Vitamin D – this is how your needs can be met and a deficiency can be identified
Vitamin D is important for bones and the immune system. Why the need cannot be met through diet alone and what indicates a deficiency.
Vitamin D plays a special role among vitamins. Strictly speaking, it is not a vitamin at all, as we know today, nor is it a nutrient in the true sense. Rather, it is the precursor to a hormone that is produced in the skin through exposure to sunlight. For this reason, some people also refer to vitamin D as the sunshine vitamin. It is actually only a nutrient if there is no solar radiation and it is absorbed through food.
Only a few foods contain vitamin D in significant amounts. No one can cover their daily vitamin D requirement through food alone. However, diet can contribute to vitamin D levels to a small extent.
In principle, it is possible to cover the entire daily requirement for vitamin D through solar radiation alone. In everyday life, however, not everyone is able to do this for many reasons.
A poor vitamin D supply is therefore more common in Germany: it is estimated that around three out of ten adults have a deficiency. Another three out of ten adults receive suboptimal care at most. And only around four out of ten adults can be assumed to have an adequate supply of vitamin D. In some cases it may therefore be advisable to take vitamin D as a dietary supplement.
What is Vitamin D?
Vitamin D is the umbrella term for a group of fat-soluble vitamins called calciferols, which are similar in structure to steroid hormones. This group includes vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol).
Vitamin D3 is found in some animal foods, such as fatty fish and egg yolks. The body also produces vitamin D3 in the skin with the help of the sun. Vitamin D2 is found in some plant foods, mushrooms and yeast.
The body can store vitamin D for a long time, especially in fatty tissue and muscles and, to a small extent, in the liver.
Vitamin D: Effects and roles in the body
Both vitamin D2 and vitamin D3 are inactive precursors of vitamin D. Both forms of vitamin D must first be converted by the body into their active form: calcitriol.
Calcitriol acts similarly to a hormone in the body. That’s why vitamin D is now also referred to as a hormone precursor, a so-called prohormone.
The conversion of vitamin D into calcitriol takes place in several steps. Vitamin D, which is absorbed through food or produced through the skin, first reaches the liver. There the body converts it into calcidiol (also called 25-hydroxyvitamin D). Calcidiol is also a hormone precursor. In this form, the body can store it in the liver.
In the next step, the body converts calcidiol into calcitriol (also called 1,25-dihydroxyvitamin D) in the kidneys. Calcitriol is the active form of vitamin D and a steroid hormone.
Vitamin D plays an important role, especially in bone health, by influencing bone metabolism. With the help of vitamin D, the body can absorb calcium and phosphate from the food in the intestine and incorporate it into the bones. It also ensures a balance between calcium and phosphate. This is important because in the bones calcium is bound to phosphate – as so-called hydroxyapatite. Vitamin D contributes to stable bones.
Vitamin D also plays a role in the muscles – there it influences muscle performance, muscle speed and muscle coordination. If the vitamin D level is too low, this can promote falls, for example.
Vitamin D also influences the function of the immune system. If the body is insufficiently supplied with it, the risk of respiratory infections such as colds increases, among other things. With a good supply of vitamin D, however, defense cells in the immune system are more easily activated and can therefore fight pathogens more quickly.
In fact, almost all cells in the body have binding sites (so-called receptors) for calcitriol. As a result, vitamin D could probably influence many processes. Accordingly, vitamin D has been and is repeatedly attributed a variety of functions and connections to diseases such as cancer, diabetes or high blood pressure are suspected. However, this has not yet been clearly proven in these cases.
Vitamin D: Daily requirement for babies, children and adults
Theoretically, the body is able to produce enough vitamin D itself as long as the skin is exposed to sufficient sunlight.
Good to know
The vitamin D content is often not given in micrograms (µg), but in international units (IU): 1 µg corresponds to 40 IU or 1 IU corresponds to 0.025 µg.
However, if it can be assumed that the body cannot produce enough vitamin D itself, the German Nutrition Society recommends 20 µg vitamin D per day for adults in order to achieve calcidiol levels of 50 nml/l in the blood serum. In such cases, the need can be compensated for with nutritional supplements.
