Vitamin D effect: the most important questions and answers

Vitamin D Wirkung: Die wichtigsten Fragen und Antworten

Vitamins are essential for our health. They supply the body with energy, help, among other things, with sharp vision and protect us from pollutants and diseases.

Vitamin D ensures a variety of movement processes in our body. If a deficiency develops, there are possible consequences associated with it, just as an overdose of the vitamin can have serious health effects.

This article is intended to give you an understanding of the main effects of this vitamin, as well as the causes and consequences of a deficiency symptom, but also those of an overdose. In the following you will receive valuable tips about the effects of vitamin D on our body and how it can help you to prevent diseases.

the essentials in brief

  • The body produces most of the vitamin D it needs itself. Most of the vitamin D requirement is produced by the body itself with the help of UV-B radiation and the rest is obtained from food.
  • A vitamin D deficiency can lead to serious health consequences, which are reflected in well-known clinical pictures. Likewise, overdosing can lead to symptoms and health effects.
  • Vitamin D affects almost all areas of the body in our body. Among other things, it affects our immune system, heart/circulatory system, as well as our nerves and brain.

Definition: What is vitamin D?

Strictly speaking, vitamin D is not a real vitamin. According to the definition, vitamins are vital organic compounds that the body has to take in regularly with food because it cannot produce them or cannot produce them in sufficient quantities.

The term vitamin D refers to a group of fat-soluble vitamins that are important for calcium balance and bone mineralization. Basically, a distinction can be made between the following variations of vitamin D.

  • Vitamin D1
  • Vitamin D2
  • Vitamin D3
  • Vitamin D4
  • Vitamin D5

If vitamin D is spoken of, vitamin D3 is usually meant. This is the natural vitamin D that is produced in the human body. Vitamin D1, D2, D4 and D5, on the other hand, are artificially produced vitamins.

Vitamin D effects: What you should know

Before we give you tips on how vitamin D works, you should learn some background information about this vitamin.

We want to answer these frequently asked questions for you in the following paragraphs.

Why does the body need vitamin D?

In contrast to all other vitamins that humans consume through food, our body can produce vitamin D itself.

The most well-known function of vitamin D is its involvement in bone metabolism. Vitamin D promotes, among other things, the absorption of calcium and phosphate from the intestine and their incorporation into the bones. It thus plays a key role in bone mineralization.

In addition, vitamin D is involved in other metabolic processes, in the formation of proteins or in the control of a large number of genes.

How can my body absorb vitamin D?

There are basically two forms of vitamin D intake. For the most part, vitamin D is formed in humans under the influence of sunlight in the skin and is ingested through food.

The second form is intake via so-called supplements, which should only be taken if there is a deficiency. In the following, however, we will show you how you can best absorb vitamin D.

Natural absorption of vitamin D

80-90% of the vitamin D requirement is produced by the body itself with the help of sunlight. Under the influence of UV-B radiation, the so-called pre-vitamin D3 is produced in the skin from a precursor of vitamin D, the pro-vitamin D3.

This previtamin D3 travels through the bloodstream to the liver and kidneys, where it is finally converted to biologically active vitamin D, also known as calcitriol.

woman sitting on the beach

Most vitamin D is absorbed when skin comes into contact with the sun. However, care should be taken not to get sunburned in order to avoid the risk of skin cancer. (Image Source: Igor Link / Pixabay)

To do this, you should let around a quarter of your body surface be exposed to the sun for around 15 minutes every day. The best time of day for vitamin production is between 10 a.m. and 2 p.m.

In the winter months, sufficient self-production of vitamin D is not always guaranteed. This is mainly due to the fact that there are fewer hours of sunshine overall in winter and the intensity of solar radiation is lower.

Other than being formed in the skin, vitamin D can also be found in some foods. 10-20% of the vitamin D requirement is met through food, but the vitamin is found in foods only to a limited extent.

The highest concentration can be found in the following foods.

  • eggs
  • liver
  • edible mushrooms
  • fatty fish (salmon, mackerel, herring)
  • butter

Although we absorb relatively little of the important vitamin through food, this amount can be decisive, especially in the rather dark seasons.

Intake of vitamin D via supplements

Depending on the type of application and dosage, vitamin D preparations are classified as medicinal or dietary supplements. These can be a useful supplement if an undersupply of vitamin D has been determined.

