What is vitamin D?
Vitamin D is a vitamin your skin makes from sunlight that works like a hormone, helping your body absorb calcium and keeping your immune system balanced.
Think of vitamin D as the delivery truck that brings calcium from your food into your body. Without enough trucks, calcium stays at the depot and your bones go short.
Why it matters for aging
Low vitamin D is common, especially in winter, in older adults, in people with darker skin and in people who spend most of the day indoors. A blood test called 25-hydroxyvitamin D shows your level.
Very high doses over time can raise blood calcium to harmful levels, so it's best to supplement based on a test and your doctor's advice rather than guessing.
A common myth
Vitamin D supplements prevent cancer and heart disease.
In the large VITAL trial, vitamin D didn't significantly reduce cancer or heart disease in healthy adults. It did reduce autoimmune disease by 22 percent. Its main value is fixing a real deficiency.
Go deeper
Deep dives
- Vitamin D and K2 together: why it matters, and what's hype
- The best supplements for longevity, graded by evidence
Related terms
Updated Nov 9, 2026 · All terms