How to recharge your solar energy

What is vitamin D?

Vitamin D is a distinctive substance that stands out among vitamins both in its origin and its effects. Strictly speaking, it is not a classic vitamin but a prohormone — a precursor of hormones that the body can produce itself under the influence of sunlight. The so‑called vitamin D group includes several related compounds, in particular vitamin D₂ (ergocalciferol), found in plant sources such as mushrooms, and vitamin D₃ (cholecalciferol), present in animal foods and mainly produced by the body.

This endogenous synthesis takes place in the skin as soon as it receives sufficient UVB radiation. Geographic location is decisive: only with adequate sunlight intensity can production occur year‑round. In many regions sunlight is sufficient only during the summer months. The body then builds reserves stored in fat tissue and mobilised during periods with little light — but usually not in adequate amounts. It is important that UVB rays reach the skin directly, as they do not penetrate window glass.

Because vitamin D is fat‑soluble, fats are required for optimal absorption. In addition, magnesium plays a central role: only with sufficient magnesium can consumed or self‑produced vitamin D be converted into its active form.

Why does the body need vitamin D?

Vitamin D fulfils central functions in the human body, particularly for bone health. It plays a key role in the absorption of calcium and phosphate from the diet and their incorporation into bone tissue. In this way it helps strengthen bones and teeth and supports the maturation and function of bone cells.

Vitamin D also has positive effects on the muscles. It enhances muscle strength and can improve movement coordination, which is especially important in older age. Vitamin D is also relevant for the immune system: it supports the defence against pathogens and may help regulate overactive immune responses. This is of particular interest in autoimmune diseases such as type 1 diabetes or multiple sclerosis.

Other possible functions under scientific investigation include protection of neurons in the brain, supportive effects on the cardiovascular system and a role in preventing vascular disease and certain cancers. It is also discussed whether vitamin D influences mental health.

How does vitamin D deficiency arise?

A vitamin D deficiency mainly results from insufficient sun exposure, since the skin produces vitamin D under UVB influence. Particularly affected are people who spend little time outdoors, older adults, care home residents and those who cover their skin for cultural or religious reasons. In winter months and at higher latitudes the sun’s rays are often too weak to enable sufficient vitamin D synthesis.

Individual factors can also limit vitamin D production: darker skin types produce less vitamin D due to higher melanin content, as do older individuals. The use of high‑factor sunscreens further reduces the skin’s vitamin D production. In addition, dietary absorption of vitamin D can be impaired, for example in conditions that affect fat absorption, since vitamin D is fat‑soluble.

A deficiency may also result from insufficient conversion into the active form, for example in liver or kidney disease or due to certain medications that affect vitamin D metabolism. Infants are particularly vulnerable because breast milk contains only small amounts of vitamin D and they are often protected with sunscreen. Overweight and low socioeconomic status are also recognised as additional risk factors for vitamin D deficiency.

What are the symptoms of vitamin D deficiency?

A vitamin D deficiency can manifest in various ways and may remain unnoticed for a long time, as symptoms do not always appear immediately. When present, symptoms often affect the musculoskeletal system, the immune system and general physical condition. These can vary by age.

In children, insufficient supply can significantly impair bone development. In severe cases rickets may occur, a disorder of bone growth that can lead to permanent skeletal deformities. Typical signs include visible changes to the spine, ribs or legs, unusual softness of the skull in infants or delayed closure of the fontanelles. Muscular weakness, delayed motor development and seizures can also occur. Other non‑specific symptoms such as excessive sweating, restlessness, sleep problems, enamel defects or frequent infections may also indicate under‑supply even in the absence of severe bone damage.

In adolescence, vitamin D supply is particularly critical as the skeleton continues to grow. During this phase a deficiency may present as fatigue, reduced performance or increased susceptibility to infections.

