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 δ¹⁵N; (3) the reliability of trophic position (TP) estimates from phenylalanine (Phe) and glutamate (Glx) δ¹⁵N 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 δ¹⁵N 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 δ¹⁵N 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 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.