Infants are an exception. Babies’ bodies cannot yet produce enough vitamin D themselves. The enzyme system required for this is not yet fully developed. In addition, infants should not be exposed to direct sunlight until the end of the 12th month of life. Babies absorb small amounts of vitamin D through breast milk, but not as much as they need.
Table: Estimates for an adequate vitamin D intake if self-synthesis is insufficient
Age group Vitamin D (µg per day) Vitamin D (IU per day)
Infants 0 to less than 12 months 10 mcg 400 IU
Children 1 to under 15 years 20 µg 800 IU
Adolescents aged 15 and over and adults 20 µg 800 IU
Pregnant and breastfeeding women 20 µg 800 IU
For this reason, experts recommend balancing the vitamin D needs of babies in their first year of life with nutritional supplements and thus preventing a deficiency: Both breastfed and non-breastfed infants should take one 10 tablet per day from the second week of life until the end of the first year of life up to 12.5 µg vitamin D (= 400 to 500 IU).
The German Society for Child and Adolescent Medicine also recommends that parents continue vitamin D prophylaxis in children from the age of two during the winter months.
It is better for babies to receive vitamin D in tablet form than in drop form. Because drops can quickly be accidentally overdosed. For example, if the child receives two drops instead of one drop. Depending on the room temperature, the drops can also be of different sizes, so it is not certain that the child will receive the same desired dose every day. If babies often receive too high a dose of vitamin D, it accumulates in the body and can lead to side effects.
Vitamin D: What role the sun plays
If you spend a short time outdoors every day without sun protection, you can theoretically cover your vitamin D needs in most cases. The body produces a good 80 to 90 percent of its daily requirements itself when sunlight hits the skin. The decisive factor is the amount of UVB radiation contained in sunlight.
In Germany, under optimal conditions, it would be sufficient to go outside for a few minutes every day between 12 p.m. and 3 p.m. in the summer months (from April to September), leaving your face and hands as well as parts of your arms and legs uncovered.
How long you should stay in the sun to meet your vitamin D needs also depends on your skin type and can be between 5 and 25 minutes. For example, darker skin types need more sunlight than light skin types to meet their vitamin D needs. Anyone who spends time outside in the morning or afternoon rather than lunchtime in the summer months needs about twice as much time.
The body is able to store vitamin D. In the best case scenario, this can even be used to largely bridge the darker winter months. From October to March, solar radiation is too weak for sufficient vitamin D production.
In fact, not everyone is able to use the sun to fill their vitamin D stores so that they last through the winter. Studies show that many people’s vitamin D status is therefore too low.
Many people also don’t get enough sun during the summer months. This has various reasons, such as:
too short stays outdoors
heavy cloud cover in the summer months
frequent use of sunscreen
darker skin type
You can find out more about which risk groups there are for vitamin D deficiency in the chapter “Risk factors for vitamin D deficiency”.
Important to know
In order to produce vitamin D, solar radiation must hit unprotected skin. However, too much sunlight can also damage the skin and promote skin cancer. Anyone who spends longer in the sun than is recommended for their personal skin type (self-protection time) should therefore protect their skin adequately with sunscreen.
Some people believe that they can cover their vitamin D needs on the tanning bed or in the solarium. However, this is a mistake. Solariums usually use UVA radiation that causes tanning quickly and filter out the necessary UVB component as much as possible. Visits to solariums can also increase the risk of skin cancer.
If daylight penetrates the skin through glass panes (e.g. in the car, at home or in the office), it has no effect in terms of vitamin D supply. The glass panes filter out the UVB portion of solar radiation.
Vitamin D: foods high in it
Not many foods have significant vitamin D content. Anyone who regularly eats such foods can only cover 10 to 20 percent of their vitamin D requirements. Vitamin D intake beyond this is hardly possible through diet alone.
A comparatively high amount of vitamin D is found primarily in cod liver oil and fatty fish (such as herring, eel, salmon or mackerel). Foods such as cheese, eggs and margarine also have a high vitamin D content. Among plant foods, mushrooms, especially chanterelles, are the main sources of vitamin D.
