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Vitamins

Your vitamin D levels: skin, season and pregnancy

Your vitamin D levels show how much vitamin D your body has in store. In the Netherlands that store depends almost entirely on the UVB your skin caught last summer. Two groups come out low unusually often. The first is women who are pregnant or breastfeeding.

The second is women who cover their skin or have darker skin. So the month of the blood draw belongs with the result. Any advice about supplementing runs through your midwife or GP.

Doctor's Assessment Included

When is this value abnormal?

See the value that applies to you:

Choose male or female — this range differs by sex.

Enter your age — this range changes with age.

This range also depends on context such as cycle phase or sample material; the highlighted rows apply to your group.

up to 69 years Your group

Decision limits per result, in nmol/l
Result Value (nmol/l)
Deficiency < 30
Sufficient ≥ 30

70 years and older Your group

Decision limits per result, in nmol/l
Result Value (nmol/l)
Severe deficiency < 35
Deficiency 35–50
Sufficient ≥ 50

Nederlandse richtlijnen zijn het niet eens over deze grens. Het Zorginstituut stelt vast: "Er ontbreekt een eenduidige grens voor vitamine D-deficiëntie" (2022). Wij volgen de Gezondheidsraad, het NHG en het Farmacotherapeutisch Kompas. De NVKC hanteert 50 nmol/l voor iedereen; de osteoporoserichtlijn hanteert 50 nmol/l bij verhoogd fractuurrisico.

Source: Gezondheidsraad Reference population: Nederlandse bevolking (Gezondheidsraad 2012)

Source: Zorginstituut Nederland Reference population: Nederlandse bevolking (Gezondheidsraad 2012)

Reference ranges may vary between laboratories. When you order a test, a BIG-registered doctor assesses your personal results in context. For treatment decisions, discuss your results with your GP.

Vitamin D (25-OH): what this test measures

What goes into the tube is 25-hydroxyvitamin D (25-OH-D), reported in nmol/l. It is the form your body stores vitamin D in. It is also the only form that says anything about the size of that store.

The store shifts slowly. Half of the 25-OH-D in your blood is still there two to three weeks later. The result therefore describes the past few months, not yesterday. A sunny week just before the draw will not pull the number straight.

The active hormone, 1,25-dihydroxyvitamin D, is deliberately left out. It survives only hours, and parathyroid hormone keeps it on a tight rein. When the store runs down, your body converts harder. You can have a perfectly normal active level with an empty shelf behind it.

Ask for total 25-OH-D, D2 and D3 together. If you take a plant-based D2 supplement, that matters. Some immunoassays recover D2 incompletely.

Vitamin D (25-OH): why this value matters

Vitamin D decides how much calcium and phosphate your gut takes out of your food. When the store empties, your gut absorbs less calcium. Parathyroid hormone rises and pulls calcium out of your bones to keep blood calcium steady. A low store therefore shows up in your bones, not in your blood calcium.

Pregnancy and breastfeeding ask more of that system. Your baby's skeleton is built through your calcium balance. The vitamin D your child is born with comes out of your store.

That is why the Gezondheidsraad lists pregnant women among the groups advised to supplement. How much that advice involves is a question for your midwife or GP.

The Gezondheidsraad, the Dutch Health Council, uses decision limits rather than a reference range:

25-OH-D in nmol/lWhat the Health Council calls it
Below 30Deficiency
30 to 50Insufficiency
50 or aboveAdequate, linked to fewer fractures

Above 50 nmol/l there is no demonstrated extra gain. A higher number is not a target to chase. Laboratories also apply their own reference values. Read your number against the limits printed on your own report.

Never read the figure apart from calcium and parathyroid hormone. A low 25-OH-D with a raised PTH tells you more than the vitamin D figure alone.

Vitamin D (25-OH): when is testing worthwhile?

Are you pregnant or breastfeeding? Discuss testing with your midwife or GP. The Dutch Health Council names pregnant women as a group that supplements.

