Normal vitamin D levels are difficult for most of us to maintain without sufficient sunshine and supplementation. Did you know that one billion people around the world suffer from a lack of vitamin D (often called the ‘sunshine vitamin’)? We can only acquire enough of this important vitamin with sufficient exposure to the sun or by taking the right supplements.
So, what does a vitamin D deficiency mean for our health? Hair loss, depression, fatigue, skin diseases and cancer are just some of the symptoms associated with vitamin D deficiency. Studies have shown that vitamin D plays a vital role in numerous processes in the body. It affects the bones, the muscles, the immune system, the blood vessels and much more. As a result, vitamin D and the effects of low vitamin D levels have been a hot and trending 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. According to a study carried out, around one-third of adults in the United Kingdom have insufficient vitamin D levels, especially during wintertime – that is, between January and March.
Find out about how tests, sunbathing sessions and supplements can ensure that you are optimally supplied with vitamin D (and what exactly your body does with the vitamin D). Read on to discover what a vitamin D deficiency or an excess of it can mean for your health and which diseases are linked to a vitamin D deficiency.
What is vitamin D?
Also referred to as a hormone – as it has a hormone-like effect and does not originate primarily from our diets (as is otherwise normal with vitamins) – vitamin D is a fat-soluble vitamin. Our body produces vitamin D itself, but requires the UV rays from 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.
Why do we need vitamin D?
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 a healthy immune system and protects blood vessels.
Other ways our bodies use vitamin D include:
- to control calcium and phosphate absorption in the small intestine;
- to regulate more than 200 genes;
- to promote heart muscle function;
- to act as a hypotensive mediator;
- to promote skeletal development in children.
Vitamin D supplementation
It is not for nothing that vitamin D is called the sunshine vitamin – our body needs the light of the sun for vitamin D 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.
How do we take in vitamin D through our diets?
The small intestine can absorb up to 80 per cent of the fat-soluble vitamin D3 consumed in food. Despite this, only relatively small amounts of vitamin D are absorbed into the body via this route; we only cover about 10 to 20 per cent 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, 3]
- 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 to even receive close to the lowest recommended daily allowance.
To summarise, 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 millilitre.
- Experts state that 30 nanograms and above per millilitre of blood represent normal vitamin D levels.
- Values below 11 nanograms per millilitre mean you have low vitamin D levels and are considered to be critical, possibly leading to disorders such as bone softening.
- Some researchers argue that 60 nanograms per millilitre or more are ideal for you to take full advantage of the health benefits vitamin D has to offer.
How can I receive a sufficient amount of vitamin D?
How long 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’s usually enough to expose your face, hands and arms to direct sunlight three times a week for ten to twenty minutes.
So, to produce 400 IU of vitamin D, a person with medium-to-light skin (one who tans slowly and rarely gets sunburnt) on the 42nd line of latitude from April to October (for example, in the south of France) must expose a quarter of their skin (for example, arms, face, chest) for about three to eight minutes at midday in the sun.
For many people around the world, it is simply not possible for them to receive enough sun exposure in the summer, and it’s even more impossible for them to do so during winter. In winter, the sun rarely shines in northern latitudes, and its light also contains too little UVB radiation. It’s a large ask for people in these locations to be able to produce an adequate amount of vitamin D.
Researchers and experts 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.
Please note: do not overdo it when it comes to 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.
Are vitamin D supplements effective?
The Mayo Clinic recommends that (in the absence of sunlight – that is, 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 supplementation is generally safe and should help people not only reach normal vitamin D levels in their blood, but also enjoy additional health benefits.
If a deficiency already exists, the researchers say, adults could correct their vitamin D levels by taking amounts of up to 10,000 IU daily for a short, fixed period of time.
Vitamin D and vitamin K
Vitamin K is thought to offer us similar benefits to vitamin D. It is primarily for the prevention and treatment of bone and vascular diseases. In nature, vitamin K1 is found, 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.
