Vitamin D Deficiency: Symptoms, Sunshine & Supplements

Sarah Vordermeier

One billion people around the world suffer from a lack of vitamin D (sometimes called the sun vitamin). We can only acquire enough of this important vitamin with sufficient sunbathing or by taking the right supplements.

Hair loss, depression, fatigue, skin diseases, and cancer are some of the symptoms associated with vitamin D deficiency. Studies have shown that vitamin D plays a role in numerous processes in the body. It impacts on the bones, the muscles, the immune system, the blood vessels, and much more. As a result, vitamin D has been a hot topic not only among scientists and physicians over recent years, but also in the media, in weekly magazines, television programs and social media.

Despite the media hype and the new awareness surrounding vitamin D, few people manage to take in enough. It is estimated that one billion people in the world are not adequately supplied with the sun vitamin. According to the National Health and Nutrition Examination Survey (NHANES), insufficient vitamin D levels were found in 41.6% of 4,495 U.S. participants in a vitamin D deficiency study[1], so it's possible that almost half of U.S. citizens are vitamin D deficient. 

This article will show you what you can do with tests, sunbathing sessions and supplements to ensure that you are optimally supplied with vitamin D (and what exactly your body does with the vitamin D). We explain what a vitamin D deficiency or excess can cause and the diseases that are linked to a vitamin D deficiency.

What Is Vitamin D?

Vitamin D is a fat-soluble vitamin, with experts also referring to it as a hormone, because it has a hormone-like effect and it does not originate primarily from the diet, as is otherwise normal with vitamins. Instead, our body produces vitamin D itself, but requires the UV radiation of the sun to generate it. The two most important forms of vitamin D are vitamin D₂ (ergocalciferol) and vitamin D₃ (cholecalciferol), also called 25-(OH)-D[2].

What functions does vitamin D have in the body?

Vitamin D is involved in numerous processes within the body, attributable particularly to its dual role as a vitamin and a hormone. Among its most important tasks is its involvement in the metabolism of bone and the development and functioning of our musculature. In addition, vitamin D promotes the immune system and protects blood vessels[2].

Other uses of vitamin D include:

  • the control of calcium and phosphate absorption in the small intestine.
  • regulation of more than 200 genes.
  • promotion of the function of heart muscle.
  • as a hypotensive mediator.
  • promotion of skeletal development in children.

Vitamin D Test

Supplementation With Vitamin D

It is not for nothing that vitamin D is called the sun vitamin - our body needs the light of the sun for its production, or more specifically UVB radiation. Initially, its precursor 7-dehydrocholesterol is formed, which is generated with the help of cholesterol in the liver and intestinal mucosa before it migrates back to the skin. There, the active form of the vitamin, namely vitamin D3, is produced, and this in turn becomes 25-hydroxy-cholecalciferol (25-OH-D), the storage form of vitamin D, which is then stored in the muscles and adipose tissue. In the kidney, 25-OH-D is activated to 1,25-(OH)2-D which is transported through the blood to the intestine, bones, muscles, immune system and cells where it exerts its effects[5].

How do we acquire vitamin D through the diet?

The small intestine can absorb up to 80 percent of the fat-soluble vitamin D3 consumed in food[5]. Despite this, only relatively small amounts of vitamin D are absorbed into the body via this route; we only cover about 10 to 20 percent of the daily requirement via dietary sources. This is partly due to the fact that few foods contain vitamin D, and when they do they only contain small quantities. Vitamin D3, which is important for the body, is found almost exclusively in animal foods, and more specifically in[2, 5]:

  • fatty fish such as herring or kippers
  • margarine and butter
  • milk and egg yolk

Mushrooms and avocados also contain vitamin D - but in a form that the body can only poorly absorb. You would in fact need to consume large amounts of these foods even to approach the lowest recommended daily allowance. For the 800 International Units (IE) recommended by the German Nutrition Society, i.e. 20 micrograms, you would need for example 2400 grams of mushrooms, four kilos of beef liver, four kilos of butter, or 80 eggs[2].

Put briefly: with food alone, it is not possible to meet your daily requirement for vitamin D. We have to produce the largest share ourselves and this simply does not work without sunlight.

How much vitamin D do I need?

With vitamin D it is important to maintain sufficient levels, and a reliable daily dose is difficult to determine because we also consume different amounts of vitamin D in different seasons. The best way is to measure the vitamin D levels in the blood. You can have your blood levels of the 25-(OH)-D measured and this might be given for instance in nanograms per milliliter[34].

