Vitamin D3 in winter: why the season counts

The short answer: the problem is not the cold, it is the angle. Between October and March the sun stands so low in Germany that the UV-B fraction of sunlight needed for vitamin D production is almost entirely absorbed in the atmosphere. The German Nutrition Society (DGE) puts it soberly: in these months the body's own production is barely possible at our latitudes. A walk on a clear frosty day is therefore good for a great many things — but it delivers no vitamin D. What counts is how much the body brought with it out of the summer.

This article explains what the sun's elevation has to do with UV-B, from when production practically stops, how long the summer stores last, and what actually still contributes something in winter — and what only looks as if it does.

Why the sun's elevation decides

Vitamin D3 forms in the skin from 7-dehydrocholesterol when UV-B radiation hits it. UV-B is a very narrow band in the solar spectrum, roughly 290 to 315 nanometres, and it is precisely this band that the ozone layer swallows most strongly. How much arrives at ground level therefore depends on how long the light's path through the atmosphere is — and that path grows rapidly longer as the sun sinks. If the sun stands half as high, the path is not twice as long but considerably more than twice.

The technical literature usually names a solar elevation of around 45 degrees as the threshold: below it, so little UV-B arrives that production practically comes to a standstill. What that means for German-speaking cities is shown by the sun's midday elevation at the two solstices:

Place	latitude	sun's elevation on 21 December (midday)	on 21 June (midday)	period above 45°
Hamburg	53.6° N	13°	60°	mid-April – end of August
Berlin	52.5° N	14°	61°	mid-April – early September
Cologne	50.9° N	16°	63°	early April – early September
Frankfurt am Main	50.1° N	16°	63°	early April – early September
Munich	48.1° N	18°	65°	late March – mid-September
Vienna	48.2° N	18°	65°	late March – mid-September
Zurich	47.4° N	19°	66°	late March – mid-September

Between Hamburg and Zurich there are a good six degrees of latitude — and two to three weeks of season at each end. Within Germany, then, the season is the big lever and the place of residence only a small one. One further point is decisive: even in summer the threshold only holds around midday. At eight in the morning and seven in the evening the sun stands just as flat in July as it does at noon in December.

The shadow rule

There is a rule of thumb that requires no looking up: at exactly 45 degrees of solar elevation, the shadow is as long as the person casting it. If your own shadow is clearly longer than your own height, the sun stands too low — and then little to nothing happens in terms of vitamin D production. If it is shorter, there is enough UV-B.

Between November and February the midday shadow in Germany is about three to four and a half times as long as the person. That is the whole winter in one image.

What the summer stores — and how long it lasts

The body lays down vitamin D in fat and muscle tissue, and the transport form in the blood, 25-OH-D, has a half-life of about two to three weeks. Supply therefore does not break off in October but sinks slowly across the autumn. The measurements of the DEGS1 population study by the Robert Koch Institute show exactly this annual pattern: highest values in August and September, lowest in February and March. Averaged over all seasons, a good 60 per cent of adults there lay below 50 nmol/l in serum — the value many professional bodies set as the lower limit of a desirable supply.

How high your own starting value is depends on more than the calendar. Anyone who spends the summer in an office tops up less than someone who works outdoors. Darker skin needs considerably longer in the sun for the same amount because of its higher melanin content. And with age the content of 7-dehydrocholesterol in the skin declines, so that the same time in the sun yields less. The calculation "I was outside a lot in summer, that will carry me to Easter" therefore rarely works out.

For the case in which the body's own production does not happen, the DGE gives an estimated value of 20 µg (800 IU) per day. That case is precisely the German winter.

What does not help in winter — although it looks as if it does

Sun behind glass. Ordinary window glass lets visible light and UV-A through but holds back UV-B almost completely. A bright winter day by the window or a long car journey in sunshine feels like sun and still delivers no vitamin D. The same applies to the conservatory.

Tanning beds. Most devices work predominantly with UV-A, which is of no use for vitamin D production. The German Federal Office for Radiation Protection expressly advises against using tanning beds for this purpose.

Dense cloud. A closed cloud cover swallows a large part of the UV-B radiation. In winter this comes on top of the low sun rather than replacing it.

What actually does contribute something is altitude: per 1,000 metres of elevation, UV exposure increases by roughly ten per cent, and fresh snow reflects a considerable share on top of that. A day's skiing in the high mountains is therefore not comparable with a walk through town — but it does not extend the season either, it merely shifts it a little.

For the months in which it works, the Federal Office for Radiation Protection recommends short periods outdoors two to three times a week with face, hands and arms uncovered — about half the time after which your own unprotected skin would turn red. Sunscreens slow production considerably on paper (SPF 30 filters around 96 per cent of UV-B radiation), in practice usually less, because too thin a layer is applied. The radiation protection advice is nevertheless: first the short unprotected period, then protection — not the other way round.