Table: Vitamin D contents in foods (examples)
Food Vitamin D (per 100 grams)
Herring 7.80 to 25.00 µg
Salmon 16.00 µg
Chicken egg yolk 5.60 µg
Mackerel 4.00 µg
Chicken egg, total 2.90 µg
Margarine 2.5 to 7.5 µg
Chanterelles 2.10 µg
Mushrooms 1.90 µg
Beef liver 1.70 µg
Gouda cheese, 45% F. i. Tr. 1.30 µg
Butter 1.20 µg
Calf liver 0.33 µg
Whole milk, 3.5% fat 0.09 µg
Vitamin D levels: Which values are normal?
A blood test can show what the vitamin D status is like when the level of calcidiol (25-hydroxyvitamin D, 25(OH)D), the precursor of vitamin D, is determined. Some laboratories give vitamin D with the Unit nanomoles per liter (nmol/l) or nanograms per milliliter (ng/ml).
Although calcitriol (i.e. 1,25-dihydroxyvitamin D) is the active form of vitamin D, it is less suitable as a standard marker for determining vitamin D. Because it only has a short half-life of around 6 to 8 hours in the blood. It is therefore more reliable to determine calcidiol – the storage form of vitamin D has a half-life of 3 to 4 weeks.
If vitamin D levels are lower than 30 nmol/l, there is a vitamin D deficiency. Values over 50 nmol/l indicate a good vitamin D status and are considered beneficial in terms of bone health from the perspective of the German Nutrition Society. However, studies show that many people have vitamin D levels below 50 nmol/l.
From a scientific perspective, it is not entirely clear how sensible the current limit values actually are. Some experts advocate 75 nmol/l as the limit for good vitamin D status, i.e. slightly higher values. However, it is questionable in this country whether this can be achieved using solar radiation alone. Many people are unable to do this with the current limits.
Table: Vitamin D values and their meaning
Calcidiol in nmol/l / Calcidiol in ng/ml / Vitamin D status
lower than 30 nmol/l lower than 11 ng/ml vitamin D deficiency; increased risk of bone diseases
such as rickets, osteomalacia and osteoporosis
30 to 50 nmol/l 12 to 20 ng/ml not sufficient; Possible consequences for bone health
50 to 75 nmol/l 20 to 30 ng/ml sufficient for bone health
75 to 125 nmol/l 30 to 50 ng/ml sufficient for bone health, no other additional health benefits
125 nmol/L or higher 50 ng/mL or higher Vitamin D levels too high; health consequences possible,
such as an increased calcium level, which causes cardiac arrhythmias
or kidney stones
If you want to have your vitamin D level tested, you should keep in mind that blood levels fluctuate throughout the year and are usually always higher in summer. Towards the end of winter, the values are rather low for almost everyone.
However, if the values are already below 50 nmol/l in September or October, i.e. at the end of summer, the body’s own vitamin D stores are already exhausted. Then taking nutritional supplements may be advisable.
As a rule, the blood test for the vitamin D test is not a health insurance service, but rather an IGeL service. So you usually have to pay for it yourself. The cost of determining the standard marker calcidiol (25-hydroxyvitamin D) is around 30 euros.
If there is already a suspicion of a vitamin D deficiency, the corresponding blood test can in principle also be billed as a health insurance benefit. However, this assessment and decision is left to the doctor.
Vitamin D deficiency: These symptoms can occur
A vitamin D deficiency occurs when the vitamin D status in the blood is too low. If this condition persists for a long time, it can cause physical symptoms and have consequences for bone health.
A possible consequence of a persistent, severe vitamin D deficiency in children is, for example, rickets. This is a bone disease that is associated with bone deformation. This disease only occurs very rarely in Germany.
In adults, osteomalacia, which causes bone softening, can develop as a result of a vitamin D deficiency. Osteoporosis (bone loss) is also promoted by vitamin D deficiency.
If you have a long-term, severe vitamin D deficiency, you may experience symptoms such as:
increased risk of bone fractures
Bone deformations (e.g. ribs, legs, spine)
Softening of the back of the head
Fontanelle in small children closes more slowly
reduced muscle strength, muscle weakness
more frequent infections
Important to know
Vitamin D levels can fluctuate greatly depending on the season. If the vitamin D status is too low, this does not say anything about how long the condition will last, as long as physical signs of a vitamin D deficiency do not already appear.
