Vitamin D deficiency: why it’s so common and how to detect it

Vitamin D deficiency is one of the most frequent findings in blood tests, and it’s striking that it should affect so many people in a country as sunny as Spain. This “paradox” has a clear explanation, and it comes down to our lifestyle. Vitamin D is far more than a nutrient for the bones: it plays a part in the immune system, in muscle function and in numerous processes throughout the body. In this article we explain why deficiency is so widespread, what symptoms it can cause and how to maintain adequate levels.

What is vitamin D and what does it do?

Unlike other vitamins, vitamin D behaves like a hormone. Its best-known role is to aid the absorption of calcium and phosphorus in the gut, which is essential for strong bones and teeth. But its effects go further than that: it is involved in the immune system’s response, in healthy muscle function and in the regulation of various cellular processes. For this reason, a sustained deficiency can have an impact on different aspects of health.

There are two main ways of obtaining it. The most important is synthesis in the skin from sunlight (UVB rays); the second is diet, although few foods are naturally rich in vitamin D (oily fish, egg yolk, liver, and some dairy products and fortified foods).

Why is vitamin D deficiency so common?

Living surrounded by sunshine does not guarantee adequate levels. There are several reasons why deficiency is so frequent:

• Living indoors: we spend most of the day at home, in the office or in the car, with little real exposure to the sun.
• Working hours: these coincide with the middle of the day, which is when the skin synthesises vitamin D most effectively.
• The use of sunscreen and clothing that covers the skin: these are necessary measures to help prevent skin cancer and ageing, but they reduce the synthesis of vitamin D.
• The time of year: in autumn and winter, UVB radiation is insufficient across much of the country.
• Older age: over the years, the skin loses its ability to produce vitamin D.
• Darker skin tone: the greater amount of melanin acts as a natural filter and reduces synthesis.
• Overweight and obesity: vitamin D becomes “trapped” in fatty tissue, reducing its availability. This is another factor that ties in with the chronic conditions associated with obesity.
• A diet low in the few dietary sources available.

There is also an important caveat: for safety reasons, prolonged, unprotected sun exposure is not recommended, particularly during the hottest hours of the day and during periods of extreme heat, as we point out in our article on the health risks of hot weather. That is why, in many cases, the vitamin D obtained from diet or supplements takes on particular importance.

Groups at higher risk of deficiency

Although anyone can have low levels, there are groups in which deficiency is especially common and which warrant closer attention:

• Older people, particularly those who rarely leave home or who live in care homes.
• Exclusively breastfed babies, since breast milk provides little vitamin D, and children in periods of rapid growth.
• Pregnant and breastfeeding women.
• People with obesity, or who have undergone weight-loss (bariatric) surgery, owing to the reduced availability and absorption of the vitamin.
• Patients with digestive conditions that impair the absorption of fats (for example, coeliac disease or inflammatory bowel disease), or with kidney or liver disease.
• People with darker skin tones or who, for various reasons, get little sun exposure.

In these cases, monitoring levels and — where the doctor considers it appropriate — supplementation become all the more important in order to prevent bone problems and safeguard general health.

Symptoms of vitamin D deficiency

Vitamin D deficiency is usually silent and is often picked up by chance in a blood test. When it does cause symptoms, the most common are:

• Tiredness and a feeling of low energy.
• Bone and muscle pain or muscle weakness.
• A greater tendency to infections.
• Changes in mood.
• Hair loss in some cases.

In children, a severe and sustained deficiency can cause rickets (weak bones and deformities), while in adults it is associated with osteomalacia (softening of the bones) and contributes to osteoporosis and an increased risk of fractures.

How is it diagnosed and what levels are adequate?

Deficiency is detected with a blood test that measures 25-hydroxyvitamin D (25-OH-D). As a rough guide, values below 20 ng/ml are usually considered deficient and those between 20 and 30 ng/ml insufficient, although the thresholds can vary depending on the laboratory and the guidelines used. You can find further information from sources such as the Spanish Society of Endocrinology and Nutrition (SEEN).

How to prevent and correct deficiency

Maintaining adequate vitamin D levels involves several fronts:

• Sensible, careful sun exposure: short periods of sun during the less intense hours can help, always avoiding burning and following photoprotection advice.
• Diet: include oily fish (salmon, sardines, mackerel), eggs and fortified foods.
• Supplementation when indicated: vitamin D supplements (usually cholecalciferol, or vitamin D3) are very effective, but the dose must be prescribed by a doctor. Just as we stress the correct use of devices and treatments in other conditions, with vitamin D it is likewise important to tailor the approach to the individual.

A note on safety: more is not always better. Taking high doses of vitamin D supplements without supervision can lead to toxicity, with a build-up of calcium and harmful effects. It is therefore best to avoid self-medication and to adjust the regimen according to each person’s levels.