Total body protein in healthy adolescent girls: Validation of estimates derived from simpler measures with neutron activation analysis

Background: Little recent and accurate information about body protein content in healthy adolescent girls is available. Objective: The objective was to assess the total body nitrogen (TBN) and total body protein (TBPr) contents of fat-free mass (P: FFM) in a group of healthy adolescent girls and to validate previously published TBN prediction equations. Design: TBN was measured with in vivo neutron activation analysis (TBN_NAA). Bone mineral density and FFM were measured with dual-energy X-ray absorptiometry (FFM_DXA), total body water and FFM were measured with bioimpedance analysis, and FFM was assessed by measuring skinfold thicknesses in 51 girls with a mean (±SD) age of 14.7 ± 0.7 y. The validity of the TBN prediction equations was assessed with Bland-Altman analysis. Results: TBN_NAA in our adolescent group was higher (1.49 kg) than values reported in earlier studies of women (1.25 and 1.31 kg), and P:FFM was slightly higher (23%) than that documented in adults (19-21%). Previously published TBN equations showed either systematic bias or wide limits of agreement. Conclusion: A predictive equation derived from the present study population based on FFM_DXA improves the prediction of TBN for groups of young girls but may not be helpful for individuals in clinical settings.