Risk factors for vitamin D deficiency
In order for the body to produce enough vitamin D, sunlight must hit the skin. A deficiency can therefore easily arise if people spend too little time outdoors during the day. This can, for example, be due to work (e.g. indoor jobs or night work) and/or leisure habits, but can also affect people who are very old, in need of care, bedridden or otherwise limited in their mobility.
Older people are often affected by vitamin D deficiency. Because with increasing age, vitamin D production in the skin decreases. In people over 65 years of age it is still around 25 percent. Many older people also no longer spend as much time outdoors during the day.
Infants are at higher risk of vitamin D deficiency because they should not be exposed to direct sunlight in the first year of life and their bodies are not yet able to produce enough vitamin D.
There is also an increased risk of vitamin D deficiency in people who only go outdoors dressed heavily and who rarely expose their skin to daylight, for example due to a sun allergy or for religious and cultural reasons.
A darker skin color can promote a vitamin D deficiency because the skin pigment melanin allows less UVB radiation to pass through. This is particularly true if those affected live in northern European regions such as Germany, where solar radiation is less intense.
Anyone who takes medications that have an unfavorable effect on vitamin D metabolism increases the risk of vitamin D deficiency. Such medications include, among others:
Glucocorticoids such as cortisone
Anticancer agents (cytostatics), such as docetaxel, carboplatin, oxaliplatin
Breast cancer agents from the group of antiestrogens, such as tamoxifen, anastrozole, letrozole, exemestane
Anti-epileptic drugs such as carbamazepine, phenytoin
Anti-HIV drugs such as efavirenz
When taking such medications, it is advisable to have your vitamin D status checked at least once a year.
The risk groups for vitamin D deficiency also include people with chronic inflammatory bowel disease, malnutrition or rheumatoid arthritis.
Does vitamin D protect against diseases such as cancer or depression?
What is certain is that good vitamin D status has a positive effect on bone health and muscle strength. This plays an important role, especially for older people. Studies show that older people who are well supplied with vitamin D
fall less often,
break your bones less often,
less likely to lose muscle strength,
are mobile for longer and
have fewer balance problems.
Older people with good vitamin D status also appear to have a lower overall risk of dying prematurely. However, from a scientific perspective, it is still unclear and rather unlikely whether vitamin D reduces the risk of cancer, type 2 diabetes, dementia or cardiovascular diseases.
According to studies, a vitamin D deficiency apparently increases the risk of developing depression. However, other research also shows that taking vitamin D as a preventive measure cannot reduce the risk of depression. Further research is needed here.
Researchers repeatedly find low vitamin D levels in people with certain illnesses. However, this does not automatically mean that a good supply of vitamin D could alleviate or prevent these diseases. While in some cases a disease may be promoted by a lack of vitamin D, in other cases the deficiency could also be a consequence of the disease – or a coincidence.
One problem with current research on vitamin D is that most of it is observational studies. Under certain circumstances, certain associations can be made using these, but not a real connection between cause, consequence and effect. It is often not possible to say from the results whether there is a causal connection. How exactly vitamin D or a deficiency of it is linked to illnesses still requires further research.
Vitamin D in pregnancy
Research shows that many pregnant women are deficient in vitamin D, even in summer. However, a good supply of vitamin D during pregnancy is important so that the embryo can develop optimally.
However, it is controversial among experts whether it makes sense to take vitamin D as a preventive dietary supplement during pregnancy. There are currently no general recommendations on this.
Pregnant women probably do not need any more vitamin D than usual. If vitamin D is not produced through the skin, experts assume that a daily intake of 20 µg (= 800 IU) is sufficient for them. But there are also voices that advocate a higher intake.
Regular vitamin D intake above 20 µg may reduce the risk of complications such as gestational diabetes. It cannot be said with certainty whether risks such as preeclampsia or low birth weight are positively influenced by vitamin D intake.
However, if pregnant women are proven to have a vitamin D deficiency, this should be compensated for. Experts then recommend a higher dose than 20 µg, namely 37 to 50 µg (= 1,480 to 2,000 IU) per day. Up to 100 µg (= 4,000 IU) of vitamin D per day is considered safe for pregnant women.
