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Vitamin D is the most discussed dietary supplement in Germany — and at the same time the most frequently missing. More than half of German adults have an undersupply, and around 15% have a pronounced shortage. Between October and March, the skin in Germany can produce virtually no vitamin D.
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High-dose vitamin D preparations (>4,000 IU/day) only under medical control. Last updated: February 2026.
Vitamin D is the most frequently supplemented dietary supplement in Germany – and at the same time one of the most misunderstood. It is not a classic vitamin, but a prohormone that the body makes itself. 80–90% of the supply comes from the skin's own production under UV-B radiation – not from the diet. This explains why in Germany practically no self-synthesis is possible between October and March, and why a large part of the population is seasonally undersupplied.
| Property | Details |
|---|---|
| Active substance | Colecalciferol (vitamin D3) / ergocalciferol (vitamin D2) |
| ATC code | A11CC05 |
| Substance class | Fat-soluble vitamin / prohormone |
| Available forms | Drops, capsules, tablets, injection solution |
| Blood measurement | 25-hydroxy-vitamin D (25-OH-D) in nmol/L or ng/mL |
| DGE recommendation | 800 IU/day (20 µg) with absent self-synthesis |
| EFSA upper limit (UL) | 4,000 IU/day (100 µg) for adults |
| Prescription status | No (as a dietary supplement); higher doses possibly as a medicine |
| Special feature | 80–90% of the supply through sunlight – not possible October–March in Germany |
The term "vitamin" is biologically imprecise: a true vitamin must be supplied through the diet, because the body cannot make it itself. Vitamin D, on the other hand, the body produces itself in the skin under UV-B radiation – and it is then converted in the liver and kidney into the active hormone calcitriol. This hormone binds to receptors in almost every tissue of the body and regulates gene expression there. Over 200 genes are under the direct or indirect influence of vitamin D.
This explains why a vitamin D deficiency has such varied consequences – and why it acts beyond the classic bone metabolism. An analysis of the VITAL study published by the DKFZ in 2024 showed: daily vitamin D supplementation lowers cancer mortality by 12%. This is not proof of cancer prevention by vitamin D alone – but a signal that deserves attention.
| Function | Effect with a deficiency |
|---|---|
| Calcium & bone metabolism | Rickets (children), osteomalacia, osteoporosis |
| Immune system | Increased susceptibility to infection, autoimmune diseases |
| Muscle function | Muscle weakness, increased fall risk in the elderly |
| Cardiovascular system | Signs of an increased risk of cardiac arrhythmias, hypertension |
| Cancer prevention | DKFZ analysis 2024: daily supplementation lowers cancer mortality by 12% |
| Mood | A link with depression, seasonal winter depression |
A vitamin D deficiency is widespread in Germany – the RKI estimates that around 30% of the population is inadequately supplied, even more in winter. Certain groups carry a particularly high risk.
| Risk group | Why? |
|---|---|
| The elderly >65 years | Reduced skin synthesis, less sun exposure, restricted kidney function |
| Patients on prednisolone/cortisone | Cortisone increases bone breakdown → vitamin D + calcium from day 1! |
| Dark skin colour | Melanin blocks UV-B → up to 6× less synthesis |
| Veiling / covering | No skin synthesis possible |
| Immobile / care-home residents | No regular sun contact |
| Obesity (BMI >30) | Vitamin D is stored (sequestered) in the fat tissue, less bioavailable |
| Pregnant & breastfeeding women | Increased need for mother and child |
| Malabsorption (Crohn's disease, coeliac disease) | Reduced uptake from the gut (a fat-soluble vitamin!) |
| Kidney impairment | Reduced activation of vitamin D (25-OH-D → calcitriol in the kidney) |
| Infants | Hardly any sun contact, little contained in breast milk |
The lab value for vitamin D is called 25-hydroxy-vitamin D (25-OH-D) – and appears in German lab findings in two different units: nmol/L or ng/mL. The conversion factor is 2.5: 50 nmol/L = 20 ng/mL. This regularly causes confusion. Both units are listed in the table.
