Vitamin D is a fat-soluble prohormone that the body primarily produces in the skin from sunlight (UV-B). There are two main forms: D3 (cholecalciferol) and D2 (ergocalciferol). The storage form 25-OH-D is measured in the blood; the active form is called 1,25-(OH)2-D, but it is not suitable for diagnosing deficiency.

Functions in the body

• Bone and tooth mineralization (calcium/phosphate metabolism)
• Muscle strength and neuromuscular functions
• Contribution to immune system function

Sources

• Sunlight (key source)
• Diet: fatty fish, egg yolk, fortified foods
• Supplements (targeted dosing possible)

A vitamin D deficiency often develops gradually and goes unnoticed for a long time. Symptoms are nonspecific and may have other causes. Only a blood test for 25-OH-D provides certainty.

Common, nonspecific signs

• Fatigue, lack of motivation, low mood
• Muscle weakness, diffuse muscle pain
• Increased susceptibility to infections

Advanced signs

• Bone/joint pain
• Impaired bone mineralization (in severe deficiency)

Clarification

If symptoms persist or risk factors are present, a lab test can help clarify the situation.

Whether sufficient vitamin D is produced depends on sun exposure , the season , and individual factors . In autumn and winter, the body's own production is severely limited in Central Europe.

Not controllable

• Latitude/Season (UV-B intensity)
• Skin type (higher melanin content = lower synthesis)
• Age (reduced synthesis capacity)

Influenceable

• Outdoor activities, clothing, sun protection
• Nutrition (few natural sources)
• Lifestyle (e.g., predominantly indoor)

Particularly at risk

People with dark skin , senior citizens , pregnant women , breastfeeding women and people with low sun exposure.

The standard test for assessing vitamin D status is the blood level of 25-OH vitamin D. It reflects the storage form and is suitable for monitoring progress.

Limit values ​​(typical orientation)

• Sufficient: e.g. ≥ 30 ng/ml (75 nmol/l)
• Insufficiency: e.g., 20–29 ng/ml
• Deficiency: e.g., < 20 ng/ml

Note: Reference ranges may vary depending on the laboratory/guideline. The physician's interpretation within the context is important.

How to test?

• Capillary blood (finger prick) – conveniently at home
• Venous blood in practice/laboratory

When to measure again?

After starting/adjusting supplementation, a check-up is often performed after 8–12 weeks .

The dosage depends on the initial value, body weight, season, and target range. Daily or weekly doses are often used. Medical advice is especially important if you have illnesses or are taking other medications.

Conversion

1 µg vitamin D = 40 IU. Example: 1000 IU = 25 µg.

Typical regimens

• Maintenance doses e.g., 800–2000 IU/day
• Loading doses for deficiency, limited in time and only after consulting a doctor

Monitoring

Check after 8–12 weeks to avoid over- or underdosing and to find the individually appropriate dose.

An overdose is rare at typical maintenance doses but can occur with very high, long-term dosing. Symptoms are often related to hypercalcemia.

Warning signs

• Nausea, vomiting, constipation
• Thirst, increased urination
• Muscle weakness, heart rhythm disturbances

Reference values

Guideline values for safe upper limits vary; high-dose regimens should be medically supervised. Regular monitoring is important.

The conditions for the body's own production of vitamin D differ greatly between summer and winter. Especially in the winter months, synthesis in Central Europe is limited.

Summer

• Short, moderate sun exposure (forearms/legs) can help
• No sunburn! Skin protection comes first.

Winter

• Low UV-B intensity—diet/supplements become more important.
• Regular outdoor exercise remains beneficial.

Nutrition

Fatty fish, egg yolks, mushrooms, and fortified products can help with intake, but usually do not fully replace the sun.

Some groups have increased needs or limited synthesis and benefit particularly from testing and needs-based administration.

Pregnancy & Breastfeeding

Daily supplementation is often recommended – please follow medical guidelines .

Seniors & those in need of care

Less sun exposure, lower synthesis capacity – ensure regular supply.

Dark skin, headscarf, indoor lifestyle

Less UV-B radiation on the skin – the risk of deficiency increases.

Vitamin D is essential for calcium-phosphate metabolism and bone health. There is also evidence for its role in muscle function and fall prevention in older adults.

Bones

Adequate vitamin D levels support mineralization.

Muscles

A deficiency can be associated with muscle weakness; correcting it can improve function.

Immune system

The research is mixed. Having an adequate status makes physiological sense; blanket promises of a cure are not justified.

There are many myths surrounding vitamin D. Here are the most common ones—and what the evidence says.

"Summer is always enough"

Not necessarily. Duration of exposure, clothing, skin type, and angle of the sun all play a role.

"Higher is always better"

Wrong. The goal is an adequate range, not maximum values. Levels that are too high can be harmful.

"Vitamin D cures colds"

It supports the immune system, but it's not a cure-all. Maintaining a healthy lifestyle remains essential.