Vitamin D deficiency: causes, symptoms and when to test

Which complaints can fit a vitamin D deficiency?

Vitamin D plays a role in building bone, in muscle function and in the immune system. A deficiency usually develops gradually, and that is exactly why the complaints are so unremarkable: fatigue, muscle complaints or muscle aches, and with a prolonged, severe deficiency also bone complaints.

Low mood in the darker months is also often linked to vitamin D. That link is weaker than it sometimes appears: a lack of daylight affects mood through other routes as well, and low mood, like fatigue, has many possible causes.

So let us be honest: none of these complaints is specific to vitamin D. If you are tired, you may have a deficiency, but you may just as well be sleeping too little, have an underactive thyroid or simply be going through a demanding period. Conversely, many people with a low level notice nothing at all.

Why a deficiency is so common in the Netherlands

The Netherlands lies at a northern latitude. From roughly October to April the sun is too low to trigger vitamin D production in the skin, however sunny a winter day may look. During those months your body draws on the stores built up in summer, and by the end of winter those stores are low in many people.

Modern life adds to this. If you work indoors during the day, you miss the hours when the sun is strong enough. Sunscreen slows production, although preventing sunburn remains sensible. And the more skin is covered, for example by concealing clothing or a veil, the less is produced.

• people with darker skin tones produce less vitamin D from the same amount of sun
• older adults produce vitamin D in the skin less efficiently
• those who spend much of the day indoors, or who keep their skin largely covered outdoors, build up little reserve
• with excess weight, part of the vitamin D is stored in fatty tissue, leaving less available for the rest of the body

These factors often stack: an office job, darker skin and a long winter together make a deficiency more the rule than the exception. That explains why a low vitamin D level is found so often in the Netherlands.

Who is advised to supplement regardless?

For a number of groups, the Dutch Health Council advises vitamin D supplementation regardless of complaints and regardless of any test. These include young children, older adults, people with darker skin and pregnant women, as well as people who spend little time outdoors or keep their skin largely covered.

If you belong to one of these groups, a blood test is not required before starting supplementation: the advice exists precisely because a deficiency is so common in these groups. Which form and dose suit you is something to discuss with your doctor or pharmacist, or you can follow the product's package leaflet.

Testing or simply supplementing?

Not everyone needs to test first. If you belong to an advice group and have no complaints, a test often adds little: the advice to supplement does not change because of it.

There are, however, situations in which a test has clear added value:

• with persistent complaints such as fatigue or muscle complaints, to find out whether a low level plays a part for you
• when you are unsure whether supplementation makes sense in your situation, for example because you do not belong to an advice group
• to follow the effect of supplementation: has the level actually risen after a few months
• if you want to stop supplementing and want to know where you stand

Bear the season in mind when timing the test. Levels are generally at their lowest at the end of winter and at their highest at the end of summer. If you already take vitamin D, you are mainly measuring the effect of that supplement; that may be exactly the question you want answered.

What do you do with the result?

If your level turns out to be low, supplementation is usually the logical next step. The right dose is not something to decide on your own based on the result: discuss it with your doctor or pharmacist, or follow the package leaflet. With a Bloodworks result you receive a doctor's interpretation, so you know how to read the value in your situation.

Do not expect a rapid change in your blood afterwards. The level rises gradually; a repeat test only becomes useful after a few months. Testing again sooner mainly shows that the body needs time, not whether the approach is working.

If the level is normal, that is information too. If the complaints persist, the explanation probably lies elsewhere, and that is a good moment to look more broadly.

Sun and food: what do they contribute?

In the months when the sun is strong enough, roughly from April to October, your skin produces vitamin D when you are outdoors with uncovered hands and face. Going outside for a while every day, a walk during your break for example, does more for production than the occasional long session in full sun. Sunburn does not produce extra vitamin D and is unwise for other reasons.

Food makes a modest contribution. Oily fish is the main source; vitamin D is also found in smaller amounts in eggs and meat, and it is added to margarines and similar spreads. Replenishing low stores through food alone rarely works in practice; it is an addition to sunlight or supplementation, not a replacement.

Vitamin D testing as part of a broader vitamin check

Vitamin D rarely stands alone. Those who want to map their vitamin levels seriously often combine the test with vitamin B12, magnesium and folic acid, markers that each play their own role in energy, concentration and the immune system. Bloodworks offers a package that combines nine vitamins, minerals and electrolytes in a single blood draw. That mainly makes sense when your complaints are broader than vitamin D alone: fatigue, tingling or trouble concentrating fit a low vitamin B12 or folic acid just as well. Having those levels determined in the same blood draw lets you read them side by side, and spares you a second appointment if the first result offers no explanation.

In short: a vitamin D deficiency is common in the Netherlands and the complaints are far from specific. If you belong to an advice group, you can supplement without testing; if you have persistent complaints or want to know where you stand, a blood test gives clarity within days, without a referral and with a doctor's interpretation.