Alfacalcidol (or 1-hydroxycholecalciferol) is an analogue of vitamin D used for supplementation in humans and as a poultry feed additive.

Alfacalcidol has a weaker impact on calcium metabolism[3] and parathyroid hormone levels[4] than calcitriol; but significant effects on the immune system, including regulatory T cells.

[5] It is considered to be a more useful form of vitamin D supplementation, mostly due to its much longer half-life and lower kidney load.

[6] It is the most commonly prescribed vitamin D metabolite for patients with end stage renal disease, given that impaired renal function alters the ability to carry out the second hydroxylation step required for the formation of the physiologically active form of vitamin D, 1,25-dihydroxyvitamin D3.

Used as a poultry feed additive, it prevents tibial dyschondroplasia and increases phytate bioavailability.