This review highlights the critical aspects of vitamin D deficiency in adults, focusing on the metabolism, biological functions, and health implications. Vitamin D, available as cholecalciferol (D3) and ergocalciferol (D2), is pivotal for bone health and the regulation of calcium and phosphorus metabolism. Although sunlight exposure can synthesize vitamin D, dietary sources and supplements are crucial due to the skin cancer risks associated with UV exposure. The activation process of vitamin D involves conversion into 25-hydroxyvitamin D in the liver and subsequently into the active form, 1,25-dihydroxyvitamin D, in the kidneys. This active form interacts with the vitamin D receptor (VDR) in various tissues, influencing over 200 genes. Deficiency in vitamin D is linked to skeletal diseases like rickets and osteomalacia, and it is associated with potential risks for non-skeletal diseases, including cardiovascular diseases, diabetes, and certain cancers. The review emphasizes dietary intake and supplementation as key strategies for preventing and treating vitamin D deficiency, advocating for a balanced approach to ensure adequate vitamin D levels to support overall health and mitigate disease risks.