  • Most publications assume that you are sufficiently supplied with blood if you show levels from 30 nanograms per milliliter of blood.
  • Values below 11 nanograms per milliliter are considered to be critical and can encourage disorders such as bone softening.
  • Some researchers argue that 60 nanograms per milliliter or more are reasonable for taking full advantage of the health benefits of the sun vitamin.

How can I get sufficient amounts of Vitamin D?

Vitamin D Test

How much we need to lie in the sun to produce enough vitamin D depends on many factors, including our skin type, the latitude we live in, the time of day, and the time of year. According to experts, over summer it usually suffices to expose the face, hands and arms to direct sunlight three times a week for ten to twenty minutes.

Here is an example: To produce 400 IU of vitamin D, a person with medium-light skin (one who tans slowly and rarely gets sunburn) on the 42nd line of latitude from April to October (for example, in southern France) must expose a quarter of his free skin (for example, arms, face, collar) for about three to eight minutes at midday in the sun.

It is not possible for many people to expose themselves to enough sun in the summer and this is even more impossible in the winter. In winter the sun rarely shines in northern latitudes and its light also contains too little UVB radiation. An adequate vitamin D production is as such barely possible[1].

Researchers and professional societies are therefore discussing how much vitamin D we should consume via dietary supplements if an adequate supply cannot be guaranteed due to insufficient sun exposure.

Attention: do not overdo it while sunbathing. A few minutes without UV protection are useful for supplying you with vitamin D, but too much direct sunlight can lead to sunburn and increase the risk of skin cancer.

Vitamin D Test

Are vitamin D supplements useful?

The Mayo Clinic recommends that (in the absence of sunlight, i.e. usually between October and February) the optimal daily amount of vitamin D through supplements should be 400 international units (IU) for children up to age 12 months, 600 IU for ages 1 to 70 years, and 800 IU for people over 70 years. However, 1,000 to 2,000 IU per day of vitamin D from a supplement is generally safe and should help people achieve an adequate blood level of vitamin D, and may have additional health benefits.[6].

Vitamin D and Vitamin K

Vitamin K is thought to have a similar protective effect to vitamin D. It is primarily for the prevention and treatment of bone and vascular diseases. In nature, vitamin K1 occurs for instance in green vegetables, while vitamin K2 is formed by intestinal bacteria. How much vitamin K we need exactly has not yet been fully researched[41].

Again and again it has been written that when consuming vitamin D, a lack of vitamin K may ensue, since both vitamins are involved in bone build-up and mutually influence one another. However, this idea has not been scientifically proven[40]. Medical experts currently recommend joint intake of vitamin D and vitamin K only as an adjunctive therapy to specifically treat osteoporosis and prevent bone fractures occurring in older people[41-43]

Vitamin D deficiency

It is estimated that about one billion people globally are affected by vitamin D deficiency[9, 10]. Various studies have linked vitamin D deficiency to chronic diseases such as osteoporosis, diabetes mellitus, cancer, depression, cardiovascular disease and immune dysfunction[14–17].

What are the causes of vitamin D deficiency?

A lack of vitamin D occurs because we do not get enough sun. While our ancestors stayed outdoors for most of human history, we spend most of our time indoors. And we also cover our bodies with clothing and protect ourselves with sunscreen. All of this reduces the effects of UVB on the skin - and that's precisely what our body needs to produce Vitamin D[19].

And this is good to know: by applying a sunscreen with a sun protection factor of 30 the synthesis of vitamin D in the skin is reduced by more than 95 percent[31].

Several other factors can also interfere with our body's endogenous vitamin D production[2]

  • E.g. diseases that impair fat digestion and ingestion, such as coeliac disease, bile acid deficiency or pancreatic insufficiency.
  • And certain medications such as antihypertensives, antioestrogens, cytostatics, antiepileptics and herbal medicines

Symptoms of vitamin D deficiency

A vitamin D deficiency is rarely manifested with clear symptoms. The symptoms are often nonspecific and insidious, and include fatigue, muscle weakness and musculoskeletal and headache pain[1]. As a result, many people do not notice their deficiency until they develop into full blown diseases. A common consequence of a long-standing vitamin D deficiency is osteomalacia (bone softening)[19].

