TL;DR: About 60 % of Germans have suboptimal vitamin D levels below 50 nmol/l. Only a blood test (25-OH vitamin D) confirms a deficiency. Typical symptoms such as fatigue, muscle weakness, and frequent infections are unspecific. A range of 75 to 125 nmol/l is often cited as the reference. Interpreting the values and any decision about intake or treatment belong in medical hands.
This article is purely informational and does not replace medical advice. If you suspect a vitamin D deficiency, a blood test and medical consultation are the right path.
How widespread is vitamin D deficiency?
The DEGS study from Germany’s Robert Koch Institute is clear: around 60 % of adults in Germany sit below 50 nmol/l. About 30 % are below 30 nmol/l — a severe deficiency. In winter the picture gets worse, in summer only slightly better. The same pattern appears across the UK, the Netherlands, and most of Northern Europe. How widespread low 25(OH)D levels are in Germany and Europe is shown in our vitamin D deficiency statistics for Europe.
The reasons are known. Germany sits between 48° and 54° northern latitude. From October to March, the sun is too low, and UV-B radiation is not strong enough to trigger vitamin D synthesis in the skin. Even in summer, most people spend lunch breaks indoors. Sunscreen with SPF 15 blocks over 90 % of vitamin D production.
A concrete example: a colleague measures her 25-OH value in late September after a summer trip to Italy — 68 nmol/l. Six months later, in late March, she sits at 34 nmol/l. Across winter, values in Central Europe typically fall by 30 to 40 nmol/l. That is why seasonal variation matters when values are interpreted by a doctor.
Symptoms of vitamin D deficiency
Symptoms are unspecific — which is why a diagnosis without a blood test is impossible. But certain patterns appear frequently, especially when multiple show up together:
- Persistent fatigue and exhaustion despite enough sleep
- Muscle weakness and muscle pain, especially in thighs and upper arms
- Frequent respiratory infections (more than 3 to 4 colds per year)
- Low mood, especially in winter (seasonal depression)
- Bone and joint pain without a clear orthopedic cause
- Hair loss, particularly in women who also have ferritin or thyroid issues
Single symptoms have many causes — fatigue can come from iron deficiency, sleep debt, or stress. If several of these complaints occur together and the last blood test is more than a year old, medical clarification can make sense. Studies show that people with 25-OH values below 30 nmol/l have roughly double the risk of respiratory infections compared with people above 75 nmol/l. That is a statistical association, not proof that a single value explains a complaint.
The key marker: 25-OH vitamin D
25-OH vitamin D (also called calcidiol) is the storage form in your blood. It reflects your vitamin D status over the past few weeks and is the value that is usually measured. The active form (1,25-OH vitamin D) is unsuitable for routine checks because it stays normal even during deficiency.
Watch the units. Germany mostly uses nmol/l, while US labs and many online studies use ng/ml. Conversion: 1 ng/ml = 2.5 nmol/l. Here are the reference ranges as they are commonly used:
| 25-OH vitamin D | Classification | What typically follows |
|---|---|---|
| below 30 nmol/l (below 12 ng/ml) | Severe deficiency | Medical clarification of the cause |
| 30–50 nmol/l (12–20 ng/ml) | Deficiency | Medical clarification and follow-up |
| 50–75 nmol/l (20–30 ng/ml) | Suboptimal | Interpretation by your doctor |
| 75–125 nmol/l (30–50 ng/ml) | Commonly cited target range | Regular checks |
| 125–250 nmol/l (50–100 ng/ml) | High but tolerable | Medical interpretation |
| above 250 nmol/l (above 100 ng/ml) | Too high | Toxicity possible, see a doctor |
In meta-analyses, a range around 80 to 100 nmol/l is discussed as a balance between infection protection, bone health, and side-effect risk. Which range counts as optimal is not conclusively settled among experts. For a wider view of lab markers, the guide on understanding blood values covers the 12 most important markers every beginner should know.
