Your vitamin D level tells you how much vitamin D your body has in store, and that store drops between October and March, exactly the months when your training load peaks. At 52 degrees north your skin makes almost none in that period, 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 differently from how it is often sold: correcting a real deficiency helps, while pushing an already adequate value higher delivers nothing measurable.
Doctor's Assessment Included
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.
| Result | Value (nmol/l) |
|---|---|
| Deficiency | < 30 |
| Sufficient | ≥ 30 |
| Result | Value (nmol/l) |
|---|---|
| Severe deficiency | < 35 |
| Deficiency | 35–50 |
| Sufficient | ≥ 50 |
Dutch guidelines disagree on this limit. The Dutch National Health Care Institute (Zorginstituut) concluded in 2022 that there is no single agreed limit for vitamin D deficiency. We follow the Health Council of the Netherlands (Gezondheidsraad), the NHG and the Farmacotherapeutisch Kompas. The NVKC uses 50 nmol/l for everyone; the osteoporosis guideline uses 50 nmol/l when fracture risk is raised.
Source: Gezondheidsraad Reference population: Dutch population (Health Council of the Netherlands 2012)
Source: Zorginstituut Nederland Reference population: Dutch population (Health Council of the Netherlands 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 rather than a snapshot. The laboratory determines 25-hydroxyvitamin D, 25-OH-D for short, the form in which your body stores vitamin D. That compound has a half-life of two to three weeks, so the result in nmol/l sums up supply over recent months. A sunny weekend barely moves it, and neither does three days of supplements before the draw. That makes the test useful for evaluating a whole training block and unsuitable for tracking an acute change.
The active form, 1,25-dihydroxyvitamin D, is deliberately not measured here. It circulates for a matter of hours and is controlled by parathyroid hormone, the hormone from the parathyroid glands that regulates your calcium. When the reserve is empty, conversion to that active form is stepped up, so a normal or even raised 1,25 value does not rule out a deficiency. Ordering it alongside this test therefore adds no information.
Ask for total 25-OH-D, meaning D2 and D3 together. If you use a plant-based D2 product, that matters, because not every immunoassay fully recognises D2 and your value can then read too low.
On your report the laboratory prints a reference range of 50 to 150 nmol/l. The Dutch Health Council uses a different yardstick: no reference range, but a floor that must be met all year round, 30 nmol/l until your seventieth birthday and 50 nmol/l after it. A value between 30 and 50 is therefore flagged as low by the lab while the Health Council regards it as sufficient under seventy; what that means for you is a conversation with your doctor.
For your muscle function this number only matters when it is too low, and that split is the point. Correcting a demonstrated deficiency improves muscle function and strength outcomes; topping up an already adequate value delivers no measurable gain. Supplementation is therefore no ergogenic aid, while a deficiency is a brake, and 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, after which calcium is pulled out of the skeleton to keep the blood level steady. With sustained deep deficiency, new bone mineralises inadequately, which under heavy loading is unfavourable for your bone strength.
The Dutch Health Council works with a floor rather than a laboratory reference. Until your seventieth birthday, at least 30 nmol/l is the value that must be met all year; below it the term is deficiency. From seventy that line sits at 50 nmol/l, and the Farmacotherapeutisch Kompas calls a value below 35 nmol/l at that age a severe deficiency. The Health Council names no higher target above that floor, so a high number is no goal to chase.
The laboratory also prints its own range of 50 to 150 nmol/l, so the two yardsticks differ below 50. Read your result against both, and judge the number alongside calcium and parathyroid hormone: a low 25-OH-D with a raised parathyroid hormone weighs more than the vitamin D figure alone.
Measure at the edges of your winter block rather than the middle. A test in September records your summer peak, and 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, alongside sleep, energy intake and iron.
Fasting is not needed and the time of day does not matter. The result is back within about 72 hours. If you take high-dose biotin, mention it at the draw, because it can interfere with some 25-OH-D immunoassays and falsely raise your value.
A low reserve often gives no signal at all, and that is exactly the problem: you will not feel it in a session. When something is noticeable, it sits in the muscles closest to the trunk, so in the thighs and hip girdle. Legs that feel heavier on unchanged programming, difficulty standing up out of a deep squat, slower recovery after hard sessions and diffuse bone and muscle pain fit that picture. With prolonged very low values, mineralisation of new bone can fall short, which is called osteomalacia and matters under heavy loading.
None of these complaints is specific. Overreaching, poor sleep, low energy availability or iron deficiency all look the same, and that is why the measurement prompts the conversation rather than the feeling.
In athletes a high 25-OH-D almost always comes from a bottle and rarely from the sun, because the skin shuts its own production down once there is enough. If you stack a multivitamin, an omega-3 product containing vitamin D and a separate D3, you are adding up without noticing. Real toxicity with a raised calcium is generally seen in the literature only above roughly 250 nmol/l, and that requires sustained very high intake.
There is an exception that weighs more than the number itself. In sarcoidosis, tuberculosis and some lymphomas, macrophages make the active form themselves, outside any regulation. Calcium can then run high at an entirely ordinary 25-OH-D. If you see a raised calcium alongside vitamin D, take it to your GP before you change anything about your supplements.
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 and note the month with every result. At roughly 52 degrees north the sun sits too low from October through March, so too little UVB of 290 to 315 nanometres reaches your skin and the reserve falls in that period even in someone who changed nothing. Without that month, a difference between two values cannot be read.
A few factors make the winter floor deeper: darker skin, because melanin absorbs UVB and more exposure is needed; covering clothing, indoor work and night shifts; a higher body fat percentage, because vitamin D is sequestered in adipose tissue; fat malabsorption; and enzyme-inducing medication such as anticonvulsants and rifampicin, or glucocorticoids, which accelerate breakdown.
The Dutch Health Council advises certain groups to supplement, including people with darker skin and people who get little daylight. What amount goes with that is a question for your GP or the Voedingscentrum, and stacking a supplement on an already adequate value does nothing for your performance.
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 therefore no ergogenic aid, while a deficiency is a brake.
Twice, at the edges of the winter block. September shows your highest point, February or March your lowest. The gap between them says more than one isolated result, and it is the only way to see whether you get through the winter.
Probably not, if the months differ. Between October and March your skin makes almost none and the reserve falls in everyone. Only compare results from the same month, and read them against the Dutch Health Council floor: 30 nmol/l until seventy, 50 nmol/l after that.
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 in the literature, but the laboratory prints 150 nmol/l as its upper limit. 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.
Medical reviewer
Dr. Naimi oversees the medical standards behind our content and assessments.
Medical policyDoctor's Assessment Included
Every result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
Vitamin D (25-OH)
€35,-
We use cookies to analyze site usage and measure ad effectiveness. Privacy Policy
