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Why Testing Vitamin D Levels?

Research shows that most people have a suboptimal level of vitamin D or even its deficiency. Due to the many health benefits of vitamin D, supplementation is encouraged if the blood levels are not optimal.

Everybody I talk to is aware of the importance of vitamin D. And most of them take a vitamin D supplement.

The questions I ask them though are what type of supplement they take and if they know their vitamin D blood levels. But almost no one knows the difference between different vitamin D supplements or their vitamin D blood levels.

Do You Have Vitamin D Deficiency?

Do you feel tired in the winter season and have difficulties getting out of your bed? Then you might have vitamin D deficiency.

Other symptoms of vitamin D deficiency can be:

  • oft colds
  • muscle pain
  • thinning hair
  • anxiety
  • depression

Why Vitamin D Is Important

Good levels of vitamin D are associated with a wide range of benefits, including:

  • increased cognition,
  • improved immune defense: rare infections, weaker symptoms, faster recovery
  • immune health, prevention of autoimmunity, reduced existing autoimmunity*,
  • bone health
  • psychological well-being
  • healthy pregnancy

* read how I achieved remission of the thyroid autoimmune disease, Hashimoto’s thyroiditis supplementing vitamin D among other interventions.

Why Vitamin D Deficiency is Bad

Vitamin D deficiency has been associated with a myriad of acute and chronic illnesses including:

CardiovascularCardiovascular disease, aortic dilatation, orthostatic hypotension
RespiratoryBronchiectasis, asthma, cystic fibrosis, bronchiolitis, obstructive sleep apnoea
GastrointestinalInflammatory bowel disease, chronic hepatitis, liver cirrhosis, pancreatitis
NeurologicalMultiple sclerosis, myasthenia gravis, meningomyelocele, depression
MusculoskeletalMuscle weakness, osteoarthritis, rheumatoid arthritis, juvenile arthritis
MetabolicMetabolic syndrome, diabetes mellitus, diabetic nephropathy, infertility (male), chronic kidney disease
CancerBreast, colorectal, ovarian, lung, prostate
SkinPsoriasis, systemic lupus erythematosus, eczema
Source: Vitamin D deficiency and associated conditions
Vitamin D deficiency | vitamin D sufficiency | symptoms and long term effects
Why Testing Vitamin D Levels? 4

Vitamin D deficiency is also linked to:

  • childhood dental caries,
  • periodontitis,
  • autoimmune disorders
  • infectious diseases, including Covid-19
  • pregnancy complications. 

Is Sun Exposure Sufficient?

It depends on the place you live and the season.

Your body produces vitamin D from cholesterol when exposed to sufficiently strong sunlight. It is only near the equator that there is abundant sunlight the whole year round for a human body to produce high enough levels of vitamin D.

If you live in a country like the Netherlands, the autumns and winters are rainy or cloudy. Then even doing outdoor sports will not cover your need to produce a sufficient amount of vitamin D.

Leading an indoor lifestyle and spending little time outside during the day will further increase your risk for vitamin D deficiency.

Your ability to produce and absorb vitamin D from foods is highly individual. The following factors may play a role:

  • skin color (Dark skin protects from the sun. If you have darker skin and live in the northern areas you will be especially prone to vitamin D deficiency.)
  • medical conditions, for example, autoimmune diseases, pregnancy, and breastfeeding
  • your body weight also affect vitamin D levels.
  • age (postmenopausal women and elderly people, in general, are lacking vitamin D even more often and experience more severe consequences)

Why Test for Vitamin D Levels

You should test your vitamin D blood levels because:

  1. the role of vitamin D in your health is extremely important.
  2. vitamin D levels depend on several factors like a living place, lifestyle, individual physiology, or specific conditions like autoimmune diseases
  3. bioavailability of vitamin D depends on the supplement form (D2 or D3), formulation, and personal gut health
  4. the deficiency symptoms are unspecific and very common at the same time
  5. in rare cases, an overdose can take place

When to Test

As I showed above Vitamin D plays such an important role in your health but the symptoms of vitamin D deficiency are not specific. Therefore I believe that vitamin D levels should be tested during the darker period of the year in order to evaluate if you need supplements or if the supplements and amounts you take are sufficient.

The testing should show how supplementation should be adjusted. A repeated test can be considered after adjusting the supplementation.