Who is at higher risk?
Some people carry a significantly higher risk for vitamin D deficiency. For these groups, it can make sense to have the value checked by a doctor more often:
- Dark skin: more melanin means less UV-B absorption. People with dark skin living in Central or Northern Europe are affected in over 80 % of cases.
- Older adults (65+): the skin produces only about 25 % as much vitamin D at age 60 compared with age 20. Kidney conversion capacity also declines.
- Low sun exposure: office workers, night shift workers, people with full body cover, and people requiring care.
- Obesity (BMI above 30): vitamin D is fat-soluble and gets stored in fat tissue, so blood levels are often lower.
- Chronic bowel conditions: Crohn’s disease, celiac disease, and ulcerative colitis impair fat absorption and therefore vitamin D.
- Certain medications: corticosteroids, anti-epileptics, and some HIV medications accelerate vitamin D breakdown.
How strongly age, body weight, other conditions, and medications affect the level differs from person to person. That is why a measured value cannot be compared with an average and should always be interpreted by a doctor in its personal context.
What doctors look at
Whether and how a low value is treated depends on the starting value, other conditions, medications, and the person’s situation. There are no blanket answers for everyone. Intake, the dose set by the doctor, and the duration of any treatment therefore belong in medical hands.
Doctors typically check:
- the cause of the low value, for example little sun exposure, bowel or kidney conditions, or medications
- other lab values, such as calcium, especially with very high values
- the course through follow-up measurements, since blood values change over weeks to months rather than immediately
The evidence overall is mixed. Single high-amount doses that some doctors prescribe are debated among experts, and the evidence is inconsistent. They belong exclusively under medical supervision and are not self-medication. Very high values are not needed and can carry risks, see the reference table above.
For general background on dietary supplements and on the quality of products, see the supplement beginners guide. It is meant as information and does not replace medical advice. For setting up a clean baseline, check the biomarker baseline checklist.
Cofactors and interactions: K2, magnesium, fat
Vitamin D does not work alone. Three connections are frequently named in the literature:
Vitamin K2 (MK-7): is linked to the activation of osteocalcin and matrix Gla protein. These proteins are involved in how calcium is distributed between bones and arterial walls. Whether very high vitamin D intake without enough K2 could theoretically promote vascular calcification is discussed. Whether a combination makes sense is something your doctor clarifies.
Magnesium: is needed as a cofactor for the enzymes that activate vitamin D in liver and kidney. With a magnesium deficiency (around 30 % of Germans), vitamin D can remain partly ineffective. Here too, whether a supplement is needed belongs in medical hands.
Fat: vitamin D is fat-soluble. Chronic bowel conditions that impair fat absorption can therefore affect vitamin D status.
Anyone who takes medications regularly should discuss possible interactions with vitamin D products with a doctor or pharmacist.
How to track your values
Measuring once is not enough. Vitamin D fluctuates with the season and your lifestyle. Late September (yearly peak) and late March (yearly low) are often named as measurement points. This shows you the full range across the year.
You need a system for this. A paper folder works, but trend analysis is painful. A spreadsheet is better but tedious. In Lab2go, you upload your lab report as a PDF or photo, the app extracts your 25-OH value automatically and plots the trend over months and years. Interpreting the values remains a matter for your doctor. For structuring your testing rhythm, the article on long-term biomarker tracking describes a proven routine.
For the setup, check the Lab2go features or compare the plans and pricing.
One final note: this article is purely informational and does not replace medical advice. Intake, dose, and therapy must be clarified by a doctor. Especially with values below 30 nmol/l, during pregnancy, or while taking certain medications, the decision belongs in the hands of a doctor.
Article FAQ
- How do I know if I have a vitamin D deficiency?
- Only a blood test gives a reliable answer. The value is called 25-OH vitamin D and is measured in nmol/l or ng/ml. Values below 50 nmol/l (20 ng/ml) count as deficiency, below 30 nmol/l (12 ng/ml) as severe deficiency. Symptoms like fatigue, muscle weakness, and frequent infections are warning signs but unspecific. A test at your GP costs around 25 to 40 euros out of pocket. Interpreting the result belongs in medical hands.
