Determining dietary lysine requirements in healthy lactating women using the indicator amino acid oxidation method

Background: Lysine is the first limiting indispensable amino acid (IAA) in cereal-based proteins. The 2019 revision of the Canada Food Guide also recommends prioritizing plant proteins. Therefore, understanding lysine needs during critical life stages, such as lactation is necessary. Current dietary reference intake (DRI) recommendations for lysine during lactation were factorially calculated by adding human milk amino acid content with determined adult requirements that were primarily derived from males. This approach inadequately addresses physiological differences during lactation. Objectives: (1) To directly determine maternal dietary lysine requirements in early lactation, when infants are exclusively breastfeeding at 3 – 4 months old and in late lactation, when infants are complimentary feeding at 9 – 10 months old; and (2) to compare the dose-response characteristics of urinary and plasma lysine metabolites to the graded test lysine intakes. Methods: Using the indicator amino acid oxidation (IAAO) method, nineteen healthy lactating participants were studied in early lactation or late lactation. Across both stages, a total of 76 study days were completed in a repeated measures design. Participants were randomly assigned lysine intakes between 12 to 90 mg·kg-¹·day-¹. Eight hourly isocaloric (1.7 × resting energy expenditure) and isonitrogenous meals were provided with protein at 1.5 g·kg-¹·day-¹. The estimated average requirement (EAR) for lysine was identified by applying a two-phasic linear mixed-effects analysis to F¹³CO₂ from L-1-¹³Cphenylalanine oxidation. Urine and plasma was also collected from participants for metabolite analysis to assess the association between dietary lysine and lysine metabolites using linear-mixed effects models. Results: Dietary lysine requirements during both lactating phases were determined to be 63.0 mg·kg-¹·day-¹ (Early: R²c = 0.85 ,Upper 95% CI = 69.9; Late: R²c = 0.91 ,Upper 95% CI = 73.4). Additionally, plasma and urine concentrations of lysine and α-aminoadipic acid were positively associated with dietary lysine, and urinary excretion was greater in late lactation despite similar plasma levels. Conclusions: Using the minimally invasive IAAO method, we directly and safely determined dietary lysine requirements in lactating women at two stages of lactation for the first time. Lysine requirements during lactation are ~50% higher than the current DRI EAR of 42 mg·kg-¹·day-¹, supporting the need to revise dietary recommendations for healthy lactating women.