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Vitamin D

Vitamin D is a vitamin your skin mainly makes under sunlight that supports your bones, muscles, and immune defence.

What is Vitamin D?

Vitamin D is mostly made by your body itself in the skin, under sunlight. Food (oily fish, eggs, fortified dairy) supplies only a small part. The blood value reflects your stores over the past few weeks. The main driver is therefore not your diet but the sun — and the lack of it. In northern climates the sunlight from October to April is too weak to make vitamin D, so stores run down every year to a trough in early spring. Darker skin, spending a lot of time indoors, covering clothing, and older age lower production further.

Why is Vitamin D relevant?

Vitamin D's best-known role is regulating calcium absorption and keeping your bones healthy. With a deficiency, calcium absorption falls and bone density declines over time. A severe deficiency noticeably weakens the bones. Vitamin D also plays a role in muscle strength and balance — supplementation in older people with a deficiency demonstrably reduces fall risk — and in your immune defence. In Northern Europe a winter deficiency is the rule rather than the exception. At-risk groups include people with darker skin, older adults, those who spend a lot of time indoors, pregnant women, and people with obesity or absorption problems. For those groups and everyone over 70, routine supplementation is advised.

Vitamin D high or low — what it means

A single measurement is a snapshot that depends heavily on the season. Always note when you test: 60 nmol/L in January without supplementation is a concerning trough, while the same value in August is a comfortable starting point before winter. With supplementation, your body needs 8–12 weeks to reach a new equilibrium — re-testing earlier says little. For most people 1,000–2,000 IU a day is enough for maintenance; correcting a deficiency quickly needs a temporarily higher dose. With absorption problems (such as coeliac disease or after stomach surgery) the value can stay low despite supplementation — that group sometimes needs a higher dose.

Vitamin D reference ranges

Deficient (general population)Deficiency per Dutch Health Council/NHG; supplementation and correction if needed advised< 30 nmol/L
InadequateInadequate stores; topping up often sensible. From age 70, < 50 nmol/L already counts as deficiency.30-50 nmol/L
SufficientTarget for the general population; from age 70 also the lower bound.≥ 50 nmol/L
OptimalCommonly used optimal band (approx. 75-100 nmol/L); labs often report 50-100 nmol/L as the reference range≥ 75 nmol/L
Toxicity riskUsually indicates excessive supplementation; toxicity typically only arises after sustained high values> 250 nmol/L

Measured as 25-hydroxyvitamin D (25-OH-D) in nmol/L. Cut-offs vary by lab and method (UMC Utrecht, for instance, uses 50-100 nmol/L from age 12) and the value depends heavily on the season — the reference range on your own lab report is leading. Clinical cut-offs follow the Dutch Health Council (2012) and NHG.

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

Read about our scientific approach →Read the guide: Supplements →

Frequently asked questions

What is a normal vitamin D level?

Vitamin D is measured as 25-hydroxyvitamin D (25-OH-D) in nmol/L. A value of 50 nmol/L or higher is considered sufficient for the general population. Below 30 nmol/L is a deficiency; 30-50 nmol/L counts as inadequate stores. Many people aim for 75 nmol/L or higher (often 75-100) as an optimal band. Cut-offs can vary slightly between labs, so the reference range on your own lab report is leading.

What does a low vitamin D level mean?

A value below 50 nmol/L means your stores are inadequate; below 30 nmol/L is a clear deficiency. In the Netherlands this often tracks the season: from October to April the sunlight is too weak to make vitamin D. Over time a deficiency can lower calcium absorption and bone density and is linked to muscle weakness. The Dutch Health Council advises maintenance supplementation of 10 µg (400 IU) a day for women aged 50-69 and 20 µg (800 IU) for everyone aged 70 and over; a higher correction dose for a confirmed deficiency should always be discussed with your doctor.

What does a high vitamin D level mean?

A high vitamin D is rare and almost only arises from excessive supplementation, not from sunlight or food. A value above 250 nmol/L counts as intoxication and can coincide with elevated calcium levels; toxicity usually only develops after sustained high values. If your level is high and you have symptoms, discuss the dose with your doctor. After a dose change it takes 8-12 weeks before a new value is stable.

Is the cut-off different for older adults?

For the blood level the lower bound is age-based: under 70 a deficiency is below 30 nmol/L, and from age 70 that threshold is 50 nmol/L — for men and women alike. Separately, the Dutch Health Council advises routine supplementation regardless of the measured value: women from age 50 (10 µg/400 IU a day) and everyone from age 70 (20 µg/800 IU), plus at-risk groups such as people with darker skin, pregnant women and those who are indoors a lot.

When is a vitamin D level concerning?

When reading your value, mind the season: 60 nmol/L in January without supplementation is a concerning trough, while the same value in August is a comfortable starting point before winter. A value below 30 nmol/L is a clear deficiency and can lower bone density over time and coincide with muscle weakness. At the top end, above 250 nmol/L counts as intoxication. If your value is abnormal and you have symptoms, discuss the dose with your doctor; with absorption problems (such as coeliac disease or after stomach surgery) the value can stay low despite supplementation.

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Folate is a B vitamin your body needs for DNA synthesis, cell division and blood production.

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