Those who have a severe vitamin D deficiency on a chronic basis are at an increased risk of[19]:

  • osteomalacia and osteoporosis
  • rickets in children
  • pain and weakness of the bones (osteoarthritis)
  • bone fractures in seniors over 65 (broken bones)

In addition, recent study results have associated a vitamin D deficiency with a number of diseases. The diseases often occur together with the deficiency, and researchers are still investigating what connections are precisely involved[20, 21]:

  • hypertension and heart disease
  • diabetes mellitus
  • depression
  • serious infections such as tuberculosis and chronic kidney disease
  • hair loss

Who is affected by vitamin D deficiency?

In general, vitamin D deficiency occurs in all age groups and social groups, and in areas as diverse as Europe, South America and the Middle East. But certain risk groups are particularly at risk. If you belong to one of these groups, it is recommended that you regularly check your vitamin D level and wherever necessary take supplements.

The risk groups include[18]:

  • Persons who are predominantly in closed rooms and who are hardly or not at all outdoors, or who cover their bodies outdoors.
  • Pregnant women, since they also have a higher requirement
  • People with a dark skin type, because they produce less vitamin D for the same amount of UV radiation received by people with fair skin
  • Seniors, since their vitamin D formation decreases significantly in old age and they are often not as frequently outdoors for mobility reasons
  • Infants, since the vitamin D content of breast milk is low and infants should not be exposed to direct sunlight

Vitamin D deficiency in old age

Vitamin D Test

From an age of 60 years onwards, vitamin D deficiency is particularly common. This is not because older people rarely go into the sun - their body in fact produces down to four times less skin-generated vitamin D than younger people. If you are over 60, it is recommended that you regularly check your vitamin D levels and make up for any deficiencies with supplements[22-25].

This makes sense because it helps with issues encountered frequently in old age. Positive effects confirmed in studies include[22, 26-29]:

  • prevention of bone fractures
  • improvement in cardiovascular health
  • reduction of cancer risk, such as colon cancer
  • improvements in balance
  • improved muscle power in old age
Vitamin D Test

Test your vitamin D

Many factors are involved in controlling the supply of vitamin D and avoiding a deficiency. Without a test, it's difficult to say how you should best optimize your intake. In order to find out if and how you should supplement vitamin D, a vitamin D Test is worthwhile - especially if you belong to one of the at-risk groups.

The most common measurements are blood tests performed by a doctor or therapist. With a self-test like the cerascreen® Vitamin D-Test, you can also check your levels from the comfort of your home. To do this, take a blood sample yourself by pricking your finger and send it to a specialist laboratory. The laboratory will then analyze the level of 25-(OH)-D in your blood serum. Afterwards you will find out in a results report where your value lies and how to get it back on track (and keep it on track) using supplements.

You should not take any vitamin D supplements without first having a blood test. Unlike other vitamins, vitamin D cannot be excreted in the urine. If you are already adequately supplied and still take long-term high-dose supplements, overdosing can be an issue. This can be associated with nausea, vomiting, cardiac arrhythmia, dysregulation, and in the long term with weight loss, kidney stone formation, and organ damage[2,5].

Vitamin D and diseases

Vitamin D Test

In recent years, scientists have spent considerable time studying how vitamin D levels impact health. We now present a series of studies that show the links between vitamin D and various diseases and health problems.

Vitamin D - depression and psyche

Studies have shown that vitamin D deficiencies impact mental health. Among other things, depression, stress, mood swings and anxiety can all be compounded by a Vitamin D shortage[48].

The relationship between a low vitamin D level and depression has already been studied. People with depression have significantly lower vitamin D levels than healthy people[48]. Some studies suggest that vitamin D supplements might improve depressive symptoms in people with vitamin D deficiency, but evidence gathered to date does not allow for any specific recommendations to be made.

Vitamin D and sleep quality

In 2017, Iranian scientists investigated the effect of vitamin D on sleep quality. 89 participants with sleep disorders aged 20 to 50 years were administered either a vitamin D supplement or a placebo. The result: The participants who received vitamin D had a significantly improved sleep quality, a longer sleep time and a lower falling to sleep time compared to individuals not receiving the vitamin D supplement. In another study, people with lower vitamin D levels endured a poorer sleep quality [49].

