September brings the blood tests people have after the summer, and one result starts almost the same conversation every time: low vitamin D. It comes back flagged, the patient walks in worried, and the question is predictable. If I live on the Costa Brava and spent the summer next to the beach, how can my level be low?
There are several reasons, and they are worth separating. One is that autumn sunlight no longer does what July sunlight does. The other, less widely known, is that what the laboratory flags as low does not always mean disease. Here is what the test actually measures, who really needs it, when something has to be done and what a supplement will not do for you.
What you will find in this article
- Why levels fall from autumn onwards
- What vitamin D does in the body
- How to read the blood test
- What symptoms it causes
- Who should be tested, and who should not
- Who is most at risk
- Sunlight: how much and how, without burning
- Where vitamin D really comes from
- Supplements: doses and limits
- What vitamin D does not do
- What else your doctor looks at
- How we study it at Clínica Eupnea
- Frequently asked questions
The key points in thirty seconds
- Most of your vitamin D is made in the skin by ultraviolet B radiation, and from November to February, at our latitude, the midday sun is no longer strong enough.
- The test measures 25-hydroxyvitamin D. Below 20 ng/ml almost every laboratory calls it deficiency; between 20 and 30 there is a grey zone with little agreement.
- The 2024 Endocrine Society guideline advises against routine testing in healthy adults, and Spanish recommendations say the same.
- Real deficiency causes bone and muscle pain and weakness. Most mildly low results cause no symptoms at all.
- A supplement is not a preventive multivitamin: the VITAL trial, with 25,871 people, reduced neither cancer, nor heart attacks, nor fractures.
- Overdoing it has consequences too: more than 4,000 IU a day over time can end in hypercalcaemia.
Why vitamin D falls from autumn onwards
Vitamin D comes mostly from your skin, not from your plate. Ultraviolet B radiation turns a cholesterol derivative in the skin into the precursor of the vitamin, and the liver and kidney finish the job. The catch is that this radiation depends on the angle of the sun: the lower it sits, the more atmosphere it has to cross and the less UVB reaches the ground.
That is why the calendar matters so much. A team at the Universitat Politècnica de València worked out how much sun was needed in Valencia to obtain the equivalent of 1,000 IU a day. In spring and summer, about 10 minutes around 1 pm with a quarter of the body uncovered was enough. In winter, from November to February, with only 10 % of the body exposed, the figure rose to roughly two hours at solar noon, which nobody actually does at 12 degrees wearing a jumper. The university published a summary of the study.
Palamós sits at 41 degrees north, slightly higher than Valencia, so the pattern here is the same or a little stricter. Summer stores run down, and the result is the one we see in autumn and winter blood tests.
What vitamin D does in the body
Its main job is to let the gut absorb calcium and phosphate from food. Without enough vitamin D, calcium does not get in properly no matter how much you eat, and bone pays the price. When the shortage is severe and long lasting, two classic conditions appear: rickets in children, with bones that deform as they grow, and osteomalacia in adults, with soft, painful bones. It also has a role in muscle, which is why muscle weakness is one of the signs of real deficiency.
Beyond bone, what is still debated
Vitamin D is involved in the immune system and in many cellular processes, which has produced a great deal of research and even more headlines. It is one thing for vitamin D receptors to be present all over the body, and quite another for a capsule to prevent disease in people who already have enough. That second part is what has not been shown, and the figures are further down this page.
How to read a vitamin D blood test
What the laboratory measures is 25-hydroxyvitamin D (you will see it as 25-OH-D or calcidiol), the form that circulates in blood and reflects your stores. It can be reported in ng/ml or nmol/l, and that is the first source of confusion, because the same result looks very different depending on the unit: 1 ng/ml equals 2.5 nmol/l.
| 25-OH-D result | Equivalent | How it is usually read |
|---|---|---|
| Below 20 ng/ml | Below 50 nmol/l | Deficiency for almost every laboratory and guideline |
| 20 to 30 ng/ml | 50 to 75 nmol/l | Grey zone: insufficiency for some guidelines, enough for others |
| Above 30 ng/ml | Above 75 nmol/l | Adequate for all of them |
Cut-offs are not universal. The 2024 Endocrine Society guideline concluded that the evidence does not allow a target level to be set for preventing disease.
That lack of agreement explains part of the noise. In Spanish studies, more than half of adults fall below 30 ng/ml, and the proportion is far higher in older people. Then again, if the threshold used to define deficiency is disputed, so is the headline that half the country is deficient. A result has to be read with the person in front of you, not as a pass or a fail.
