Sunlight and vitamin D

The risks of sunlight for the development of skin cancer are repeatedly highlighted in the media. Children in particular should not go out into the sun without sunscreen with a high sun protection factor or suitable protective clothing, the health authorities say. On the other hand, people are warned about the dangers of vitamin D deficiency which in turn can be caused by a lack of sunlight. Vitamin D deficiency has been shown to increase the risk of certain cancers and leads to bone growth disorders in young children. It is also known that unborn babies whose mothers suffer from vitamin D deficiency are more likely to develop certain autoimmune diseases such as diabetes as children. Which recommendations should be followed, then? And what can a healthy approach to sunlight and vitamin D look like? This fact sheet gives you pointers for orientation in advance of or in addition to a medical consultation.

How is vitamin D formed and for what do we need it?

Unlike other vitamins, vitamin D can be produced by the human body itself, but only under the influence of sunlight: the cholesterol built up in the skin is converted into provitamin D when exposed to sunlight. This "light hormone" reaches the liver via the blood where it is stored as vitamin D3. In a further step, it can be converted into active vitamin D3 in the kidneys and other organs. Sunlight as well as age and the health of the liver and kidneys play a decisive role in vitamin D formation.

As early as a hundred years ago it became known that children who received too little sunlight (in the light-deprived and densely built-up working-class neighbourhoods) were prone to rickets which manifested itself in growth disorders and severe bone deformities. It was discovered that the absorption of calcium, which is essential for bone formation, depends on the body's vitamin D level. Since vitamin D is present in hardly any or only small amounts in food (such as some types of fish), the body has to produce it itself – and is dependent on sunlight to do so.

Vitamin D has a restorative and formative effect not only on bone formation. It is essential for a well-functioning immune system. Thus vitamin D deficiency can lead to the increased occurrence of immune deficiencies and autoimmune diseases. Moreover vitamin D shows a certain protective effect against cancer, particularly breast and bowel cancer. Sufficient vitamin D is also necessary for a healthy pregnancy. It could be put like this: the light of the sun has a formative effect through the formation of vitamin D (bones), it helps us to arrive on earth (pregnancy) and to develop healthily (immune system, prevention of cancer).

However, the effect of sunlight on humans cannot be reduced to vitamin D formation. The sleep-wake rhythm, which is likewise of great importance for the immune system, but also for learning and concentration, is also stimulated by the absorption of sunlight by the eye. The internal regulation of hormones is similarly strongly dependent on sunlight. There is evidence that sunlight/vitamin D deficiency promotes certain forms of depression, some of which can be treated with light therapy. Sufficient outdoor exercise cannot therefore simply be replaced by vitamin D tablets. On the contrary, many of the effects attributed to vitamin D are derived from comparing people with high and low vitamin D levels. Here the positive effects are not solely due to vitamin D. As a rule, however, people with high vitamin D levels also show a comparatively healthier lifestyle with sufficient outdoor exercise.

According to the latest research results, nitrogen monoxide is produced in the epidermis under the influence of sunlight. It is absorbed into the blood and from there has a harmonising effect on a tendency to high blood pressure and improves the blood flow to the heart and brain. This is a nice additional example of the great value of sunlight for health (1).

How much sun do we need and can we tolerate?

In Central Europe, the sun does not rise very high in the sky in winter. Moreover, it is usually cold and we are dressed warmly, so that little sun reaches our skin. Therefore it is good that vitamin D can be produced in the body from spring to autumn and then stored for the winter. The best time for the formation of vitamin D is from March to October, between 10am and 3pm. At the height of summer and in southern countries, the hot midday hours can be omitted. Here even 15-30 minutes are usually enough, without sunscreen. The more skin is exposed, the better. At the same time, the more sensitive the skin is to sunlight, the more quickly it produces vitamin D. People with dark, less sensitive skin types therefore need more sunlight, light-skinned people correspondingly less.

Sunscreen suppresses vitamin D formation to a great extent even with low sun protection factors! That is why it is better to go out in the sun for a short time (without cream) and more often, instead of spending the whole time on the beach for just two weeks a year. If you go outdoors regularly where you live, most people's skin (except for very fair-skinned people) adapts to the season and the risk of sunburn is greatly reduced. It is not possible to be more precise about the time, as both the required and the tolerable exposure time depends very much on the individual skin type, the location and the light intensity.

In general, if you let the sun shine for example on your hands, arms and face three times a week for just the length of time to make sure to avoid (mild) sunburn, you will produce enough vitamin D without putting yourself at risk of skin cancer. There are, however, people who constitutionally only produce small amounts of vitamin D, due to their age or due to illness. This should be determined in each individual case by taking a blood sample and possibly be treated. Conversely, appropriate sunscreen remains important when travelling to places with high sun exposure, unavoidable over-exposure to the sun (birthday party on the beach) or very high sensitivity to light.

Preventing skin cancer is important. This includes avoiding sunburn, most sensibly by wearing appropriate clothing that covers the skin and thereby protects it. The ingredients of the vast majority of sunscreens, on the other hand, are not harmless and neither has their protective effect against melanoma been proven. A well-dosed and considered exposure to the sun, which is necessary for a sufficient vitamin D supply, does not lead to an increased risk of skin cancer. Today we know that with moderate sun exposure, the protective function of vitamin D in the skin outweighs carcinogenic effects of sunlight.

Vitamin D determination, standard ranges and tablets

Given the importance attributed to vitamin D with regard to various health problems, the vitamin D content in the blood of many people is determined and assessed against a so-called standard range. Unfortunately, this standard range has been set independently of season, skin colour and latitude and therefore only provides a guide. The majority of people in Central Europe, for example, show lower values at the end of winter. This does not mean that everyone who is otherwise healthy but lies slightly below the standard range must now start taking vitamin D tablets. It can, however, be an encouragement to get outside into the sun a little more and change lifestyle in that direction.

If, on the other hand, there is a medical reason, vitamin D treatment can be very helpful. This applies, for example, to people who are ill and those whose forebears come from more southern countries and whose skin absorbs sunlight more slowly. They are at risk of developing vitamin D deficiency, especially in the winter months. Women who strongly shield themselves from sunlight should be careful to avoid vitamin D deficiency, particularly during pregnancy. Vitamin D should be taken in consultation with a doctor.

Sunlight is the source of life on earth. The relationship of humans with the sun is of central importance to them. The current debate about sunscreen and vitamin D reminds us that every human being is called upon to actively and consciously shape this relationship.