Vitamin D in winter: why plenty of sunshine isn't always enough

• 6 min read
• Updated: October 22, 2026
• 4 sources cited

In winter, your skin makes very little vitamin D if you live above roughly the 35th parallel: during the cold months the sun reaches you at too low an angle to produce it. That's why low levels are common even in Spain, one of the sunniest countries in Europe — and the same geometry applies to much of the United States.

The sunny-country paradox

It's hard to believe. Much of Spain gets between 2,500 and 3,000 hours of sunshine a year, and yet population studies find low vitamin D levels uncomfortably often.

The explanation isn't the hours of sun. It's geometry.

Why the angle matters more than the amount of light

Your skin makes vitamin D when ultraviolet B radiation reaches it. UVB has a short wavelength, and short wavelengths are the ones that pass through the atmosphere least easily.

When the sun is low — in winter, early in the morning or late in the day — its rays travel through a much thicker layer of atmosphere before reaching you. Along the way, most of the UVB is scattered and absorbed. Light gets through, warmth gets through, even enough UVA to tan you a little. But not the UVB you need.

That is the deciding factor, and as the Revista Española de Cardiología summarizes, it depends on the solar zenith angle: latitude, season and time of day.

What the data show in Spain

The same review is explicit about Spain: most of the country lies above the 35th parallel, "where the possibility of synthesizing vitamin D is low in winter."

And the prevalence figures are striking. In people over 65, the share with 25(OH)D levels below 20 ng/ml is between 80% and 100%. In younger people, around 40%.

It's not a statistical oddity: it's the pattern. In the US, the 35th parallel runs roughly through Memphis, Oklahoma City and Albuquerque — so everyone living north of that line faces the same winter geometry.

Who is most likely to run low

Beyond latitude, some factors reduce synthesis or increase need:

• Age. Skin loses some of its ability to make vitamin D over the years.
• Skin tone. More melanin means more natural UVB filtering and more exposure time needed.
• Indoor work. An office schedule in winter can mean not seeing the midday sun all week.
• Clothing and sunscreen. Necessary for other reasons, but they reduce synthesis.

What can be said about vitamin D

We're a European company, so these are the claims authorized in the EU:

"Vitamin D contributes to normal absorption/utilisation of calcium and phosphorus."
"Vitamin D contributes to the maintenance of normal bones."
"Vitamin D contributes to the maintenance of normal teeth."
"Vitamin D contributes to the maintenance of normal muscle function."
"Vitamin D contributes to the normal function of the immune system."
"Vitamin D has a role in the process of cell division."

Health claims authorized in the European Union

D2 or D3

D2 (ergocalciferol) comes from fungi; D3 (cholecalciferol) is the form your own skin produces and the one found in animal sources, although lichen-derived vegan D3 also exists. D3 is the most common form in supplements.

How much, and how to read the label

Labels can be confusing here. The EU labeling reference value for vitamin D is 5 µg (200 IU), so European labels show huge percentages: a 2,000 IU product is 50 µg, or 1,000% of the reference value. In the US, the Daily Value is 20 mcg (800 IU), so the same 2,000 IU reads as 250% DV.

Neither number means the dose is extreme. As a ceiling, both the European Food Safety Authority and the US National Institutes of Health set the tolerable upper intake level for adults at 100 µg (4,000 IU) a day.

What to take it with

Vitamin D is fat-soluble: it's absorbed better alongside some fat. Taking it with a meal that contains fat — not on an empty stomach with a glass of water — is a small habit with a real effect.

So what about the sun?

It's still the main source the rest of the year. In spring and summer, short exposures to the midday sun on your arms and legs are enough to make meaningful amounts; there's no need to chase a tan or get burned, which carries its own well-documented risk.

The problem is purely seasonal. From November to February, across most of these latitudes, the sun warms you but doesn't make vitamin D.

Frequently asked questions

How much sun do you need to make vitamin D?

In spring and summer, short midday sun exposure on arms and legs is usually enough, without burning. From November to February, above roughly the 35th parallel, the sun isn't at a high enough angle to produce it.

Do you need to take vitamin D all year round?

It depends on your levels, age and sun exposure. The shortfall is mostly a winter problem; a 25(OH)D blood test and a healthcare professional can tell you whether you need it and for how long.

Is 2,000 IU of vitamin D a day a lot?

2,000 IU is 50 µg, half the tolerable upper intake level set for adults (100 µg or 4,000 IU a day). It shows as 250% DV on US labels and 1,000% of the NRV on EU labels because those reference values are low.

Vitamin D2 or D3?

D3 (cholecalciferol) is the form your skin makes and the one most used in supplements. Lichen-derived D3 is also available for vegans.

Sources

• Regulation (EU) 432/2012 — list of permitted health claims
• EFSA — Tolerable upper intake level for vitamin D (2023)
• Regulation (EU) 1169/2011 — food information and nutrient reference values (NRV)
• Revista Española de Cardiología — review on vitamin D and cardiovascular disease, cited in the text