Vitamin D: what the 25-hydroxy number means
The sunlight hormone, the blood test that stands in for it, and an honest read of a hyped literature. Plain numbers, both directions.
PUBLISHED 2026-08-16 · UPDATED 2026-08-17 · 4 SOURCES
01What does the test actually measure?
Vitamin D from sun or food gets converted in the liver to 25-hydroxyvitamin D — the storage form, with a weeks-long half-life. That's what labs report: a slow-moving reservoir level, not the active hormone itself. It reads like a season average of your sun, food, and supplement inputs.
The reference-range argument is real and unresolved: the IOM's review put 20 ng/mL as sufficient for bone outcomes in nearly everyone [1]; the Endocrine Society's older guideline argued for 30 [2]. A number between the two isn't a crisis — it's a question for your doctor, and an honest article says so instead of picking the scarier cutoff.
02What did the big trial actually find?
For years, low vitamin D was associated with nearly every bad outcome in observational data — and the obvious confounder was always lurking: people with low D are often people who are indoors, older, heavier, sicker. VITAL was the test: 25,871 adults randomized to 2,000 IU daily or placebo, followed about five years [4].
The headline: no overall association with fewer major cardiovascular events or invasive cancers. That's not “vitamin D doesn't matter” — bone biology is settled science, and subgroup questions remain live. It's narrower and more useful: for broadly supplementing already-replete adults, the big outcomes didn't move. The reservoir matters; overfilling it didn't.
03Where does a lifter with a full calendar land?
Muscle tissue carries vitamin D receptors, and deficiency states are associated with weakness and falls in older adults — the mechanical case for caring about the reservoir is real, particularly for anyone training indoors at 5:40 AM through a northern winter.
The honest position: know your number before you buy anything. Sun exposure, fortified foods, and fatty fish move it; if a test comes back low, dosing is a conversation with your doctor or pharmacist — vitamin D is fat-soluble, and it accumulates. “We sell nothing” applies here with special force: this is the most-sold supplement in the argument, and the strongest trial we have says the blanket version of that sale outran its evidence [4].
