Vitamin D deficiency: the most important questions and answers

Vitamin D Mangel: Die wichtigsten Fragen und Antworten

Do you often feel weak and tired? A lack of vitamin D can be responsible for this. Vitamin D plays important roles in our body. If a vitamin D deficiency develops, some functions in your body are restricted and deficiency symptoms can occur.

In this article we will show you how you can prevent a vitamin D deficiency and how you should deal with a deficiency. We also answer frequently asked questions. We have compiled the most important background information on vitamin D deficiency for you and summarized the most important scientific findings.

the essentials in brief

  • Vitamin D plays an important role in our body and contributes significantly to health. A vitamin D deficiency can be harmful over a longer period of time and lead to diseases.
  • Symptoms of a vitamin D deficiency are, for example, tiredness, muscle weakness or a weakened immune system. You can also do a vitamin D blood test to detect a deficiency.
  • A vitamin D deficiency can usually be remedied by consuming sunlight. Other treatment options include changing your diet or taking vitamin D supplements

Definition: What is vitamin D?

Vitamin D belongs to the group of fat-soluble vitamins. Strictly speaking, however, vitamin D is not a vitamin at all, but a prohormone (1). Unlike other vitamins, the body can produce vitamin D itself with the help of sunlight. An endogenous, i.e. endogenous, synthesis takes place.

Approximately 80% of vitamin D absorption occurs via the UV-B radiation of the sun. Only 20% is ingested through food.

Woman standing in the sun

Vitamin D plays an essential role in numerous functions in our body. The majority of our daily dose of vitamin D is absorbed through the sun's rays. The UV-B radiation of the sun is required for the synthesis of endogenous vitamin D. (Image source: Alexander Shustov / unsplash)

Vitamin D plays an important role in central bodily functions. For example, the D hormone is responsible for regulating calcium, which is important for bone formation. It also has an impact on the muscles, the kidneys, the immune system (2) or well-being (3). Vitamin D deficiency is treated either through exposure to sunlight, diet, or with supplements.

Background: What you should know about vitamin D deficiency

In this section we explain everything you need to know about vitamin D deficiency. We give you information about the functions of vitamin D, the daily requirement and the causes and consequences of vitamin D deficiency.

What effect does vitamin D have in the body?

Vitamin D performs important functions in our body. You can read about which areas vitamin D affects in the following part.

immune system

The immune system is a complex network consisting of various components and mechanisms of action. It protects our body from pathogens such as bacteria and viruses. A weakened immune system usually results in a higher susceptibility to infections.

Various researches have found a connection between a vitamin D deficiency and a weakened immune system. Among other things, vitamin D can influence the adaptive and innate immune response and plays an important role in the functioning of the immune system.

This is also shown by the observation that immune cells react to vitamin D in an autoimmune disease (4).


Not only is vitamin D produced in the skin, but vitamin D also has positive effects on the skin. Vitamin D contributes significantly to skin health.

The vitamin is involved in cell division and differentiation and is therefore responsible for skin regeneration. It also supports wound healing and can work on skin diseases such as neurodermatitis and acne. Since vitamin protects the skin, it contributes to a functional immune response of the skin (5).


The muscular system is an essential part of our body. It is important that you take care of his health so that you can move around without problems.

A 2011 scientific study looked at the link between vitamin D and muscles.

It was found that vitamin D both supports muscle function and can improve muscle strength.

This is especially important for older people, because vitamin D supplementation can reduce the risk of falls (6).

nervous system

Vitamin D performs important tasks in the central nervous system. The vitamin ensures that the brain and nervous system remain healthy and can continue to develop.

Among other things, vitamin D protects the nerves from damage, is involved in detoxification and controls regulation. In addition, vitamin D contributes to the formation of nerve connections and has an effect on signal transmission in the nervous system (7).

heart and circulatory system

Vitamin D contributes to the proper functioning of the cardiovascular system. It regulates the functions of smooth muscle cells, thereby promoting heart health (8).