Time and again, it has been stated 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 theory has not been scientifically proven. Medical experts currently recommend a combined intake of vitamin D and vitamin K only as an adjunctive therapy to specifically treat osteoporosis and prevent bone fractures occurring in older people.[7-9]
Vitamin D deficiency
It is estimated that about one billion people globally are affected by vitamin D deficiency.[11, 12] Various studies have linked vitamin D deficiency to chronic diseases, such as osteoporosis, diabetes mellitus, cancer, depression, cardiovascular disease and immune dysfunction.[13–16]
Vitamin D deficiency causes
A lack of vitamin D occurs when we do not get sufficient sun exposure. While our ancestors lived outdoors for the most part, we spend a large amount of our time indoors. Not only this, but 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.
Did you know that 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 per cent?
Several other factors can also interfere with our body’s endogenous vitamin D production:
- various diseases that impair fat digestion and ingestion, such as coeliac disease, bile acid deficiency or pancreatic insufficiency;
- certain medications such as antihypertensives, anti-oestrogens, cytostatics, antiepileptics and herbal medicines.
Vitamin D deficiency symptoms
A vitamin D deficiency rarely reveals itself with clear symptoms. The symptoms are often non-specific and hidden and include fatigue, muscle weakness and musculoskeletal and headache pain. As a result, many people do not notice their deficiency until they develop into more significant diseases. A common consequence of a long-standing vitamin D deficiency is osteomalacia (bone softening).
Those who have a severe chronic vitamin D deficiency are at an increased risk of:
- osteomalacia and osteoporosis
- rickets (in children)
- pain and weakness of the bones (osteoarthritis)
- bone fractures (in seniors over 65)
In addition, recent study results have associated low vitamin D levels 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
- 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 people are particularly at risk. If you belong to one of these groups, you should regularly check your vitamin D levels and, if necessary, take supplements.
The risk groups include:
- people 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, as they produce less vitamin D for the same amount of UV radiation absorbed by people with fair skin;
- elderly people, 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
From the 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 up to four times less vitamin D than younger people. If you are over 60, you should regularly check your vitamin D levels and make up for any deficiencies with supplements.[23-26]
Taking these supplements also helps with issues encountered frequently in old age. Positive health benefits of supplementation found in studies include:[23, 27–30]
- 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
How can I check my vitamin D levels at home?
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 optimise your intake. In order to find out if and how you should take vitamin D supplements, a vitamin D blood test is absolutely worth your while – especially, if you belong to one of the risk groups stated above.
The most common methods of performing a vitamin D test are through blood tests taken by a doctor. With a home test kit like the cerascreen® Vitamin D Test, you can also check your levels within the comfort of your own home. To do this, take a blood sample yourself by pricking your finger and send this sample to a specialist laboratory. The laboratory will then analyse the level of 25-(OH)-D in your blood sample. Afterwards, you will find out in a results report where your vitamin 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 have adequate or normal vitamin D levels, and you take long-term high-dose supplements, you may overdose. 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, 31]
Our tip: We recommend purchasing our Sun Vitamin D Value Pack – containing two vitamin D test kits, so that you are able to test your vitamin D levels again after a few months. This way, you can see for yourself if there has been an improvement in your vitamin D levels.
Vitamin D and diseases
In recent years, scientists have spent a considerable amount of time studying how vitamin D levels impact health. Luckily for you, we’ve broken down a series of studies that reveal links between vitamin D deficiency and various diseases and health problems.
Vitamin D and depression
Studies have shown that low vitamin D levels impact mental health. Among other things, depression, stress, mood swings and anxiety can all arise from vitamin D deficiency.
The relationship between low vitamin D levels and depression has already been studied. People with depression have significantly lower vitamin D levels than healthy people. 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.
Due to its hormone-like effect, vitamin D may also support the functionality of the brain. It helps our brain make decisions, process information and store it properly.
Vitamin D and sleep quality
In 2017 Iranian scientists investigated the effect of vitamin D on sleep quality. A total of 89 participants with sleep disorders aged between 20 and 50 years were administered either a vitamin D supplement or a placebo. The study revealed that participants who received vitamin D experienced a significant improvement in their sleep quality, they slept for longer and took less time to fall asleep compared to individuals not receiving the vitamin D supplement. In another study, people with low vitamin D levels endured a poorer sleep quality.
Vitamin D and hair loss
For some time, it has been acknowledged that vitamins and minerals affect hair growth. As examples, iron, biotin and zinc are important for promoting healthy hair roots. 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 theory.
Can vitamin D help with migraines?
