Vitamin D is one of the most economical nutrients which can be easily derived from the direct exposure to sun. That’s why it is known as “sunshine vitamin.” It is the most powerful vitamin ubiquitously present on earth but still most of the population are deficient in vitamin D. It is the ultimate fat-soluble vitamin significant for proper growth and development of the body.
There are various ways through which vitamin D is viable to us like sun, dietary sources (seafood, egg, milk and milk products), vitamin D fortified foods and supplements. There are only a few food sources through which people can accomplish their daily requirements. So, Vitamin D supplementation along with daily diet helps in achieving the optimal level.
Table: Dietary Reference Intake for vitamin D (Institute of Medicine, 2010)
*Adequate Intake in infants is 400 IU/day for 0-6 months of age and 400 IU/day for 6-12 months of age.
Kidney’s role in vitamin D metabolism
Vitamin D is a pre-hormone acquired from diet or synthesised in the skin from the cholesterol-derived precursor (7-dehydrocholesterol) via sun exposure. Generally, vitamin D occurs in two biologically inactive forms i.e. vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). The former one is derived from either skin or animal products while the latter is derived exclusively from plant origin.
Both liver and kidney take part in the vitamin D activation process which is a sequential two-step hydroxylation process. The first step occurs in the liver where 25-hydroxylation takes places and produces 25-hydroxyvitamin D or 25(OH)D. Then, the addition of another hydroxyl group is carried out in the kidney which converts 25(OH)D to 1,25-dihydroxy vitamin D (Calcitriol) by the enzyme 1α-hydroxylase (Dusso AS et al., 2005). Therefore the kidney plays a crucial role in the metabolism of vitamin D and also regulates the circulating levels. Thus, compromised kidney function may result in vitamin D deficiency as commonly observed in chronic kidney disease (CKD) patients.
Hyperparathyroidism may occur in patients during the advancement of CKD. However, persistent high level of parathyroid hormone and low level of Calcitriol is related to bone loss, cardiovascular disease, immune suppression and increased mortality in patients with end-stage kidney failure (Andress, 2006).
Vitamin D is also known as the sunshine vitamin. It is one of the most important fat-soluble vitamins which can be obtained via sun exposure or by dietary sources (egg, milk, seafood and fortified foods). Nowadays, vitamin D deficiency is prevalent among most of the population. The kidney plays a central role in the metabolism of vitamin D i.e. kidney participates in the conversion of inactive form of vitamin D (cholecalciferol) to its active form (Calcitriol). This active compound circulates in the bloodstream and maintains the calcium homeostasis in the body. Vitamin D deficiency and chronic kidney diseases are interlinked means vitamin D deficiency can occur due to CKD or vice-versa. Therefore, vitamin D supplementation can be an alternative way to achieve the required vitamin D status.
- Institute of Medicine (2010). Dietary Reference Intakes for Calcium and Vitamin D. Retrieved from http://www.nationalacademies.org/hmd/~/media/Files/Report%20Files/2010/Dietary-Reference-Intakes-for-Calcium-and-Vitamin-D/Vitamin%20D%20and%20Calcium%202010%20Report%20Brief.pdf
- Dusso, A.S., Brown, A.J., & Slatopolsky, E (2005). Vitamin D. Am J Physiol Renal Physiol, 289(1), F8–F28.
- Andress, D.L. (2006). Vitamin D in chronic kidney disease: a systemic role for selective vitamin D receptor activation. Kidney Int, 69(1), 33-43.