- What counts as a normal or optimal vitamin D level?
- A range between 75 and 125 nmol/l (30 to 50 ng/ml) is often cited. Studies examine whether the risk of respiratory infections, bone fractures, and low mood is lower in this range; the evidence is partly inconsistent. Values above 250 nmol/l (100 ng/ml) count as too high and can be associated with hypercalcemia if very high intake is sustained. Which range applies to you is decided by your treating doctor.
- What does the research say about vitamin D and complaints such as fatigue?
- Studies examine the link between 25-OH vitamin D and respiratory infections, bone health, muscle function, and mood. Observational studies show associations, for example roughly double the risk of respiratory infections at values below 30 nmol/l compared with values above 75 nmol/l. Such associations alone say nothing about cause and effect. Fatigue, muscle weakness, or mood changes have many possible causes, and vitamin D is only one of several factors.
- Does vitamin D only matter in winter?
- No. From October to March, the sun in Germany and most of Central Europe is too low, and the skin barely produces vitamin D. In summer, sun alone can theoretically be enough if face, arms, and legs are uncovered outdoors for around 15 to 20 minutes per day. Many office workers do not reach that. Studies show that even in August, 30 to 40 % of Germans sit below 50 nmol/l. That is why measured values vary considerably over the year.
- What role do vitamin K2 and magnesium play?
- Vitamin K2 (menaquinone-7, MK-7) is linked to osteocalcin and matrix Gla protein, proteins involved in how calcium is distributed between bones and arterial walls. Magnesium is needed as a cofactor for the enzymes that activate vitamin D in liver and kidney. Whether a supplement comes into question and which interactions exist is something your doctor clarifies.
- How much does a vitamin D blood test cost?
- At a GP as a private service, 25 to 40 euros in Germany. Online labs offer home tests from around 30 euros, where a fingerprick sample is collected and mailed in. Statutory health insurance only covers the test with a clear medical reason (osteoporosis, malabsorption, etc.). Two measurements per year, late summer and late winter, show the range across the year.
- What risks come with very high vitamin D levels?
- Vitamin D toxicity is rare but can occur with sustained very high intake or unmonitored single high-amount doses. Symptoms are nausea, loss of appetite, muscle weakness, and elevated blood calcium. Values above 250 nmol/l (100 ng/ml) count as too high. With abnormal values or symptoms, clarification belongs in medical hands.
- When should I have a low value checked by a doctor?
- With values below 30 nmol/l, during pregnancy, with chronic bowel or kidney conditions, and when taking certain medications (for example corticosteroids or anti-epileptics), medical clarification is important. The doctor also looks for the cause and plans a follow-up measurement. Single symptoms such as fatigue or hair loss have many causes and are not proof of a deficiency.
- Is sunshine alone enough?
- In theory yes, in practice for most people no. On paper, about 15 to 20 minutes of midday sun in summer with uncovered arms and legs and no sunscreen, several times a week, would be needed. Above 40° northern latitude (most of Germany, Austria, Switzerland, UK), the skin produces no vitamin D from October to March. Dark skin needs 3 to 5 times longer in the sun. Office workers and regular sunscreen users often do not reach sufficient levels even in summer.
This article is for general information only and is not a substitute for individual medical advice, diagnosis, or treatment. Discuss any changes to your diet, supplementation, or medication with a qualified healthcare professional.
Maritta Schmid, Heilpraktikerin (licence under the German Heilpraktikergesetz; non-medical health practitioner), Licence under the German Heilpraktikergesetz, issued by Gesundheitsamt Heilbronn (February 2010), Supervisory authority: Landratsamt Ostalbkreis – Gesundheitsamt Aalen
Heilpraktikerin & Founder
Schwäbisch Gmünd, Germany
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