Migraines are severe headaches that can often reoccur. Experts now believe that migraine attacks develop as a result of inflammation in the nerves and blood vessels. 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.
However, at present, there are still relatively few studies in this area, and the research results remain inconsistent. Some have shown a connection between vitamin D and migraines, and in one study it was shown that taking vitamin D could reduce the frequency of such headache attacks. In other studies, however, vitamin D did not have an impact on migraines.
Vitamin D for skin
Vitamin D also seems to play a role in our skin’s health. 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).
Are vitamin D supplements good for skin? 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 low vitamin D levels were particularly likely to suffer from such infections. Researchers are also investigating how vitamin D supplements can positively impact the course of psoriasis and vitiligo (white spot disease).
Vitamin D for heart health
Vitamin D, according to research, can strengthen heart muscles. In addition, vitamin D fulfils important tasks in calcium and phosphate metabolism. The all-important sunshine vitamin ensures that calcium and phosphate supplies are 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, which can lead to calcification.
A study published in the American Journal of Cardiology in 2012 revealed that vitamin D deficiency may increase mortality rates from cardiovascular disease. Vitamin D supplements reduced this risk in the same study. The study authors concluded that vitamin D deficiency is a risk factor for vascular disease, cardiac muscle problems and high blood pressure.
These results were confirmed by another study involving over 40,000 patients. Subjects with low vitamin D levels of less than 15 nanograms per millilitre were more likely to experience hypertension, elevated blood lipid levels, heart defects and strokes than those with vitamin D levels of 30 nanograms per millilitre.Read more about heart health in our Health Portal here.
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 low vitamin D levels and the risk of colon cancer and breast cancer.
Current large-scale meta studies, however, found no influence of vitamin D intake on the development of tumours. Many scientists say that more research is required before anything conclusive can be stated. Studies dealing with this topic are currently under way, 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 fed up 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-found 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 has meant that fewer people have been able to go on their usual summer holiday – those vital annual vitamin D reserves might therefore be lower than normal for some people.
So, what about the importance of vitamin D supplements 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 has revealed that there is no evidence of vitamin D being protective against Covid-19.
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 per cent of vitamin D all by itself, but to do this it needs 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 per cent we can obtain from our diets – for example, from fatty fish, eggs, dairy products and 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 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 migraines.
How can I check 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 test. Most scientific sources recommend levels of at least 30 nanograms per millilitre 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.
 Crowe, Francesca L et al. ‘Trends in the incidence of testing for vitamin D deficiency in primary care in the UK: a retrospective analysis of The Health Improvement Network (THIN), 2005–2015.’ BMJ open, vol. 9,6 e028355. 4 Jun. 2019, doi:10.1136/bmjopen-2018-028355
 Kasper, H., Burghardt, W. Ernährungsmedizin und Diätetik. Elsevier, Urban & Fischer, Munich (2014)
