Vitamin D: from October to March, the sun is no longer enough

Journal · Vitamins & seasons

Vitamin D: from October to March, the sun is no longer enough

Vitamin D is the only vitamin our skin makes itself, thanks to the sun. But from October to March, at our latitudes, the sun barely has the means to do so. Here is why, who is most affected, and how to choose your vitamin D well.

Every autumn, vitamin D comes back into the conversation, often with more noise than explanation. At Biologyst, we prefer to start from the mechanism: what the sun does, why it is no longer enough in winter, what vitamin D really provides according to the claims recognised in Europe, and how to read a label.

How does the body make vitamin D?

When the sun’s ultraviolet B rays reach the skin, they convert a molecule derived from cholesterol, 7-dehydrocholesterol, into previtamin D3, which then becomes vitamin D3. The liver and then the kidneys convert it into active forms. Food provides some too, but few foods are rich in it: oily fish, egg yolk, cod liver.

Why the sun is no longer enough in winter

It all depends on the angle of the sun. In winter, it is low on the horizon, and its rays pass through a thicker layer of atmosphere that filters out almost all UVB. You can have a fine, clear day and make practically no vitamin D.

Mainland France lies between 42° and 51° north latitude, between these two cities. From October to March, most vitamin D must therefore come from stores built up over the summer and from food.

On a fine January day, the skin makes practically no vitamin D.

Who is most affected?

• People who spend their days indoors, at the office or working from home.
• Older people, whose skin makes vitamin D less efficiently.
• Olive and dark skin, which needs longer exposure.
• People who cover their skin or always use sunscreen.
• Overweight people, in whom more vitamin D is stored in fat tissue.

To find out your status, your doctor can prescribe a 25-hydroxyvitamin D blood test.

What vitamin D does, according to recognised claims

In Europe, several health claims are authorised for vitamin D. The most useful to know in winter:

• Vitamin D contributes to the normal function of the immune system.
• Vitamin D contributes to the maintenance of normal bones.
• Vitamin D contributes to the maintenance of normal muscle function.
• Vitamin D contributes to normal absorption/utilisation of calcium and phosphorus.

Vitamin D3 or D2: which to choose?

There are two main forms: vitamin D2 (ergocalciferol), of plant or fungal origin, and vitamin D3 (cholecalciferol), the form our skin makes.

Oil-based softgel or tablet?

Vitamin D is fat-soluble: it dissolves in fats. This is why it is often found in an oil-based softgel, and why it is advisable to take it with a meal that contains fat.

D3 Active+: 120 vitamin D3 softgels, one softgel a day.

Our BIOLOGYST grid: how to compare vitamin D supplements

On 2 October 2026, we recorded the official product pages of four vitamin D3 supplements sold in France and assessed them, together with D3 Active+, against five criteria anyone can check on a label: D3 form, presence of an oil (vitamin D is fat-soluble), a single daily intake, no declared additive apart from the shell, at least four months per box. A criterion is only ticked if the brand’s product page shows it. Prices per day are calculated at the dosage indicated by each brand.

Recorded on 2 October 2026 from the brands’ official product pages. D3 Active+: 75 µg per softgel, €0.19 per day.

Sources recorded on 2 October 2026: Dynveo, Vitamine D3 2000 UI, 60 capsules, €17.90; Novoma, Vitamine D3 2000 UI, 120 capsules, €19.90; Arkopharma, Arkogélules Vitamine D3, 60 capsules, €16.40; Nutri&Co, Vitamine D végétale en gouttes, €19.90. Prices and compositions may have changed since this record.

How to take D3 Active+?

One softgel a day, preferably with a meal containing fat, for several months according to your needs, particularly during periods of low sun exposure. Do not exceed the recommended daily dose.

Vitamin D in our rituals

D3 Active+ is part of two rituals: our ritual combining vitamin D, lactoferrin and nigella, offered at €80.90, and Preserving the Foundations, with Omega Active+ and Vital Flow, offered at €88.90. In these sets, it is vitamin D that carries the claim on the immune system.

To discover the other two formulas in the first ritual, read our guides Lactoferrin: what the science really says and Nigella oil: what the science really says.

Frequently asked questions

Why do we lack vitamin D in winter?

Because the winter sun is too low for its UVB to reach the skin in sufficient quantity. A scientific study from 1988 showed that at 52° north latitude, the skin produced no vitamin D from October to March, even in clear weather.

Vitamin D3 or D2: which to choose?

Vitamin D3 (cholecalciferol) is the form the skin makes. A 2012 meta-analysis observed that it raised the blood level of vitamin D more effectively than vitamin D2.

What is vitamin D for?

Vitamin D contributes to the normal function of the immune system, to the maintenance of normal bones and normal muscle function, and to normal absorption/utilisation of calcium and phosphorus.

When should you take vitamin D?

With a meal that contains fat, because vitamin D is fat-soluble. For D3 Active+: one softgel a day, without exceeding the recommended daily dose.

Can you take too much vitamin D?

Yes, which is why you should respect the recommended dose and add up the intake from all supplements that contain it. In case of hypercalcaemia, kidney disease or medication, ask your doctor for advice.

Further reading

• D3 Active+: the product page
• The vitamin D, lactoferrin and nigella ritual
• The Preserving the Foundations ritual
• Lactoferrin: what the science really says
• Nigella oil: what the science really says