AbstractAimPhosphorus is a critical constituent of bone as a component of hydroxyapatite. Bone mineral content accrues rapidly early in life necessitating a positive phosphorus balance, which could be established by a combination of increased renal reabsorption and intestinal absorption. Intestinal absorption can occur via a transcellular pathway mediated by the apical sodium‐phosphate cotransporter, Slc34a2/NaPiIIb or via the paracellular pathway. We sought to determine how young mammals increase dietary phosphorus absorption from the small intestine to establish a positive phosphorus balance, a prerequisite for rapid bone growth.MethodsThe developmental expression profile of genes mediating phosphate absorption from the small intestine was determined in mice by qPCR and immunohistochemistry. Additionally, Ussing chamber studies were performed on small bowel of young (p7–p14) and older (8‐ to 17‐week‐old) mice to examine developmental changes in paracellular Pi permeability and transcellular Pi transport.ResultsBlood and urinary Pi levels were higher in young mice. Intestinal paracellular phosphate permeability of young mice was significantly increased relative to older mice across all intestinal segments. NaPiIIb expression was markedly increased in juvenile mice, in comparison to adult animals. Consistent with this, young mice had increased transcellular phosphate flux across the jejunum and ileum relative to older animals. Moreover, transcellular phosphate transport was attenuated by the NaPiIIb inhibitor NTX1942 in the jejunum and ileum of young mice.ConclusionOur results are consistent with young mice increasing phosphate absorption via increasing paracellular permeability and the NaPiIIb‐mediated transcellular pathway.