 Skypala, I. ‘Adverse food reactions--an emerging issue for adults.’ J. Am. Diet. Assoc. vol. 111,12 (2011): 1877–91. doi:10.1016/j.jada.2011.09.001
 Atherton, D. J. et al. ‘A double-blind controlled crossover trial of an antigen-avoidance diet in atopic eczema.’ Lancet (London, England) vol. 1,8061 (1978): 401–3. doi:10.1016/s0140-6736(78)91199-6
 Mayo Clinic ‘Vitamin D’. Mayo Foundation for Medical Education and Research, 18 October 2017, https://www.mayoclinic.org/drugs-supplements-vitamin-d/art-20363792, [accessed 4 February 2021]
 Graham-Rowe, D. ‘Lifestyle: When allergies go west’. Nature. 479, S2–S4 (2011). doi:10.1038/479S2a
 Steeb, D. med S. Lebensmittelunverträglichkeiten So testen Sie sich selbst: Schritt für Schritt zur richtigen Diagnose. Über 60 neue Rezepte - auch für Mehrfachintoleranzen. Schlütersche (2015)
 Zhang, Y., Chen, Y., Zhao, A., et al. ‘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 = Journal of Hygiene Research. vol. 44, 226–231 (2015)
 Turnbull, J. L., et al. ‘Review article: the diagnosis and management of food allergy and food intolerances.’ Aliment. Pharmacol. Ther. vol. 41, 3–25 (2015). doi:10.1111/apt.12984
 Shakoor, Z., et al. ‘Prevalence of IgG-mediated food intolerance among patients with allergic symptoms.’ Ann Saudi Med. vol. 36, 386–390 (2016). doi:10.5144/0256-4947.2016.386
 Naleway, A. L. ‘Asthma and Atopy in Rural Children: Is Farming Protective?’ Clin Med Res. vol. 2, 5–12 (2004). doi:10.3121/cmr.2.1.5
 Sepp, E., et al. ‘Intestinal microflora of Estonian and Swedish infants.’ Acta Paediatr (Oslo, Norway : 1992), vol. 86, 956–961 (1997) doi:10.1111/j.1651-2227.1997.tb15178.x
 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)
 Worm, M., Reese, I., Ballmer-Weber, B. et al. ‘Guidelines on the management of IgE-mediated food allergies’. Allergo J Int 24, 256–293 (2015). https://doi.org/10.1007/s40629-015-0074-0
 Ho, M. H.-K. et al. ‘Clinical spectrum of food allergies: a comprehensive review’. Clin Rev Allergy Immunol., vol. 46, 225–240 (2014). doi:10.1007/s12016-012-8339-6
 Robert Koch-Institut, Number of the Month, April 2017: Allergies
 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. vol. 132, 1216–1219.e5 (2013). doi:10.1016/j.jaci.2013.07.018
 Worm, M., et al. ‘Food allergies resulting from immunological cross-reactivity with inhalant allergens: Guidelines from the German Society for Allergology and Clinical Immunology (DGAKI), the German Dermatology Society (DDG), the Association of German Allergologists (AeDA) and the Society for Pediatric Allergology and Environmental Medicine (GPA).’. Allergo J Int., vol. 23, 1–16 (2014). doi:10.1007/s40629-014-0004-6
 Parva, N. R. et al. ‘Prevalence of Vitamin D Deficiency and Associated Risk Factors in the US Population (2011-2012).’ Cureus vol. 10,6 (2018). doi:10.7759/cureus.2741
 Sicherer, S. H. ‘Clinical implications of cross-reactive food allergens’. J Allergy Clin Immunol. vol. 108, 881–890 (2001). doi: 10.1067/mai.2001.118515
 Sampson, H. A. et al. ‘Food allergy: a practice parameter update-2014’. J Allergy Clin Immunol., vol. 134, 1016–1025 (2014). doi:10.1016/j.jaci.2014.05.013
 Burks, A. W., et al. ‘ICON: food allergy’. J Allergy Clin Immunol., vol. 129, 906–920 (2012). doi:10.1016/j.jaci.2012.02.001
 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. vol. 9, 234–237 (2009). doi:10.1097/ACI.0b013e32832b88e7
 Nowak-Wegrzyn, A. et al. ‘Tolerance to extensively heated milk in children with cow’s milk allergy’. J Allergy Clin Immunol, vol. 122, 342–347, 347.e1-2 (2008). doi:10.1016/j.jaci.2008.05.043
 Osborne, N. J. et al. ‘Prevalence of challenge-proven IgE-mediated food allergy using population-based sampling and predetermined challenge criteria in infants’. J Allergy Clin Immunol. vol. 127, 668–676.e1–2 (2011). doi:10.1016/j.jaci.2011.01.039
 Peters, R. L., et al. ‘Natural history of peanut allergy and predictors of resolution in the first 4 years of life: A population-based assessment’. J Allergy Clin Immunol., vol. 135, 1257–1266.e1–2 (2015). doi:10.1016/j.jaci.2015.01.002