Vitamin D and hair loss

For some time it has been known that vitamins and minerals affect hair growth. As examples, iron, biotin and zinc are important for the health of the hair roots[50]. Studies in test tubes suggest that vitamin D might also be involved in active hair growth. Vitamin D as such promotes the production of receptors in the hair roots which then stimulate growth. Up until now, however, no meaningful and informative clinical studies have been published that can confirm this idea.

Vitamin D and migraine

Migraine is a severe headache that keeps on coming back. Experts now believe that migraine attacks develop as a result of inflammation in the nerves and blood vessels[51]. Researchers are currently investigating whether vitamin D supplementation can inhibit the inflammatory factors involved in migraine development. The fact that vitamin D is anti-inflammatory has been confirmed in other studies[52].

However, at present there are still relatively few studies in this area and the research results are still inconsistent. Some have shown a connection between vitamin D and migraine, and in one study it was shown that taking vitamin D could reduce the frequency of headache attacks[52]. In other studies, however, vitamin D did not have an impact on migraine.

Vitamin D and skin diseases

Vitamin D also seems to play a role in the skin. The vitamin clearly contributes to wound healing and allowing the protective skin barrier to develop properly. A vitamin D deficiency can therefore most likely contribute towards the development of skin diseases such as eczema (atopic eczema), psoriasis and white spot disease (vitiligo)[53].

Studies have shown promising results regarding vitamin D supplementation and atopic (neuro)dermatitis. Eczema patients are very susceptible to bacterial skin infections - in one study, patients with a low vitamin D level were particularly likely to suffer from such infections[54]. Researchers are also investigating how vitamin D supplements can positively impact on the course of psoriasis and vitiligo (white spot disease)[55].

Vitamin D and cardiovascular disease

Vitamin D according to research can strengthen the heart musculature. In addition, vitamin D fulfils important tasks in calcium and phosphate metabolism. The sun vitamin ensures that calcium and phosphate stay stored in the bones. If there is a vitamin D deficiency, calcium in particular is not stored properly and instead settles in the blood vessels so that calcification can then occur[46].

A study published in the American Journal of Cardiology in 2012 revealed that vitamin D deficiency may have the capacity to increase mortality from cardiovascular disease. Vitamin D supplements reduced this risk in the same study. The study authors assumed that vitamin D deficiency is a risk factor for vascular disease, cardiac muscle problems and high blood pressure[56].

These results were confirmed by another study involving over 40,000 patients. Subjects with vitamin D levels of less than 15 nanograms per milliliter were more likely to experience hypertension, elevated blood lipid levels, heart defects, and strokes than those with vitamin D levels of 30 nanograms per milliliter[56].

Vitamin D and cancer

Vitamin D is considered by many to be a ray of hope when it comes to preventing cancer. Study results until now, however, have proven ambiguous. Individual studies found associations for instance between vitamin D levels and the risk of colon cancer and breast cancer[57].

Current, large-scale metastudies, however, found no influence of vitamin D intake on the development of tumours. Many scientists say that more research is required before anything rock solid can be stated. Studies are currently underway dealing with this topic, some of which are also investigating the effects of high-dose vitamin D supplements on cancer development.

Vitamin D and Covid-19

You’ve no doubt heard a multitude of times from various sources that the global pandemic has transformed our lives in unprecedented ways. As frustrated as we may be with hearing this phrase, it is certainly true when it comes to our vitamin D levels. 

Dermatologists have warned that our new routines require vitamin D supplementation. With almost everybody working from home these days, and with no commuting and little social plans giving us an excuse to step outside our front doors – even if just for 20 minutes – it is essential for us to make sure we are getting that all-important exposure to the sun. Even during the winter months! It is also crucial to consider that travel restrictions and general fears to venture abroad have meant that fewer people have been able to go on their usual summer vacation – those vital annual vitamin D reserves might therefore be lower than normal for some people.

So, what about the importance of vitamin D supplementation in the fight against the coronavirus itself? Studies are currently being conducted to inspect in further detail just how effective vitamin D could be in terms of fighting Covid-19. Currently, there only seems to be reported and unverified links between a vitamin D deficiency and the severity of the virus; however, a study published in January 2021 by BMJ Nutrition, Prevention and Health has revealed that there is no evidence of vitamin D being protective against Covid-19.[58]

From science: vitamin D deficiency increases the risk of mortality

Vitamin D: At a glance

What is vitamin D?

