Vitamin D is a fat-soluble hormone precursor that your skin makes from UVB sunlight and that you absorb in smaller amounts from food, and its main job is to regulate how much calcium and phosphate your gut takes up. It is called a vitamin for historical reasons; biochemically it behaves like a hormone, converted first in the liver and then in the kidney into an active form that binds a receptor found in bone, gut, muscle and immune cells. When people talk about “checking their vitamin D”, they mean a blood test for the storage form, 25-hydroxyvitamin D. Low levels are common in winter, at northern latitudes, in people with darker skin, and in older adults, whose skin synthesizes less.
What it measures
The standard test is serum 25-hydroxyvitamin D, written 25-OH D or 25(OH)D, from an ordinary blood draw. It reflects your combined supply from sun, food and supplements over the preceding weeks, because this form circulates bound to a carrier protein and turns over slowly. The active form, 1,25-dihydroxyvitamin D, is not used for routine screening: it is held in a narrow range by Parathyroid hormone (PTH) and can read normal, or even high, while stores are empty. Results come back in ng/mL in the United States and in nmol/L elsewhere; 1 ng/mL equals 2.5 nmol/L, which is the usual source of confusion when people compare results. A fuller reference on testing and correcting Vitamin D covers the practical side.
Typical values
Expert bodies genuinely disagree about where “enough” starts, so reports differ depending on which guideline the lab follows. The bands below are the ones in common use.
| 25-OH D | Common interpretation |
|---|---|
| Below 12 ng/mL (30 nmol/L) | Clear deficiency; the level associated with rickets in children and osteomalacia in adults |
| 12-20 ng/mL (30-50 nmol/L) | Inadequate by most definitions; many clinicians treat this range |
| 20-30 ng/mL (50-75 nmol/L) | Adequate for bone health in the Institute of Medicine framework, but labeled insufficient by the Endocrine Society |
| Above 30 ng/mL (75 nmol/L) | Sufficient under every common definition |
| Well above 100 ng/mL (250 nmol/L) | The range where toxicity and high blood calcium become a concern |
Why it matters for longevity
Low 25-OH D is associated in large observational cohorts with higher all-cause mortality, more fractures, more infections and worse cardiovascular outcomes. That association is weaker than it looks: illness, inactivity and obesity all push levels down, so a low reading is partly a marker of poor health rather than its cause. Randomized trials in already-replete adults, VITAL being the largest, did not show the expected reductions in cancer incidence or major cardiovascular events, which is the strongest argument against treating vitamin D as a general-purpose longevity supplement. What is solid is the bone and muscle case in people who are actually deficient: correcting deficiency, usually alongside adequate Calcium (total and ionized), prevents osteomalacia and supports fracture prevention in frail older adults.
What changes it
Sunlight on bare skin is the main lever, and the one most constrained by where you live. In winter at higher latitudes the UVB reaching the ground is too weak to make much of anything, no matter how long you stay outside. Food contributes modestly: oily fish, egg yolk, liver, and fortified milk or cereal are the realistic sources. Supplements work, and cholecalciferol (D3) raises blood levels more reliably than ergocalciferol (D2). Take it daily with a meal containing fat rather than as occasional megadoses, which have performed no better in trials and, at very high single doses, worse. Vitamin D also distributes into fat tissue, so people with obesity typically need more to reach the same blood level. Conditions that impair fat absorption, and drugs such as glucocorticoids and some anticonvulsants, lower it as well.
Related reading
- Vitamin D and Longevity: Who Needs a Supplement
- Vitamin D Slowed Telomere Shortening in VITAL Trial
- How to Strengthen Bones and Prevent Osteoporosis
- How to Support Your Immune System as You Age
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.