Take vitamin D: tablets, capsules, oil & drops
Whether in the form of tablets, capsules, oil or drops – if you plan to take vitamin D on your own as a dietary supplement over a long period of time, you should have your vitamin D status checked by a doctor beforehand. This makes it possible to determine whether there is actually a defect that needs to be remedied.
In some cases, a vitamin D deficiency can be easily compensated for by those affected going outdoors more, thereby giving the skin more opportunity to produce vitamin D itself. However, this is not always possible.
If there is a proven vitamin D deficiency, the doctor will recommend appropriate vitamin D preparations in the form of tablets, oil or drops if it can be assumed that
that vitamin D production via the skin is not sufficient
and/or risk factors for a deficiency exist.
In many cases, those affected by a vitamin D deficiency have to pay for the corresponding preparations themselves. An exception are people with certain diseases, such as osteoporosis, osteomalacia, rickets, hypoparathyroidism or severe kidney dysfunction. You can get vitamin D supplements as a health insurance benefit on prescription.
When it comes to prescribing vitamin D, it is also important to ask whether a vitamin D preparation is a dietary supplement or a medication. Preparations up to 1,000 IU (= 25 µg) are considered dietary supplements and therefore must be taken from a pharmacy. So they can be purchased freely. All higher-dose vitamin D preparations are classified as medications – they are only available with a prescription.
In certain cases, however, vitamin D preparations up to 1,000 IU can also be prescribed, namely at
People with osteoporosis who have already suffered bone fractures due to the disease,
People who have been treated with certain cortisone preparations for at least six months
People who are being treated with bisphosphonates (e.g. for osteoporosis).
Taking vitamin D for preventative purposes may also be advisable for people who are at increased risk of vitamin D deficiency.
This includes, for example, older people aged 65 and over, especially if they have limited mobility and therefore cannot get enough outdoors. People who only go outdoors heavily covered or who have dark skin are also among the risk groups.
According to the German Nutrition Society, anyone who takes vitamin D in the form of dietary supplements should not take more than 20 µg (= 800 IU) per day. From a nutritional perspective, a higher dose is not necessary. However, if the vitamin D deficiency is due to an illness, the doctor may still recommend a higher vitamin D dosage.
Important to know
Taking vitamins has a positive effect on health if there was previously a deficiency. However, if the vitamin D status is normal, the health benefits of taking it seem questionable.
People who regularly take medications such as cardiac glycosides should only take vitamin D supplements after consulting a doctor. Because these can increase the effect of cardiac glycosides. However, under certain circumstances the risk of interactions can be avoided if you take them at the appropriate time interval.
Take vitamin D and vitamin K together?
It is often read that vitamin D preparations should always be taken together with vitamin K. The reason for this is that taking vitamin D alone, especially at higher doses, could otherwise lead to a deficiency of vitamin K and thus counteract the positive effect on bone health.
However, it is scientifically unclear whether the combination of vitamin D and vitamin K is actually better for bone health than the individual administration of vitamin D or protects against calcium deposits in the blood vessels. Therefore, general recommendations on vitamin D and vitamin K cannot currently be made.
The doctor must decide whether a combined intake of vitamin D and vitamin K may be advisable in the case of osteoporosis. The guideline for the treatment of osteoporosis advises against taking high doses of vitamin K, but recommends ensuring adequate vitamin K intake through your diet.
Vitamin D: Signs of an overdose
A vitamin D overdose is only possible if vitamin D supplements are taken in too high doses over a long period of time. A long-term vitamin D intake of more than 100 µg (= 4,000 IU) daily is considered questionable.
If the vitamin D overdose lasts longer, hypercalcemia can occur. The calcium levels in the blood rise sharply because the body absorbs more calcium from the food in the intestine due to the high amounts of vitamin D.
If calcium levels are persistently high, calcium is deposited in the kidneys and the risk of kidney problems such as kidney stones or kidney calcification increases. The deposited calcium damages the kidneys in a way that cannot be reversed.
However, there is no need to worry about an overdose of vitamin D due to too much sun or diet. In any case, there are only a few foods that have any significant vitamin D content.
With the help of the sun (in summer), 90 percent of your daily requirements can theoretically be covered after just a short time. However, if you spend longer in the sun, you don’t have to worry about a vitamin D glut. The body does not produce vitamin D in the skin in excess of what is needed and reduces production after a while.