Vitamin D

What Is Vitamin D?

Vitamin D is a fat-soluble vitamin that functions more like a hormone than a classical vitamin because the body can synthesize it endogenously when skin is exposed to ultraviolet B (UVB) radiation (290-315 nm). The two main dietary forms are vitamin D2 (ergocalciferol, from fungi and yeast) and vitamin D3 (cholecalciferol, from animal sources and synthesized in skin). Both forms are converted in the liver to 25-hydroxyvitamin D [25(OH)D], the main circulating form and biomarker of status, and then in the kidney to the active hormone 1,25-dihydroxyvitamin D (calcitriol).

The RDA is 600 IU (15 mcg)/day for adults aged 19-70 and 800 IU (20 mcg)/day for adults over 70. The UL is 4,000 IU/day. Serum 25(OH)D levels of at least 20 ng/mL (50 nmol/L) are generally considered adequate.

Key Functions

• Calcium and phosphorus homeostasis: Calcitriol binds the vitamin D receptor (VDR) in the small intestine, increasing active calcium absorption from roughly 10-15% to 30-40%. It also promotes calcium reabsorption in the kidney and stimulates bone remodeling via osteoblast activity and RANKL expression.
• Bone health: Adequate vitamin D prevents rickets in children (soft, deformed bones) and osteomalacia in adults (bone pain, muscle weakness, increased fracture risk).
• Immune modulation: VDR is expressed on nearly all immune cells; calcitriol modulates innate and adaptive immunity, with evidence linking deficiency to increased susceptibility to respiratory infections.
• Muscle function: Low vitamin D is associated with muscle weakness and increased fall risk in older adults.

Sources and Deficiency Risk

Few foods naturally contain significant vitamin D. Fatty fish (salmon: approximately 447 IU/85 g, mackerel, sardines), cod liver oil, and egg yolks are the primary natural sources. Fortified foods (milk, orange juice, breakfast cereals) are major contributors in many countries. Widespread deficiency (serum 25(OH)D below 20 ng/mL) affects an estimated 1 billion people globally, driven by limited sun exposure, darker skin pigmentation, obesity, and aging.