Exploring Early Childhood Diet, Stress, Trophic Position and Dietary Protein Quality Using Amino Acid Nitrogen Isotope Compositions of Fingernail Keratin

Proline δ¹⁵N reliably tracks nutritional transitions, likely due to its role in arginine synthesis during infancy. The unexpected variability in δ¹⁵N_Phe complicates its use in TP and dietary protein quality assessments. This variability may result from phenylalanine's slow turnover and delayed dietary incorporation during endogenous catabolism. TP is an unreliable marker of breastfeeding or weaning. TEF_Glx-Phe for infants seems indicative of high dietary protein quality, but interpretations must consider the influence of non-dietary factors on δ¹⁵N_Phe.