How UVB Turns Sunlight Into Vitamin D
Vitamin D is made through a chemical chain reaction in the skin, not absorbed from sunlight. When ultraviolet B (UVB) rays reach the skin, they change a cholesterol-like substance called 7-dehydrocholesterol into a precursor of vitamin D₃. Body heat then helps convert that precursor into vitamin D₃. The liver and kidneys process it further into a form the body can use.
That active vitamin D helps the intestine absorb calcium and phosphorus, minerals needed for normal bone formation. If vitamin D is too low, the body has a harder time maintaining healthy bones.
Why Sun Exposure Varies
The skin’s production depends on how much UVB actually reaches it. Season, location, time outdoors, cloud cover, skin pigmentation, clothing, and age can all affect the amount made. Window glass blocks most UVB, so sitting in a sunny room generally does not provide the same stimulus as being outdoors.
Sunlight is only one source. Some foods are naturally rich in vitamin D, and some—such as certain milks—are fortified with it; supplements can also provide it. The useful distinction is that sunlight triggers production, while food and supplements supply vitamin D directly. Because UV exposure can damage skin, getting more sun is not a risk-free way to address a shortfall.