Essentially, vitamin D preparations can be differentiated according to the following characteristics:

  • Active ingredient (vitamin D2, vitamin D3, vegan vitamin D3)
  • Dosage (200 to 20,000 IU)
  • Ingredients (combined preparation with other micronutrients, additives, allergens)
  • Dosage form (drops, capsules, tablets)
  • Purpose (nutritional supplement or drug)

However, caution is required here. In addition, it is expressly warned against overdosing. The body alone cannot produce too much vitamin D, but supplements can increase the vitamin D value to a level that is harmful to health.

A daily intake without the advice of the doctor can therefore harm the body.

What is the daily requirement of vitamin D?

Vitamin D occupies a special position among the vitamins because, as already mentioned, it is supplied through nutrition and is also formed by humans through exposure to the sun.

However, since the formation of sun exposure does not occur regularly in some people, the German Society for Nutrition (DGE) calculates the following estimates for an appropriate vitamin D intake using a vitamin D preparation.

Old vitamin D per day
Infants (0 to under 12 months) 10 micrograms
Children (1 to under 15 years) 20 micrograms
Adolescents and adults (15 to under 65 years) 20 micrograms
Adults (from 65 years) 20 micrograms
pregnant women 20 micrograms
breastfeeding 20 micrograms

To prevent rickets, infants are regularly given a vitamin D supplement. This ensures that the vitamin D requirement is actually covered. In older children, adolescents and adults it may also be necessary to supplement vitamin D separately.

However, this is more likely to be used in the months with less sun, because the sun is too weak and the bare skin is hardly exposed to the light due to the low temperatures.

What is a vitamin D deficiency?

One speaks of a vitamin D deficiency when there is a lack of vitamin D in the body over a longer period of time and clinically relevant symptoms occur. Long-term vitamin D deficiency can have serious consequences.

According to a study by the Robert Koch Institute, around 30% of adults in Germany are deficient in vitamin D based on their serum blood levels.(1)

This inadequate supply can be reflected in the following clinical pictures.

  • heart diseases (2)
  • stroke
  • high blood pressure
  • Diabetes (3)

Above all, older people and people who rarely spend time outdoors or only spend time outdoors with covered skin are at increased risk of vitamin D deficiency.

They also include people who have chronic diseases, especially gastrointestinal, liver or kidney diseases. As well as people taking medications that affect vitamin D metabolism.

What is the effect of vitamin D on various diseases?

In the therapy of diseases, vitamin D has not turned out to be very effective. Only in the case of rickets is vitamin D regarded as an effective therapeutic agent. (4)

Otherwise, vitamin D is only used as a supplementary or supportive therapy measure. For example, in tuberculosis, influenza and viral diseases of the upper respiratory tract. (5)

Mugs, glasses and handkerchiefs

Vitamin D can help prevent diseases like the flu. Low vitamin D levels in particular are often associated with colds and flu. (Image Source: Kelly Sikkema / Unsplash)

However, vitamin D also plays a major role in the immune system and other areas of the body, which can help prevent many diseases.

Conditions for the action of vitamin D?

In order for vitamin D to have the best effect, there must be an adequate supply of both vitamin D and some cofactors.

Here is a summary of the most important points.

  • Adequate blood levels: Vitamin D is most effective at blood levels between 35 and 60 ng/ml
  • Supply from the sun: It is best to let around a quarter of the body surface be exposed to the sun for about 15 minutes. However, do without sunscreen so that the body can absorb the sunlight well.
  • Sufficient dosage: The best active ingredient is vitamin D3, which is used with vitamin D preparations. Average doses are 400-1000 IU in summer and 3000-4000 IU in winter. IE refers to the international unit in which vitamin D is measured. 1 IU corresponds to 0.025 micrograms.
  • Sufficient supply of cofactors: Vitamin D can only fulfill some effects if there is an adequate supply of vitamin A, vitamin K and magnesium.

What is the normal vitamin D value?

A study published in November 2019 dealt with determining the vitamin D normal value. This was a randomized, double-blind, placebo-controlled study in which gene activity was observed after various levels of vitamin D administration. (6)

As a result, the researchers found that the vitamin D level should always be above the 30 micrograms/milliliter deficiency limit. Values ​​between 40 and 60 micrograms/milliliter are optimal.