Vitamin D is both a fat-soluble vitamin and a hormone at the same time. It is involved in many processes in the body, including bone metabolism, muscle function, the immune system and vascular protection.

Where do people get vitamin D from?

Our body produces 80 to 90 percent of vitamin D all by itself - but to do this it needs the UVB radiation from sunlight. To maintain our vitamin D levels, we must expose our face, hands and arms to the sun three times a week for 10 to 20 minutes during the summer months. The remaining 10 to 20 percent we get from the diet, e.g. from fatty fish, eggs, dairy products and edible mushrooms.

Who is affected by vitamin D deficiency?

Around one billion people across the world are not adequately supplied with vitamin D. The risk groups include the elderly, pregnant women, people with darker skin types, and people who hardly ever go outdoors or cover most of their body when doing so.

What are the consequences of a vitamin D deficiency?

Vitamin D deficiency can have a particularly negative impact on bone health where it may promote osteomalacia and osteoporosis. Other illnesses associated with a deficiency include depression, cardiovascular disease, hair loss, skin diseases and migraine.

How can I test my vitamin D levels?

Using a blood test, you can determine the concentration of 25-(OH)-D in your blood. This is the most meaningful parameter that describes your supply of vitamin D. Such a measurement can also be taken as a home self-test. Most scientific sources recommend levels of at least 30 nanograms per milliliter of blood.

What can I do about a vitamin D deficiency?

If you have a vitamin D deficiency, it is difficult to counteract it with diet and sunbathing alone, especially during autumn and winter. Dietary supplements are more effective. A daily dose of 1,000 to 2,000 International Units (IU) is recommended to maintain its levels. To correct a deficiency, higher doses may be useful.


