Evaluation of dietary protein and amino acid requirements: a systematic review

Background: Protein intakes are set by the dietary reference intakes, which were published in 2005 and have not been updated since. Objectives: This review assesses the evidence on requirements for average daily dietary protein and individual indispensable amino acid intake for healthy individuals by life stage and sex. Methods: Literature was searched from January 2000 through May 2024 in Medline, EMBASE, AGRICOLA, and Scopus. Two reviewers independently screened titles, abstracts, and full-text publications using predetermined inclusion/exclusion criteria. We extracted data, assessed risk of bias (RoB), qualitatively synthesized results from low to moderate RoB studies, and evaluated the strength of evidence supporting conclusions. Results: We identified 11,408 studies, of which 68 articles reporting on 66 unique studies were eligible for the review and 45 were assessed as low or moderate RoB. For infants, 6 studies examined requirements for isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. For children/adolescents, 7 studies examined requirements for protein, lysine, methionine, phenylalanine, and total branched-chain amino acids. For pregnant people, 4 studies examined requirements for protein, lysine, and phenylalanine. For adults 19–50 y, 16 studies examined requirements for protein, leucine, lysine, methionine, phenylalanine, threonine, and valine and for adults 51–>70 y, and 6 studies examined requirements for protein, leucine, and phenylalanine. Both males and females were studied for all requirements except valine for infants (males only), phenylalanine for children/adolescents (not reported), protein, leucine, methionine, phenylalanine, threonine, and valine for adults 19–50 y (males only) and in pregnant people (females only). Commonly used methods included indicator amino acid oxidation, 24-h indicator amino acid oxidation/indicator amino acid balance, and nitrogen balance. Conclusions: Our systematic review of human indispensable amino acid and protein requirement research from January 2000 to May 2024 finds limited studies across the lifecycle, particularly in pregnancy, lactation, infancy, childhood, and adolescence. Trial registration: This review was registered at PROSPERO with CRD42023446618 (https://www.crd.york.ac.uk/PROSPERO/display_record.php?RecordID=446618).