The DGE gives a value from 50 nmol/L (20 ng/mL) as an adequate supply. Many experts, however, see the optimal range at 75–125 nmol/L (30–50 ng/mL) – a range in which bones, muscles, and the immune system are well supplied without the risk of toxic levels. International guidelines (JCEM 2024) recommend focusing measurement and supplementation on risk groups – no routine screening in the healthy.
| 25-OH-D (nmol/L) | 25-OH-D (ng/mL) | Assessment |
|---|---|---|
| <30 | <12 | Severe deficiency → treatment required! |
| 30–50 | 12–20 | Undersupply → supplementation recommended |
| 50–125 | 20–50 | Adequate – optimal: 75–125 nmol/L (30–50 ng/mL) |
| 125–250 | 50–100 | Increased, but mostly not yet toxic |
| >250 | >100 | Potentially toxic → hypercalcaemia risk! |
The right dose depends on the starting situation, the risk profile, and whether a deficiency is present. For most adults in Germany the rule is: 800–1,000 IU daily are safe and effective, especially between October and March, when the sun is too weak for self-synthesis.
| Group | Recommended dose | Note |
|---|---|---|
| Healthy adults (prophylaxis) | 800 IU/day (DGE) | With absent sun synthesis (Oct–March) |
| Adults with risk factors | 1,000–2,000 IU/day | With a proven deficiency or risk factor |
| Severe deficiency (<30 nmol/L) | Initial dose: 5,000–50,000 IU/week | Only under medical control! Then a maintenance dose |
| On cortisone (prednisolone) | 800–1,000 IU/day + calcium 1,000 mg | From day 1 of the cortisone therapy! |
| Infants (until the 2nd early summer) | 400–500 IU/day | Combined with fluoride prophylaxis |
| Premature babies | 800–1,000 IU/day | The first months of life |
| The elderly >65 years | 800–1,000 IU/day | Bone and fall prevention |
| EFSA upper limit (UL) | 4,000 IU/day | Do not exceed without medical control! |
On the market there are vitamin D preparations with 20,000 IU per tablet – for a weekly intake. Sounds practical – but according to current evidence it is not optimal. DKFZ data show that daily low doses (800–2,000 IU) are more effective for the long-term supply than rare high-dose administrations. The body can metabolise only a limited amount of vitamin D at once. On top of that: vitamin D is fat-soluble – it must be taken with a fat-containing meal, otherwise a considerable part is not absorbed. Anyone who takes vitamin D on an empty stomach gives away effect.
This is one of the most important facts about vitamin D: the skin has a built-in protection mechanism. As soon as enough vitamin D3 has been formed in the skin, excess provitamin is broken down again – an overdose through sunlight is biologically ruled out. Vitamin D toxicity can arise exclusively through dietary supplements or medicines.
It becomes dangerous from a permanent intake over 4,000 IU daily – or with a single massive overdose (e.g. a mix-up of IU and µg, or a decimal-point error when measuring out drops). The result is a hypercalcaemia: too much calcium in the blood through excessive calcium uptake from the gut.
| Dose | Risk assessment |
|---|---|
| Up to 800 IU/day | Safe – no check needed |
| 800–2,000 IU/day | Safe for most adults |
| 2,000–4,000 IU/day | The EFSA upper limit. A check if needed with long-term intake |
| >4,000 IU/day | Only under medical control! Hypercalcaemia risk rises |
| >10,000 IU/day long-term | Toxic! Severe hypercalcaemia threatens |
This is the clinically most important vitamin D indication after bone health: anyone who takes prednisolone or other glucocorticoids must supplement vitamin D and calcium from the start. Not optional advice – a guideline-based must.
In practice this is overlooked frighteningly often: brite data show that a considerable proportion of prednisolone patients do not supplement vitamin D. Anyone who gets a prescription for prednisolone should also ask about vitamin D and calcium at the same doctor's appointment – or use the interaction check.
