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Using targeted liquid chromatography-tandem mass-spectrometry, this study identified early-life determinants of fecal AAs, analyzing fecal samples collected at week 1 to 4 in controls. Additionally, AA profiles prior to LOS onset were assessed to improve pathophysiological understanding, analyzing samples at t0 and t-3\u00a0days from LOS-affected infants and controls.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Results<\/jats:title>\n                    <jats:p>\n                      The most common LOS pathogens were\n                      <jats:italic>Escherichia coli<\/jats:italic>\n                      (n\u2009=\u20098),\n                      <jats:italic>Serratia<\/jats:italic>\n                      species (n\u2009=\u20094), and\n                      <jats:italic>Streptococcus agalactiae<\/jats:italic>\n                      (n\u2009=\u20093). Postnatal age, full-enteral feeding status, probiotic administration, and delivery mode significantly influenced fecal AAs. In the discovery cohort, threonine and glutamine were significantly decreased in affected infants (median [Q1-Q3], threonine: 43.8 [29.3\u201366.3] vs. 64.5 [48.1\u2013107.3] \u03bcmol\/L, p\u2009=\u20090.02; glutamine: 19.0 [9.0\u201338.3] vs. 34.1 [27.0\u201369.8], p\u2009=\u20090.01), while the validation cohort solely showed a non-significant decrease of threonine. When combining both cohorts, threonine achieved an area under the curve of 0.65 (95%-CI: 0.53\u20130.76).\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Conclusions<\/jats:title>\n                    <jats:p>These findings suggest several clinical characteristics influence fecal AAs, and altered threonine metabolism may contribute to LOS pathophysiology, warranting evaluation of related metabolites and multi-marker approaches in future studies.<\/jats:p>\n                  <\/jats:sec>","DOI":"10.1007\/s11306-026-02483-9","type":"journal-article","created":{"date-parts":[[2026,6,16]],"date-time":"2026-06-16T14:48:37Z","timestamp":1781621317000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Longitudinal fecal amino acid profiles in extremely preterm infants: early-life determinants and associations with late-onset sepsis"],"prefix":"10.1007","volume":"22","author":[{"ORCID":"https:\/\/orcid.org\/0009-0007-6957-3850","authenticated-orcid":false,"given":"A. 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