Biomarker 101: Vitamin D

Vitamin D is unusual among vitamins: most of it is not obtained from food at all, but manufactured in your skin from sunlight.

What vitamin D does

Its central role is helping the body absorb calcium and phosphate from the diet, which is why it matters so much for bones and teeth. Without enough, bones soften and repair less effectively. It also contributes to muscle function and immune activity.

There are two forms. D2 comes from plant sources including fortified foods and UV-exposed mushrooms. D3 makes up the great majority of your supply and is produced in the skin on sun exposure, with smaller amounts from oily fish, egg yolks and fortified dairy.

Why the test measures 25(OH)D

Vitamin D arrives inactive. The liver converts it to 25-hydroxyvitamin D, and the kidneys then convert that into the active form when it is needed.

Blood tests measure the middle step, 25(OH)D, because it circulates in larger amounts and reflects overall supply. The active form is tightly regulated and can look normal even when stores are depleted, so it is measured only in specific clinical situations.

Why it's measured

Testing is used to check for deficiency, particularly in people with limited sun exposure, and when calcium, phosphate or bone results are abnormal. It is also used where absorption may be affected, and to monitor people already taking supplements.

Higher risk applies to anyone with little sun exposure, people with naturally darker skin, those who cover up for cultural reasons, people who avoid sun because of skin cancer risk, and people with conditions affecting absorption.

What the numbers mean

Results are reported in nmol/L. A level of 50 nmol/L or above is generally considered sufficient for bone health, with the range beneath that describing degrees of deficiency. Laboratories differ, so read your own report.

Levels vary predictably by season — higher at the end of summer, lower through winter. In Australia and New Zealand that swing can be substantial, so the time of year a sample was taken is worth knowing.

What vitamin D testing won't tell you

• It isn't a routine test for everyone. Testing is targeted at people with reason to suspect deficiency.
• The season affects it. A winter result and a summer result are not directly comparable.
• More isn't better. Very high levels, almost always from supplementation, can cause harm.
• Sun protection still applies. Skin cancer risk in Australia and New Zealand is not a trade-off worth making, which is precisely why testing exists.

The bottom line

Vitamin D sits between sunlight, diet and bone health. Because production depends so heavily on sun exposure, and because that varies by season, skin type and lifestyle, testing tells you something you genuinely cannot infer.