Discussions about the intake of vitamin D are particularly prevalent during the sunless season. Many Sports doctors recommend that their athletes take vitamin D as a dietary supplement - also as a prophylaxis against infections. Vitamin D fulfils key functions for performance. The vitamin regulates calcium and phosphate metabolism and thus ensures the development and maintenance of healthy bones. Muscle strength is strengthened and the immune system is supported. An adequate supply of vitamin D is particularly important for endurance athletes.
The body's own vitamin D production takes place in the skin under the influence of sunlight. With sufficient sunlight exposure, autosynthesis contributes to 80 to 90 per cent of the vitamin D supply. As a rule, the intake of vitamin D through the normal diet is low, as vitamin D is only found in significant quantities in a few foods. Fatty fish such as salmon or herring are rich in vitamin D. Although the amount of sunlight in Germany in autumn and winter is not sufficient to produce enough vitamin D through the skin, the body stores vitamin D in fat and muscle tissue. It can be released from these stores through physical activity and contribute to the vitamin D supply in winter. The Federal Institute for Risk Assessment (BfR) does not recommend taking it on suspicion - but if serum levels are low, it makes sense to take it.
Vitamin D supply situation
According to the BfR, around 44 per cent of adults in Germany have desirable vitamin D serum levels of 50 nmol/L (20 ng/ml) or more. There is a risk of vitamin D deficiency in 15 per cent of adults. Around 41 per cent of adults are in the suboptimal range. Sufficient vitamin D levels are not always achieved through the body's own production. In addition to sun exposure, this also depends on age and skin type. The risk groups for an insufficient vitamin D supply include people who hardly spend any time outdoors or only go outside with their bodies completely covered. People with dark skin colour also belong to the risk groups, as they can produce less vitamin D than people with light skin due to the high content of the skin pigment melanin.
Older People form another important risk group because vitamin D production decreases significantly with age. The elderly population often includes people with restricted mobility, chronic illnesses and people in need of care who can hardly or not at all move outdoors. According to the BfR, general supplementation with vitamin D up to 20 µg (800 IU) per day should be considered for these risk groups.
Safe dosing and risks of high doses
If you want to supplement vitamin D, the BfR recommends taking food supplements containing up to 20 µg of vitamin D (800 international units) per day. This dose is not associated with any adverse health effects, even if taken over the long term and taking other sources of vitamin D into account. With this dose, an adequate vitamin D serum concentration of 50 nmol/L (20 ng/ml) can generally be achieved without exposing the skin to sunlight.
However, high-dose preparations are not recommended. The European Food Safety Authority has derived a tolerable upper limit for vitamin D intake of 100 µg (4,000 IU) per day for adults. High-dose vitamin D supplements containing 4000 international units or more have the potential to cause total vitamin D intake levels that are harmful to health if taken over a long period of time. In clinical studies, daily administration of 4,000 IU of vitamin D over a longer period of time was associated with a greater decrease in bone density in older women, an increase in the risk of falls and a deterioration in heart function in people with heart disease.
Warning against high-dose preparations
The BfR explicitly warns against preparations in which very high single doses ("bolus doses") of vitamin D are to be taken at intervals of several days to weeks. Some preparations also contain vitamin K2. - With high bolus doses of vitamin D (e.g. 500 micrograms (µg) every 20 days), vitamin D concentrations can be reached in the blood that studies have shown to be associated with health risks, especially in people who are already very well supplied with vitamin D.
Practical recommendation for athletes
A general recommendation to take vitamin D supplements to prevent diseases cannot be justified on the basis of the current scientific data. The best way to achieve a good supply of vitamin D is through self-synthesis in the skin - cyclists who spend a lot of time outdoors in summer normally top up their stores. A good supply of vitamin D can be achieved by spending enough time outdoors and exposing the skin to sufficient sunlight, physical exercise and activity as well as a balanced diet with oily fish once or twice a week if possible.
Natural sources of vitamin D:
- Body's own formation: 80-90% with sufficient sunlight
- Food sources: Oily fish (salmon, herring)
- Storage: fat and muscle tissue
- Release: Through physical activity