What symptoms low vitamin D causes
The honest answer is that mildly low results usually cause none. Established deficiency does give signals: bone pain, muscle pain and weakness, and in young children with severe deficiency, muscle spasms as the first sign of rickets. The MedlinePlus page from the US National Library of Medicine sets out the basics.
Tiredness is a different matter. It is the symptom most often blamed on low vitamin D, and also one of the least specific in medicine: anaemia, thyroid problems, poor sleep, anxiety or simply not sleeping enough all produce it. Pinning it on vitamin D straight away sometimes means the real cause is never looked for.
Who should have their vitamin D measured (and who should not)
This has changed in recent years. The Endocrine Society guideline published in June 2024 advises against screening 25-OH-D in healthy adults, including people with obesity or dark skin, because no level has been shown to deliver a specific benefit. Spanish scientific societies and health services take the same line and do not recommend population screening.
Testing when there is a reason is another matter. It makes sense with osteoporosis or a fragility fracture, suspected osteomalacia, a condition that prevents fat absorption (coeliac disease, Crohn's disease, bariatric surgery), chronic kidney or liver disease, abnormal calcium or parathyroid results, or treatments that interfere with vitamin D, such as some antiepileptic drugs or rifampicin.
Why your doctor does not test everyone
Because a number with no plan behind it generates more appointments, more supplements and more follow-up tests without the person being any better. In groups with a clinical reason, by contrast, knowing the figure changes what happens next: you treat, you correct the cause and you check the result.
Who is most at risk of low vitamin D
The risk profile is fairly recognisable, and it has more to do with how you live than with the country you live in:
- Older adults, especially if they rarely go out or live in a care home. Skin makes less vitamin D with age and the gut absorbs less.
- People with dark skin: melanin filters ultraviolet radiation, so more time outdoors is needed for the same effect.
- Anyone who stays covered up, whether for work, habit or belief.
- People with obesity or who have had bariatric surgery.
- Conditions with fat malabsorption, and some kidney or liver problems.
- Exclusively breastfed babies, because breast milk contains very little. That is why a supplement is recommended during the first year.
Sunlight: how much and how, without burning
The advice that has circulated for years is to expose arms and legs, or face, arms and hands, for 5 to 15 minutes about three times a week, and longer for darker skin or older people. It also has to be said that many dermatologists do not recommend chasing more sun, because the same ultraviolet that makes the vitamin is what raises the risk of skin cancer.
A sensible balance is simple: short, frequent spells outdoors, never to the point of redness. Burning does not make more vitamin D, it only damages skin. And in midwinter, however hard you look for it, midday sun will not give you much.
Does sunscreen leave you short of vitamin D?
It is a widespread fear and the short answer is no. A review of 75 studies published between 1970 and 2017, which appeared in the British Journal of Dermatology in 2019, found no significant effect of regular sunscreen use on vitamin D production, and a trial in Tenerife using factor 15 creams applied properly showed excellent synthesis. There is more on using it properly in our article on why sunscreen matters.
Food: where vitamin D really comes from
Few foods contain it naturally, which is why diet alone struggles to reach the recommended amounts. The best natural sources are oily fish (salmon, sardines, mackerel, trout, herring), egg yolk and liver. Then there are fortified products: milk, yoghurts and breakfast cereals often have vitamin D added. The NHS vitamin D page lists the same sources and reminds readers that from October to early March, at British latitudes, sunlight is not enough.
Oily fish is worth putting on the table two or three times a week anyway, quite apart from vitamin D. We covered that in detail when we explained how to read a cholesterol blood test.
Supplements: doses, schedules and limits
The recommended daily amounts are modest: 600 IU for most adults, 800 IU from age 71 and 400 IU during the first year of life. The NHS simplifies this to 10 micrograms a day for everyone over one year old, and points out that 1 microgram equals 40 IU. When there is a genuine deficiency, a doctor may prescribe higher doses for a few weeks and then move to a maintenance dose.
The ceiling matters too. Taking more than 4,000 IU a day over a long period can cause hypercalcaemia, too much calcium in the blood, which weakens bone and can damage the kidneys and the heart. For children aged 1 to 10 the limit is 2,000 IU a day, and 1,000 IU for babies under one. You cannot overdose from sunlight; you can from capsules.
Why a small daily dose beats a monthly mega-dose
For people aged 50 and over who have a reason to take it, the 2024 guideline prefers low daily doses to large, spaced-out ones. The monthly or quarterly ampoule is convenient and still widely used, but current evidence does not favour it.
