ANSWER
The principal function of Vitamin D in calcium homeostasis is to increase calcium absorption from the intestine and colon. Calcium is absorbed by both an active transcellular pathway, which is energy dependent, and by a passive paracellular pathway through tight junctions. 1,25 Dihydroxyvitamin D3 (1,25(OH)2D3) is the hormonally active form of Vitamin D.
In the intestine, (1,25(OH)2D3) affects the process of transcellular calcium transport which has been proposed to involve the entry of calcium via the apical calcium channel, transient receptor potential vanilloid type 6 (TRPV6), translocation of calcium through the interior of the enterocyte and basolated extrusion of calcium by the intestinal plasma membrane pump PMCA1b (Christakos et al., 2011).
The paracellular pathway functions throughout the entire length of the intestine and predominates in the more distal regions. (1,25(OH) 2D3) has been shown to convert regions of net secretion of calcium in ileum and the colon to net absorption (Christakos et al., 2011).
References
Christakos, S., Dhawan, P., Porta, A., Mady, L. J. & Seth, T. (2011). Vitamin D and Intestinal Calcium Absorption. Molecular and Cellular Endocrinology, 347(1-2), 25-29. https://doi.org/10.1016/j.mce.2011.05.038
Christakos, S., Lieben, L., Masuyama, R. & Carmeliet, G. (2014). Vitamin D endocrine system and the intestine. BoneKEy Reports, 3. https://doi.org/10.1038/bonekey.2013.230