Otherwise, there is a compensatory increase in parathyroid hormone, which mobilizes calcium from the bones to maintain the required blood calcium levels.

However, the optimal value of 40-60 micrograms/milliliter is not to be regarded as a permanent figure and could shift upwards again in the next few years.

What happens with a vitamin D overdose?

In addition to a deficiency, poisoning or overdose with vitamin D can also occur. This is because vitamin D, as a fat-soluble vitamin, can be stored in fat and muscle tissue.

While poisoning cannot be caused by the body's own vitamin D formation and natural nutrition, it can be caused by excessive intake of dietary supplements, high-dose medication or a high consumption of fortified foods

According to a 2018 study, the most commonly identified clinical symptoms of vitamin D toxicity are as follows. (7)

  • confusion
  • apathy
  • increased vomiting
  • stomach pain
  • Polyuria (increase in urine volume)
  • dehydration

Although vitamin D toxicity is rare, the health effects can be serious if not recognized in time. Since vitamin D increases calcium absorption in the gastrointestinal tract, more serious diseases can also develop.

This potentially leads to muscle weakness, high blood pressure, neuropsychiatric disorders, gastrointestinal disorders, kidney stones and, in extreme cases, kidney failure, cardiac arrhythmia, calcification of the coronary arteries and heart valves and ultimately death. (8th)

vitamin capsule

Vitamin D supplements are available at almost every drug store. However, an overdose of vitamin D can actually cause harm (Image source: Michele Blackwell / Unsplash)

You should be particularly careful when using supplements to counteract a vitamin D deficiency. Due to manufacturing defects or self-administration, you can also take too much and thus poison your body.

How to improve the body's absorption of vitamin D?

In order to improve the absorption of vitamin D in the body, there are a few factors that should be considered. In the following we will show you what can inhibit absorption and what you should look for in order to achieve optimal vitamin D intake.

  • Sunscreens: Sunscreens block the absorption of the sunlight spectrum responsible for the synthesis of vitamin D. Long-term use can lead to low body stores of vitamin D in some individuals.(9)
  • Latitude: Increasing the path that solar radiation has to travel through the ozone layer results in a reduction in the number of UVB photons that reach the Earth's surface. So if you live above and below the 33rd parallel, very little or no vitamin D3 can be produced in the skin by exposure to the sun. (10)
  • The color of the skin : Due to the constant exposure to sunlight, man has developed an efficient natural sunscreen melanin with evolution. This is why people with a lighter skin type absorb vitamin D more quickly from the sun.
  • UV index: The UV index indicates the intensity of the sun's radiation and what sun protection measures are required. The UV index ranges from 0 to more than 11. 0 to 2 means a weak radiation intensity. A value of 3 to 5 is already stronger. Sun protection is already recommended here. Values ​​of 8 or more advise against staying outdoors. In general, the lower the UV index, the less vitamin D is produced.
  • Showering after sunbathing: The skin needs up to 48 hours to absorb the provitamin D formed in the outer skin areas during sunbathing and to transport it into the bloodstream. Therefore, one should not shower, at least not with soap, for at least the first few hours after sunbathing. Otherwise the newly formed provitamin could be released from the skin.(11)

Now you have got a small overview of how you can improve your vitamin D intake. By taking the above points into account, but still taking care of your health and not being exposed to the sun without protection for too long.

How does vitamin D affect the body?

The effects of vitamin D have been intensively researched for decades. Despite everything, new effects of the vitamin become known almost every year. It is becoming increasingly apparent that vitamin D has an effect on almost all areas of the body.

In the following we have summarized the most important points for you and explained what effect vitamin D has on these parts of the body and how they influence each other.

Effect on the genome/DNA

The effect of the vitamin D hormone occurs via so-called vitamin D receptors, which can be found in almost all body cells. Via these, vitamin D influences how the genetic material of these cells is read and thus regulates the synthesis of numerous proteins, enzymes and messenger substances, which in turn control and influence numerous bodily processes. (12)

In addition to the direct activation of genes, vitamin D also has an influence on so-called epigenetics, which describes that certain genes can be permanently switched on or off through methylation. (13)

Generally speaking, vitamin D is therefore a genetic modulator that does not change the genome directly, but can control and activate a large number of genes through various mechanisms.

