Vitamin D: importance, deficiency and how to improve it

Vitamin D is a hormone that the body produces in the skin through sunlight and that plays a key role in bones, the immune system, and metabolism. Vitamin D deficiency is very common, even in sunny places, and can usually be corrected with a straightforward strategy: measuring your levels, reviewing your habits, and, if necessary, supplementing under medical supervision.

At Clínica Valorian, in Marbella, we approach vitamin D from an evidence-based medicine perspective, integrating it into the assessment of overall health and long-term wellbeing. In this guide we explain what it does, why it is so frequently lacking, what signs may indicate low levels, how it is measured, and what you can do to improve it safely.

What vitamin D does in the body

The best-known function of vitamin D is to enable the intestinal absorption of calcium and phosphorus, the minerals that keep bones strong. Without sufficient vitamin D, the intestine absorbs calcium poorly and the body extracts it from the skeleton itself, which over time weakens bone structure.

Its role extends beyond bone. Vitamin D acts on many tissues because its receptors are present throughout much of the body:

• Bones and muscles. It promotes bone mineralisation and contributes to muscle strength, an important factor in preventing falls as we age.
• Immune system. It modulates the immune response, playing a role both in defence against infections and in the regulation of inflammation.
• Energy and wellbeing. Many people with deficiency report fatigue and weakness, symptoms that often improve once levels are corrected.

Because of its wide-ranging influence, vitamin D is taken into account in preventive medicine and in longevity medicine, which aims to maintain the function of the body's systems over the years.

Why vitamin D deficiency is so common, even in sunny climates

It seems logical to assume that no one on the Costa del Sol should have a vitamin D deficiency. The reality is different. European studies show that vitamin D insufficiency is a widespread problem across the entire continent, not only in northern countries.

A standardised analysis of 55,844 individuals from 18 European studies found that 13.0% had 25-hydroxyvitamin D concentrations below 30 nmol/l — the threshold for severe deficiency — and that 40.4% fell below 50 nmol/l, according to data published in the American Journal of Clinical Nutrition. That same study observed a marked seasonal variation: the proportion of deficient individuals rose to 17.7% in winter compared with 8.3% in summer.

Living in a sunny region helps, but several factors explain why deficiency remains common:

• Indoor lifestyle. Office work and leisure spent in enclosed spaces drastically reduce the actual hours of sun exposure.
• Sun protection. Covering the skin and applying sunscreen — both advisable for preventing sun damage — also reduce vitamin D synthesis. It is a matter of balance, which we explain in our sun protection guide for the Costa del Sol.
• Sun angle and season. During the colder months the sun strikes at a more oblique angle and the skin produces less vitamin D.
• Skin tone and age. Darker skin requires longer exposure times, and the capacity for synthesis decreases with age.

The result is that a sunny geography does not guarantee adequate levels if one's lifestyle keeps the person away from the sun for most of the day.

Symptoms that may indicate low levels

Vitamin D deficiency is often silent for a long time, and that is part of what makes it risky. When symptoms do appear, they tend to be non-specific and easy to attribute to other causes. Among the most commonly described are:

• Fatigue and a persistent lack of energy.
• Diffuse muscle aches and weakness.
• Bone discomfort, especially in the back, hips, or legs.
• A greater tendency toward respiratory infections.
• Low mood, particularly during the months with less daylight.

None of these symptoms alone confirms a deficiency, as they can result from many other conditions. They are, rather, a signal to consider a proper assessment. Because these complaints overlap with other common conditions, the only way to resolve any doubt is to measure.

How vitamin D is measured and what values are appropriate

Vitamin D status is assessed with a blood test that measures 25-hydroxyvitamin D, abbreviated as 25(OH)D. This is the circulating form that best reflects the body's reserves, and results are usually expressed in nanograms per millilitre (ng/mL) or nanomoles per litre (nmol/L).

As a general guide, laboratories and clinical guidelines typically distinguish the following situations:

• Deficiency. Clearly low values, associated with increased risk to bone health.
• Insufficiency. Intermediate values below the recommended level.
• Sufficiency. The range considered adequate for most people.
• Excess. Very high figures that may indicate an excessive intake and a risk of toxicity.

The exact thresholds vary slightly between scientific societies and according to each person's situation, so interpretation always falls to the doctor. An isolated value says little without context: a low figure in a young, healthy person is not the same as in someone with osteoporosis or undergoing treatment. This is why vitamin D testing fits naturally within a preventive medical check-up, where it is assessed alongside all other parameters.

Strategies to optimise your levels safely

Improving vitamin D is not about taking the highest possible dose, but about combining several approaches sensibly and in line with your test results. These are the strategies with the strongest evidence.

Sensible sun exposure

The skin synthesises vitamin D when it receives ultraviolet B light. Brief periods of exposure on areas such as the arms and legs, several times a week, are usually sufficient to stimulate that production. The key lies in balance: never to the point of sunburn, always protecting more sensitive areas such as the face, and keeping track of total sun exposure time. Getting excessive sun to raise vitamin D levels is not a good idea, as it increases the risk of skin damage and long-term lesions.

Diet

Few foods contain vitamin D naturally, but it is worth including those that do. Particularly notable are oily fish, such as salmon, mackerel and sardines, egg yolk, and certain fortified foods, such as some dairy products and cereals. Diet alone rarely corrects a significant deficiency, although it does help to maintain levels once they have been normalised.

Supplementation under medical supervision

When test results show a deficiency, supplementation is usually the most effective way to correct it. The dose and regimen must be individualised according to each person's baseline level, age and clinical situation. Taking supplements on one's own initiative and at high doses over a long period can cause toxicity, as excess vitamin D raises calcium in the blood and can damage the kidneys and other organs. This is why health authorities consistently recommend not exceeding safe limits without professional supervision.

Monitoring and follow-up

Optimising vitamin D is a process that needs to be measured. The sensible approach is to start with a blood test, apply the recommended changes and check the response with a follow-up test within the timeframe set by the doctor. This measure, act and measure again approach is the same one that guides the assessment of biological age and, more broadly, personalised preventive medicine.

Conclusion

If you have compatible symptoms, risk factors, or simply want to know where you stand, the most useful step is straightforward: measure it. A 25-hydroxyvitamin D blood test, interpreted by a doctor, will tell you whether you need to take action and how to do so safely. If you would like an individualised assessment of your levels and a plan tailored to you, at Clínica Valorian, with Dr. Florian A. Vallecillo Cabrera, we can help you integrate it into a long-term health strategy.