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Total body protein in healthy adolescent girls: validation of estimates derived from simpler measures with neutron activation analysis^1,2,3

¹ From the Departments of Adolescent Medicine (VKH and MRK), Nuclear Medicine (JNB), and Psychological Medicine (SM), and the James Fairfax Institute of Pediatric Nutrition (JRA, MG, SZ, and KJG), the Children's Hospital at Westmead, Sydney, Australia; the Centre for Research into Adolescent's Health, Westmead Hospital, Sydney, Australia (SDC); the University of Sydney, Sydney, Australia (KJG); the Meridian Clinic, Sydney, Australia (MRK); and the Institut für Humanernährung und Lebensmittelkunde der Christian–Albrechts–Universität zu Kiel, Kiel, Germany (VKH and MJM)

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.

Key Words: Body composition • adolescents • total body protein • total body nitrogen • fat-free mass • neutron activation analysis • dual-energy X-ray absorptiometry • anthropometric measures • bioimpedance analysis