 Bock, S. A., et al. ‘Fatalities due to anaphylactic reactions to foods’. J Allergy Clin Immunol., vol. 107, 191–193 (2001). doi:10.1067/mai.2001.112031
 Bock, S. A., et al. ‘Further fatalities caused by anaphylactic reactions to food, 2001-2006’. J Allergy Clin Immunol., vol. 119, 1016–1018 (2007). doi:10.1016/j.jaci.2006.12.622
 Chen, J. L., Bahna, S. L. ‘Spice allergy’. Ann Allergy Asthma Immunol., vol. 107, 191-265 (2011). doi:10.1016/j.anai.2011.06.020
 Muraro, A. et al., ‘EAACI food allergy and anaphylaxis guidelines. Primary prevention of food allergy’. Allergy. vol. 69, 590–601 (2014). doi:10.1111/all.12398
 American College of Allergy, Asthma & Immunology, ‘Food Allergy’, http://acaai.org/allergies/types/food-allergy, [accessed 4 February 2021]
 Cuomo, A., Giordano, N. Goracci, A. and Fagiolini, A. ‘Depression and Vitamin D Deficiency: Causality, Assessment, and Clinical Practice Implications’. Neuropsychiatry, (2017). doi:10.4172/Neuropsychiatry.1000255
 Majid, M. S., et al. ‘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’. Nutr Neuroscie, vol. 21, 511–519, (2018). doi:10.1080/1028415X.2017.1317395
 Almohanna, H. M., et al. ‘The Role of Vitamins and Minerals in Hair Loss: A Review. Dermatol Ther (Heidelb)., vol. 9, 51–70, (2019). doi:10.1007/s13555-018-0278-6
 Malhotra, R. ‘Understanding migraine: Potential role of neurogenic inflammation’. Ann Indian Acad Neurol., vol. 19, 172–182, (2016). doi: 10.4103/0972-2327.182302
 Yin, K. and Agrawal, D. K. ‘Vitamin D and inflammatory diseases’. J Inflamm Res., vol. 29, 69–87, (2014). doi:10.2147/JIR.S63898.
 Wadhwa B., et al. ‘Vitamin D and skin diseases: A review’. Indian J Dermatol Venereol Leprol., vol. 81, 344–355 (2015). doi:10.4103/0378-6323.159928.
 Palmer D. J. (2015). ‘Vitamin D and the Development of Atopic Eczema’. J Clin Med., vol. 4, 1036–1050, (2015). doi:10.3390/jcm4051036
 AlGhamdi, K., et al. ‘The role of vitamin D in melanogenesis with an emphasis on vitiligo’. Indian J Dermatol Venereol Leprol., vol. 79, 750–758, (2013). doi:10.4103/0378-6323.120720
 Berni Canani, R., et al. ‘Diagnosing and Treating Intolerance to Carbohydrates in Children’. Nutrients., vol. 8, 157 (2016). doi:10.3390/nu8030157
 Kheiri, B., et al. ‘Vitamin D deficiency and risk of cardiovascular diseases: a narrative review’. Clin Hypertens., vol. 24, 9 (2018), [published correction appears in Clin Hypertens. 2018 Dec 24;24:19]. https://doi.org/10.1186/s40885-018-0094-4
 Garland, C. F., et al. ‘The role of vitamin D in cancer prevention’. Am J Public Health, vol. 96, 252–261, (2006). doi:10.2105/AJPH.2004.045260
 Amin H. A., ‘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
 Roitt, I.M., Brostoff, J., Male, D.K. eds: Kurzes Lehrbuch der Immunologie. Thieme, Stuttgart (1995)
 Björkstén, B.: Genetic and environmental risk factors for the development of food allergy. Curr. Opin. Allergy Clin. Immunol. 5, 249–253 (2005)
 S3-Leitlinie Allergieprävention - Update 2014. Leitlinie der Deutschen Gesellschaft für Allergologie und klinische Immunologie (DGAKI) und der Deutschen Gesellschaft für Kinder- und Jugendmedizin (DGKJ)
 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)
 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)
 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)
 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).
 Niggemann, B., Beyer, K.: Factors augmenting allergic reactions. Allergy. 69, 1582–1587 (2014)
 Werfel, T., Breuer, K.: Role of food allergy in atopic dermatitis. Curr. Opin. Allergy Clin. Immunol. 4, 379–385 (2004)
 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)
 Review article: the aetiology, diagnosis, mechanisms and clinical evidence for food intolerance - Lomer - 2014 - Alimentary Pharmacology & Therapeutics
 Authority, N.F., Allergy and intolerance, /foodsafetyandyou/life-events-and-food/allergy-and-intolerance
 Lactose - Fructose - Sorbitol: DAAB, http://www.daab.de/lebensmittel-allergietag/laktose-fruktose-sorbit/
 Food intolerance, https://www.nhs.uk/conditions/food-intolerance/