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 are fatigue, muscle weakness, and frequent infections. For a proven deficiency, 2000 to 4000 IU per day for 8 to 12 weeks is a solid protocol, followed by a retest. Target: 75 to 125 nmol/l.
If you suspect a vitamin D deficiency, a blood test and medical advice are the right path — do not self-prescribe high doses blindly.
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.
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: your 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. Without supplementation, the typical central European loses 30 to 40 nmol/l across winter. That is why year-round supplementation makes sense for most people.
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. But if you check three or more of these boxes and your last blood test is more than a year old, a measurement is worth it. 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.
The key marker: 25-OH vitamin D
25-OH vitamin D (also called calcidiol) is the storage form in your blood. It reflects your actual vitamin D status over the past few weeks and is the only value you should measure. 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:
| 25-OH vitamin D | Rating | Action |
|---|---|---|
| below 30 nmol/l (below 12 ng/ml) | Severe deficiency | Treatment required, see a doctor |
| 30–50 nmol/l (12–20 ng/ml) | Deficiency | Start supplementation |
| 50–75 nmol/l (20–30 ng/ml) | Suboptimal | Raise or maintain dose |
| 75–125 nmol/l (30–50 ng/ml) | Optimal | Maintenance dose, regular checks |
| 125–250 nmol/l (50–100 ng/ml) | High but tolerable | Reduce dose slightly |
| above 250 nmol/l (above 100 ng/ml) | Too high | Toxicity possible, stop immediately |
Aim for 80 to 100 nmol/l. This range shows the best balance in meta-analyses between infection protection, bone health, and side-effect risk. 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. If you belong to one of these groups, measure your level at least twice a year:
- 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, lowering blood levels. People with obesity need up to 50 % higher doses.
- 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.
A concrete example: a 70-year-old man with a BMI of 32 taking corticosteroids often reaches only 40 nmol/l on 1000 IU per day. The same man on 4000 IU reaches 75 nmol/l. The dose needs to fit your individual situation, not an average.
Treatment: how much vitamin D should you take?
The right dose depends on your starting value. Blanket recommendations like “1000 IU works for everyone” are too low for people with severe deficiency and too high for those who are already optimal. Here is an evidence-based dosing table:
| Current 25-OH value | Dose | Duration | Retest |
|---|---|---|---|
| above 75 nmol/l (optimal) | 1000–2000 IU/day | Ongoing | Every 6–12 months |
| 50–75 nmol/l (suboptimal) | 2000–3000 IU/day | Ongoing | After 3 months |
| 30–50 nmol/l (deficiency) | 2000–4000 IU/day | 8–12 weeks | After 12 weeks |
| below 30 nmol/l (severe) | 4000–5000 IU/day or bolus | 8–12 weeks | Under medical supervision |
Important: After the loading phase (8 to 12 weeks), switch to a maintenance dose. Permanent 5000 IU per day will eventually push you above 150 nmol/l — not harmful, but not useful. Retest 12 weeks after starting therapy and adjust.
Bolus therapy: some doctors prescribe single doses of 20,000 to 50,000 IU. This works, but the evidence is mixed. Daily doses of 4000 IU are just as effective and easier to control. Only use bolus dosing under medical supervision, never on your own.
If you are still building your supplement routine, the supplement beginners guide covers the basics. For starting a clean baseline, check the biomarker baseline checklist.
Cofactors: K2, magnesium, fat
Vitamin D does not work alone. Three cofactors determine whether the absorbed vitamin D actually ends up in your bones without causing problems:
Vitamin K2 (MK-7): activates osteocalcin and matrix Gla protein. These proteins direct calcium into bones and keep it out of arterial walls. Without K2, high-dose vitamin D could theoretically promote vascular calcification. Dose: 100 to 200 µg MK-7 per day, ideally combined with D3 in a single capsule.
Magnesium: acts as a cofactor for all eight enzymes that activate vitamin D in liver and kidney. With a magnesium deficiency (around 30 % of Germans), even high-dose vitamin D stays partly ineffective. Dose: 300 to 400 mg magnesium per day, preferably citrate, glycinate, or malate.
Fat with the dose: vitamin D is fat-soluble. Taking it on an empty stomach cuts absorption by up to 50 %. Take vitamin D with the biggest meal of the day, ideally with 10 to 15 grams of fat.