Effect on the mineral balance

Vitamin D ensures a sufficient supply of phosphate and calcium and prevents this from having to be mobilized from the bones. It thus maintains the health of bones, teeth and muscles and supports cell function.

Without vitamin D, the two minerals calcium and phosphate cannot be absorbed from food.

Stable calcium levels are of the utmost importance, since calcium, in addition to building bones, also plays a central role in cellular communication and also significantly controls muscle function.

effect on the immune system

One of the most important tasks of vitamin D is the control of the immune system. Vitamin D has several functions here.

Vitamin D Mechanism effect
release of endogenous antibodies Vitamin D controls the release of cathelicidin and other antibodies that are effective against bacteria, viruses and fungi. Thus, it prevents infections and can also treat some of them.
detection of pathogens Vitamin D helps identify and fight pathogens
immune response Vitamin D down-regulates inflammatory and autoimmune defense processes. In this way, chronic inflammation and autoimmune reactions are prevented and in some cases also treated

Vitamin D is therefore helpful in combating acute diseases and at the same time prevents the development of autoimmune diseases and chronic inflammation. (14)

effect on the heart and circulation

Cardiovascular diseases are the leading cause of death worldwide. Therefore, vitamin D also plays an important role in maintaining the health of the cardiovascular system.

The most important cell types in the vessels and also in the heart have vitamin D receptors and their function is controlled by vitamin D.

Vitamin D also modulates the renin-angiotensin-aldosterone system (RAAS), which in turn controls blood pressure and salt and water balance.

It has only recently become known that vitamin D also has a direct effect on the endothelium and stabilizes and protects it. (15)

The endothelium is a thin layer of endothelial cells on the inside of blood vessels.

Vitamin D, through the sum of its effects in the cardiovascular system, ensures that vessels are protected, clogging plaques are prevented, the heart function is guaranteed and the blood is kept free-flowing.

Effect on nerves and brain

Vitamin D is important for protecting and maintaining the nerves and the brain. Vitamin D even fulfills several functions which we have summarized for you here:

  • Production of Neurotrophins: Neurotrophins control the wiring of the brain and the survival of neuronal connections. Vitamin D is therefore important for building and maintaining these neuronal connections.
  • The synthesis of important neurotransmitters: Neurotransmitters are messenger substances in the central nervous system. Vitamin D is therefore important for the transmission of chemical signals in the central nervous system, which also affect mood and cognition.
  • Intracellular calcium homeostasis: Calcium is another important cellular messenger. Vitamin D is therefore particularly important for the transmission of cellular signals.
  • Important antioxidants: Vitamin D protects the nerves from oxidative damage by controlling important antioxidants such as glutathione.
  • Reduces inflammation in nerves and in the brain: Vitamin D thus protects the nerves and ensures that nerve health is maintained.

Because of these central functions in protecting the nerves, vitamin D has been implicated in almost all diseases of the central nervous system.

Effect on proliferation and differentiation of cells

Proliferation is the term for rapid growth or multiplication of tissue. This cell proliferation is expressed in cell division and cell growth.

Studies have found that vitamin D is even involved in controlling cell division, cell growth, cell death and the cell cycle in nerve cells. (16) As a result, vitamin D could also act on the following areas.

  • Cancer: In animal studies, many forms of cancer can be both prevented and treated by vitamin D. So far, however, corresponding human studies have been too small or methodologically inadequate. (17)
  • Organ health: There are vitamin D receptors in almost all organs and tissues of the body, so this hormone has an influence in many places.
  • Health of skin and hair: Since vitamin D has an effect on all areas where cell division plays an important role, it is of course also important for the tissue cells that regenerate the fastest, the cells of the skin and hair. (18)


Since vitamin D affects almost all areas of the body and also helps to recognize and fight pathogens, it is all the more serious when there is a deficiency or an overdose.

Therefore, one should always pay attention to what signs there are to maintain the vitamin D level, and thus to grant the effect of it. Since many areas in particular, such as the immune system but also our cardiovascular system, work together and thus serious diseases as well as chronic diseases can be prevented.


