Vitamin D – sun and exercise promote a good supply

Vitamin D occupies a special position among vitamins: unlike other vitamins that must be taken up with food, vitamin D can be formed from precursors present in the body. This requires exposure of the skin to sunlight. Compared to vitamin D intake through food, the body's own production makes a significantly greater contribution to the human supply of this vitamin.

Vitamin D is involved in numerous metabolic processes in the body. It regulates calcium and phosphate metabolism, thereby promoting bone density. It also influences muscle strength and contributes to a well-functioning immune system.

Vitamin D is the generic term for various calciferols. A distinction is made between ergocalciferol (vitamin D2), which is prevalent in mushrooms, and cholecalciferol (vitamin D3), which is prevalent in animals and can also be produced by humans. When exposed to sunlight (UVB radiation), the skin first produces the unstable precursor previtamin D3, which is then quickly converted into vitamin D3 and transported to the liver via the blood.

Vitamin D2 and D3 supplied through food are taken up in the small intestine and also transported to the liver via the bloodstream. There, the two forms of vitamin D are converted into 25-hydroxyvitamin D precursors, also known as calcidiol. Only in the kidneys is calcidiol converted into the active form calcitriol (1α,25-dihydroxyvitamin D), which then has the actual effect on the target organs.

Under physiological conditions, the body can also convert excess vitamin D into inactive forms of vitamin D, thereby preventing oversupply.

In addition to the main pathway via the liver and kidneys, numerous other body tissues can also convert vitamin D into its active form.

Biologically active calcitriol (1α,25-dihydroxyvitamin D) reaches the target organs via the blood, where it binds to the vitamin D receptor within the cells and is transported to the cell nucleus. There, the vitamin receptor complex binds to specific sites in the genome (deoxyribonucleic acid, DNA), causing certain genes to be read and various proteins to be formed. These can regulate different processes in the cells. For example, this leads to the formation of more calcium channels in the kidneys and small intestine, which ensure that calcium is absorbed into the blood. Bone mineralisation is also regulated via the vitamin D receptor.

The fact that the vitamin D receptor is found throughout almost the entire body indicates that vitamin D may have many other effects in addition those on bone health.

Cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2) are generally permitted in the European Union (EU) for in the purpose of food fortification and for use in food supplements. In Germany, the fortification of food for general consumption with vitamin D is subject to approval. To date, there are no legally defined maximum levels for the fortification of common foods or food supplements. The BfRshort forGerman Federal Institute for Risk Assessment has already derived and published proposals for maximum levels for Germany. Uniform maximum levels are also currently being developed at the EU level, taking into account the BfRshort forGerman Federal Institute for Risk Assessment's proposals.

Furthermore, various foods are authorised in the EU in which the vitamin D concentration is increased by irradiation. These include mushrooms, baker's yeast, yeast-leavened bread and pastries, and milk. In addition, UV-irradiated mushroom powder may be added to numerous different food categories. UV irradiation increases the ergocalciferol concentration in mushrooms and yeast, and increases the cholecalciferol concentration in milk.

As UV irradiation of food is a new method, these foods are considered Novel Foods. Novel Foods are subject to a health risk assessment by the European Food Safety Authority (EFSAshort forEuropean Food Safety Authority), and to authorisation, and the manufacturer must comply with the maximum levels for certain ingredients approved under the novel food procedure, in this case vitamin D.

Furthermore, since May 2024, the addition of calcidiol (25-hydroxyvitamin D) to food supplements has been permitted within the EU. Calcidiol is also considered a Novel Food because it has not yet been used for human consumption. Calcidiol is more easily absorbed from the intestine into the body and increases serum vitamin D levels (based on 25-hydroxyvitamin D levels) more quickly and efficiently than cholecalciferol taken up orally. However, in the range of high doses, the same amount of calcidiol carries a higher risk of adverse health effects than the same amount of cholecalciferol. The maximum permitted amount of calcidiol as a novel food in food supplements is 10 μg per day for children aged 11 and above and adults, and 5 μg per day for children aged 3 to 10.

The addition of biologically active calcitriol (1α,25-dihydroxyvitamin D) to common foods or food supplements is not permitted. It is used as a medicine, for example in patients with kidney failure, to treat dysfunctional calcium and phosphate metabolism, as these individuals are unable to produce calcitriol or can only produce very small amounts. Calcitriol is highly potent. Medically indicated use requires close medical supervision, as there is a significant risk of excessive serum calcium levels (hypercalcaemia), which can be harmful to health.

