Schematic of altered metabolic pathways associated with calcium deficiency based on urinary metabolites analyzed by ultra-performance liquid chromatography and quadrupole time-of-flight tandem mass spectrometry (UPLC/Q-TOF MS/MS). (A) Metabolism of calcium and phosphorus metabolism; (B) Krebs cycle and metabolism of bone collagen, purines and other biomarkers. Biomarkers are shown in red. Red arrows indicate increased levels and green arrows indicate decreased levels of these metabolites in the low-calcium group (LCG) rats compared with the control (normal-calcium group; NCG) rats. Black arrows indicate the reaction direction. The results of steps 1, 2, 3, and 4 steps lead to disorders of bone metabolism and decreased bone-mineral density.