Your vitamin D levels drop between October and March. Those are exactly the months when your training load peaks. At 52 degrees north your skin makes almost none. So you live off what you built up over summer.
The test measures 25-OH-D, the storage form. That number does say something about muscle function, but not the way it is often sold. Correcting a real deficiency helps. Pushing an already adequate value higher does not.
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| Result | Value (nmol/l) |
|---|---|
| Deficiency | < 30 |
| Sufficient | ≥ 30 |
| 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.
What you measure is a reserve, not a snapshot. 25-hydroxyvitamin D (25-OH-D) has a half-life of two to three weeks. The result in nmol/l therefore sums up supply over recent months. A sunny weekend barely moves it.
Nor does three days of supplements before the draw. That makes the test useful for evaluating a whole training block. It is unsuitable for tracking an acute change.
The active form, 1,25-dihydroxyvitamin D, does not belong here. It circulates for a matter of hours and is controlled by parathyroid hormone. When the reserve is empty, the body steps that conversion up.
A normal or raised 1,25 value therefore does not rule out a deficiency. Ordering it alongside this test adds no information.
Ask for total 25-OH-D, meaning D2 and D3 together. If you use a plant-based D2 product, that matters. Not every immunoassay fully recognises D2, so your value can read too low.
If you train seriously, muscle function is the reason to know this number. An honest split comes with it. Correcting a demonstrated deficiency improves muscle function and strength outcomes. Topping up an already adequate value delivers no measurable gain.
That asymmetry is the point. Supplementation is not an ergogenic aid. A deficiency is a brake. By the end of a Dutch winter the odds of one are considerable.
The mechanism runs through bone and calcium. Vitamin D drives the absorption of calcium and phosphate from the gut. As the reserve falls, that absorption drops and parathyroid hormone rises.
Calcium is then pulled out of the skeleton. With sustained deep deficiency, new bone mineralises inadequately. Under heavy loading that is unfavourable for bone strength.
The Dutch Health Council does not use a laboratory reference, but decision thresholds. Below 30 nmol/l is deficiency. Between 30 and 50 nmol/l is insufficiency.
From 50 nmol/l the value is associated in research with fracture prevention. Above 75 nmol/l there is no demonstrated benefit for healthy adults. So it is not a number to chase.
Reference values also differ between laboratories. Read your result against the limits your own lab states. Judge the number alongside calcium and parathyroid hormone.
Measure at the edges of your winter block, not the middle. A test in September records your summer peak. A second in February or March shows how far you dropped.
That difference is the only way to know whether you get through the winter. Without the month attached, a single number says little.
Test too when your programming stayed the same but recovery got noticeably worse. Vitamin D is then one thing you can rule out.
A low reserve often gives no signal at all. That is exactly the problem: you will not feel it in a session. When something is noticeable, it sits in the proximal muscles.
So in the thighs and hip girdle. Think of:
With prolonged very low values, mineralisation of new bone can fall short. That is called osteomalacia and it matters under heavy loading.
None of these complaints is specific. Overreaching, poor sleep, low energy availability or iron deficiency all look the same. So the measurement prompts the conversation, not the feeling.
In athletes a high 25-OH-D almost always comes from a bottle, not the sun. The skin shuts its own production down once there is enough. Are you stacking a multivitamin, an omega-3 product containing vitamin D and a separate D3?
Then you are adding up without noticing. Real toxicity with hypercalcaemia is generally seen only above roughly 250 nmol/l. That requires sustained very high intake.
There is an exception that weighs more than the number itself. Sarcoidosis, tuberculosis and some lymphomas fall under this. In those conditions macrophages make the active form themselves, outside any regulation.
Hypercalcaemia can occur at an entirely ordinary 25-OH-D. A raised calcium alongside vitamin D therefore belongs with the GP.
Low vitamin D may cause fatigue, muscle weakness, and bone pain. Consider 2000-5000 IU daily supplementation and increase sun exposure.
High vitamin D may indicate excessive supplementation. Reduce supplement dose and retest in 3 months.
Low vitamin D may cause fatigue, muscle weakness, and bone pain. Consider 2000-5000 IU daily supplementation and increase sun exposure.
High vitamin D may indicate excessive supplementation. Reduce supplement dose and retest in 3 months.
Time the measurement around your training block. A draw in September shows your summer peak. A draw in February or March shows the floor.
Want to know whether you get through the winter? Measure at the start of the winter block and again near the end. Note the month with every result.
Without that month, a difference between two values cannot be read. At roughly 52 degrees north the sun sits too low from October through March. Too little UVB (290 to 315 nm) then reaches the skin. So the reserve falls in that period even in someone who changed nothing.
A few factors make the floor deeper:
The Dutch Health Council advises certain groups to supplement. Those include people with darker skin and people who get little daylight. What amount goes with that is a question for your GP or the Voedingscentrum.
Do you take high-dose biotin? Mention it before the draw. It can interfere with some 25-OH-D immunoassays and falsely raise your value.
Only if you are correcting a demonstrated deficiency. Then muscle function and strength outcomes improve. If your value is already adequate, topping up delivers no measurable gain. Supplementation is not an ergogenic aid, but a deficiency is a brake.
Twice, at the edges of the winter block. September shows your highest point. February or March shows your lowest. The gap between them says more than one isolated result.
Probably not, if the months differ. Between October and March your skin makes almost none. The reserve falls in everyone then. Only compare results from the same month.
That is the usual cause in athletes. A multivitamin, an omega-3 containing vitamin D and a separate D3 all add up. Real toxicity is far off, generally above roughly 250 nmol/l. Talk to your GP before stacking.
This marker is included in the following test panels.
Measure your vitamin D, important for strength, recovery and energy. No referral needed, results within a few working days.
A 5-biomarker inflammation panel inspired by InsideTracker's Inflammation category — hs-CRP, a complete blood count with differential, vitamin D and ferritin behind your inflammation score.
A 10-biomarker recovery panel inspired by InsideTracker's Recovery category — muscle enzymes, liver values, electrolytes, magnesium and vitamin D behind your recovery score.
A 10-biomarker sleep panel inspired by InsideTracker's Sleep category — blood sugar regulation, magnesium, vitamin D, inflammation, testosterone and cortisol behind your sleep score.
Articles that explain this value in context.
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Vitamin D (25-OH)
€35,-