Medicines are intended to cure, alleviate or prevent diseases or pathological conditions. They are subject to approval and manufacturers must prove their efficacyClose dialogefficacyPositive Predictive ValueTo glossary and safety to an authority (German Federal Institute for Drugs and Medical Devices, BfArM, or the European Medicines Agency, EMA). The quantities stated on the packaging of prescription-only pharmaceutically manufactured medicinal products may not deviate by more than five per cent from the actual dosage of the substance.

Food supplements are foodstuffs and serve to supplement the general diet. They are not subject to approval but only have to be notified to the German Federal Office of Consumer Protection and Food Safety (BVLshort forGerman Federal Office of Consumer Protection and Food Safety). Their nutritional or physiological effects and safety do not have to be determined by an authority – the manufacturer or food business operator is responsible for safety. In contrast to the narrow ranges of variation for the amounts of ingredients in prescription-only pharmaceutically manufactured medicinal products, the amounts stated on the packaging of food supplements – with the exception of technological additives – may deviate by up to 50 per cent from the actual amount in the product, according to External Link:the German Federal Office of Consumer Protection and Food Safety (BVL).

As there are no maximum quantities for food supplements, some food supplements containing vitamin D have (much) higher concentrations of vitamin D than certain medicines containing vitamin D.

In clinical studies with high-dose bolus doses of vitamin D administered once a year, an increase in falls and fractures was reported in the vitamin D group compared to the control group. Monthly high-dose bolus doses also led to an increased risk of falls compared to lower doses. Therefore, if vitamin D supplements are taken, high-dose bolus doses should be avoided. Higher doses prescribed by a doctor are not assessed here.

A suitable biomarker for checking the supply status is the total serum concentration of 25-hydroxyvitamin D, i.e. the sum of 25-hydroxyvitamin D2 and D3. 25-hydroxyvitamin D occurs in relatively high concentrations in serum and has a long half-life compared to other vitamin D metabolites. As it is biologically inactive, it is not subject to any hormonal feedback mechanism. The serum concentration is therefore only influenced by the body's own synthesis in the skin and the amount of vitamin D absorbed from food.

The measurement of the active metabolite 1α,25-dihydroxyvitamin D (calcitriol) is usually only used in medical diagnostics to clarify congenital or acquired vitamin D metabolic disorders.

Despite refinements in analytics, measuring vitamin D status remains challenging, and results often vary depending on the method used, between individual tests or even between laboratories. Methods such as automated immunoassays or mass spectrometry methods are used. The advantage of automated assays is that they are simple and easy to use and often less expensive than mass spectrometry. However, most automated immunoassays are less effective at detecting vitamin D2. There is therefore a risk that vitamin D levels may be underestimated in people who consume mainly vitamin D2. This may be the case, for example, in people who primarily follow a plant-based diet and consume enriched food or food supplements that mainly contain vitamin D2.

When interpreting the measurement results, it should also be taken into account that laboratories sometimes use different limit values to determine vitamin D deficiency. Some laboratories define vitamin D deficiency as 25-hydroxyvitamin D levels below 75 nmol/l (30 ng/mlshort formillilitre), while others classify serum levels below 50 nmol/l (20 ng/mlshort formillilitre) as insufficient.

According to the German Nutrition Society (DGE), the estimated value for an adequate vitamin D intake in the absence of the body's own production is 20 micrograms per day. In other words, anyone who does not spend any time outdoors in daylight and therefore does not produce vitamin D themselves should consume 20 micrograms of vitamin D daily. This value, derived from studies, applies to all age groups from the age of one.

However, this scenario is unrealistic for most people: with regular outdoor exposure and sufficient sunlight, the body's own (endogenous) production in the skin contributes around 80 to 90 per cent to vitamin D supply. Vitamin D intake through nutrition with normal food only accounts for a relatively small proportion (10 to 20 per cent) of the supply.

Therefore, measuring vitamin D intake from nutrition is not a suitable way to assess actual vitamin D status. Instead, the concentration of 25-hydroxyvitamin D in blood serum is used as a marker for assessing vitamin D status. It reflects both vitamin D intake from nutrition and the body's own vitamin D production.

However, there has been a long-standing debate as to whether a 25-hydroxyvitamin D level of 75 nmol/l (30 ng/mlshort formillilitre) or 50 nmol/l (20 ng/mlshort formillilitre) is adequate. Since 2024, the Society for Endocrinology has, due to a lack of scientific evidence, no longer recommended a specific value for defining vitamin D status.

