L-serine dose-dependently ameliorates high-selenium-induced insulin resistance in mice.

Supranutritional selenium intake, while potentially oncoprotective, paradoxically elevates the risk of type 2 diabetes through metabolic hijacking of the one-carbon pool. Se-detoxification exhausts S-adenosylmethionine (SAM) and its precursor L‑serine, triggering a maladaptive surge in de novo serine synthesis that impairs insulin sensitivity. Identifying a precise nutritional countermeasure to decouple Se-driven metabolic risks from its therapeutic benefits remains a critical challenge. We systematically evaluated the dose-response efficacy of dietary L‑serine (0.7%-3.5% of dietary protein) in C57BL/6J mice under high-Se stress (0.8 mg/kg). Systemic glucose homeostasis, tissue-specific Se distribution, hepatic S-adenosylmethionine/S-adenosylhomocysteine (SAM/SAH) methylation index (LC-MS/MS), hepatic malondialdehyde (MDA), PI3K-AKT-mTOR signaling, and plasma hepatic and renal function markers were assessed. The pharmacological PHGDH inhibitor NCT503 was included as a mechanistic comparator rather than as an alternative therapeutic strategy. L‑serine supplementation dose-dependently ameliorated high-Se-induced glucose intolerance and insulin resistance, with a functional threshold between 0.7% and 1.4% dietary serine (≈96-182 mg/kg/d). Direct LC-MS/MS quantification demonstrated a dose-dependent increase in the hepatic SAM/SAH methylation index, providing direct biochemical evidence of one-carbon pool replenishment; parallel reduction in hepatic MDA indicated attenuation of oxidative stress. Recovery of PI3K-AKT-mTOR signaling was observed across hepatic, muscular, and pancreatic tissues in a tissue-specific, dose-dependent manner. Plasma ALT, AST, and creatinine did not differ significantly across groups at all doses. NCT503 induced hyperhomocysteinemia and produced the lowest hepatic SAM/SAH ratio, indicating that PHGDH suppression under basal serine availability depletes rather than replenishes the one-carbon pool. Dietary L‑serine supplementation ameliorates high-selenium-induced insulin resistance with a functional threshold between 0.7% and 1.4% dietary serine (≈96-182 mg/kg/d), mechanistically anchored to dose-dependent elevation of the hepatic SAM/SAH methylation index. These findings provide a preclinical framework for precision nutritional interventions in populations chronically exposed to supranutritional selenium environments.