Some studies link vitamin D deficiency to an increased risk of cardiovascular disease, ischemic heart disease, high blood pressure, and heart failure (9).

A meta-analysis found that low vitamin D levels could be associated with early death (10).


The skeleton supports our body and makes life possible. Healthy and strong bones increase the quality of life and they are especially important in old age.

Vitamin D is often associated with healthy bone formation and maintenance. The vitamin regulates calcium and phosphate absorption as well as bone mineralization.

Did you know that vitamin D can increase calcium and phosphorus absorption by 30% to 40% and 80%, respectively (11)?

Calcium and phosphate are important for our bone structure. Calcium contributes to the formation of bones and teeth, healthy blood clotting, normal enzyme function, and heart rhythm (12).

Phosphate is also important for our bones. Most of it is stored in the bone. But phosphate is also contained in the genetic material or in numerous chemical compounds in our body (13).

Important proteins for bone formation are also regulated by vitamin D (14).

What is the daily requirement of vitamin D?

The German Society for Nutrition lists the daily requirements for different groups of people. In this table we show you how much vitamin D you should take per day.

Approximately 80% to 90% of the daily amount of vitamin D is produced by endogenous synthesis in the body. Only 20% to 10% has to be added through the diet.

group of people Vitamin D µg / day*
baby 10
Children up to 15 years 20
teenagers and adults 20
Pregnant and lactating women 20

*In the absence of endogenous synthesis

An overdose of vitamin D from the sun is not possible. A protective mechanism in the body ensures that not too much vitamin D is synthesized. Overdosing can occur when taking supplements (15).

What are the causes of a vitamin D deficiency?

There are many causes of a vitamin D deficiency. The main cause is usually too little sunlight. You can see what other causes a lack of vitamin D can have in this overview (16):

  • too little vitamin D in the diet
  • Skin diseases that interfere with absorption through the skin (e.g. psoriasis)
  • Intestinal diseases that interfere with absorption from the diet
  • Kidney and liver diseases that affect the conversion of vitamin D into the active form
  • certain medications that affect the absorption and utilization of vitamin D

There are some groups of people who are more prone to vitamin D deficiency. The following risk groups are known (17):

risk group Description
Individuals with insufficient sunlight consumption Individuals who do not consume enough sunlight belong to this group. This includes immobile persons or persons wearing covering clothing for personal or religious reasons.
Elderly people Older people are more at risk because the efficiency of vitamin D synthesis decreases with age. In addition, there is often a risk that they do not eat enough or are immobile and therefore do not go out in the sun often.
Dark-skinned people People with a dark skin color belong to the risk group because the synthesis in the skin is less efficient (high melanin content).
Pregnant and lactating women During pregnancy and lactation there is an increased need for vitamin D.
baby Infants are often deficient in vitamin D because breast milk is low in vitamin D and because the skin's protective mechanisms do not allow them to be exposed to direct sunlight (18).

If you belong to the risk group, you should pay particular attention to the symptoms of a vitamin D deficiency. In the case of a severe deficiency, treatment by a doctor can help you identify the causes and bring vitamin D levels back to a healthy level.

What are symptoms of a vitamin D deficiency?

A vitamin D deficiency manifests itself in various forms. In the following list we have summarized the most common symptoms of a vitamin D deficiency.

  • Depression (19)
  • fatigue (20)
  • sleep disorders (21)
  • weakened immune system (22)
  • hair loss
  • Cramps, muscle weakness and pain
  • Bone Pain (23)

If the symptoms are severe or do not improve after self-treatment, you should definitely seek medical treatment. Some of these symptoms can have serious causes that should be investigated.

woman in bed

Symptoms of a vitamin D deficiency can be constant tiredness, depression or a sleep disorder. A vitamin D deficiency can affect the entire body and should therefore be examined by a doctor at an early stage. (Image source: Kinga Cichewicz / unsplash)

Long-term vitamin D deficiency can have serious health consequences. These will only be examined in detail in research. There are some references to these episodes:

  • Cardiovascular Diseases (24)
  • Rickets in infants and children (25)
  • Osteomalacia (softening of the bones) and osteoporosis (brittle bones) (26)
  • Nerve Damage and Disorders (27)

The consequences of a vitamin D deficiency affect the entire body and can damage your body. Therefore, you should have your vitamin D deficiency treated.

