What is Vitamin D?

Vitamin D, also known as the "sunshine vitamin," is a fat-soluble vitamin that plays a unique role in the human body – because it acts not only as a vitamin but also as a hormone precursor . It is crucial for bone health, the immune system, hormonal balance, and overall vitality .

The body can produce vitamin D itself when the skin is exposed to direct sunlight (UV-B radiation). Nevertheless, many people, especially in northern regions or with a predominantly indoor lifestyle, suffer from a vitamin D deficiency – particularly in autumn and winter.

There are two main forms:

  • Vitamin D₂ (Ergocalciferol) – plant origin

  • Vitamin D₃ (cholecalciferol) – of animal origin and the more biologically active form

Foods such as fatty fish (salmon, mackerel, herring), eggs, cod liver oil or fortified dairy products contain small amounts of vitamin D, but are usually not sufficient to meet daily requirements.

How does vitamin D work?

Vitamin D is produced in the skin, then converted in the liver to 25-hydroxyvitamin D (calcidiol) and in the kidneys to the active form 1,25-dihydroxyvitamin D (calcitriol) . This hormone-like substance regulates over 2000 genes and has profound effects throughout the body.

  1. Bone and muscle health:
    Vitamin D regulates the absorption of calcium and phosphorus in the intestines and ensures their incorporation into bones. It helps prevent osteoporosis, muscle weakness, and bone diseases (e.g., rickets).

  2. Immune system:
    It activates immune cells (T cells and macrophages) and strengthens the body's defenses against infections. Studies show that optimal vitamin D levels can reduce the frequency of respiratory infections.

  3. Hormone balance and mood:
    Vitamin D influences the production of serotonin and can therefore contribute to improved mood. A deficiency is associated with depressive moods, lack of motivation, and chronic fatigue.

  4. Cell protection and anti-inflammatory properties:
    It inhibits inflammatory processes and plays a role in protecting against autoimmune diseases (e.g., Hashimoto's thyroiditis, multiple sclerosis).

  5. Performance and regeneration:
    Vitamin D supports muscle function, improves strength development and, in combination with magnesium and vitamin K2, can promote regeneration after physical exertion.

Benefits of Vitamin D

  • Promotes strong bones and healthy teeth

  • Supports the immune system and the body's defenses

  • Contributes to normal muscle function

  • Regulates calcium and phosphorus metabolism

  • Supports hormone and serotonin production

  • Can reduce fatigue and depressive moods

  • Protects against inflammatory processes and strengthens cell balance.

Possible side effects and interactions

Vitamin D is fat-soluble and stored in the body. Overdosing is rare, but extremely high doses over a long period (over 10,000 IU daily) can lead to hypercalcemia – an elevated calcium level in the blood, which puts a strain on the kidneys and heart. Symptoms can include nausea, thirst, weakness, or heart rhythm disturbances.

People taking medication for high blood pressure, heart disease, or epilepsy should discuss vitamin D supplementation with their doctor.

Optimal supply is generally between 40–80 ng/ml (100–200 nmol/l) serum value.

Conclusion

Vitamin D is far more than just a bone vitamin – it's a holistic health factor. It strengthens the immune system, supports muscles and bones, regulates hormones, and improves overall well-being. Since many people are deficient due to lack of sunlight or lifestyle factors, targeted supplementation, especially during the winter months, can be extremely beneficial. Vitamin D is particularly effective and safe when combined with vitamin K2 and magnesium .

Sources

  1. Holick MF. (2007): Vitamin D deficiency. N Engl J Med , 357(3), 266–281.

  2. Bike DD. (2014): Vitamin D metabolism, mechanism of action, and clinical applications. Chem Biol , 21(3), 319–329.

  3. Martineau AR, et al. (2017): Vitamin D supplementation to prevent acute respiratory infections. BMJ , 356, i6583.

  4. Pilz S, et al. (2019): Vitamin D and muscle function. Nutrients , 11(4), 866.

  5. Grant WB, et al. (2020): Health benefits of higher serum 25(OH)D concentrations. Nutrients , 12(4), 988.