    1. Prevalence of Vitamin D Deficiency and Associated Risk Factors in the US Population,
    2. Kasper, H., Burghardt, W.: Nutritional medicine and dietetics. Elsevier, Urban & Fischer, Munich (2014)
    3. Skypala, I.: Adverse food reactions--an emerging issue for adults. J. Am. Diet. Assoc. 111, 1877–1891 (2011).
    4. Roitt, I.M., Brostoff, J., Male, D.K. eds: Kurzes Lehrbuch der Immunologie. Thieme, Stuttgart (1995)
    5. American College of Allergy, Asthma & Immunology: Food Allergy,
    6. Mayo Clinic - Vitamin D.
    7. Graham-Rowe, D.: Lifestyle: When allergies go west. Nature. 479, S2–S4 (2011).
    8. Björkstén, B.: Genetic and environmental risk factors for the development of food allergy. Curr. Opin. Allergy Clin. Immunol. 5, 249–253 (2005)
    9. Naleway, A.L.: Asthma and Atopy in Rural Children: Is Farming Protective? Clin. Med. Res. 2, 5–12 (2004)
    10. Sepp, E., Julge, K., Vasar, M., Naaber, P., Björksten, B., Mikelsaar, M.: Intestinal microflora of Estonian and Swedish infants. Acta Paediatr. Oslo Nor. 1992. 86, 956–961 (1997)
    11. S3-Leitlinie Allergieprävention - Update 2014. Leitlinie der Deutschen Gesellschaft für Allergologie und klinische Immunologie (DGAKI) und der Deutschen Gesellschaft für Kinder- und Jugendmedizin (DGKJ)
    12. Molloy, J., Allen, K., Collier, F., Tang, M.L.K., Ward, A.C., Vuillermin, P.: The Potential Link between Gut Microbiota and IgE-Mediated Food Allergy in Early Life. Int. J. Environ. Res. Public. Health. 10, 7235–7256 (2013)
    13. Nwaru, B.I. et al., EAACI Food Allergy and Anaphylaxis Guidelines Group: The epidemiology of food allergy in Europe: a systematic review and meta-analysis. Allergy. 69, 62–75 (2014)
    14. Boyce, J.A. et al., NIAID-sponsored Expert Panel: Guidelines for the diagnosis and management of food allergy in the United States: summary of the NIAID-Sponsored Expert Panel Report. Nutr. Burbank Los Angel. Cty. Calif. 27, 253–267 (2011)
    15. Guideline Management of IgE Mediated Food Allergies S2k LL Allergo Journal 11-2015
    16. Ho, M.H.-K., Wong, W.H.-S., Chang, C.: Clinical spectrum of food allergies: a comprehensive review. Clin. Rev. Allergy Immunol. 46, 225–240 (2014)
    17. RKI - Number of the Month - April 2017: Allergies
    18. Burks, A.W., Tang, M., Sicherer, S., Muraro, A., Eigenmann, P.A., Ebisawa, M., Fiocchi, A., Chiang, W., Beyer, K., Wood, R., Hourihane, J., Jones, S.M., Lack, G., Sampson, H.A.: ICON: food allergy. J. Allergy Clin. Immunol. 129, 906–920 (2012)
    19. McGowan, E.C., Keet, C.A.: Prevalence of self-reported food allergy in the National Health and Nutrition Examination Survey (NHANES) 2007-2010. J. Allergy Clin. Immunol. 132, 1216–1219.e5 (2013)
    20. Sicherer, S.H.: Clinical implications of cross-reactive food allergens. J. Allergy Clin. Immunol. 108, 881–890 (2001)
    21. Sampson, H.A. et al.: Food allergy: a practice parameter update-2014. J. Allergy Clin. Immunol. 134, 1016–1025.e43 (2014)
    22. Nowak-Wegrzyn, A., Fiocchi, A.: Rare, medium, or well done? The effect of heating and food matrix on food protein allergenicity. Curr. Opin. Allergy Clin. Immunol. 9, 234–237 (2009)
    23. Nowak-Wegrzyn, A. et al.: Tolerance to extensively heated milk in children with cow’s milk allergy. J. Allergy Clin. Immunol. 122, 342–347, 347.e1–2 (2008)
    24. Osborne, N.J. et al., HealthNuts Investigators: Prevalence of challenge-proven IgE-mediated food allergy using population-based sampling and predetermined challenge criteria in infants. J. Allergy Clin. Immunol. 127, 668-676.e1–2 (2011)
    25. Peters, R.L., Allen, K.J., Dharmage, S.C., Koplin, J.J., Dang, T., Tilbrook, K.P., Lowe, A., Tang, M.L.K., Gurrin, L.C., HealthNuts Study: Natural history of peanut allergy and predictors of resolution in the first 4 years of life: A population-based assessment. J. Allergy Clin. Immunol. 135, 1257-1266.e1–2 (2015)
    26. Bock, S.A., Muñoz-Furlong, A., Sampson, H.A.: Fatalities due to anaphylactic reactions to foods. J. Allergy Clin. Immunol. 107, 191–193 (2001)
    27. Bock, S.A., Muñoz-Furlong, A., Sampson, H.A.: Further fatalities caused by anaphylactic reactions to food, 2001-2006. J. Allergy Clin. Immunol. 119, 1016–1018 (2007)
    28. Chen, J.L., Bahna, S.L.: Spice allergy. Ann. Allergy Asthma Immunol. Off. Publ. Am. Coll. Allergy Asthma Immunol. 107, 191-199; quiz 199, 265 (2011)
    29. Muraro, A. et al., EAACI Food Allergy and Anaphylaxis Guidelines Group: EAACI Food Allergy and Anaphylaxis Guidelines. Primary prevention of food allergy. Allergy. 69, 590–601 (2014)
    30. Leitlinie_Management_IgE-vermittelter_Nahrungsmittelallergien-S2k
