Importance of vitamin D for the skin and general health

In this article, we explain the importance of vitamin D for the skin and general health, why deficiency can occur even in countries such as Spain, and the balance between sun protection and vitamin D synthesis.

Vitamin D is unusual among vitamins because the body can produce it when the skin is exposed to ultraviolet B radiation.

It is also present in certain foods, although dietary sources alone may not always provide sufficient amounts.

Vitamin D plays an essential role in the absorption and metabolism of calcium and phosphorus, which are fundamental for maintaining healthy bones and teeth. Some of the strongest evidence for its clinical importance relates to bone health, including the prevention of rickets in children and the maintenance of bone strength in adults.

Vitamin D also acts as a hormone and is involved in several physiological processes beyond bone metabolism. Research has explored its potential role in cardiovascular health, reproductive health, pregnancy, immune function, neurological conditions and several other areas.

Vitamin D deficiency has also been associated with a number of health problems, although the nature and clinical relevance of these associations vary considerably. In older adults, low vitamin D levels may be associated with muscle weakness and impaired physical function. Vitamin D status has also been studied in relation to wound healing and recovery after surgery.

Associations have additionally been reported between low vitamin D levels and conditions such as depression, cardiovascular disease and diabetes, although in many cases it remains difficult to establish whether vitamin D deficiency is a cause, a consequence or simply a marker of poorer health.

Because vitamin D is synthesised in the skin and is involved in immune and inflammatory pathways, its relationship with several dermatological conditions has also been investigated. These include acne, rosacea, psoriasis, hair disorders, vitiligo, hidradenitis suppurativa, alopecia areata, melanoma, lupus and atopic dermatitis.

Lower vitamin D levels have been reported in some patients with these conditions, and in certain diseases deficiency has been associated with greater severity. However, this does not necessarily mean that vitamin D deficiency causes the condition or that supplementation will improve it in every patient.

For selected patients and conditions, clinicians may consider measuring vitamin D levels and correcting a confirmed deficiency where appropriate.

Why can vitamin D deficiency occur in Spain?

In theory, sunlight can contribute substantially to vitamin D synthesis. However, vitamin D deficiency is common worldwide, including in Mediterranean countries.

Studies in Spain have reported low vitamin D levels in substantial proportions of the population, including older adults.

Several factors may contribute to this apparent paradox. These include age, skin phototype, limited outdoor exposure, dietary patterns, obesity, certain metabolic, renal or liver conditions and some medicines. Geographical latitude and seasonal variation also affect the amount of ultraviolet B radiation that reaches the skin.

During autumn and winter, vitamin D synthesis from sunlight may be reduced, particularly at higher latitudes.

Sun protection and vitamin D: is there really a dilemma?

The relationship between sunlight, vitamin D and photoprotection has generated considerable debate. If sunlight contributes to vitamin D synthesis, should people reduce sunscreen use? Could rigorous sun protection increase the risk of vitamin D deficiency?

From a dermatological perspective, vitamin D deficiency should not be used as a reason to encourage unprotected or excessive sun exposure. Ultraviolet radiation is a well-established cause of sunburn, photoageing and skin cancer.

Only ultraviolet B (UVB) radiation contributes significantly to vitamin D synthesis in the skin. However, deliberate unprotected exposure is not a risk-free strategy, and tanning devices should not be used as a source of vitamin D. It is also important to note that UVB radiation does not pass effectively through ordinary window glass, so sunlight received through a closed window does not provide the same stimulus for vitamin D synthesis as direct outdoor exposure. The amount of sun exposure required to maintain adequate vitamin D levels varies greatly between individuals and depends on factors such as season, latitude, age, skin phototype, clothing and the area of skin exposed. For this reason, it is not appropriate to recommend a single universal exposure time for everyone.

If vitamin D deficiency is suspected or confirmed, the safest approach is to discuss it with a healthcare professional. The underlying cause can then be assessed and, when appropriate, vitamin D supplementation can be considered. Supplements should not be started indiscriminately, as excessive intake can be harmful and may interact with other medicines or supplements.

Excessive vitamin D intake can lead to abnormally high calcium levels. Symptoms may include nausea, vomiting, loss of appetite, constipation, weakness and weight loss. More severe toxicity may cause kidney stones, kidney injury and hypercalcaemia, which can lead to dehydration, confusion and heart rhythm abnormalities.

For this reason, people who believe they may be at risk of vitamin D deficiency because of limited outdoor exposure, dietary factors or other health issues should consult their doctor rather than self-supplement. Diet may also contribute to vitamin D intake, although the amount obtained naturally from food is usually limited.

Dietary sources of vitamin D

• Oily fish, such as salmon, mackerel, sardines and tuna. This is perhaps the most significant dietary source of vitamin D, given the eating patterns of the general population.

• Cod liver oil. It is in fact the richest source of vitamin D. This remedy was quite popular a few decades ago and is generally considered to have the same benefits as omega-3 fatty acids. However, because of its unpleasant taste, it has fallen out of use.

• Milk.

• Orange juice.

• Cheese.

• Natural yoghurt.

• Egg yolk.

• Avocado.

• Mushrooms.

• Fortified dairy products and functional foods, such as breakfast cereals and juices, have been enriched with vitamin D because obtaining it from foods that naturally contain it is difficult, and impossibly large quantities of them would be needed to reach an acceptable level.

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