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

Objectives Evaluate the effectiveness of compound‐specific nitrogen isotope analysis of amino acids (CSIA‐AA) in reconstructing early childhood diets and detecting episodes of stress. Examine (1) proline's potential for identifying breastfeeding and weaning; (2) the influence of physiological and pathological stress on AA δ 15 N; (3) the reliability of trophic position (TP) estimates from phenylalanine (Phe) and glutamate (Glx) δ 15 N during dietary transitions; and (4) mother‐infant trophic enrichment factors (TEF Glx‐Phe ) as indicators of infant dietary protein quality. Materials and Methods Three mother‐infant dyads provided fingernail clippings ( n = 43) for CSIA‐AA analysis pre‐ and post‐birth, alongside dietary and health surveys. Results Proline δ 15 N was elevated by 2.4‰–3.5‰ in exclusively breastfed infants compared to their mothers and decreased by 2.2‰–4.1‰ during weaning. Phenylalanine δ 15 N showed large positive shifts (e.g., by 6.7‰) during maternal stress, despite being a source AA expected to remain stable. TP differences between mother‐infant pairs were minimal (−0.2 to 0.1), except for one pair with higher infant TP (by 0.6–1.5). The calculated TEF Glx‐Phe for infants ranged from −1.4‰ to 11.3‰. Discussion Proline δ 15 N reliably tracks nutritional transitions, likely due to its role in arginine synthesis during infancy. The unexpected variability in δ 15 N 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 δ 15 N Phe .