How is a vitamin D deficiency diagnosed?

With a vitamin D blood test you can measure your vitamin D level. The concentration of 25-hydroxyvitamin D in the blood serum is measured.

You can have a blood test done by your doctor. Alternatively, you can carry out a self-test to check your vitamin D values ​​for the first time. For this you take your blood at home.

Depending on the model, you can then analyze the vitamin D content in your blood yourself or send your sample to an external laboratory.

One speaks of a vitamin D deficiency when the 25-hydroxyvitamin D value is below 30 nmol/l in the blood. An optimal supply is achieved with 50 nmol/l (18).

A self-test does not replace a visit to the doctor. In the case of severe deficiency symptoms in particular, you should consult your doctor immediately.

Vitamin D deficiency: 3 approaches to effective treatment

There are numerous ways you can maintain or increase your vitamin D levels. In the following we will show you 3 effective treatment options to remedy a vitamin D deficiency.

consume sunlight

Sunlight is the most important component in the synthesis of vitamin D in the body. About 80% of the amount of vitamin D is formed in the skin through UV-B radiation (28). Some factors such as skin pigmentation, age, clothing, and sunscreen can make it difficult for skin to absorb vitamin D.

Even a sunscreen with a factor of 30 can reduce vitamin D synthesis by 95% (29).

It is important that you get enough direct sunlight during the day. You should take into account that window glass completely blocks UV-B radiation (30).

change nourishment

Food covers about 20% of vitamin D requirements. If you have a vitamin D deficiency, you should switch your diet to vitamin D-rich meals (28). Some of the foods richest in vitamin D include the following (17):

  • High-fat fish (such as tuna, mackerel, and salmon)
  • egg yolk
  • beef liver
  • Cheese
  • certain types of fungus, especially when exposed to ultraviolet light
  • foods fortified with vitamin D (such as milk and margarine)

A balanced diet is essential for your health and your body. This also prevents other deficiency symptoms and can increase your well-being.

taking preparations

There are many forms of vitamin D supplements. These include tablets, capsules and drops. The bioavailability, i.e. the absorption of vitamin D, improves if the preparations are taken with oil or fat. That is why preparations are often offered in oil form.

If you have a severe vitamin D deficiency, your doctor can also give you a vitamin D injection (31).

Vitamin D supplements

One way to treat a vitamin D deficiency is to take supplements. This variant is advisable if there is no way to absorb vitamin D from the sun. It is possible to overdose on vitamin D with supplements. (Image source: Kayla Maurais / unsplash)

Vitamin D in supplements, like in food, is poorly absorbed by the body. However, if absorption through the skin and diet is not possible or insufficient, supplements can raise your vitamin D level.

However, caution is required. Because unlike the natural intake through the skin and diet, vitamin D can be overdosed through supplements (30). Symptoms of vitamin D overdose include weakness, nausea, vomiting, loss of appetite, and constipation (16).

Vitamin D toxicity can damage the kidneys, raise calcium levels, and lead to calcium stones (32).

We therefore advise you to discuss taking vitamin D supplements with your doctor.


Vitamin D is an important component in our body and assumes central functions. The vitamin has an effect on the skin, nerves, muscles, psyche, bones and the immune system. A lack of vitamin D has very physical effects in both young and old people.

The causes of the deficiency are too little sunlight, a diet poor in vitamin D, illnesses or medication. A vitamin D deficiency can be diagnosed with a blood test. Treatment options include consuming sun rays, changing diet, or taking vitamin D supplements.


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