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Are you constantly tired, even though you get enough sleep? Do you always catch every cold first? And do your bones sometimes ache for no apparent reason? Many people experience exactly this, and a vitamin D deficiency is often the culprit. The surprising thing is that most people don't realize how important this vitamin is for key bodily functions and how quickly a deficiency can develop.
In this article, we'll show you what really matters: what tasks vitamin D performs in the body, which warning signals you should take seriously, and how you can easily and reliably improve your supply.
Table of contents
The most important points in brief
Over 90% of Germans have insufficient vitamin D levels.
Typical symptoms of vitamin D deficiency include persistent fatigue, increased susceptibility to infections, and unexplained bone or muscle pain.
Sunlight is the main source: In Germany, UVB radiation is only sufficient for the body's own production from April to September.
Targeted supplementation can ensure an adequate vitamin D status in at-risk individuals.
What is vitamin D and why is it so important?
Vitamin D occupies a special position among nutrients. Unlike other vitamins, the body can produce it itself under suitable conditions – therefore, it is technically also referred to as a prohormone . Vitamin D is a fat-soluble vitamin and is primarily stored in fat and muscle tissue, as well as in the liver.
The two most important members of the vitamin D group are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D3 is the more biologically active and effective form for humans. It is either synthesized in the skin through UVB radiation or ingested through animal-based foods.
However, the body's supply of vitamin D depends significantly on the amount of UVB radiation that reaches the skin (1).
The role of vitamin D in health
Vitamin D is involved in numerous vital functions in the body:
- Bone health : Vitamin D regulates calcium and phosphate metabolism and is essential for bone mineralization. It promotes calcium absorption in the intestines and its incorporation into bone structure.
- Muscle function : Adequate vitamin D levels contribute to normal muscle function and can reduce the risk of falls, especially in older people.
- Immune system : Vitamin D is considered one of the most important vitamins for the immune system. It can strengthen a weak immune system and at the same time downregulate excessive immune responses, such as those that occur in allergies or autoimmune diseases (1, 2).
Studies show that people with low vitamin D levels (10-20 nmol/l) are over 80% more susceptible to infections than people with good vitamin D levels (60-90 nmol/l) (2). Adequate vitamin D status is therefore essential for the body's immune system.
How can you recognize a vitamin D deficiency?
The symptoms of a vitamin D deficiency are varied and are often mistaken for other causes. An insufficient vitamin D status can manifest itself both physically and mentally.
Common symptoms of vitamin D deficiency:
- Persistent tiredness and exhaustion, even after sufficient sleep
- Increased susceptibility to infections and frequent colds
- Bone and muscle pain without a discernible cause
- Muscle weakness and reduced physical performance
- Mood swings, low mood or depressive moods•
- Slowed wound healing
- Increased risk of multiple sclerosis, psoriasis, heart failure, non-alcoholic fatty liver disease and bacterial pneumonia (3)
In children, severe vitamin D deficiency can lead to rickets – a disease characterized by impaired bone mineralization that can cause growth retardation and permanent bone deformities (4). In adults , a severe deficiency manifests as osteomalacia, characterized by general bone and muscle pain. This also significantly increases the risk of osteoporosis, as well as falls and fractures (1).
Causes of vitamin D deficiency
Vitamin D deficiency is most often caused by insufficient sun exposure . Germany lies between the 47th and 54th parallels north, which means that UVB radiation is only sufficient for effective vitamin D synthesis for about six months of the year – mainly from April to September.
During the winter months (October to March) the solar radiation is too weak and the angle of incidence too shallow to ensure sufficient production.
People at risk for vitamin D deficiency include:
- People who spend little time outdoors
- People over 65 years of age, as the skin produces less vitamin D with increasing age.
- People with dark skin, as melanin filters UVB radiation more effectively.
- Pregnant and breastfeeding women who have an increased need for vitamin D
- People who regularly use sunscreen or wear long clothing (5)
- People with chronic diseases of the digestive system such as Crohn's disease, celiac disease, chronic hepatitis, chronic kidney disease (6).
Taking certain medications can also increase vitamin D breakdown and thus lead to a deficiency. Furthermore, advanced liver disease, gastric bypass surgery, or alcohol abuse can impair the conversion of vitamin D to its active form (6).
How is a vitamin D deficiency diagnosed?
The most reliable method for assessing vitamin D status is the measurement of 25-hydroxyvitamin D (25(OH)D) in blood plasma. This metabolite serves as a storage form and provides information about the availability of vitamin D for metabolism (4).