As a guideline value for the healthy population, the BfRshort forGerman Federal Institute for Risk Assessment uses the 25-hydroxyvitamin D serum value of 50 nmol/l (20 ng/mlshort formillilitre) derived by the former Institute of Medicine (IOM, now: National Academy of Medicine, NAM), which reflects an adequate supply for bone health for almost all individuals in a population. This value is also considered appropriate by the German Nutrition Society (DGE) and the European Food Safety Authority (EFSAshort forEuropean Food Safety Authority).

However, it should be noted that the physiological requirement for vitamin D varies from person to person. This means that a serum level of 50 nmol/l is sufficient to meet the needs of even those with very high requirements. For most people, this level is already above the actual individual requirement for good bone health.

Serum levels between 30 nmol/l (12 ng/mlshort formillilitre) and 50 nmol/l (20 ng/mlshort formillilitre) are considered suboptimal, although not everyone with these serum levels is necessarily undersupplied, depending on individual needs.

A risk of vitamin D deficiency only occurs y when serum levels fall below 30 nmol/l (12 ng/mlshort formillilitre). In the long term the deficiency also leads to an increased risk of osteomalacia (bone softening) and rickets.

A study has shown that almost all participants with serum levels above approximately 50 nmol/l (20 ng/mlshort formillilitre) broke down vitamin D, suggesting that their requirements – even beyond bone health – were more than adequately met.

In the absence of the body's own vitamin D production, an adequate vitamin D serum concentration of 50 nmol/l is achieved with an intake of 20 µgshort formicrogram of vitamin D per day.

In Germany, based on the derived values of the former IOM, approximately 54 per cent of children and adolescents and 44 per cent of adults have desirable vitamin D serum levels of 50 nmol/l (20 ng/mlshort formillilitre) and above, while around 13 per cent of children and adolescents and 15 per cent of adults have serum levels below 30 nmol/l (12 ng/mlshort formillilitre), putting them at risk of vitamin D deficiency. Around 33 per cent of children and adolescents and 41 per cent of adults are in the suboptimal domain with serum levels of 30 to 50 nmol/l or 12 to 20 ng/mlshort formillilitre. Sixteen per cent of children and 9 per cent of adults in Germany have serum levels above 75 nmol/l.

The body's own vitamin D production in the skin via sunlight (UVB radiation) depends on latitude, time of year and time of day, weather conditions, clothing, length of time spent outdoors, and skin type. The use of sunscreen also affects vitamin D supply, as it reduces the body's own production. This means that the contribution of the body's own production to vitamin D supply varies greatly from person to person and cannot be determined quantitatively for individuals or for the general population as a whole.

During the summer months, it is possible to achieve the desired serum concentration of 25-hydroxyvitamin D of 50 nmol/l through the body's own production, including in Germany. For about half of the year, it is sufficient to expose about a quarter of the body surface (face, hands and parts of the arms and legs) to the sun for 5 to 25 minutes several times a week, depending on skin type and season. If you are exposed to the sun for longer periods of time, appropriate measures should be taken (e.g. sunscreen, protective clothing) to avoid sunburn. This is because frequent sunburn increases the risk of developing skin cancer.

In contrast to the summer months, the sun's rays in Germany are not strong enough between October and March to ensure sufficient vitamin D production. However, the body stores vitamin D in fat and muscle tissue. Physical activity can release it and contribute to the body's supply in winter.

As vitamin D is fat-soluble, it can be stored very well in the body. The main storage locations for vitamin D are the fat and muscle tissue of the human body; smaller amounts are also found in the liver.

There are only a few foods, mostly of animal origin, that contain significant amounts of vitamin D. These include, in particular, fatty fish (e.g. eel, herring, mackerel, salmon) and, to a lesser extent, eggs, mushrooms, beef liver, cheese, butter and milk. In Germany a normal diet provides only 2 to 4 micrograms of vitamin D per day.

Table1 : Natural vitamin D concentration of some common food (German Food Code and Nutrient Data Base (BLS) 3.02)

Long-term severe vitamin D deficiency in infants and children increases the risk of insufficient mineralisation of the growth plates – a condition known as rickets. Rickets is always accompanied by insufficient mineralisation of the bones (osteomalacia). In adults, a vitamin D deficiency no longer leads to growth disorders (rickets) due to closed growth plates, but only to osteomalacia. This mineralisation disorder forms soft bones that can be deformed under mechanical stress.

Vitamin D deficiency can also contribute to the formation of osteoporosis, especially in older people. However, osteoporosis is a multifactorial disease, meaning that numerous other factors are involved in its development.

