Nickan Research Institute Journal of Parathyroid Disease 2345-6558 8 1 2020 01 01 Average levels of vitamin D in different continents e01 e01 EN Mahzad Irandoust https://orcid.org/0000-0003-4795-1295 Hashem Nayeri https://orcid.org/0000-0002-4334-9064 Hossein Irandoust https://orcid.org/0000-0003-1193-6798 Journal Article 2019 02 27 2019 08 07 Vitamin D is a group of fat-soluble hormone precursor that enters the body through food and nutritional supplements. Vitamin D has two important forms which are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). This vitamin is activated in the body through two hydroxylation reactions and produces calcitriol (1,25-dihydroxycholecalciferol), which is its active form. The metabolites and other analogues of these substances also refer to vitamin D. Vitamin D plays a significant physiological and essential role in maintaining the levels of extracellular calcium ions. The most important role of extracellular calcium is its contribution to muscle contractions. Around 10% to 20% of vitamin D is supplied by the foods. Vitamin D sources include salmon, cod liver oil, wild mushroom, milk, and meat and also egg yolk. However, 80% of vitamin D comes from sunlight. Vitamin D has an important role in the process of bone metabolism, regulation of proliferation and cellular differentiation, as well as the regulation of immune responses. The purpose of this review article is to study the importance of vitamin D and its use in the body as well as examining the vitamin D levels in different continents.