Human embryo metabolomic profiling under constant 5% vs physiological 8%–5%–2% oxygen: a preliminary prospective sibling-embryo study

Abstract Oxygen concentration declines from 5–8% in the fallopian tube to ~ 2% in the uterine cavity. However, clinical studies investigating ultra-low O₂ concentrations (2–3%) during human embryo culture have reported conflicting developmental outcomes. Whether these developmental differences are accompanied by metabolic adaptation in human embryos remains unknown. In this preliminary prospective sibling-split oocyte study, 111 embryos from 13 patients were cultured under constant 5% O₂ or a physiological 8%–5%–2% O₂ gradient. Spent culture media (n = 222) were analysed by nuclear magnetic resonance spectroscopy and high-performance liquid chromatography–tandem mass spectrometry against matched incubated control media. Spent-media analyses confirmed embryo-derived metabolic activity, characterized by pyruvate and glutamic acid depletion together with alanine and glutamine secretion. Neither patient-level analyses nor embryo-level linear mixed-effects models identified statistically significant metabolomic differences between the two O₂ culture conditions, whereas metabolite-turnover patterns showed significant inter-patient variability. Blastocyst-forming embryos exhibited lower total amino acid turnover than arrested embryos in cleavage-stage media (P = 0.037), whereas a similar trend in blastocyst-stage media was not significant (P = 0.106). These findings suggest that patient-specific and developmental factors contribute to embryo metabolic variability, while subtle O₂-associated metabolic effects cannot be excluded. Given the limited sample size, these findings are preliminary and hypothesis-generating.