In addition, there is a link between low vitamin D levels and increased susceptibility to infections, muscle cramps, and muscle and bone pain. People with low vitamin D levels also often experience symptoms such as fatigue, weight gain, mood swings and hair loss; however, a clear cause-and-effect relationship has not been proven for these mostly non-specific symptoms.

Risk groups for deficiency include people who spend little or no time outdoors (or are unable to do so) or who, for cultural or religious reasons, only go outside with their bodies completely covered. People with dark skin are also at risk, as they are less able to produce vitamin D than people with light skin due to the higher concentration of melanin in their skin.

Another major risk group is older people, as vitamin D production decreases significantly with age and the older population also includes people with limited mobility, chronic illnesses and those in need of care who spend little or no time outdoors.

Finally, infants are among the risk groups for vitamin D deficiency, as the concentration of vitamin D in breast milk is very low and infants should not be exposed to direct sunlight because their skin's own protective mechanism has yet to develop. To ensure that infants receive sufficient vitamin D, the daily administration of vitamin D tablets (400–500 international units, corresponding to 10 to 12.5 µgshort formicrogram per day; vitamin D prophylaxis) is recommended in Germany until the child's second early summer.

In old age, the skin's ability to produce vitamin D decreases significantly and can be reduced to less than half compared to younger ages. If older people also spend less time outdoors and thus have limited exposure to sunlight on their skin, the body's own vitamin D production decreases even further. This is particularly common in older people with limited mobility, chronic illnesses and those in need of care (nursing home residents, geriatric patients). Vitamin D deficiency is often found in these individuals. Therefore, additional vitamin D intake may be beneficial for these groups of people, especially during the winter months. This does not generally apply to (older) people who spend a lot of time outdoors.

Due to the potential health risks, it is not advisable to go to a solarium to improve your vitamin D supply. The UV radiation used in solariums can increase the risk of skin cancer, among other things. More information on this can be found, for example, at External Link:the Federal Office for Radiation Protection.

No, vitamin D overdose from the body's own synthesis is not possible. Vitamin d overdoses and thus undesirable effects are only possible from excessive oral intake (long-term > 100 micrograms per day), for example by using vitamin D supplements.

Since part of the population has insufficient vitamin D levels, the conditions for fortifying food with vitamin D in accordance with the European Union (EU) Fortification Regulation are generally met. This is because fortification can improve the supply to the population or parts of it. At the same time, care must be taken to ensure that the population is not oversupplied, as this also poses health risks.

In the opinion of the BfRshort forGerman Federal Institute for Risk Assessment, general fortification of food with vitamin D is not recommended. Instead, targeted fortification of food categories that are consumed in relatively consistent amounts by large sections of the population should be carried out. The BfRshort forGerman Federal Institute for Risk Assessment has developed a vitamin D fortification concept in which it proposes maximum levels for the addition of vitamin D to certain food groups. The aim of the concept is to design the possible fortification of food with vitamin D in such a way that excessive total intake levels that are harmful to health are avoided in all age groups, while at the same time making a meaningful contribution to the population's supply.

From the BfRshort forGerman Federal Institute for Risk Assessment's point of view, the addition of vitamin D to the following food is suitable for achieving this goal:

• Milk and dairy produce (including cheese): maximum 1.5 µgshort formicrogram per 100 g
• Bread and baked goods (except pastries) and breakfast cereals: maximum 5 µgshort formicrogram per 100 g
• Spreadable fats and edible oils (as these are already authorised) (including liquid vegetable fat preparations and vegetable creams): maximum 7.5 µgshort formicrogram per 100 g

The BfRshort forGerman Federal Institute for Risk Assessment's vitamin D fortification concept is a recommendation; it is not legally binding.

At EU level, uniform maximum levels for vitamins and minerals (including vitamin D) for the fortification of food and for the addition to food supplements are currently being developed, taking into account the proposals of the Member States. These are to be regulated by law in the future and will thus be legally binding in all EU Member States.

The results of large clinical studies on vitamin D published in recent years show that people with adequate vitamin D levels generally do not benefit from taking additional vitamin D. Based on the current scientific data, a general recommendation to take vitamin D supplements to prevent disease is therefore not justified.

However, sufficient vitamin D levels are not always achieved through the body's own production, which depends not only on sunlight exposure but also on age and skin type, among other factors. Therefore, additional intake of vitamin D via supplements may be beneficial for certain groups of people, especially during the winter months. The relevant risk groups are listed under the question "Which groups are at risk of vitamin D deficiency?"

