VitaminDSynthese

VitaminDSynthese, commonly referred to as vitamin D synthesis, describes the body's endogenous production of vitamin D, predominantly in the skin after exposure to ultraviolet B (UVB) radiation from sunlight. It can also be obtained from dietary sources and supplements.

In the skin, UVB photons convert 7-dehydrocholesterol to previtamin D3, which then spontaneously isomerizes to cholecalciferol (vitamin D3). Vitamin D2 (ergocalciferol) is produced by some plants and fungi and can be ingested as part of the diet.

The liver and kidneys carry out two hydroxylation steps: first to 25-hydroxyvitamin D [calcidiol], then to 1,25-dihydroxyvitamin D [calcitriol], the hormonally active form. Calcitriol binds to the vitamin D receptor (VDR) and regulates the expression of many genes involved in calcium and phosphate balance.

Factors affecting synthesis include geographic latitude, season, time of day, skin melanin content, age, sunscreen use, clothing, air pollution and cloud cover. Aging reduces skin 7-dehydrocholesterol levels; higher melanin decreases UVB penetration; sunscreen and indoor living reduce cutaneous production.

Health relevance: calcitriol supports calcium and phosphate homeostasis and bone mineralization and may influence immune function and cell differentiation. Deficiency can cause rickets in children and osteomalacia in adults. Serum 25-hydroxyvitamin D is the common measure of vitamin D status.

Safety and intake: guidelines vary by country. Excessive UV exposure raises skin cancer risk, while insufficient sun exposure may be addressed with diet or supplements. Typical adult recommendations range from 600 to 800 international units (15–20 micrograms) per day, depending on age and circumstance.