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Vitamins

Your vitamin D levels after six months indoors

Your vitamin D levels show how much vitamin D you have in store. In the Netherlands that mostly reflects the sun of the past few months. Do you train indoors and work indoors? Do you commute in the dark?

Then a low result in February says nothing about your effort. The number follows your exposure, not your discipline. The cut-off also differs between laboratories. Read your result next to the range printed on your own report.

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 the laboratory measures is 25-hydroxyvitamin D, or 25-OH-D, reported in nmol/l. That is the storage form. It describes your reserve, not your day.

Its half-life is two to three weeks. The result therefore lags months behind your behaviour. A sunny weekend barely moves it. Half a year indoors moves it a lot.

The active form, 1,25-dihydroxyvitamin D, does not belong in this test. It shifts within hours and parathyroid hormone keeps it in check. It can read normal or even high while your reserve is empty.

So do not measure the active form when you suspect a shortage. You measure the wrong thing and get a reassuring number back.

Ask for total 25-OH-D, meaning D2 and D3 together. That counts if you use a plant-based D2 product. Not every immunoassay recovers D2 fully. Your result then comes out below your actual status.

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

If you lift, you will meet the testosterone question sooner or later. Observational studies do find low vitamin D status and lower testosterone together. That observation is real. It does not show that one causes the other.

Men with a low value also tend to carry more fat mass. They get outdoors less and are more often in poorer health. Those three lower testosterone on their own. The season moves all of them at once.

In controlled trials men are given vitamin D. The effect on testosterone there is inconsistent and usually absent. So learn your status because a shortage matters in itself. Not because you are pulling a hormonal lever.

What the value does govern directly is calcium and bone. Too little vitamin D means less calcium absorbed from your food. Parathyroid hormone rises and the skeleton is used as a buffer.

The Dutch Health Council calls below 30 nmol/l a deficiency. Between 30 and 50 nmol/l the reserve is insufficient. From 50 nmol/l it has been linked to fewer fractures.

Stacking higher is not a goal. So read the result alongside calcium and parathyroid hormone. A low reserve with a rising PTH weighs more than the vitamin D number alone.

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

Measure at the end of winter, not in August. August shows your summer peak, and that says nothing about the rest of the year.

Do you train indoors, work indoors and commute in the dark? Then February or March is when your value sits lowest.

Test too if strength or recovery slips while your programming stayed the same. Vitamin D is one thing you can tick off.

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

Low Levels

Most of the time you notice nothing. A reserve can sit low for months without anything feeling different. The number therefore turns up before the complaint does.

When something does show, it sits in the muscles closest to the trunk:

  • heavy legs coming out of a deep squat
  • stairs that feel harder than they should
  • a dull ache in bones and muscles

A deep, long-standing shortage leaves new bone poorly mineralised. In adults that is called osteomalacia.

A lifter puts exactly these things down to volume or sleep. Usually that is right. They point at vitamin D and at dozens of other causes. So the blood result decides here, not how you feel.

High Levels

A high result almost always comes out of a bottle, not out of the sun. Once the skin has made enough, it shuts its own production down. Plenty of men stack several sources without noticing:

  • vitamin D on its own
  • a multivitamin
  • a fish oil blend with D in it
  • a sports formula

Real toxicity with a raised calcium is seen only well above 250 nmol/l. Prolonged very high intake comes first. The exception matters more. In granulomatous disease such as sarcoidosis and tuberculosis, and in some lymphomas, something else happens.

Macrophages make the active form themselves, outside the normal control. Calcium can then climb at a perfectly ordinary 25-OH-D. If your calcium is climbing too, ask your GP.

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

Male

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

From October through March the sun sits too low here. The UVB your skin needs (290 to 315 nm) barely reaches the ground. You make almost none of your own in those months.

You live off the summer reserve. A gym full of daylight does not help, because glass blocks UVB.

So write the month of the draw next to every result. Put March beside September without that date and you see a drop. That drop is only the season.

Expect a lower value with:

  • indoor work and night shifts
  • darker skin, because melanin soaks up UVB
  • covering clothing
  • a higher fat mass, because vitamin D accumulates in fat tissue and less stays in the blood
  • older age, because the skin holds less of the precursor
  • impaired fat absorption
  • anticonvulsants, rifampicin and glucocorticoids, which speed up breakdown

The Dutch Health Council names groups who are advised to supplement. They include people who spend little time outdoors and people with darker skin. How much that means is a question for your GP or the Voedingscentrum.

If you take high-dose biotin, say so at the blood draw. It can falsely raise some 25-OH-D assays.

Vitamin D (25-OH): frequently asked questions

Does vitamin D raise my testosterone?

There is an observed association between vitamin D status and testosterone. An association is not a cause. The studies are confounded by season, body fat and general health. There is no evidence that raising your vitamin D raises your testosterone.

I have trained hard for six months and my value is low. How?

Training barely matters here. Your skin makes vitamin D under UVB. If you train indoors and commute in the dark, you never get that UVB. Your effort is not the problem, your exposure is.

When should I measure?

At the end of winter. February or March shows your lowest point. A test in August flatters you and says little about the rest of the year.

Can my value get too high?

Not from the sun. Your skin shuts production down once there is enough. From stacking, yes: a multivitamin, an omega-3 with vitamin D and a separate D3 all add up. Real toxicity generally sits above roughly 250 nmol/l.

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Vitamin D (25-OH)

€35,-