Classification of vitamin D levels:
- Severe deficiency: below 30 nmol/l
- Moderate deficiency: 30-50 nmol/l
- Adequate supply: from 50 nmol/l
- Optimal range: 75 nmol/l or higher (1)
According to a 2016 study by the Robert Koch Institute (RKI), 30.2% of Germans have a severe vitamin D deficiency, while 61.5% of study participants have a moderate deficiency. These alarming figures illustrate how widespread the problem is (2).
Vitamin D foods: Can the need be met through diet?
Although the most important source of vitamin D is the body's own production through sunlight, a certain amount can also be obtained through food. However, only a few foods contain significant amounts of vitamin D.
Vitamin D content in foods:
- Fatty sea fish (salmon, herring, mackerel): 4-25 µg per 100 g
- Egg yolk: approximately 5.6 µg per 100 g
- Mushrooms (button mushrooms, chanterelles): 1.9-2.1 µg per 100 g
- Beef liver: 1.7 µg per 100 g
- Cheese: approximately 3 µg per 100 g
Important: Oral intake of vitamin D from food cannot fully replace the beneficial effects of endogenous production through UVB radiation. UV-induced synthesis in the skin produces additional biologically active metabolites that are not formed when vitamin D is ingested through food (4).
How much sun does the body need for sufficient vitamin D?
Regular, moderate sun exposure is crucial for sufficient vitamin D production. The best time for sunbathing is between 12 and 3 p.m., as the sun's rays then take the shortest path through the atmosphere and the UVB intensity is highest. Before 9 a.m. and after 5 p.m., even in summer, the UVB radiation is too weak for effective vitamin D synthesis.
Recommendations for optimal sun exposure in summer:
- At least 25% of the body should be exposed (e.g., face, hands, arms).
- Do not use sunscreen during vitamin D production.
- Exposure 4 to 6 times per week
- Duration: 10 to 20 minutes in the summer months (April to September) (4)
Practical tips for the winter months:
Even though your body cannot produce vitamin D from the sun in winter, there are simple and practical ways to keep your vitamin D levels stable.
- Make the most of every hour of sunshine : Daylight supports the biorhythm, improves mood and strengthens general well-being.
- Supplement thoughtfully: From October to March you can take vitamin D3 daily in the form of a dietary supplement – ideally in combination with vitamin K2.
- Plan your winter holiday strategically : In southern regions (above the 35th parallel, e.g., Crete, southern Spain, the Canary Islands), UVB radiation is often sufficient even in winter. A winter holiday there can replenish your vitamin D stores.
- Vitamin D-rich diet : Especially in winter, you should focus more on fatty fish, fortified foods and eggs.
- Get your status checked: A blood test in October/November and February/March will show whether your supplementation is sufficient or needs to be adjusted.
Vitamin D - yes! Sunburn? No, thank you!
The fear of skin cancer is justified, but with responsible and moderate sun exposure, the risk is minimized. The health benefits of adequate vitamin D intake clearly outweigh the risks. However, it is important to avoid sunburns – these significantly increase the risk of skin cancer.
Sunscreen should be used with care: Even a sun protection factor (SPF) of 15 blocks approximately 99% of UVB rays, thus almost completely preventing vitamin D production. A sensible strategy is to spend the first 10-20 minutes without sunscreen and then apply it.
Vitamin D dosage: How much is appropriate?
The correct dosage of vitamin D depends on several factors: current vitamin D levels, body weight, age, and sun exposure. The German Nutrition Society recommends a daily intake of 20 µg (800 IU) for adolescents and adults who cannot synthesize enough vitamin D on their own.
In cases of diagnosed deficiency, a higher dosage or initial therapy with high-quality preparations may be advisable. These should only be taken after consulting a doctor.
Vitamin D tablets or drops: Which is better?
Vitamin D is available as a dietary supplement in various forms: vitamin D tablets, capsules, and drops. But which form is the best choice?
Since vitamin D is a fat-soluble vitamin, the form of administration plays a crucial role in its absorption by the body. A direct comparison reveals the most important differences:
|
Vitamin D drops |
Vitamin D capsules & tablets |
| ✓ Higher bioavailability due to liquid dosage form |
✗ Bioavailability depends on the quality |
| ✓ Efficient and rapid absorption via the oral mucosa |
✗ Nutrient absorption only occurs after dissolution in the stomach |
| ✓ Flexible, individual dosage |
✗ Fixed dosage per tablet/capsule |
| ✓ Fewer additives |
✗ Often contains many additives (fillers, binders) |
| ✓ Can be taken directly or mixed in |
✗ Must be swallowed whole |
Our recommendation
What happens in the case of a vitamin D overdose?
