What is Vitamin D?
Vitamin D is not a single compound but a group of vitamins, being the prohormones ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3), the main sources for the hydroxylated metabolites which are active in animals.
Ergocalciferol is widely available from plants and fungi while cholecalciferol is formed from UV irradiation in the animal skin.
Vit D is important for maintaining calcium levels and promoting bone health and development, but a large and still growing amount of research has found that Vitamin D3 also stimulates immune system and is beneficial in respiratory tract infections (e.g. COVID).