The usefulness of total body protein mass models for adolescent athletes

The present study aimed to assess the utility of a less laborious technique for estimating total body protein (TBPro) in young athletes, using a multicomponent model as the criterion method. A total of 88 (49 boys and 39 girls) adolescent athletes (age: 15.2 ± 1.5 years; body mass index: 21.2 ± 2.7 kg/m²) participated. A 6-compartment model was used as the reference method (TBPro_Reference) involving air displacement plethysmography for body volume, dual-energy X-ray absorptiometry (DXA) for bone mineral content, and deuterium dilution for total body water (TBW). Alternatively, DXA TBPro models were used as TBPro = lean-soft mass (LSM) - HF_FFM × fat-free mass (FFM) - Ms. - G, where LSM and FFM were assessed using DXA, HF_FFM is the hydration fraction of the FFM using measured TBW or assumed TBW (adult fraction of 0.732; Lohman's constants or mean observed HF_FFM), Ms. is soft tissue minerals (Ms = 0.0129 × HF_FFM × FFM), and G is glycogen calculated as 0.044 × (LSM - HF_FFM × FFM - Ms). The maturation level was determined by self-assessment. TBPro obtained from DXA using the assumed HF_FFM explained 73% to 77% of the variance compared to TBPro_Reference. Meanwhile, using the mean values of measured HF_FFM, the DXA model explained 53 and 36% for boys and girls, respectively. Larger bias (8.6% for boys and 25.8% for girls) and limits of agreement were found for the DXA model using measured HF_FFM (boys for 66.9% and girls for 70%) compared to an assumed HF_FFM (bias ranged from 1.5% to 22.5% and limits of agreement ranged from 31.3% to 35.3%). Less complex and demanding TBPro DXA models with the assumed HF_FFM are valid alternatives for assessing this relevant FFM component in groups of adolescent athletes but are less accurate for individual results. Though future studies should be conducted to test the usefulness of these models in longitudinal and experimental designs, their potential to provide an estimation of protein mass after exercise and diet interventions in young athletes is anticipated.