Vitamin D

Vitamin D refers to a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate. It exists mainly as vitamin D2 (ergocalciferol) and D3 (cholecalciferol). Endogenously, 7-dehydrocholesterol in the skin is converted to cholecalciferol upon exposure to UVB radiation. Both dietary and skin-derived vitamin D undergo two hydroxylations: first in the liver to 25-hydroxyvitamin D (calcidiol), then in the kidneys to the active hormone 1,25-dihydroxyvitamin D (calcitriol). Calcitriol binds vitamin D receptors in bones, intestines, kidneys, immune cells, and other tissues, regulating gene expression to maintain calcium-phosphate balance, support bone mineralization, modulate immune responses, and promote muscle function. Deficiency can result from inadequate sun exposure, malabsorption, liver or kidney disease, or low dietary intake, leading to rickets, osteomalacia, muscle weakness, and immune compromise. Dietary sources include fatty fish, fortified foods, and supplements, which together help maintain optimal levels for health.

Vitamin D is hydroxylated in the liver to 25-hydroxyvitamin D, then converted in the kidneys to the active form, calcitriol. Calcitriol binds to vitamin D receptors in target cells, forming a complex that enters the nucleus and influences gene transcription. This process enhances intestinal absorption of calcium and phosphate, regulates bone remodeling, modulates immune cell activity, and influences cellular differentiation and proliferation.

Few natural foods provide vitamin D. Rich sources include fatty fish (salmon, mackerel, tuna), fish liver oils, and UV-exposed mushrooms. Fortified products like milk, plant-based milks, cereals, and juices also contribute significantly.

Vitamin D deficiency can cause rickets in children (bone deformities, growth retardation) and osteomalacia in adults (bone pain, muscle weakness). It also increases risk of osteoporosis and may impair immune function.

Recommended Dietary Allowances (RDAs) for vitamin D vary by age and life stage. Guidelines from the NIH (US) and EFSA (EU) serve as references for sufficient intake.

Observational studies link adequate vitamin D levels to lower all-cause mortality. Some trials suggest slight reductions in cancer mortality, but evidence on lifespan extension is inconclusive.

Vitamin D is available as D2 (ergocalciferol) and D3 (cholecalciferol). D3 is more effective at raising and sustaining serum 25(OH)D levels. Because it is fat-soluble, absorption improves when taken with dietary fat.