Vitamin D deficiency can be particularly widespread among residents of care facilities. According to the BfRshort forGerman Federal Institute for Risk Assessment, general supplementation with vitamin D up to 20 µgshort formicrogram (800 IU) per day should therefore be considered for this risk group. With 20 µgshort formicrogram (800 IU) of vitamin D per day, an adequate supply can generally be ensured without any exposure of the skin to sunlight.

From a medical point of view, higher doses may be prescribed in certain cases. However, serum levels should be monitored regularly alongside supplementation.

Special conditions apply to infants, as they should not be exposed to direct sunlight. To ensure that this group receives sufficient vitamin D, daily vitamin D tablets are recommended in Germany until the child's second early summer (400–500 international units, corresponding to 10 to 12.5 µgshort formicrogram per day; vitamin D prophylaxis).

In 2023, the European Food Safety Authority (EFSAshort forEuropean Food Safety Authority) updated the Tolerable Upper Intake Level (ULshort forTolerable Upper Intake Level) for vitamin D per day. The ULshort forTolerable Upper Intake Level refers to intake from all food sources, including food supplements containing vitamin D and enriched food. The ULshort forTolerable Upper Intake Level is not a recommended intake.

Table 2: Tolerable Upper Intake Level (ULshort forTolerable Upper Intake Level) for vitamin D intake (EFSAshort forEuropean Food Safety Authority, 2023)

Regular daily intake of vitamin D above the ULshort forTolerable Upper Intake Level increases the risk of adverse effects. Given normal dietary habits and the fact that food naturally contains only small amounts of vitamin D, this is currently only possible through the intake of high-dose vitamin D preparations.

Vitamin D overdoses and the resulting adverse effects are only caused by excessive oral intake and not by the body's own production. In particular, studies have shown that additional long-term use of high-dose food supplements containing 100 micrograms or 4,000 international units (IU) of vitamin D or more per daily dose may be associated with an increased risk of adverse health effects. These include a decrease in bone density in older women, an increased risk of falls, and a deterioration in heart function in people with heart disease.

Case reports of poisoning in children and adults have been reported after intake of excessive doses of vitamin D. These were characterised by a marked increase in serum calcium levels. Symptoms of such hypercalcaemia can include fatigue, muscle weakness, nausea, cardiac arrhythmia and weight loss. If hypercalcaemia persists for a longer time, it can lead to kidney stones and kidney calcification, and even to an (irreversible) decline in kidney function.

In a number of observational studies, low vitamin D levels were correlated with an increased risk of extraskeletal diseases such as cancer, cardiovascular disease, type 2 diabetes, depression, asthma and respiratory infections. Based on these study results, there are high expectations that taking vitamin D preparations, which are available as medicines and also over the counter as a food supplement, could protect against these diseases or alleviate their progression. However, this has not yet been scientifically proven. It is also conceivable, for example, that the low vitamin D levels measured are a consequence of the diseases identified – and not the cause.

In recent years, results from large placebo-controlled studies on the potential benefits of vitamin D supplements involving more than 2,000 participants and lasting between 2.5 and 5.3 years have been published. In almost all of these studies, the participants already had adequate vitamin D serum levels (approximately 55 to 80 nmol/l or 22 to 32 ng/mlshort formillilitre) at the start of the study. While vitamin D levels remained almost constant with placebo administration, the administration of vitamin D supplements in these studies led to a clear increase in serum levels. However, the further increase in vitamin D serum levels (from already adequate to supra-physiological domain) showed no additional benefit for the prevention of the diseases or health impairments investigated. These included cancer, cardiovascular disease, bone fractures, falls, growth and bone density. In contrast, a positive effect was observed in autoimmune diseases.

Based on the current scientific data, a general recommendation to take vitamin D supplements for the prevention of diseases cannot be justified. However, vitamin D deficiency should be avoided at all costs. A detailed opinion on the suitability of food supplements containing vitamin D can be found External Link:here.

Anyone who wants to supplement their diet with vitamin D via a food supplement should use products containing up to approximately 20 µgshort formicrogram of vitamin D (800 IU) per day, as this dose is not associated with any adverse health effects, even when taken over a long period of time and taking into account other sources of vitamin D (e.g. enriched food).

Those who want to do something for their health and support a good vitamin D supply should go out into fresh air often, both in summer and in winter. Physical exercise and outdoor activities as well as sport strengthen muscles and bones. It is also recommended to eat oily sea fish once or twice a week, which contains omega-3 fatty acids and iodine in addition to vitamin D. With sufficient time spent outdoors and sufficient sun exposure on the skin, as well as a balanced diet, a good vitamin D supply can be achieved without taking vitamin D supplements.