Adults often experience vitamin D deficiency as persistent muscle and bone pain, declining muscle strength and general fatigue. With prolonged deficiency the structure and stability of bones may deteriorate. Experts refer to this as osteomalacia, a softening of the bones. Symptoms such as muscle twitches, spontaneous bone fractures, painful limbs or balance disorders (e.g. frequent stumbling) are warning signs. Psychological complaints such as depressive moods, concentration problems or persistent insomnia may also occur. Changes in the skin, brittle nails or white spots on nails can also be related.

Vitamin D products – when are they useful?

An additional intake of vitamin D can be useful when adequate supply cannot be ensured either by sunlight or diet. In particular, when low blood levels are confirmed, a supplement can help to correct the deficiency in a targeted manner.

The same applies to persistently limited living conditions or health issues that hinder natural vitamin D production. In such cases supplementation should ideally be done in consultation with a physician. A daily intake of up to 20 micrograms is often recommended.

When choosing a product, attention should always be paid to the correct dosage. Higher‑strength products are not necessary for regular use without medical supervision and may lead to problems in the long term. If uncertain whether a deficiency exists, one should first have their status tested. This allows a targeted decision on whether a vitamin D supplement is really necessary or whether other measures might suffice.

Can vitamin D be overdosed?

An overdose of vitamin D is possible, since it is a fat‑soluble vitamin that accumulates in fat and muscle tissue and is not rapidly excreted by the body. Excessively high vitamin D levels can lead to increased calcium in the blood and cause unpleasant symptoms such as nausea, vomiting, loss of appetite and intense thirst. In severe cases heart rhythm disturbances, kidney damage up to kidney failure, loss of consciousness or even death can occur.

Such an excess is, however, not caused by sunlight or a natural diet but mainly by taking too many dietary supplements or fortified foods. A daily intake of more than 100 micrograms of vitamin D is particularly risky.

How to improve your vitamin D supply: useful tips

• Have your vitamin D level professionally measured. A vitamin D test at the doctor or as a self‑test gives clear guidance on whether and how much vitamin D you need. Values below 30 ng/ml indicate a deficiency, while a value of around 40 ng/ml is considered a good target.
• Spend some time in the sun daily, ideally with uncovered arms and legs. Check the UV index beforehand to optimise the duration of your sun exposure. Start slowly if your skin is unaccustomed to avoid sunburn.
• Include vitamin D‑rich foods in your diet. Regularly eat fatty fish such as salmon, herring or eel, and mushrooms, especially wild varieties or those dried in the sun.
• Vitamin D is heat‑stable, so foods containing vitamin D lose little potency when heated up to about 180 °C.
• In summer place cultivated mushrooms like button mushrooms with the gills facing up in the sun and dry them. Once dried they retain a lot of vitamin D and can contribute to winter nutrition.
• Choose high‑quality vitamin D supplements. Select drops or capsules that are easy to dose and suited to your needs. Drops are often more practical for individual adjustment. Follow manufacturer instructions and dosage recommendations.
• Because vitamin D is fat‑soluble, simultaneous intake of fat (e.g. oil in a salad dressing or butter on bread) improves utilisation. Drops usually contain oil and can therefore also be taken undiluted.
• Since vitamin D is stored in fat tissue, overdosing can lead to health problems. Measure your level again after two to three months and adjust the dose if necessary.
• Combine vitamin D with vitamins K₂ and A: vitamin K₂ supports the incorporation of calcium into bone, while vitamin A enhances the effect of vitamin D.
• Vitamin D is recommended to be taken in the morning or at midday after a meal, as this promotes absorption. Avoid taking it shortly before bedtime to minimise sleep disturbances.
• Consider interactions with medications. Drugs such as corticosteroids or cholesterol‑lowering agents can affect vitamin D metabolism. Discuss appropriate vitamin D management with your doctor if you are taking such medications.

Good vitamin D status is crucial for overall wellbeing and many bodily functions. It is therefore worth checking your vitamin D status regularly and, if necessary, taking targeted action.