What vitamin D does not do
This part tends to surprise people. VITAL, the largest trial so far, gave 2,000 IU a day or placebo to 25,871 adults for a median of 5.3 years. It did not reduce cancers or major cardiovascular events, and the fracture analysis published afterwards found no reduction in fractures either. Those taking part were mostly healthy with adequate starting levels, which is precisely the group that most often buys supplements on its own.
None of this means vitamin D is useless. It means it is useful for what it is useful for: correcting a real deficiency, protecting bone when there is a reason, and supporting osteoporosis treatment. As a universal preventive pill, the numbers are not there.
What else your doctor looks at when vitamin D comes back low
A single figure says very little. With low vitamin D plus symptoms or risk factors, the next step is usually to check calcium, phosphate, alkaline phosphatase, kidney function and, depending on the case, the parathyroid. That is how a dietary or sunlight problem is told apart from an absorption or hormonal one.
A full blood test also picks up other things behind symptoms that people blame on vitamin D: iron and ferritin, vitamin B12 and thyroid function. In fact, when someone comes in about autumn hair loss, those three usually explain more than vitamin D does.
How we study low vitamin D at Clínica Eupnea
In Palamós we do it as part of our metabolic studies, which include the glucose profile, the lipid profile, thyroid function, liver and kidney function, and the minerals and vitamins section, where iron, calcium, vitamin D, vitamin B12 and folic acid sit. The blood draw requires a 10 to 12 hour fast and the result is interpreted alongside the rest of the panel.
Who interprets your low vitamin D result
The metabolic studies are signed by Dr David Pérez Asensio, a specialist in endocrinology. The appointment covers whether there was a reason to order the test, what the rest of the panel shows, whether treatment is needed and for how long, and when it is worth repeating. It does not always end with a prescription, and that is a valid conclusion too.
To book, call 872 21 75 91, Monday to Friday, 9.00 to 13.30 and 15.30 to 19.30.
Frequently asked questions
That the 25-hydroxyvitamin D in your blood is below the level the laboratory flags, usually 20 or 30 ng/ml. Below 20 ng/ml (50 nmol/l) is treated as deficiency almost everywhere. Between 20 and 30 guidelines disagree, and the 2024 Endocrine Society guideline concluded that the evidence does not allow a target to be set. A slightly low result in a healthy person with no symptoms is not the same as a disease.
Significant deficiency causes bone pain, muscle pain and weakness, and in adults it can progress to osteomalacia. In children, severe deficiency causes rickets, and muscle spasms are often the first sign. Most mildly low results cause no symptoms. Tiredness is so unspecific that anaemia, thyroid problems, poor sleep and anxiety should be ruled out before blaming vitamin D.
It depends on the season, the time of day and your skin. Calculations made in Valencia, at a latitude close to ours, give about 10 minutes around 1 pm in spring and summer with a quarter of the body uncovered, and roughly two hours at midday in winter with only 10 % exposed. The classic advice is short spells of 5 to 15 minutes about three times a week, never to the point of redness.
Oily fish is the best natural source: salmon, sardines, mackerel, trout and herring. Egg yolk and liver also contribute. The rest comes mainly from fortified foods such as milk, yoghurts and breakfast cereals. Diet alone rarely reaches the recommended amounts, which is why sunlight remains the main source.
The recommended daily amounts are 400 IU in the first year of life, 600 IU from one to 70 and 800 IU from 71 onwards. If deficiency is confirmed, a doctor may prescribe higher doses for a few weeks followed by a maintenance dose. In people aged 50 and over with a reason to take it, the 2024 guideline prefers low daily doses to monthly mega-doses.
Yes. Taking more than 4,000 IU a day over time can raise blood calcium (hypercalcaemia), which weakens bone and can damage the kidneys and heart. For children aged 1 to 10 the limit is 2,000 IU a day, and 1,000 IU for babies under one. Sun exposure carries no risk of overdose, because the body regulates how much it makes.
Not under real conditions of use. A review of 75 studies published between 1970 and 2017 found no significant effect of sunscreen on vitamin D production, and a trial using factor 15 creams applied properly showed excellent synthesis. Protecting your skin from burns and skin cancer and keeping your vitamin D up are not incompatible goals.
Has your blood test come back with low vitamin D?
At Clínica Eupnea in Palamós we review it as part of our metabolic studies, and the endocrinologist explains whether anything needs doing and for how long.
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