I live in the sun. Why is my vitamin D low?

The confusion usually arrives with the paperwork. Twenty-two nanograms per millilitre, sometimes fourteen, and a doctor writing out a prescription for a monthly ampoule while you sit there thinking about the walk from the car park, which was bright enough that you squinted the whole way.

The lab is rarely the problem. A population study across Catalonia, a coastal Mediterranean region with the kind of climate people relocate countries for, found roughly three in four residents below optimal levels, and among younger adults the figure came out closer to four in five. So the paradox is real and well documented, which leaves the more useful question of what sunlight is actually doing and what you are doing while it does it.

What your skin is waiting for

Vitamin D synthesis depends on a narrow band of ultraviolet light, UVB, between roughly 290 and 315 nanometres. It is the scarcest and most easily scattered part of what reaches the ground, and it only arrives in useful quantities when the sun sits high overhead. Once solar elevation drops below about 45 degrees, the light travels through enough atmosphere that most of the UVB is absorbed before it gets to you. Heat still comes through, and so does the glare, though neither has anything to do with the reaction happening in your skin.

A model built specifically for the Mediterranean basin puts the working window at March through mid-October, between around ten in the morning and four in the afternoon. Across November, December and January the sun at these latitudes never climbs high enough on any day of the month, which means three hours on a terrace in Málaga in December will warm you thoroughly and accomplish nothing else. We ran the same geometry for 38 European cities and published the result as the European Light Deficit Index, which puts Barcelona above 45 degrees on 205 days of the year and in vitamin D winter for 72 of them.

Glass ends it entirely. UVB does not pass through window glass, so a car with the windows up, an office with a view, and a living room with the balcony door shut against the heat all count as indoors for this purpose.

The Spanish day is built around missing it

That makes this a question of habit rather than geography, which is better news than it sounds.

Everything about a Spanish summer is organised around getting through the middle of the day indoors. Ten until four is work, shade, persianas down, the flat kept dark while the heat peaks. The streets refill around eight in the evening, when the light turns low and golden and completely useless for this particular purpose. The sun people move here for is the sun that shows up after the window has already closed.

Then layer on the rest of an ordinary week. Commuting behind car glass. Long sleeves and a hat. Sunscreen applied at breakfast and topped up on instinct. Skin that has never built any tolerance, so that the rare occasions it does get real exposure it burns, which teaches you to avoid exposure altogether. Northern Europeans who relocated for the light and then spent five summers looking at it through a window.

None of that is carelessness, it is what a sensible life looks like in a country where August afternoons reach forty degrees. The cumulative effect is somebody surrounded by sunshine whose bloodwork reads like they live in Helsinki.

None of this makes early light a waste, because it does a different job. Morning sun sets circadian timing, which governs sleep, alertness and how the rest of the day feels, and we worked through how much of it a European summer actually asks for in a separate guide. What morning light cannot do is make vitamin D, since the sun has not yet climbed high enough to deliver any UVB.

Your skin stops at enough

There is a ceiling built into the reaction itself. Once the precursor in your skin reaches equilibrium, further UVB stops producing vitamin D and begins converting that precursor into inactive compounds instead, which is why sunlight has never caused vitamin D toxicity in anyone, and why the second hour on the beach contributes nothing the first twenty minutes had not already delivered.

The quantities involved are small. That same Mediterranean model estimates around five minutes for the palest skin and up to twenty-five for the darkest, with roughly a quarter of the body uncovered and no sunscreen, to maintain about a thousand international units a day. Somewhere between five and twenty-five minutes, most days, in the middle of the day, from spring through autumn.

Where sunscreen fits

Public health messaging compresses two separate ideas into one instruction, which is to cover up before you go outside, every time. It is easy to remember, and it is part of why deficiency is so widespread in the sunniest parts of Europe.

What causes harm is burning. Redness, peeling, and repeated injury to skin that was given no chance to adapt are what raise long-term risk, and a short exposure that stops well before any of that is a different situation altogether. Treating the two as identical costs people something measurable.

The practical version works like this. Go out in the middle of the day with arms and legs uncovered, for less time than your skin needs to react. When that is done, put a shirt on, move into shade, or apply sunscreen for however long you plan to stay out. The protection is there for the rest of the afternoon, once the useful part is behind you.

Treat it as something you build across the season. Skin adapts when it is given a gradual reason to, so a routine that starts in April at a few minutes and lengthens through May and June leaves you in a very different position by August than arriving at a beach in July straight out of a winter office.

Some people should raise this with a doctor before changing anything, including anyone with a history of skin cancer, numerous or changing moles, photosensitising medication, an immune condition, or skin fair enough to burn in under ten minutes.

The parts sunlight cannot solve

Body composition moves the number. Vitamin D is fat soluble, so a larger fat mass distributes the same total quantity through more tissue and the serum reading comes out lower even when synthesis is working properly.

Magnesium matters more than most people expect. What your skin produces is biologically inert until the liver and then the kidneys convert it, and both of those enzymatic steps depend on magnesium. Low magnesium status is common enough that it accounts for a fair number of cases where somebody supplements diligently for months and watches the number barely move.

Food contributes at the margins. A portion of salmon carries around 570 IU, a tablespoon of cod liver oil more than 1,300, an egg about 44. Spain fortifies very little compared with the Nordics or the UK, so there is no accidental intake happening quietly in the background, and diet on its own rarely closes a real deficit.

If you supplement through the winter, get retested after about three months rather than assuming the dose was correct.

What to do to raise vitamin D naturally

1. Move one habit into the ten-to-four window. Coffee on the balcony at eleven, lunch on a bench, a walk before the afternoon rather than after it.
2. Uncover more surface area. Arms and legs for fifteen minutes achieves more than face and hands for an hour, because the amount of skin exposed drives the total.
3. Start short in spring and lengthen through the season. A few minutes in April, longer by June, so that August finds you with skin that has already adapted.
4. Stop before any redness appears. Once you reach that point, put a shirt on, move into the shade, or apply sunscreen for the rest of your time outside.
5. Get past the glass. Balcony doors open, or step outside properly, since a bright room contributes nothing at all.
6. Eat oily fish twice a week. Salmon, sardines and mackerel carry real amounts, and egg yolks add a little on top.
7. Check magnesium alongside vitamin D. Without it, the conversion steps in the liver and kidneys cannot proceed properly.
8. Supplement across the dark months, with food. October through February produces almost nothing at Spanish latitudes, and vitamin D absorbs best in a meal containing fat.

When this belongs with a doctor

Very low readings, bone pain, muscle weakness or fractures need proper medical attention, as does a deficiency that persists despite supplementation. High oral doses are worth supervising, since vitamin D taken as a supplement does have an upper limit that sunlight does not. What this article covers is the ordinary version of the problem: a normal life in a bright country, lived mostly on the wrong side of a pane of glass.

Common questions