current stage of development is therefore timely and necessary. We reviewed the current D₃-creatine dilution method as now widely applied, its foundational assumptions, available validation studies comparing estimates against magnetic resonance imaging (MRI), and sources of measurement error, including variability in the assumed skeletal muscle creatine concentration and isotope spillage correction. Only 4 studies with 102 total adult participants varying in sex (78 males), age, activity level (49 athletes), and health status have evaluated the current D₃-creatine dilution SMM prediction model against MRI as the reference; mean group differences ranged from -13.8% to 20.9%, lending scant support for the validity of the current approach. The one longitudinal validation study (n = 14) reported wide limits of agreement, ±7 kg for D₃-creatine compared with ±1.5 kg for MRI, raising concerns about the method's sensitivity to detect small changes in SMM. Isotope spillage correction algorithms, developed in 1 sample (n = 36), similarly require additional development and validation. Although D₃-creatine dilution has potential as a new approach that can add to the multiple currently available SMM estimation methods, characterization as a reference standard for quantifying SMM is premature. Validation studies across well-defined cross-sectional and longitudinal samples varying widely in sex, age, diet, and health characteristics are needed to establish the D₃-creatine dilution method's accuracy and generalizability.