The principal action of vitamin D is maintenance of calcium absorption from the intestine. In the traditional model 1,25(OH)2D3 acts by regulating calcium entry through TRPV6, transcellular movement of calcium by calbindin D and extrusion of calcium from the cell by the CaATPase PMCA2b. While intestine specific transgenic expression of TRPV6 markedly increases intestinal calcium absorption, studies in Trpv6 and S100g null mice have challenged this traditional view since there were no phenotypic differences between S100g or Trpv6 null mice and wildtype mice, suggesting compensation by other channels or proteins. Recent studies suggest that vitamin D sensitive calcium uptake does not affect a single entity but rather is achieved via a complex network of active calcium regulating components. The duodenum has been the focus of research related to vitamin D regulated calcium absorption. However, studies in which VDR is expressed specifically in the distal region of the intestine of Vdr null mice resulted in the rescue of VDR dependent rickets and an induction by 1,25(OH)2D3 of genes associated with active calcium transport. These findings indicate the importance of both the distal as well as the proximal segments of the intestine in vitamin D mediated calcium homeostasis. Although it had been a matter of debate, our studies using mouse intestine as well as human enteroids noted 1,25(OH)2D3 regulation of target genes in both villus and crypt. With age there is decreased basal and 1,25(OH)2D3 induction of classic (eg. Trpv6) and non-traditional intestinal target genes despite normal VDR expression. Since 1,25(OH)2D3 genomic action depends on protein partner/coactivator interactions RIME (Rapid Immunoprecipitation Mass Spectrometry) was used to compare VDR binding partners in young and old female mice. Protein interactions with VDR significantly diminished with increased age. ChIP/re-ChIP analysis of mouse intestinal chromatin noted a 2-3 fold decrease in VDR and coactivator SRC1 binding at Trpv6 VDREs which may contribute to the decrease of Trpv6 expression with age. To determine which intestinal cell populations may be contributing to the decline in intestinal calcium regulation with age single cell-RNAseq analysis of small intestinal epithelial cells was done. A significant decline in the stem/progenitor pool was observed in older mice. Across all epithelial cells populations Vdr expression was stable but altered patterns of expression of classic and non-traditional VDR target genes were observed with aging. Our data further indicate the complexity of 1,25(OH2D3 action in the regulation of calcium homeostasis and beyond. The principal action of vitamin D is maintenance of calcium absorption from the intestine. In the traditional model 1,25(OH)2D3 acts by regulating calcium entry through TRPV6, transcellular movement of calcium by calbindin D and extrusion of calcium from the cell by the CaATPase PMCA2b. While intestine specific transgenic expression of TRPV6 markedly increases intestinal calcium absorption, studies in Trpv6 and S100g null mice have challenged this traditional view since there were no phenotypic differences between S100g or Trpv6 null mice and wildtype mice, suggesting compensation by other channels or proteins. Recent studies suggest that vitamin D sensitive calcium uptake does not affect a single entity but rather is achieved via a complex network of active calcium regulating components. The duodenum has been the focus of research related to vitamin D regulated calcium absorption. However, studies in which VDR is expressed specifically in the distal region of the intestine of Vdr null mice resulted in the rescue of VDR dependent rickets and an induction by 1,25(OH)2D3 of genes associated with active calcium transport. These findings indicate the importance of both the distal as well as the proximal segments of the intestine in vitamin D mediated calcium homeostasis. Although it had been a matter of debate, our studies using mouse intestine as well as human enteroids noted 1,25(OH)2D3 regulation of target genes in both villus and crypt. With age there is decreased basal and 1,25(OH)2D3 induction of classic (eg. Trpv6) and non-traditional intestinal target genes despite normal VDR expression. Since 1,25(OH)2D3 genomic action depends on protein partner/coactivator interactions RIME (Rapid Immunoprecipitation Mass Spectrometry) was used to compare VDR binding partners in young and old female mice. Protein interactions with VDR significantly diminished with increased age. ChIP/re-ChIP analysis of mouse intestinal chromatin noted a 2-3 fold decrease in VDR and coactivator SRC1 binding at Trpv6 VDREs which may contribute to the decrease of Trpv6 expression with age. To determine which intestinal cell populations may be contributing to the decline in intestinal calcium regulation with age single cell-RNAseq analysis of small intestinal epithelial cells was done. A significant decline in the stem/progenitor pool was observed in older mice. Across all epithelial cells populations Vdr expression was stable but altered patterns of expression of classic and non-traditional VDR target genes were observed with aging. Our data further indicate the complexity of 1,25(OH2D3 action in the regulation of calcium homeostasis and beyond.