Review Reminder: Sources of and Roles of Vitamin D and Calcitriol – Important in Maintaining Blood Calcium Levels and Bone Density

*Vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) can be formed in the skin by epithelial cells upon exposure to UV.  Alternatively dietary Vitamins D2 and D3 can be ingested in the form of oily fish, meat, supplemented dairy and plant-based products (e.g., milk, yogurt, soy milk), daily vitamins as well as various plant, fungi (e.g., some mushrooms) and yeast sources.

Once synthesized by the skin, or ingested by the digestive tract, vitamin D3 is carried to the liver, where hepatocytes convert it to a calcitriol precursor, which is converted to calcitriol by proximal tubular cells in the kidney.  Calcitriol is a hormone which is able to help regulate blood calcium levels.  Specifically, calcitriol is released and active when blood calcium levels are low.  Calcitriol helps to increase blood calcium levels and maintain homeostasis by performing 3 functions:

1. Calcitriol facilitates the absorption of dietary calcium and phosphate in the intestine, which leads to more calcium and phosphate entering the bloodstream.
2. Calcitriol increases the reabsorption of calcium within the nephron, which leads to less calcium being excreted in the form of urine, and therefore more calcium being retained in the bloodstream.
3. Calcitriol promotes osteoblasts to release RANKL, a signalling protein, which activates osteoclasts to dissolve bone matrix and release the calcium and phosphate which is taken up by the bloodstream, increasing blood calcium levels.