Vitamin D deficiency: why even sunny countries are running low

Spain gets more than 2,500 hours of sunshine a year. It is one of the sunniest countries in Europe. And yet, a study by the Universitat Oberta de Catalunya published in Scientific Reports found that 75% of the Spanish population has insufficient vitamin D levels. Worse still: levels are lower than in Norway, Iceland or Sweden. The paradox is real. And it is not just a Spanish problem — it stretches across most of southern Europe.

If this sounds odd, it is because the equation "sun = vitamin D" is a half-truth. There is a long way between sunshine existing outside and your skin actually making the vitamin D you need. And along that path are sunscreens, indoor jobs, latitudes that are unforgiving in winter and a diet that delivers a tiny fraction of what you actually need.

Let's unpack the paradox. What vitamin D really is, what it does in your body, why your blood test probably shows you are deficient and when supplementation actually makes sense.

What vitamin D really is (and why it is not exactly a vitamin)

Vitamin D is, technically, a prohormone. We call it a "vitamin" out of historical inertia, but its biological behaviour is hormonal. Your skin produces it when exposed to ultraviolet B (UVB) radiation, the liver converts it into 25-hydroxyvitamin D (calcidiol, the metabolite measured in your blood test) and the kidney activates it into 1,25-dihydroxyvitamin D (calcitriol). It is this active form that acts on receptors found in nearly every tissue in the body.

That alone is the first clue that vitamin D does much more than "strengthen bones". If that were its only role, it would not have receptors in the brain, the pancreas, the immune system or the heart. But it does.

What it does in your body: beyond bones

The classic role of vitamin D, the one everyone knows, is to facilitate the absorption of calcium and phosphorus in the gut. Without enough vitamin D, you can eat all the calcium in the world and your gut will not absorb it properly. It is that direct. That is why prolonged deficiency weakens bones — not because there is no calcium in the diet, but because the body cannot use it.

But research from the past two decades has shown vitamin D has roles that go well beyond bones:

Immune system. Vitamin D regulates both innate and adaptive immunity. It stimulates the production of antimicrobial peptides (cathelicidins and defensins) that act as natural antibiotics, and at the same time modulates the inflammatory response to keep it in check. A meta-analysis published in the BMJ with over 11,000 participants found that vitamin D supplementation was associated with a lower risk of acute respiratory tract infections.

Muscle function. Vitamin D plays a role in muscle contraction and strength. Deficiency is linked to proximal muscle weakness (the kind you notice when climbing stairs or getting up from a chair) and a higher risk of falls, especially in older adults.

Cardiovascular and metabolic health. Low vitamin D levels have been associated with higher risk of hypertension, insulin resistance and cardiovascular disease. The causal relationship is still being studied, but the epidemiological correlation is consistent.

Mood. Vitamin D receptors are present in brain regions involved in emotional regulation. Several studies have found an association between low vitamin D levels and depressive symptoms — though it is not an antidepressant on its own.

The southern European paradox: why sunshine is not enough

This is the point where most articles fall short. They say "go outside" and that is it. But the reality has nuances that change the whole picture.

Latitude matters, a lot. For your skin to synthesise vitamin D, UVB rays must hit it at a steep enough angle. In southern Europe, above 35°N (Madrid is at 40°N, Rome at 41°N, Lisbon at 38°N), between November and February the solar angle is so low that cutaneous synthesis is essentially zero. It does not matter that the sun is shining. The angle is wrong.

We work indoors. Peak synthesis hours (between 10:00 and 15:00) coincide with working hours. Most of the population spends that window inside offices, schools, factories or shopping centres. And glass blocks UVB.

Sunscreen. Here there is a real tension. SPF 30 sunscreen reduces vitamin D synthesis by 95-99%. But sunscreen is necessary to prevent skin cancer. It is not reasonable to ask people to choose between vitamin D and protection against melanoma. Supplementation resolves that tension.

Diet does not compensate. The richest food sources of vitamin D are oily fish (salmon, mackerel, sardines), cod liver oil, eggs and fortified dairy. But the amounts are small. A serving of salmon provides around 400-600 IU. You need significantly more than that per day if there is no skin synthesis.

Nordic countries, with far less sunshine, actually have better vitamin D status than southern Europe. The reason? They have spent decades fortifying foods and recommending systematic supplementation. In southern Europe, that population-level preventive policy still does not exist.

8 signs you might be vitamin D deficient

The catch with vitamin D deficiency is that in its early stages it is silent. It does not hurt, it is not visible, it does not warn you. But as it progresses, certain signs appear that often get blamed on "stress" or "getting older":

If three or more of these ring a bell, it is worth asking for a 25-hydroxyvitamin D blood test at your next check-up. It is a routine, inexpensive test. And the result will tell you exactly where you stand.

How much vitamin D you actually need

Here there is debate, and it is fair to acknowledge it. Recommendations vary by source:

What is clear is that the minimum consensus puts the threshold at 20 ng/mL, below which there is frank deficiency with bone consequences. But many endocrinologists and scientific societies advocate for at least 30 ng/mL to cover extra-skeletal functions. SEIOMM explicitly recommends maintaining 30-50 ng/mL in patients at risk of osteoporosis or fracture.

A dose of 4,000 IU per day (EFSA's tolerable upper intake level) is a safe and effective dose for most healthy adults who want to maintain optimal levels, especially if there is limited sun exposure, indoor work or risk factors such as older age, darker skin or obesity.

D3 vs D2, when to supplement and how to choose

There are two forms of vitamin D used in supplementation: D3 (cholecalciferol) and D2 (ergocalciferol). They are not the same.

D3 is the form your skin produces. It has higher affinity for the vitamin D binding protein in blood, is metabolised more efficiently and is significantly more effective than D2 at raising and maintaining serum 25(OH)D levels. D2 comes from plant sources (irradiated mushrooms) and may have a place in strictly vegan diets, but in terms of clinical effectiveness D3 is clearly superior.

What to look for in a vitamin D supplement

Active form D3 (cholecalciferol). Make sure the supplement specifies D3, not D2.

Adequate dose. For preventive maintenance in healthy adults, 1,000 to 4,000 IU/day is the most well-supported range. If your blood test shows severe deficiency (<12 ng/mL), your doctor may prescribe higher temporary doses.

Clean formulation. Vitamin D3 itself needs very little: a base excipient and the capsule. The fewer additives, the better. A good supplement does not need sugars, colourings or artificial flavours.

Take it with fat. Being fat-soluble, vitamin D is much better absorbed alongside foods that contain some fat. With breakfast or a main meal, not on an empty stomach.

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Vitamin D and magnesium are synergistic: magnesium is a required cofactor for vitamin D activation, and vitamin D facilitates intestinal magnesium absorption. If you supplement one, the other works better. We have a complete guide on types of magnesium if you want to dig deeper.

Frequently asked questions about vitamin D

Sources and scientific references