    31. Worm, M., et al.: Food allergies resulting from immunological cross-reactivity with inhalant allergens. Allergo J. Int. 23, 1–16 (2014)
    32. Beaudouin, E., Renaudin, J.M., Morisset, M., Codreanu, F., Kanny, G., Moneret-Vautrin, D.A.: Food-dependent exercise-induced anaphylaxis--update and current data. Eur. Ann. Allergy Clin. Immunol. 38, 45–51 (2006)
    33. Patterson, A.M., Yildiz, V.O., Klatt, M.D., Malarkey, W.B.: Perceived stress predicts allergy flares. Ann. Allergy Asthma Immunol. Off. Publ. Am. Coll. Allergy Asthma Immunol. 112, 317–321 (2014).
    34. Niggemann, B., Beyer, K.: Factors augmenting allergic reactions. Allergy. 69, 1582–1587 (2014)
    35. Werfel, T., Breuer, K.: Role of food allergy in atopic dermatitis. Curr. Opin. Allergy Clin. Immunol. 4, 379–385 (2004)
    36. Ellman, L.K., Chatchatee, P., Sicherer, S.H., Sampson, H.A.: Food hypersensitivity in two groups of children and young adults with atopic dermatitis evaluated a decade apart. Pediatr. Allergy Immunol. Off. Publ. Eur. Soc. Pediatr. Allergy Immunol. 13, 295–298 (2002)
    37. Atherton, D.J. et al.: A double-blind controlled crossover trial of an antigen-avoidance diet in atopic eczema. Lancet Lond. Engl. 1, 401–403 (1978)
    38. Review article: the aetiology, diagnosis, mechanisms and clinical evidence for food intolerance - Lomer - 2014 - Alimentary Pharmacology & Therapeutics
    39. Turnbull, J.L., Adams, H.N., Gorard, D.A.: Review article: the diagnosis and management of food allergy and food intolerances. Aliment. Pharmacol. Ther. 41, 3–25 (2015)
    40. Shakoor, Z., et al.: Prevalence of IgG-mediated food intolerance among patients with allergic symptoms. Ann. Saudi Med. 36, 386–390 (2016)
    41. Steeb, D. med S.: Food intolerance: How to test yourself step by step to the correct diagnosis. Over 60 new recipes - also for multiple intolerances. Schlütersche (2015)
    42. Zhang, Y., Chen, Y., Zhao, A., Li, H., Mu, Z., Zhang, Y., Wang, P.: [Prevalence of self-reported food allergy and food intolerance and their associated factors in 3 - 12 year-old children in 9 areas in China]. Wei Sheng Yan Jiu. 44, 226–231 (2015)
    43. Turnbull, J.L., Adams, H.N., Gorard, D.A.: Review article: the diagnosis and management of food allergy and food intolerances. Aliment. Pharmacol. Amp Ther. 41, 3–25 (2015)
    44. Authority, N.F., Allergy and intolerance, /foodsafetyandyou/life-events-and-food/allergy-and-intolerance
    45. Lactose - Fructose - Sorbitol: DAAB,
    46. Berni Canani, R., Pezzella, V., Amoroso, A., Cozzolino, T., Di Scala, C., Passariello, A.: Diagnosing and Treating Intolerance to Carbohydrates in Children. Nutrients. 8, 157 (2016)
    47. Food intolerance,
    48. Cuomo, A., Giordano, N. Goracci, A. and Fagiolini, A. Depression and Vitamin D Deficiency: Causality, Assessment, and Clinical Practice Implications.
    49. Majid, M.S., Ahmad, H.S., Bizhan, H., Hosein, H. Z. M., Mohammad, A. The effect of vitamin D supplement on the score and quality of sleep in 20-50 year-old people with sleep disorders compared with control group.
    50. Almohanna, H.M., Ahmed, A. A., Tsatalis, J. P., and Tosti, A. The Role of Vitamins and Minerals in Hair Loss: A Review.
    51. Malhotra, R. Understanding migraine: Potential role of neurogenic inflammation.
    52. Yin, K. and Agrawal, D. K. Vitamin D and inflammatory diseases.
    53. Wadhwa B, Relhan V, Goel K, Kochhar AM, Garg VK. Vitamin D and skin diseases: A review. Indian J Dermatol Venereol Leprol. 2015 Jul-Aug;81(4):344-55. doi: 10.4103/0378-6323.159928. PMID: 26144849.
    54. Palmer D. J. (2015). Vitamin D and the Development of Atopic Eczema. Journal of clinical medicine, 4(5), 1036–1050.
    55. AlGhamdi, K., Kumar, A., & Moussa, N. (2013). The role of vitamin D in melanogenesis with an emphasis on vitiligo. Indian journal of dermatology, venereology and leprology, 79(6), 750–758.
    56. Kheiri, B., Abdalla, A., Osman, M., Ahmed, S., Hassan, M., & Bachuwa, G. (2018). Vitamin D deficiency and risk of cardiovascular diseases: a narrative review. Clinical hypertension, 24, 9.
    57. Garland, C. F., Garland, F. C., Gorham, E. D., Lipkin, M., Newmark, H., Mohr, S. B., & Holick, M. F. (2006). The role of vitamin D in cancer prevention. American journal of public health, 96(2), 252–261.
    58. Amin H. A., (2021) No evidence that vitamin D is able to prevent or affect the severity of COVID-19 in individuals with European ancestry: a Mendelian randomisation study of open data’ BMJ Nutrition, Prevention & Health, 7 January 2021. doi: 10.1136/bmjnph-2020-000151