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Low Vitamin D: Symptoms, Causes and When to Get Tested

Oct. 21, 2026 · 4 min ·

Low Vitamin D: Symptoms, Causes and When to Get Tested

There is a paradox that surprises many people: Spain is a sunny country and yet vitamin D deficiency is very common. The reason is that sunshine alone is not enough; lifestyle, age, the use of sun protection and other factors all play a role. Vitamin D is important for bones, muscles and the immune system, and its deficiency often gives no obvious symptoms, so it goes unnoticed. In this article we explain what it is for, what symptoms its lack can give, why it is so common and how the test is interpreted, without going into doses, which the doctor always indicates.

What Vitamin D Is For

Vitamin D fulfils important functions in the body:

  • It helps fix calcium and maintain bone health.

  • It participates in muscle function.

  • It has a role in the immune system.

It is worth keeping expectations in their place: vitamin D is necessary, but it is not a miracle remedy for everything. The sensible thing is to ensure adequate levels, not to chase high figures thinking that "more is better".

Symptoms of Vitamin D Deficiency

The first thing to know is that, often, the deficiency is asymptomatic: it is not noticed, and it is only detected with a test. When symptoms do appear, they can be:

  • Tiredness and a feeling of weakness.

  • Muscle and bone pain.

  • A greater tendency to infections (low defences).

  • Muscle weakness, especially in older people.

These are non-specific symptoms, that is, they can be due to many other causes. That is why they do not serve to diagnose on their own: a test and an assessment are needed.

Causes: Why Vitamin D Is Lacking Even in Spain

Most vitamin D is produced in the skin with exposure to the sun, so almost all the causes have to do with receiving or making use of less sun:

  • Indoor life: office work and little time outdoors.

  • Sun protection and clothing: necessary for skin health, but they reduce the synthesis of vitamin D.

  • Age: over the years the skin produces less vitamin D.

  • Excess weight: can affect its availability in the body.

  • Skin pigmentation and latitude/season: in winter and with fewer hours of sun, less is produced.

  • Some diseases of the digestive or renal system that affect its absorption or activation.

Low Vitamin D in Women

In women there are stages when it is worth paying particular attention, such as the menopause, because of its relationship with bone health. Maintaining adequate levels of vitamin D is part of looking after the bones at that stage, along with other measures that the doctor assesses.

The Vitamin D Test (25-OH)

The deficiency is measured with a blood test of 25-hydroxyvitamin D (25-OH vitamin D), which is the form that best reflects the body's status.

How the Result Is Interpreted

The thresholds vary according to the scientific society, so they should be interpreted with the doctor and not on your own. As a reference, the Spanish Society of Endocrinology and Nutrition (SEEN) suggests maintaining concentrations of 25-OH vitamin D between 30 and 50 ng/ml (75–125 nmol/L) to obtain its benefits; below those values one speaks of insufficiency or deficiency, with cut-off points that change according to the reference (for example, deficiency below 20 ng/ml).

What matters is not to memorise figures, but that whoever interprets your result takes your situation into account. And a word of warning: always look at the units in the report (ng/ml or nmol/L) before comparing it with any table.

How Often to Repeat It

If a deficiency is detected and treated, the doctor usually indicates a follow-up check to confirm that the levels have been corrected. The frequency depends on each case; it is not a test to be repeated without a criterion.

Consequences of Vitamin D Deficiency

Maintained over time, the deficiency is associated above all with bone problems, due to the poorer fixing of calcium (from bone pain to poorer bone health). That is why it is not advisable to ignore a deficiency, even if it gives no clear symptoms.

A note on a frequent myth: does a lack of vitamin D make you gain weight? Not directly. There is no simple cause-and-effect relationship by which the deficiency makes you gain weight. What is observed is that the deficiency is more common in people with excess weight, but that does not mean that one causes the other. Correcting vitamin D is not a method for losing weight.

How to Raise Vitamin D

There are three routes, and all three have nuances:

  • Sun, with common sense: moderate exposure helps, always looking after the skin and avoiding burns.

  • Diet: some foods provide vitamin D (fatty fish, eggs, fortified dairy), although with diet alone it is usually difficult to correct a marked deficiency.

  • Supplements: they are effective when needed, but only on medical indication. Here we deliberately do not give doses: the dose is indicated by the doctor according to your test.

An important warning: taking vitamin D supplements on your own and in excess is not harmless. Excessive and prolonged consumption can cause levels that are too high and adverse effects. That is why supplementation is done with a test and follow-up, not "just in case".

Vitamin D Within a Health Check-Up

Vitamin D is one of the parameters that it makes sense to review within a general check-up, along with other markers, especially if you have risk factors or symptoms. Integrating it into a complete assessment makes it possible to interpret it in context and act only when necessary. You can see how we approach the complete medical check-up within our preventive medicine approach.

Frequently Asked Questions

Can I take vitamin D without a test? It is not advisable. The prudent thing is to measure the levels first and let the doctor indicate whether supplementation is needed and in what amount. Taking it "just in case" and in excess can be counterproductive.

Does a lack of vitamin D make you gain weight? Not directly. The deficiency is more common in people with excess weight, but there is no simple cause-and-effect relationship, and correcting it does not serve to lose weight.

How much sun do I need? Moderate exposure helps produce vitamin D, but there is no single rule: it depends on the season, the time, the skin type and the place. Always looking after the skin and without burning. If you are worried about your level, the most reliable thing is the test.

Book Your Assessment

If you have symptoms that concern you or want to review your state of health, an assessment with a test lets you know how your vitamin D levels (and other markers) are and act with judgement.