While vitamin D deficiency is widespread, the question of a possible vitamin D overdose is occasionally raised. In fact, an overdose through natural sun exposure is practically impossible, as the body regulates its own production. However, an overdose can occur with uncontrolled, high-dose supplementation.
Possible symptoms of a vitamin D overdose:
- Nausea, vomiting and loss of appetite
- Elevated blood calcium levels (hypercalcemia)
- Increased thirst and frequent urination
- Kidney stones or kidney damage from long-term overdose
- Cardiac arrhythmias in severe cases
Vitamin D overdose only occurs with very high doses over extended periods – generally only with daily intakes exceeding 10,000 IU for several months. There is no risk with normal dosages as recommended.
Tips for the long-term prevention of vitamin D deficiency
- Get regular sun exposure: Spend 10-20 minutes outdoors 4-6 times a week between April and September – ideally without sunscreen between 12 and 3 pm.
- Check your levels : A blood test in late winter and late summer shows how well your stores are filled.
- Choose the right form : Vitamin D3 drops are absorbed better than tablets. Products containing vitamin K2 can also be beneficial.
- Take with fat : Since vitamin D is fat-soluble, it is best to take it together with healthy fats such as avocado, nuts or fatty fish.
- Include vitamin D-rich foods : Fatty fish, eggs, and fortified products support your supply.
Conclusion
The importance of vitamin D extends far beyond bone health. It plays a role in key bodily processes – from muscle strength and the immune system to your mood. Especially during periods of increased need, such as growth or old age, an adequate supply can significantly contribute to preventing health problems and supporting overall well-being.
The crucial point is: you can do a lot yourself to keep your vitamin D levels within a healthy range. Regular sun exposure, a well-balanced supplement, ideally in the form of vitamin D drops , and occasional blood tests are among the simplest and most effective measures that can make a noticeable difference in the long run.
Our expert
Robert Appuhn
Alternative medicine practitioner, nutritionist
Key areas:
Micronutrients | Orthomolecular therapy
What is a vitamin D deficiency?
A vitamin D deficiency occurs when the body is not adequately supplied with vitamin D. This can be caused by insufficient sun exposure, diet, or certain risk factors.
What symptoms indicate a vitamin D deficiency?
Common signs of a vitamin D deficiency include fatigue, muscle weakness, mood swings, or increased susceptibility to infections.
How does a vitamin D deficiency develop?
Vitamin D deficiency is usually caused by insufficient sunlight, especially during the winter months. Certain lifestyle factors, skin types, or illnesses can also impair absorption.
Who is particularly at risk of vitamin D deficiency?
Risk groups include older people, people with dark skin, people with low sun exposure, and people with certain chronic diseases.
Can a vitamin D deficiency be prevented through diet?
Only to a limited extent – vitamin D is present in small amounts in some foods such as fatty fish or egg yolks. The body's own production through sunlight remains the most important source.
Are vitamin D drops or tablets better in case of deficiency?
Vitamin D drops usually have a higher bioavailability and are well suited for individually adjustable dosages.
Can a vitamin D deficiency have long-term consequences?
A persistent vitamin D deficiency can impair various bodily processes. Adequate intake supports health at all stages of life, whether during growth, pregnancy, or old age.
Why is vitamin K2 often combined with vitamin D?
It is believed that vitamins D and K2 complement each other effectively: While vitamin D promotes calcium absorption, vitamin K2 helps to bind it specifically to bones and teeth. Therefore, taking them together can be beneficial. However, taking them should always be done in consultation with a doctor, especially if you have pre-existing medical conditions or are taking long-term medication.
References for further reading
- Biesalski HK, Grimm P, Nowitzki-Grimm S. Pocket Atlas of Nutrition. 8th, completely revised edition. Stuttgart, New York: Georg Thieme Verlag; 2020.
- Schmiedel V. How we can specifically strengthen the immune system. Uro-News 2021; 25(9):44–7.
- Fang A, Zhao Y, Yang P, Zhang X, Giovannucci EL. Vitamin D and human health: evidence from Mendelian randomization studies. Eur J Epidemiol 2024; 39(5):467–90.
- Reichrath J. Vitamin D in focus. Berlin, Heidelberg: Springer Berlin Heidelberg; 2024.
- Rabenberg M, Mensink GB. Vitamin D status of adults in Germany. Journal of Health Monitoring 2016; 1(2).
- Pfotenhauer KM, Shubrook JH. Vitamin D Deficiency, Its Role in Health and Disease, and Current Supplementation Recommendations. J Am Osteopath Assoc 2017; 117(5):301–5.
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