Ultraviolet (UV) radiation—specifically UVB radiation (wavelengths between 290–315 nm)—plays a crucial role in the natural synthesis of vitamin D in the human body. Here’s how the process works:
1. Skin Exposure to UVB Radiation
- When bare skin is exposed to sunlight, UVB photons penetrate the outer layers (epidermis).
- The radiation interacts with a cholesterol-derived compound in the skin called 7-dehydrocholesterol.
2. Conversion to Pre-Vitamin D3
- UVB energy converts 7-dehydrocholesterol into pre-vitamin D3 through a photochemical reaction.
- This is a non-enzymatic process triggered purely by the energy from UVB light.
3. Thermal Conversion to Vitamin D3
- Within hours, body heat converts pre-vitamin D3 into cholecalciferol (vitamin D3), the biologically inactive form of vitamin D.
4. Activation in the Liver and Kidneys
- Vitamin D3 travels through the bloodstream to the liver, where it is converted to 25-hydroxyvitamin D.
- It is then sent to the kidneys, where it becomes the active form, 1,25-dihydroxyvitamin D (calcitriol).
- This active form helps regulate calcium and phosphate in the body, supporting bone health, immune function, and more.
Factors Affecting UVB-Induced Vitamin D Synthesis
- Time of day (midday sun is strongest)
- Latitude and altitude
- Skin pigmentation (darker skin requires more exposure)
- Age (older skin produces less)
- Clothing and sunscreen (reduce UVB penetration)