Why we measure it

Vitamin D deficiency is endemic. Surveys across multiple countries consistently find that 40–60% of adults have 25(OH)D below 30 ng/mL (75 nmol/L), and a substantial proportion fall below 20 ng/mL (50 nmol/L) — the level associated with clear physiological impairment in calcium absorption, bone mineralisation, and immune regulation. The consequences of deficiency extend well beyond the classic rickets and osteomalacia of severe insufficiency: low vitamin D status is associated in prospective epidemiological studies with increased all-cause mortality, cardiovascular disease, type 2 diabetes, autoimmune conditions, respiratory infections, cognitive decline, and several cancers. The VITAL trial — the largest randomised controlled trial of vitamin D supplementation (25,871 participants, 2,000 IU vitamin D3 daily, five-year follow-up), published in the New England Journal of Medicine in 2019 — found no primary prevention effect for major cardiovascular events or cancer incidence in a population with baseline mean 25(OH)D of approximately 31 ng/mL. But it did find a significant reduction in cancer mortality and potential metabolic benefits in subgroup analyses. The VITAL result is important for calibrating expectations: it suggests that correcting frank deficiency has measurable benefit, but supplementation above sufficiency threshold in a population that is already broadly sufficient does not proportionally reduce disease risk. This makes knowing your actual baseline 25(OH)D level — and targeting sufficiency rather than arbitrary high doses — the rational approach.

Why every 6 months

Vitamin D status has a marked seasonal pattern in populations without year-round strong UV-B exposure: 25(OH)D peaks in late summer and early autumn, reaching its nadir in late winter and early spring. For someone supplementing year-round, the seasonal fluctuation is attenuated but not eliminated; the nadir and peak still provide useful information about whether the supplementation dose is achieving sufficiency across the full annual cycle. The six-month testing interval captures both the seasonal high and low points, and provides sufficient resolution to confirm that a supplementation adjustment has achieved its target — vitamin D levels respond measurably to supplementation changes within 8–12 weeks, reaching a new steady state by approximately 16–20 weeks.

What movement means

The Endocrine Society's 2011 clinical practice guideline defines deficiency as 25(OH)D below 20 ng/mL (50 nmol/L), insufficiency as 20–29 ng/mL (50–74 nmol/L), and sufficiency as ≥ 30 ng/mL (75 nmol/L). Some researchers and clinicians advocate for a higher optimal target — typically 40–60 ng/mL (100–150 nmol/L) — based on observational associations with reduced cancer and cardiovascular risk; but this range is not universally adopted in clinical guidelines and is not supported as a primary prevention target by the VITAL RCT. Toxicity (hypercalcaemia) is rare from supplementation alone but has been reported at sustained levels above 150 ng/mL (375 nmol/L), typically only achievable with excessive supplementation doses.

In the protocol