Vitamin D in winter: needs, intake and sensible amounts

There's that moment somewhere in November: daylight is just enough that you don't leave the house in the dark in the morning, and by 5 p.m. it's over again. That's exactly when winter starts — and for your vitamin D level, it's the toughest half of the year.

Here's the background — and what you can do with it concretely. If you're also feeling tired a lot, it's worth a parallel look at iron and tiredness, since the vitamin D and iron stories like to overlap in winter.

Vitamin D is the special case among vitamins

Unlike all other vitamins, you can produce vitamin D yourself. When UV-B radiation of the right wavelength hits uncovered skin, a small synthesis runs in the skin's surface, at the end of which is active vitamin D. In the EU list of authorised health claims (Regulation 432/2012), vitamin D is listed among other things for the normal function of the immune system, the maintenance of normal bones, normal muscle function and the absorption of calcium and phosphorus.

For this self-synthesis to work at all, three things have to come together: a high sun position (UV index from about 3), uncovered skin and no sunscreen on the exposed areas. In Germany that's only roughly given between April and September — and even then, only around midday.

Why winter is so difficult

Between October and March, in Germany — north of the 50th parallel — the sun stands so low in the sky that UV-B loses most of its energy in the atmosphere. Even several hours of a sunny walk in January deliver practically nothing for your vitamin D synthesis.

What helps you is your own reservoir. Vitamin D is fat-soluble, so it accumulates in liver and fat tissue. Anyone who spends time outdoors regularly through the summer builds up reserves that bridge part of the winter. The point is: for many people, those reserves don't last all the way — studies by the Robert Koch Institute show that larger parts of the population fall below the desired blood levels in winter.

What DGE recommends

For vitamin D, DGE works with an estimated value rather than a fixed recommendation — because part of the supply comes from sunlight, which can't be prescribed. If no endogenous synthesis occurs, the following applies:

Group	Estimated value (without endogenous synthesis)
Infants up to 12 months	10 µg / day
Children, adolescents, adults	20 µg / day
Pregnant and breastfeeding	20 µg / day
Older adults from 65	20 µg / day

20 µg corresponds to 800 IU (international units) — that's the unit you'll see on most packs.

Your DGE reference value — Vitamin D

Age groupInfants (up to 12 months)1–64 years65 years and olderLife stageNonePregnancyBreastfeeding

Your DGE reference value: 20 µg/day

Children, adolescents, adults

Good to know

• From roughly October to March the sun in central Europe sits too low for the body's own production — the estimated value applies exactly to such periods without endogenous synthesis.
• If you rarely spend time outdoors or usually keep your skin covered, your body produces less vitamin D itself.
• With increasing age, the skin produces less vitamin D.

Shown is the published DGE reference value for the selected group — not an individual assessment. Orientation value — it does not replace a medical examination or advice. Source: DGE reference values for vitamin D (estimated values in the absence of endogenous synthesis)

Vitamin D from foods — difficult but not hopeless

Reaching 20 µg per day from diet alone is possible, but it takes planning:

Food (100 g)	Vitamin D
Cod liver oil	~ 300 µg
Herring	~ 25 µg
Salmon	~ 16 µg
Mushrooms (UV-treated)	~ 10 µg
Egg yolk	~ 5.6 µg
Mackerel	~ 4 µg

Anyone who eats fatty fish once a week reaches a good range on a weekly average. People who don't like fish have it harder — egg yolk and UV-treated mushrooms contribute a bit, but rarely enough.

Does supplementing in winter make sense?

For many people who spend little time outside between October and March, supplementation is the simplest way to stay near the DGE estimate. Three practical points:

• Vitamin D is fat-soluble. Take it with a meal that contains some fat — uptake in the gut improves noticeably.
• 20 µg per day equals 800 IU. Higher-dose products (1,000–4,000 IU) exist but aren't sensible for everyone. Before high-dose supplementation, it's worth having your 25-OH-vitamin D blood level measured at your GP — that way you know where you stand and can dose deliberately.
• Supplementation is not "more is better". EFSA cites 100 µg (4,000 IU) per day as the tolerable upper amount for adults. Anyone who stays well above that long-term risks elevated calcium levels.

Who pays particularly close attention?

There are life situations in which the self-synthesis argument doesn't hold — and supplementation across the winter can be particularly sensible:

• Office work, barely any daylight in everyday life
• People with care needs without regular access to the outdoors
• Low UV self-synthesis, e.g. with darker skin types or permanently covering clothing
• Older adults — skin synthesis declines with age

In all these cases, we're not talking about high-dose courses but about quietly reaching the DGE recommendation over several months.

Vitamin D rarely works alone

Calcium and phosphorus are absorbed efficiently from the gut only thanks to vitamin D — that's where the bone link comes from. Magnesium, in turn, is needed by the body to enzymatically activate vitamin D in the liver. So anyone with "vitamin D" on their radar should keep an eye on their overall diet — see, for example, the article on Magnesium and muscle function.

If you'd like to find out whether your vitamin D intake fits your daily life — and which small adjustments would be worthwhile — take a look at the free Vital-Check. Three minutes, personal assessment, no sales pressure.