What diet contributes in winter

Little, and that is not carelessness but a property of the foods. According to the nutritional data of Souci/Fachmann/Kraut, only a few supply appreciable amounts:

Food	vitamin D per 100 g	corresponds to about
Herring	7.8–25 µg	up to 1,000 IU
Eel	20 µg	800 IU
Salmon	16 µg	640 IU
Sardine	11 µg	440 IU
Hen's egg	2.9 µg	116 IU
Button mushrooms	1.9 µg	76 IU
Butter	1.2 µg	48 IU
Emmental	1.1 µg	44 IU

According to DGE figures, diet contributes only about 10 to 20 per cent of the supply overall; the rest normally comes from the skin. Anyone wanting to reach the DGE estimated value of 20 µg through food would have to eat a good piece of oily sea fish every day on average. As a winter strategy that is unrealistic for most people. Which portion sizes lie behind these figures, and how far fortified products carry, is worked through in detail in Vitamin D in food.

What that means for a supplement

Four figures stand side by side in Germany, and they do not contradict each other — they answer different questions:

Value	amount per day	origin
Nutrient reference value (NRV)	5 µg (200 IU)	Regulation (EU) No 1169/2011, Annex XIII — reference figure for the percentage on the label
Estimated value without the body's own production	20 µg (800 IU)	German Nutrition Society
Proposed maximum per daily portion	20 µg (800 IU)	German Federal Institute for Risk Assessment (BfR), proposal without the force of law
Tolerable Upper Intake Level (UL)	100 µg (4,000 IU)	EFSA — upper limit for permanent intake from all sources, adults and adolescents from 11 years

For winter this means in practice: the relevant range lies between the DGE estimated value and the EFSA upper limit, and which strength is printed on the tin is secondary — what counts is the amount per day that results from strength and interval. How the strengths in our range can be reduced to that one column is set out in Vitamin D3 dosage: 1000 IU to 10000 IU; where the upper limit runs and which arithmetical error really does exceed it, in Can you take too much vitamin D?.

Converting strength and rhythm into the same column

The winter half of the year — October to March inclusive — has 182 days. Anyone who knows what daily amount they want to aim for can work backwards from it to how often they reach for the tin over the winter and how many tablets that takes:

Strength	rhythm	arithmetically per day	doses October–March
1,000 IU	daily	1,000 IU	182
4,000 IU	every fourth day	1,000 IU	46
4,000 IU	every second day	2,000 IU	91
5,000 IU	every five days	1,000 IU	36
10,000 IU	every ten days	1,000 IU	18
20,000 IU	every two weeks	around 1,400 IU	13

All six rows stay well below the EFSA upper limit of 100 µg (4,000 IU) per day on average. The calculation does, however, assume that the interval is actually kept — the commonest mistake with depot strengths is not the strength itself but a forgotten interval.

For winter there are thus essentially two routes. One is the daily tablet in a small to medium strength: Vitamin D3 4,000 IU as a year's supply covers several winters with its 365 tablets if you take one only every second or fourth day. The other is the depot strength at a wider interval — more on that below.

Anyone who would rather not think about a daily tablet in winter will find the other route in the depot strengths: Vitamin D3 10,000 IU every ten days is 36 appointments a year instead of 365. An overview of all strengths and combinations is in the Vitamin D3 category. Among the authorised claims for vitamin D are that Vitamin D contributes to the normal function of the immune system. Vitamin D contributes to the maintenance of normal bones (Regulation (EU) No 432/2012). Food supplements are not a substitute for a balanced and varied diet and a healthy lifestyle.

Frequently asked questions

From exactly when does production stop? There is no date, it is a transition. The DGE names October to March as the time in which production is barely possible in Germany; by the 45-degree rule it already becomes tight in early to mid-September, depending on latitude. October is more the end of a ramp than an edge.

Is a winter holiday in the south enough? For the days on site yes, for the winter hardly. In Tenerife or Egypt the sun stands high enough in January. After returning, though, the level sinks again with the same half-life of two to three weeks — a week of sun does not carry three months.

Does the summer then not matter, if I supplement in winter anyway? It does. The starting value in October decides how deep the winter goes at all. Anyone who was in the sun regularly and briefly through the summer starts higher — and the winter intake builds on that instead of filling a hole.

Do I have to take more in winter than in summer? The question is rather whether anything is needed in summer at all. The DGE estimated value of 20 µg (800 IU) applies expressly to the case in which the body's own production does not happen — that is, to winter. Anyone who is in the sun regularly and briefly from April to September covers their needs through the skin during that time. A winter amount higher than 800 IU is therefore not automatically justified; it follows, if at all, from a measured starting value and not from the calendar. What such a laboratory value says is set out in Vitamin D blood level: reading 25-OH-D correctly.

What about children? For children from 1 to 10 years the EFSA upper limit is 50 µg (2,000 IU) per day, for infants considerably lower. The recommendations for the first year of life run through the paediatrician anyway; that is where the question belongs, not on the shelf.