Dietary xenoglycan Neu5Gc and the self-glycan Neu5Ac inhibit human sperm function at physiological concentrations

N-glycolylneuraminic acid (Neu5Gc), a dietary xenobiotic sialic acid acquired primarily from red meat and dairy products, cannot be synthesized by humans due to the inactivation of the CMAH gene. In contrast, its endogenous homologue N-acetylneuraminic acid (Neu5Ac) is abundant in human reproductive tissues. Although anti-Neu5Gc antibodies have been associated with infertility, the direct impact of free sialic acids on sperm function remains unclear. This study investigated the dose-dependent effects of physiologically relevant concentrations (0-10 μM) of Neu5Gc and Neu5Ac on human sperm function in vitro and elucidated the underlying mechanisms. After treatment, key sperm functions were comprehensively evaluated: motility by computer-assisted analysis, penetration capacity through methylcellulose, and acrosome reaction (AR) via chlorotetracycline staining. Sperm ATP concentration and mitochondrial membrane potential were evaluated by ELISA and JC-1. Intracellular Ca^2 + levels and protein tyrosine phosphorylation were measured fluorometrically and by western blotting, respectively. Both Neu5Gc and Neu5Ac significantly and dose-dependently inhibited sperm total motility, progressive motility, penetration capacity, and both spontaneous and progesterone-induced AR. Neu5Gc and Neu5Ac compromised sperm ATP generation without impairing mitochondrial membrane potential. Mechanistically, these impairments were accompanied by a marked suppression of total protein tyrosine phosphorylation and a reduction in resting intracellular Ca²⁺ levels. In conclusion, our in vitro results demonstrate that free Neu5Ac and Neu5Gc, at physiologically relevant micromolar concentrations, impair essential sperm functions by disrupting capacitation-associated tyrosine phosphorylation and calcium signaling pathways, suggesting a possible diet-environment-reproductive health axis in male fertility.