A concrete example: you take 3000 IU of vitamin D each morning with black coffee only. Over 12 weeks, your value rises from 40 to 55 nmol/l. You switch to taking it at lunch with whole-grain bread, avocado, and olive oil — same product, but your value climbs to 75 nmol/l in the same 12 weeks. Intake conditions make the difference.
How to track your values
Measuring once is not enough. Vitamin D fluctuates with the season, your supplementation, and your lifestyle. The best measurement points are late September (yearly peak) and late March (yearly low). 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. You can instantly see whether your current dose is enough or needs adjusting. For structuring your testing rhythm, the article on long-term biomarker tracking describes a proven routine.
Start today: order a 25-OH vitamin D test from your GP or an online lab, document your baseline, pick the matching dose from the table above, and retest in 12 weeks. For the setup, check the Lab2go features or compare the plans and pricing.
One final note: this article does not replace medical advice. If you suspect a vitamin D deficiency, a blood test and medical consultation are the right path — do not self-prescribe high doses blindly. Especially with values below 30 nmol/l, during pregnancy, or while taking certain medications, treatment 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.
- What is the optimal vitamin D level?
- Between 75 and 125 nmol/l (30 to 50 ng/ml). In this range, studies show the lowest risk for respiratory infections, bone fractures, and low mood. Values above 250 nmol/l (100 ng/ml) add no benefit and can cause hypercalcemia if sustained. Aim for 80 to 100 nmol/l — that is the sweet spot.
- How much vitamin D should I take per day?
- For healthy adults without a deficiency, 1000 to 2000 IU per day is a solid maintenance dose. With a proven deficiency, 2000 to 4000 IU for 8 to 12 weeks makes sense, followed by a retest. Most public guidelines recommend 800 IU, which only covers bone needs, not the optimal levels for immune and muscle function. Dose according to your current 25-OH value, not a blanket number.
- Do I only need vitamin D 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 work — if you spend 15 to 20 minutes outside per day with uncovered face, arms, and legs. Most office workers do not hit that. Studies show that even in August, 30 to 40 % of Germans sit below 50 nmol/l. Year-round supplementation makes sense for most people.
- Should I combine vitamin D with K2?
- At doses of 2000 IU per day or higher: yes. Vitamin K2 (menaquinone-7, MK-7) directs calcium into bones instead of arteries. Typical dose: 100 to 200 µg MK-7 per day. At small maintenance doses up to 1000 IU, K2 is less critical, but it does not hurt. Magnesium also matters, because vitamin D cannot be activated without enough magnesium.
- How much does a vitamin D blood test cost?
- At a GP as a private service, 25 to 40 euros in Germany. Online labs like Cerascreen or Medivere offer home tests from 30 euros — you collect a fingerprick sample and mail it in. Statutory health insurance only covers the test with a clear medical reason (osteoporosis, malabsorption, etc.). For useful tracking, aim for two measurements per year: late summer and late winter.
- How fast does vitamin D supplementation work?
- Blood values rise measurably within 6 to 8 weeks — at 4000 IU per day, on average 20 to 30 nmol/l. Subjective symptoms like fatigue and muscle weakness typically improve within 4 to 8 weeks. Mood and infection frequency take longer, often 2 to 3 months. Retest after 12 weeks before adjusting the dose. Without a retest, you dose blind.
- Can you take too much vitamin D?
- Yes, but rarely. Toxicity only occurs with sustained doses above 10,000 IU per day or unmonitored bolus injections. Symptoms are nausea, loss of appetite, muscle weakness, and elevated blood calcium. Values above 250 nmol/l (100 ng/ml) count as too high. Stick to doses up to 5000 IU per day without medical supervision — safe for almost everyone.
- Which symptoms go away with treatment?
- Fatigue, muscle weakness, and frequent respiratory infections often improve within 4 to 6 weeks. Bone pain and hair loss take 2 to 3 months. Low mood and mood swings improve for many but not all — vitamin D plays a role but is not the only factor. Long-term deficiency can affect bone density, and recovery takes 6 to 12 months.
- Is sunshine alone enough?
- In theory yes, in practice for most people no. You need 15 to 20 minutes of midday sun in summer with uncovered arms and legs, no sunscreen, three times a week. 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 rarely hit sufficient levels even in summer.
Discussion
Community comments coming soon. Until then, we welcome feedback and questions via email.
E-Mail anzeigen
