Background We previously identified the urinary biomarkers to diagnose calcium deficiency and nutritional rickets by ultra-performance liquid chromatography/quadrupole time-of-flight tandem mass spectrometry (UPLC/Q-TOF MS/MS).To further confirm these biomarkers in vivo, we performed serum metabolomics analysis of calcium deficiency. Methods A calcium-deficient rat model was established with a low-calcium diet for 12 weeks. Serum-metabolomics-based UPLC/Q-TOF MS/MS and multivariate statistical

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... nalysis was performed to identify the alterations in metabolites associated with calcium deficiency in rats. Results Bone mineral density, serum parathyroid hormone and alkaline phosphatase were significantly decreased in the low-calcium diet group (LCG) compared to the normal calcium diet group (NCG). Serum metabolic-profiling analysis could definitively distinguish between the LCG and NCG andidentified25 calcium-deficient biomarkers. Three metabolites (indoxyl sulfate, phosphate, and taurine) of the 25 biomarkers were found in our previous urinary metabolomics study of rats with a calcium deficiency and nutritional rickets. The areas under the curve (AUCs) of these three biomarkers were greater than 0.8, and the combination of any two biomarkers was higher than 0.95. Conclusion Dietary calcium deficiency induced the alterations of metabolites in the serum of rats, and the three identified biomarkers had relatively high diagnostic values for calcium deficiency in rats.