At the end of a long, grey winter you feel listless, your muscles are tired and your mood is low. Coincidence? Maybe. But this is exactly the moment when the vitamin D level of many people in the Netherlands is at its lowest, and those broad, vague complaints fit that strikingly well.
My view: vitamin D is no miracle cure, but with this symptom pattern at the end of winter it is one of the first values I would have checked.
Why is vitamin D important?
Strictly speaking, vitamin D is a hormone and plays a role in the absorption of calcium and your bone health (preventing osteoporosis), your immune system, your muscle function, your mood and in cell division and tissue growth. You mainly make it in your skin under the influence of sunlight, and to a lesser extent get it from food.
So most of your vitamin D comes not from your plate but from the sun. Under the influence of UVB radiation, your skin converts a compound into vitamin D, which your liver and kidneys then activate into the form your body uses. This explains why your location, your skin colour and your time outdoors have so much influence: less sun on the skin means less production, however healthily you otherwise eat.
Vitamin D deficiency symptoms
The symptoms are often subtle and not always recognised. Because the complaints are broad, they give no certainty without a blood test. Common signs:
- Fatigue, one of the most common complaints, which can also point to a vitamin B12 deficiency
- Muscle weakness or muscle pain, especially in legs and back
- Bone pain, diffuse and mainly in back, hips and legs
- Low mood, because vitamin D affects serotonin production
- Getting ill frequently, from reduced resistance
- Slow wound healing
- Hair loss in severe deficiencies
Vitamin D reference values
Vitamin D is measured as 25-hydroxyvitamin D, expressed in nmol/L. The following classification is often used:
| Value (nmol/L) | Interpretation | What it means |
|---|---|---|
| Above 75 | Optimal | More than sufficient |
| 50 to 75 | Sufficient | No deficiency |
| 30 to 50 | Insufficient (insufficiency) | Consider supplementing |
| Below 30 | Deficiency | Consult your doctor |
Who is at risk of a vitamin D deficiency?
- Everyone in the Netherlands: from October to April sunlight is insufficient for production
- People with dark skin, because melanin reduces production in the skin
- Older adults, whose skin makes vitamin D less efficiently
- People who are outdoors little, such as home workers and office staff
- People with overweight, because vitamin D is stored in fat tissue and less available
- People who cover their skin
A prolonged deficiency is not harmless. Read more about how serious a vitamin D deficiency is.
Why the GP does not test everyone
You might think: just measure vitamin D in everyone, since the deficiency is so widespread. Yet the Gezondheidsraad advises against large-scale screening. The reason is that the advice for the known risk groups is already clear, namely simply to supplement, and a test result changes little there. For you personally a test can still be useful: if you have symptoms you want to explain, if you doubt whether your supplementation is working, or if you belong to a risk group and want certainty. The value of a test lies in answering a concrete question, not in routine measuring without a reason.
Correcting a vitamin D deficiency
- Sunlight: 15 to 30 minutes per day in summer (arms and face exposed, without sunscreen). Insufficient in the Dutch winter.
- Food: oily fish, eggs and fortified dairy. Food alone is rarely enough.
- Supplements: the Gezondheidsraad advises 10 ug per day for adults up to 70 and 20 ug for those over 70. Also read how much vitamin D you need per day. With a proven deficiency your doctor may advise a higher dose.
Frequently asked questions
When is the best time to test my vitamin D?
At the end of winter (February and March) your level is at its lowest. That is the most informative time to test. After summer your level is at its highest.
Can I take too much vitamin D?
Not through sunlight and food. Through supplements an overdose is possible at very high doses (prolonged above 100 ug per day). Stick to your doctor's advice and have your level checked.
Does a tanning bed help for vitamin D?
Tanning beds mainly produce UVA radiation, while your skin needs UVB. They are discouraged as a vitamin D source and increase the skin cancer risk.
Getting your vitamin D measured
If you want your vitamin D measured, view your vitamin D value. Because vitamin D works closely with calcium, it can be useful to include it. To place vitamin D in a broader picture, the basic health checkup covers your key values. This article is part of our vitamin deficiency overview.
Every blood test result at Vitalcheck includes a professional assessment by a BIG-registered doctor. A blood value is not a diagnosis: always discuss symptoms and treatment decisions with your GP.
Sources
- Gezondheidsraad. Dietary reference values for vitamins and minerals. 2018.
- RIVM. Vitamin D deficiency in winter among the Dutch population. 2023.
- Thuisarts.nl / NHG. Vitamin D. Accessed 2026.
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