Vitamin D deficiency: causes, symptoms and when to test

Why vitamin D is so often deficient in the Netherlands

The body produces most of its vitamin D itself, via sunlight on the skin. In the Netherlands, this process is barely possible from October to April: the sun is not strong enough to trigger production. Food provides a small supplement, oily fish, eggs, fortified products, but rarely enough to prevent a deficiency without sun exposure.

Certain groups face extra risk:

• people with darker skin tones produce less vitamin D from the same sun exposure
• older adults produce vitamin D in the skin less efficiently
• people who work indoors a lot, or who cover up when outdoors, miss the exposure that others build up naturally
• with excess weight, vitamin D is stored in fatty tissue, leaving less available for the rest of the body

Which complaints can be associated with a low vitamin D level?

Vitamin D plays a role in bone mineralisation, muscle function, and the functioning of the immune system. A low level does not always come with noticeable complaints, but it can contribute to fatigue, muscle aches, a weakened immune system, and in the long term, bone loss. The relationship is rarely one-to-one: fatigue has dozens of causes, and not everyone with a low level experiences symptoms.

When does a vitamin D test make sense?

Testing makes sense when you want to know where you stand, not to prove anything, but as a starting point. That applies in these situations, for example:

• people taking supplements can check after two to three months whether their level has actually risen
• those who spent little time outdoors last winter can take a snapshot in late February or March
• those who systematically track how their values change across seasons will have a personal pattern after two measurements

A test is also useful if you want to stop supplementing and see how your level develops, or to determine whether a higher dose is worthwhile. Restarting earlier than necessary wastes money; waiting too long does too. A test gives that insight.

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 and can each independently affect energy, concentration, or the immune system. Bloodworks offers a package that combines nine vitamins, minerals and electrolytes in a single blood draw, for €159 plus a one-off €29 blood draw fee (waived from €225).