In one sentence
The storage form of the hormone your skin makes from sunlight and you take in from a few foods and supplements, measured as 25-hydroxyvitamin D, which sums both sources over the past several weeks.
Why it matters
Bone is the outcome the evidence settles. The US Institute of Medicine's 2011 review found the evidence for calcium and vitamin D in skeletal health consistent with cause and effect, set the adult allowance at 600 IU a day to age 70 and 800 after, which corresponds to a serum level of at least 50 nmol/L, found the evidence for cancer, cardiovascular, diabetes and autoimmune outcomes inconsistent and inconclusive, noted U-shaped associations for some outcomes with higher levels no better, and concluded that the prevalence of inadequacy in North America had been overestimated (Ross 2011). The Endocrine Society's 2011 guideline took a different view of what sufficiency means and suggested testing people likely to be short of it and treating them, while finding insufficient evidence to screen everyone else or to prescribe vitamin D for cardiovascular benefit (Holick 2011). The largest trial since then, 25,871 US adults aged 50 and over given 2,000 IU a day for a median of 5.3 years, found no reduction in invasive cancer (hazard ratio 0.96) or in major cardiovascular events (0.97) against placebo (Manson 2019), and the Endocrine Society's 2024 guideline for people without an established indication, built on the randomized trials, took the position that routine 25-hydroxyvitamin D testing and supplementation above the dietary allowance are not supported for most healthy adults (Demay 2024). The field agrees on bone and disagrees on the threshold, and the entry shows the 50 and the 75 nmol/L figures as the two bodies' positions. New Zealand is the reason the test is on VT's recommended tier: in a national sample of 2,946 New Zealanders aged 15 and over, the mean was 47 nmol/L in women and 52 in men, 48 percent were at or below 50 nmol/L, levels were lower in Māori and Pacific people and in older women, and the swing from spring to summer was about 30 nmol/L (Rockell 2006); a late-winter and a late-summer value tell a member where their year actually sits.
What moves it
Season and latitude most of all; time outdoors, clothing and sunscreen; skin pigmentation; body fat, which stores the vitamin away from the blood (obese women in the national sample ran 6 nmol/L lower); age, as the skin's synthesis slows; oily fish and fortified foods; supplements, which take about three months to settle at a new level. High-dose biotin interfered with 25-hydroxyvitamin D immunoassays in the crossover study of healthy adults (Li 2017).
How to measure it well
Once at the end of winter and once at the end of summer in your first year, then yearly in late winter. No fasting needed. Pause high-dose biotin for three days. Ask for nmol/L; VT converts ng/mL by multiplying by 2.496. Same laboratory, because immunoassays and mass-spectrometry methods can differ by a tenth or more on the same sample. In New Zealand the test is funded only for specific indications and is usually self-pay.
How Vitality Tracker uses it
Stored as reported with the laboratory's range and flag, never overridden; typical laboratory reference above 50 nmol/L, with the New Zealand categories of deficient below 25 and insufficient from 25 to 50 shown as the laboratory's. VT's working optimal (v0.1) is 75 to 125 nmol/L, shown beneath in smaller type and marked working, with the note that the 2024 guideline questions routine testing in healthy adults; whether the test stays on the recommended tier is on the clinical advisor's list. The winter and summer results are shown as a pair, a vitamin D supplement on the Supplement Tracker is shown beside the result with its dose, and when calcium is on the register as well, corrected calcium from the same draw is shown with it. A dose change needs three months before a re-test.
How far to trust the number
Laboratory, with a method caveat: assays for this analyte vary between makers and methods more than most on the panel, so compare results within one laboratory. The seasonal swing is larger than any assay error, which is why the timing of the draw is part of the measurement.
The ranges
The laboratory’s range and flag are shown with your result and never overridden; the working optimal stays beneath, in smaller type, until the clinical advisor has reviewed it.
Sources
- Ross AC, et al. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know. J Clin Endocrinol Metab 2011;96:53–8. doi:10.1210/jc.2010-2704
- Holick MF, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab 2011;96:1911–30. doi:10.1210/jc.2011-0385
- Manson JE, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med 2019;380:33–44. doi:10.1056/NEJMoa1809944
- Demay MB, et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab 2024;109:1907–47. doi:10.1210/clinem/dgae290
- Rockell JE, Skeaff CM, Williams SM, Green TJ. Serum 25-hydroxyvitamin D concentrations of New Zealanders aged 15 years and older. Osteoporos Int 2006;17:1382–9. doi:10.1007/s00198-006-0118-x
- Li D, et al. JAMA 2017. doi:10.1001/jama.2017.13705
- Vitality Tracker, Recommended baseline lab panel v0.1; optimal set v0.1 (working).