  1. Martina Rabenberg, Gert BM Mensink Journal of Health Monitoring 2016 1(2) DOI 10.17886/RKI-GBE-2016-036 Robert Koch Institute, Berlin
  2. Danik JS, Manson JE. Vitamin d and cardiovascular disease. Curr Treat Options Cardiovasc Med. 2012;14(4):414-424. doi:10.1007/s11936-012-0183-8
  3. Parker J, Hashmi O, Dutton D, et al. Levels of vitamin D and cardiometabolic disorders: systematic review and meta-analysis. Maturitas. 2010;65(3):225-236. doi:10.1016/j.maturitas.2009.12.013
  4. Shirvani, A., Kalajian, TA, Song, A. et al. Disassociation of Vitamin D's Calcemic Activity and Non-calcemic Genomic Activity and Individual Responsiveness: A Randomized Controlled Double-Blind Clinical Trial. Sci Rep 9, 17685 (2019).
  5. Yamshchikov AV, Desai NS, Blumberg HM, Ziegler TR, Tangpricha V. Vitamin D for treatment and prevention of infectious diseases: a systematic review of randomized controlled trials. Endocr Pract. 2009;15(5):438-449. doi:10.4158/EP09101.ORR
  6. Khajavi A, Amirhakimi GH. The rachitic lung. Pulmonary findings in 30 infants and children with malnutritional rickets. Clin Pediatr (Phila). 1977;16(1):36-38. doi:10.1177/000992287701600106
  7. Marcinowska-Suchowierska E, Kupisz-Urbańska M, Łukaszkiewicz J, Płudowski P, Jones G. Vitamin D Toxicity-A Clinical Perspective. Front Endocrinol (Lausanne). 2018;9:550. Published 2018 Sep 20. doi:10.3389/fendo.2018.00550
  8. Galior K, Grebe S, Singh R. Development of Vitamin D Toxicity from Overcorrection of Vitamin D Deficiency: A Review of Case Reports. nutrients. 2018;10(8):953. Published 2018 Jul 24. doi:10.3390/nu10080953
  9. Matsuoka LY, Wortsman J, Hanifan N, Holick MF. Chronic sunscreen use decreases circulating concentrations of 25-hydroxyvitamin D. A preliminary study. Arch Dermatol. 1988;124(12):1802-1804.
  10. Wacker M, Holick MF. Sunlight and vitamin D: A global perspective for health. dermatoendocrinol. 2013;5(1):51-108. doi:10.4161/derm.24494
  11. Binkley N, Novotny R, Krueger D, et al. Low vitamin D status despite abundant sun exposure. J Clin Endocrinol Metab. 2007;92(6):2130-2135. doi:10.1210/jc.2006-2250
  12. Kamen DL, Tangpricha V. Vitamin D and molecular actions on the immune system: modulation of innate and autoimmunity. J Mol Med (Berl). 2010;88(5):441-450. doi:10.1007/s00109-010-0590-9
  13. Gibson CC, Davis CT, Zhu W, et al. Dietary Vitamin D and Its Metabolites Non-Genomically Stabilize the Endothelium. PLoS One. 2015;10(10):e0140370. Published 2015 Oct 15. doi:10.1371/journal.pone.0140370
  14. Yang CY, Leung PS, Adamopoulos IE, Gershwin ME. The implication of vitamin D and autoimmunity: a comprehensive review. Clin Rev Allergy Immunol. 2013;45(2):217-226. doi:10.1007/s12016-013-8361-3
  15. Gibson CC, Davis CT, Zhu W, et al. Dietary Vitamin D and Its Metabolites Non-Genomically Stabilize the Endothelium. PLoS One. 2015;10(10):e0140370. Published 2015 Oct 15. doi:10.1371/journal.pone.0140370
  16. HAP Pols, JC Birkenhäger, JA Foekens, JPTM van Leeuwen, Vitamin D: A modulator of cell proliferation and differentiation, The Journal of Steroid Biochemistry and Molecular Biology, Volume 37, Issue 6, 1990, Pages 873-876, ISSN 0960-0760,
  17. Bike DD. Vitamin D and cancer: the promise not yet fulfilled. endocrine. 2014;46(1):29-38. doi:10.1007/s12020-013-0146-1
  18. Mostafa WZ, Hegazy RA. Vitamin D and the skin: Focus on a complex relationship: A review. J Adv Res. 2015;6(6):793-804. doi:10.1016/j.jare.2014.01.011
Back to blog
Vorheriger Beitrag

Nächster Beitrag

Leave a comment

Please note, comments need to be approved before they are published.