Vitamin D is structurally a steroid, and belongs to the group of fat-soluble vitamins. There are two main forms with very similar activity: cholecalciferol or vitamin D3, synthesised in the human body, and ergocalciferol or vitamin D2, synthesised by plants. Both forms can be taken with the diet.
Vitamin D3 is of fundamental importance in the human body as it maintains serum calcium and phosphate homeostasis by promoting the mineralisation of the bone matrix. Produced by the skin in an inactive form, it is activated by the action of sunlight.
- A first mechanism of action is related to vitamin D3's ability to directly promote the reabsorption of calcium and phosphorus in the Gut.
- A second mechanism is related to the stimulation of RANKL production, a known activator of osteoclastogenesis.
- A third way involves Vitamin D3, in association with increased parathormone production, stimulating calcium reabsorption at the level of the distal renal tubule, reducing its urinary elimination.1
- In addition, Vitamin D3 inhibits parathormone synthesis by acting directly in the parathyroid glands. Indeed, when calcium levels in the body are optimal, Vitamin D3, by binding to its receptor VDR (Vitamin D Receptor), suppresses gene expression leading to parathyroid cell proliferation.2
When, as with the menopause, there is a reduction in bone mass due to the reduction of oestrogen, vitamin D3 supplementation is recommended, as it promotes calcium absorption and its presence in the bone3 and prevents osteoporosis.4 Vitamin D3 excretion is faecal.3
1. B.G. Katzung, General and Clinical Pharmacology, Third Edition, Piccin publisher. Pharmacology of Bone Mineral Homeostasis, p. 592-593.
2 were identified. A. Catharine Ross et al. Calcium Vitamin D. D.R.I. Dietary reference contents, Committee to Review Dietary Reference contents for Vitamin D and Calcium Food and Nutrition Board. 2011.
3 were identified. B.G. Katzung, General and Clinical Pharmacology, Third Edition, Piccin publisher. Gonads and inhibitors, p. 550.
4 were identified. Sahni S, Hannan MT, Gagnon D, Blumberg J, Cupples LA, Kiel DP,Tucker KL, High vitamin C content is associated with lower 4-year bone loss in elderly men. J Nutr. 2008 Oct; 138(10):1931-8.