Test too if you cover your skin or have darker skin. Both shrink your summer reserve, and that reserve has to carry you through winter.

The month of your draw decides what you see. September is your highest point, March your lowest. Only compare two results if they come from the same month.

Vitamin D (25-OH): symptoms of a high or low value

Low Levels

An empty store is quiet for a long time. Plenty of women with a value below 30 nmol/l feel fine. The number usually surfaces through a blood test rather than a complaint.

When symptoms do appear, they are vague and unspecific:

  • weakness in your thighs
  • trouble getting up out of a low chair or a squat
  • aching bones and muscles
  • a less steady walk

Dozens of other causes fit that same picture. During pregnancy and the months after birth they barely stand out. They get filed under tiredness or recovery.

A long, deep shortage leaves new bone tissue poorly mineralised. That is osteomalacia in adults and rickets in children. So the number starts the conversation, not the symptom.

High Levels

A high 25-OH-D almost always comes out of a jar, not out of the sun. Your skin stops making more once it has enough. In pregnancy there is a practical wrinkle.

A pregnancy multivitamin usually contains vitamin D already. Take a separate preparation alongside it and two sources quietly add up. Bring a list of everything you swallow to your midwife or GP.

Poisoning is generally only seen above roughly 250 nmol/l. There is one exception that weighs more heavily. In granulomatous disease such as sarcoidosis and tuberculosis, something else happens. Some lymphomas do the same.

Macrophages convert 25-OH-D to the active form without regulation. Calcium can then run high at a completely ordinary 25-OH-D. Raised calcium alongside vitamin D is a question for your GP.

Vitamin D (25-OH): what to do about an abnormal value

Female

If Low

Low vitamin D may cause fatigue, muscle weakness, and bone pain. Consider 2000-5000 IU daily supplementation and increase sun exposure.

If High

High vitamin D may indicate excessive supplementation. Reduce supplement dose and retest in 3 months.

Vitamin D (25-OH): lifestyle and this value

The Netherlands sits at about 52 degrees north. From October through March the sun stands too low. Almost no UVB (290 to 315 nm) reaches the ground.

Your skin makes next to nothing in those months. You live off what you built up in summer.

How large that summer store gets varies a lot. Melanin absorbs UVB. Darker skin needs more time in the same sun for the same synthesis. Covering clothing works the same way.

Where no UVB reaches skin, nothing happens. None of this is about lifestyle or about doing something wrong. It is physics at a latitude where the sun is too weak for half the year. That is why low values are measured more often here in women with darker skin.

The same holds for women who cover their skin. They go into the winter with a smaller reserve.

Other reasons for a low result:

  • being indoors most of the day, or working nights
  • obesity, because vitamin D is sequestered in fat tissue
  • fat malabsorption
  • drugs that speed up its breakdown, such as anticonvulsants, rifampicin and glucocorticoids

The Gezondheidsraad advises several groups to supplement. Pregnant women are on that list, as are people with darker skin. So are people who are rarely outdoors. The amount is a question for your midwife, GP or the Voedingscentrum.

Vitamin D (25-OH): frequently asked questions

Does vitamin D matter during pregnancy?

The Dutch Health Council names pregnant women as a group that supplements. What amount goes with that is a conversation with your midwife or GP. We deliberately give no figure for it.

Why is my value lower than a friend's?

Skin and clothing explain a lot. Melanin absorbs UVB, so darker skin needs more exposure for the same synthesis. Covering clothing works the same way. That is physics, not lifestyle.

Does the month of my blood draw count?

Strongly. From October through March the sun sits too low for synthesis. You live off your summer reserve then. A March result should sit lower than a September one.

My value is high. Is that my pregnancy vitamins?

Often yes. A prenatal multivitamin usually contains vitamin D. If you also take a separate product, the two add up. Talk to your midwife or GP before changing anything.

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Vitamin D (25-OH)

€35,-