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Abstract

Tryptophan, an indispensable amino acid, is important during pregnancy because of increased demand for maternal protein synthesis and fetal growth and development. Tryptophan serves as a biosynthetic precursor to the formation of serotonin, and kynurenic acid, which impact physiological and psychological processes, including postpartum depression. Current dietary reference intake (DRI) recommendations, for tryptophan during pregnancy are estimated based on factorial calculations. Tryptophan requirements have never been directly determined during human pregnancy. Our primary objective was to apply the indicator amino acid oxidation (IAAO) method to estimate the dietary requirement of tryptophan during early (11–21 weeks) and late (31–40 weeks) stages of gestation. Our secondary objective was to compare plasma metabolites (tryptophan, kynurenine and serotonin) in response to different tryptophan test intakes and compare between early and late stages of pregnancy. Twenty-eight healthy singleton pregnant females (20–40y) participated in the study. Across both stages, a total of 73 oxidation study days were performed in a repeated measure design. Participants received graded tryptophan intakes (1–14 mg·kg⁻¹·day⁻¹). The diets were provided as isocaloric and isonitrogenous meals that consist of a liquid protein shake with (L-[1–¹³C] phenylalanine) tracer and protein-free cookies. Breath samples were collected at baseline and isotopic steady state and analyzed for tracer (¹³C) enrichment. The estimated average requirement (EAR) of tryptophan was statistically determined using breakpoint analysis on the effect of different tryptophan intakes on breath ¹³CO₂ enrichments with a two-phase linear mixed effect model. Tryptophan requirements were determined to be 7 mg·kg⁻¹·day⁻¹ in early gestation and 8 mg·kg⁻¹·day⁻¹ in late gestation. These values are 40% and 60% higher than the current DRI EAR of 5 mg·kg⁻¹·day⁻¹. For plasma, linear mixed‐effects modeling showed that increasing dietary tryptophan intake was significantly associated with higher plasma tryptophan and kynurenine levels but not serotonin. To the best of our knowledge, this was the first study to use a stable isotope–labeled amino acid technique to determine tryptophan requirement during two stages of pregnancy.