Sources of vitamin D

The primary and most efficient source of vitamin D is cutaneous synthesis triggered by exposure to ultraviolet B (UVB) radiation from sunlight. When UVB rays penetrate the skin, they convert 7-dehydrocholesterol into pre-vitamin D3, which is then thermally isomerized into cholecalciferol (vitamin D3). This process is influenced by several factors, including geographic location, season, skin pigmentation, age, and sunscreen use. Individuals with limited sun exposure, such as those living at high latitudes or who predominantly stay indoors, are at higher risk of vitamin D deficiency. Vitamin D can also be obtained through dietary sources, although natural food sources are relatively scarce. Fatty fish (e.g., salmon, mackerel, and sardines), fish liver oils, and egg yolks are among the richest natural sources of vitamin D3. Additionally, fortified foods such as dairy products, cereals, and plant-based milk alternatives contribute significantly to dietary vitamin D intake, especially in populations with inadequate sun exposure. Once absorbed through diet or synthesized in the skin, vitamin D undergoes two hydroxylation steps: first in the liver to form 25-hydroxyvitamin D (25(OH)D), the major circulating form, and then in the kidneys to form 1,25-dihydroxyvitamin D (1,25(OH)2D), the active form responsible for its physiological functions. Balancing both sources: adequate sun exposure and proper dietary intake are essential to maintaining optimal vitamin D levels and preventing deficiency-related disorders.