Vitamin D

What is Vitamin D?

Vitamin D (measured as 25-hydroxyvitamin D, abbreviated 25(OH)D) is technically not an ordinary vitamin but a precursor to a steroid hormone — your body makes it through a reaction that starts in the skin. UV-B radiation converts cholesterol in the skin to previtamin D3; that travels via the blood to the liver for the first processing step to 25(OH)D, the stable circulating storage molecule measured in the bloodstream. From there it moves to the kidneys — and other tissues — for the second step to the biologically active 1,25-dihydroxyvitamin D (calcitriol). Dietary sources (oily fish, eggs, vitamin D-fortified dairy and margarine) contribute only a modest fraction of what the skin can produce with adequate sun exposure. The value is reported in nanomoles per litre (nmol/L) or, in international literature, nanograms per millilitre (ng/mL). The conversion factor is 2.5: 1 ng/mL equals 2.5 nmol/L. As a general guideline (guideline- and lab-dependent): values below 30 nmol/L (12 ng/mL) are typically severe deficiency; 30–50 nmol/L (12–20 ng/mL) insufficient; above 50 nmol/L (20 ng/mL) adequate for most purposes, with 75–125 nmol/L (30–50 ng/mL) often considered optimal. The Dutch Health Council (Gezondheidsraad) uses 50 nmol/L as the practical adequacy threshold for adults; above 250 nmol/L (100 ng/mL) toxicity can occur, though this is rarely a problem with standard oral supplementation. The most important driver of the value is not diet but sun — and more specifically the lack of it. In the Netherlands and Belgium, UV-B radiation is too weak from October to April to make meaningful amounts of vitamin D (the sun angle is too low), causing stores to run down every year, reaching their trough in January–March. Skin pigmentation plays an equally important role: people with more melanin need more UV-B exposure for the same production. Covering clothing, sunscreen, spending all day indoors, and older age (production capacity declines after middle age) all suppress the value further.

Why is Vitamin D relevant?

The most established role of vitamin D is regulating calcium absorption in the intestine and supporting bone mineralisation. Without sufficient vitamin D, calcium absorption falls, the parathyroid glands respond with higher PTH output to mobilise calcium from bone, and bone density declines over time. Severe deficiency causes rickets in children and osteomalacia in adults — above a broad threshold that varies between tissues. For muscle function there is robust evidence: vitamin D receptors are present in muscle tissue, deficiency is associated with loss of muscle strength and balance, and supplementation in deficient older adults reduces fall risk — one of the best-supported applications of vitamin D supplements. For people who train intensively or are recovering from injury, an adequate vitamin D level may therefore be worthwhile beyond its skeletal role. The immune system is a third domain: vitamin D receptors play a role in both innate and adaptive immunity. Consistent associations have been found with susceptibility to respiratory infections and certain autoimmune conditions — the evidence for causality across all those conditions is not uniformly strong, but it is part of why vitamin D is broadly tracked. In Northern Europe, deficiency during winter and early spring is the rule rather than the exception. At-risk groups are: people with darker skin pigmentation, older adults, those who spend all day indoors, pregnant and breastfeeding women (increased requirements), people with obesity (vitamin D accumulates in fat tissue and circulates less), and those with malabsorption conditions (coeliac disease, IBD, gastric bypass). The Dutch Health Council advises routine supplementation for those groups — and for everyone above age 70.

Vitamin D high or low — what it means

A single vitamin D measurement is a snapshot that depends heavily on the season and any current supplementation. Always note which season the test is taken in: a value of 60 nmol/L in January in someone not currently supplementing is a concerning trough; the same 60 nmol/L in August is a comfortable starting point before the winter months. With supplementation, the system takes 8–12 weeks to reach a new equilibrium — re-testing earlier gives an unreliable picture of the effect of a dose change. As a rule of thumb, values in most adults rise by roughly 10–25 nmol/L per 25 µg (1,000 IU) of extra daily supplementation, but this varies considerably with body weight, starting level, and absorption. Higher doses (75–100 µg/day, 3,000–4,000 IU) are sometimes needed to correct a deficiency quickly; for maintenance 25–50 µg/day (1,000–2,000 IU) is sufficient for most people. Combine vitamin D with calcium and corrected calcium when bone markers are in question — that shows whether a low vitamin D is also affecting calcium metabolism. Conversely, a high 25(OH)D alongside hypercalcaemia calls for investigation of toxicity. In people with malabsorption conditions (coeliac, IBD, gastric bypass), unexpectedly low values despite supplementation should prompt a review — this group sometimes needs higher doses or may do better with intramuscular administration. An elevated PTH alongside normal or low vitamin D is an indication for additional evaluation of parathyroid function and calcium balance.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.