Role of Skin in Vitamin D Synthesis

UVB radiation from sunlight reacts with provitamin D3, a cholesterol-based steroid located in the stratum basale and stratum spinosum layers of the epidermis. This reaction converts provitamin D3 into previtamin D3, which rapidly transforms into cholecalciferol, or vitamin D3. The vitamin then enters the bloodstream for transport throughout the body.

Vitamin D3 attaches to vitamin D-binding protein in the serum and travels to the liver, where it converts into calcidiol, a biologically inactive primary circulating form. Calcidiol then reaches the kidneys and undergoes conversion to calcitriol, the active form of vitamin D that regulates calcium and phosphorus absorption.

People with darker skin have higher levels of melanin, which reduces the skin's ability to synthesize vitamin D compared to individuals with lighter skin. Melanin absorbs UVB radiation, limiting the amount available to convert provitamin D3 into vitamin D3. This difference means individuals with darker skin may require longer sun exposure to produce equivalent vitamin D levels.

Severe vitamin D deficiency causes rickets in children, characterized by impaired bone development and mineralization. In adults, deficiency leads to osteomalacia, a softening of bones, and osteoporosis, a condition of decreased bone density. These conditions compromise skeletal health and increase fracture risk due to inadequate calcium and phosphorus absorption.

Calcitriol, the active form of vitamin D, stimulates calcium and phosphorus absorption from the gastrointestinal tract, raising their serum levels. This enhanced absorption supports skeletal health by ensuring adequate mineral availability for bone formation and maintenance. Proper vitamin D levels are essential for preventing bone disorders and maintaining structural integrity.

Latitude, season, time of day, skin pigmentation, clothing, sunscreen use, and age all influence the skin's ability to produce vitamin D. Sunscreen blocks UVB radiation, while higher latitudes and winter months reduce UVB intensity. Older individuals produce less vitamin D, and protective clothing limits sun exposure, collectively reducing synthesis capacity.

Vitamin D3 is lipid soluble, allowing adipocytes, or fat cells, to store it during periods of excessive production. This storage capacity enables the body to maintain vitamin D reserves and draw upon them when dietary intake or sun exposure is insufficient. Vitamin D also has both inhibitory and stimulatory effects on adipocyte differentiation depending on the cell stage.