After you learned about your personal needs for supplementation with vitamin D there is no need for frequent testing. But changes in your health condition, like any major disease, pregnancy, breastfeeding, or aging are a good reason to re-evaluate vitamin D levels and its supplementation.

Reference Range

The reference range is set based on the test results of a group of healthy individuals. Usually, hundreds to thousands of normal, healthy patients are tested.

These ranges may vary from country to country. In the Netherlands, the reference range is set from 50 to 150 nmol/L (or 20 to 60 ng/ml).

Normal versus Optimal

Is normal the same thing as optimal? Not necessarily.

Most organizations accept values of vitamin D levels above 50 nmol/L as normal. Whereas the Endocrine Society has the highest norm starting from 75 nmol/L and higher.  

Such a higher range of the Endocrine Society is supported by studies related to chronic conditions like diabetes, autoimmune disease, or cancer. 

How to Treat Vitamin D Deficiency

  1. Sun exposure helps keep up vitamin D levels in the summertime. Nevertheless, you should protect the skin from UV light damage and premature aging.
  2. Foods containing significant levels of vitamin D are fatty fish like salmon, herring, cod liver oil, as well as eggs. Nevertheless, Vitamin D deficiency can be hardly treated by increasing the consumption of these products.
  3. Therefore, vitamin D supplementation is crucial during the darker seasons for preventing the adverse effects of vitamin D deficiency mentioned above. 

Supplementation of Vitamin D

Intake Recommendation

Recommended Dietary Allowance (RDA) is the average amount of a nutrient a healthy person should consume daily. Consuming an RDA dietary intake level for each nutrient is supposed to be sufficient to meet the nutrient requirements in nearly all (97–98 percent) healthy individuals.

RDA varies from country to country but also depends on age, gender, and state of pregnancy or breastfeeding for women. Nevertheless, it does not take into account your personal specifics like skin color, lifestyle, or conditions like autoimmune diseases.

Below you can find a daily vitamin intake calculator based on RDA values, so it is an RDA calculator. Enter your age, and choose your sex and vitamin type.

You will find out the RDA for adults is 15 mcg daily and for the elderly 20 mcg of vitamin D daily.

In Case of Deficiency

The NutriProfiel project, a collaboration between the Gelderse Vallei Hospital and Wageningen University (both in the Netherlands), developed a protocol for the treatment of vitamin D deficiency and for the maintenance of optimal status. The NutriProfiel project revealed that in addition to the RDA 1 μg/d of vitamin D is required for each 1 nmol/l increase of 25(OH)D. Here you can find their recommendations.

Vitamin D council also gives suggestions on how much vitamin D to take depending both on your lab results and on the level you want to achieve.

What Type of Supplement to Take

There are two forms of vitamin D. D3 is found in fish or eggs, whereas D2 is extracted from yeast and plants. Vitamin D (D2 and D3) is modified in the liver and further in the kidneys to turn it into the biologically active form of vitamin D (1,25(OH)2D) or calcitriol). D3 vitamin (cholecalciferol) is preferred over D2 form (ergocalciferol).

Vitamin D is fat-soluble. So, the formulation should be preferably oil-based or taken with oil for the best uptake. However, studies have shown that the uptake of vitamin D does not depend on the presence of oil.

Vitamin D effect is highest when it is taken in combination with another fat-soluble vitamin, vitamin K. Both vitamins contribute to bone and cardiovascular health. Their joint supplementation might be more effective than the consumption of either alone.

infographics | increased needs | decreased needs | wellbeing blog

How much Vitamin D do I Take, and Which

Side Effects of Vitamin D Supplementation

Scientific research shows that vitamin D intoxication is rare.

The reason for an overdose can be:

  • manufacturing errors,
  • overdosing by patients or prescribers

The toxic levels of vitamin D in the blood are those above 150 nmol/L. Some typical symptoms of vitamin D toxicity are vomiting, dehydration, pain, and loss of appetite.

To lower your levels, you should stop taking vitamin D supplements and re-adjust the supplementation doses after your levels normalize.


  • Vitamin D is vital and has many positive effects on your health, whereas its deficiency has multiple adverse effects on your health
  • Vitamin D deficiency is frequent, but symptoms are not specific
  • Your vitamin D blood values depend on multiple factors and are, therefore, impossible to predict
  • Vitamin D testing helps you know your needs and responses to supplementation and allows you to profit from the sound levels and avoid the adverse effects of deficiency.


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