Short answer: in a northern European summer, most people with lighter skin can cover their vitamin D needs with a few minutes of midday sun on bare arms and legs, and in winter almost nobody can. Between roughly October and March the sun sits too low for your skin to make any vitamin D at all, no matter how long you stand in it. That is why European guidance lands on 10 to 20 micrograms a day (400 to 800 IU) from food or a supplement during the dark months, and why summer advice is mostly about sensible sun rather than pills.
Vitamin D is the one nutrient where the season genuinely changes the answer. Iron does not care what month it is. Vitamin D does, because most of your supply is made in your skin by ultraviolet B radiation, and the amount of UVB reaching the ground swings enormously with the sun angle. This article walks through what actually changes between June and January, what the numbers behind official recommendations are, and how to read a supplement label honestly, including our own.
Why the season changes everything: the vitamin D winter
Vitamin D3 is made when UVB photons, in a narrow band around 290 to 315 nanometres, hit 7-dehydrocholesterol in your skin. UVB is the part of sunlight most aggressively filtered by the atmosphere. When the sun is low, its light travels a longer path through the ozone layer, and the UVB fraction is stripped out almost completely while visible light and warmth still arrive. That is the whole mechanism: a bright, cold January afternoon can feel sunny and deliver essentially zero vitamin D.
The classic measurement of this is Webb, Kline and Holick in 1988 [1]. They exposed human skin and 7-dehydrocholesterol to real sunlight on cloudless days at four latitudes. In Boston, at 42.2 degrees north, no previtamin D3 was produced from November through February. In Edmonton, at 52 degrees north, that dead period stretched from October through March. At 34 and 18 degrees north, sunlight still worked in the middle of winter. Researchers now call that dead period the vitamin D winter [2].
Read those latitudes against a map of Europe and the practical picture is blunt. Amsterdam sits at about 52 degrees north, Berlin at 52.5, Copenhagen at 55.7, Helsinki at 60.2. Every one of them is at or beyond Edmonton. Madrid, at about 40 degrees north, sits at the edge. A northern European winter is not a season of reduced vitamin D synthesis. It is a season of no synthesis.
What official European guidance actually says
Because the skin route closes in winter, dietary reference values are deliberately set as if you got no sun at all. Three numbers are worth knowing, and they are all in the same range.
- EFSA set an Adequate Intake of 15 micrograms per day for everyone over one year of age, including pregnant and breastfeeding women [3].
- The UK NHS, following the SACN review, advises 10 micrograms a day for adults and children over one, and says plainly that "between October and early March we do not make enough vitamin D from sunlight" [4][5].
- Germany's DGE sets an estimated value of 20 micrograms per day for adolescents and adults when endogenous synthesis is absent, and notes the conversion: 1 microgram equals 40 International Units [6].
So the European consensus band is 10 to 20 micrograms a day, or 400 to 800 IU, for the months when your skin cannot help you. There is no serious disagreement about the direction of the advice, only about where in that band to land.
Those are intakes, not claims. Under EU law the only things a vitamin D supplement may say about what the nutrient does come from Regulation (EU) 432/2012 [7]: vitamin D contributes to the normal absorption and utilisation of calcium, to the maintenance of normal bones, teeth and muscle function, and to the normal function of the immune system. Anything beyond that list is not an authorised claim, however often you see it online.
How much summer sun is actually enough
The most useful modern answer comes from the Manchester group, who combined ten years of UV climatology with controlled exposure studies. Their conclusion for white Caucasian adults in the UK: about nine minutes of daily lunchtime sunlight from March to September is enough to keep blood 25-hydroxyvitamin D above 25 nmol/L right through the following winter [8].
The caveat matters more than the headline. Those nine minutes assume forearms and lower legs are exposed from June to August, and hands and face in the cooler months either side. Exposing only hands and face throughout the summer does not meet requirements. Surface area is doing as much work as time.
Two more things quietly cancel summer sun. Window glass blocks most UVB, so a sunny office or car seat contributes nothing. And sun exposure behaviour in real life is modest: monitoring in Greater Manchester at 53.5 degrees north found healthy white adults receive only about 3 percent of the available ambient UVR [9]. Living in a sunny month is not the same as being in the sun.
Skin tone, age and cover change the maths
Melanin is a very effective natural sunscreen. It absorbs UVR to protect the skin underneath, and in doing so it also reduces the UVR available for vitamin D synthesis, so darker skin needs substantially longer exposure for the same yield [10]. The same Manchester methodology applied to Fitzpatrick skin type V adults showed the simple nine minute rule does not transfer across skin types.
The population data agree. In a standardised analysis of 55,844 Europeans, dark-skinned ethnic subgroups had 3 to 71 times the prevalence of very low vitamin D status compared with white populations [11]. Age matters too, because ageing skin contains less 7-dehydrocholesterol, and so does anything that covers skin, whether that is clothing, indoor work, or high factor sunscreen used correctly.
How common is low vitamin D in Europe, really
This is where the seasonal effect stops being theoretical. Using one standardised laboratory method across 14 European population studies, Cashman and colleagues found that 13.0 percent of 55,844 people had serum 25(OH)D below 30 nmol/L averaged over the year. Split by season, that was 17.7 percent in the extended winter (October to March) against 8.3 percent in summer (April to November). Against the more generous 50 nmol/L threshold, 40.4 percent of Europeans fell short [11].
Roughly a doubling of deficiency from summer to winter, in the same people, in the same countries. That is the seasonal swing quantified.
A practical summer versus winter framework
| Period | What your skin can do | Sensible approach |
|---|---|---|
| Late spring to early autumn (roughly April to September) | Real synthesis around midday, if skin is uncovered | Short regular midday exposure with arms and legs bare, well short of burning. Supplement only if sun exposure is genuinely low |
| October to March at northern latitudes | Effectively none, the vitamin D winter | Cover the 10 to 20 micrograms a day from food or a supplement |
| Any season, if you have darker skin, cover up, work indoors or are older | Reduced, sometimes markedly | Treat supplementation as a year round question, not a winter one |
Diet rarely closes the gap on its own. Oily fish, egg yolk and fortified products are the meaningful sources, and typical northern European diets deliver only a few micrograms a day, which is why the guidance above exists in the first place.
Reading a label: IU, micrograms and how often
Two conversions solve most confusion. 1 microgram equals 40 IU, so 400 IU is 10 micrograms and 800 IU is 20 micrograms [6]. And a per-serving number only means something once you know how often the serving is taken.
Our own Vitamin D3 + K2 drops are a good worked example. One serving is 0.25 ml of MCT oil and delivers 125 micrograms of vitamin D3 (5000 IU) and 120 micrograms of vitamin K2 as MK-7. The label instruction is 0.25 ml every three days, not daily, and a 20 ml bottle holds 80 servings. Averaged out that is about 41.7 micrograms, roughly 1,667 IU, per day.
Stated plainly: that is above the 15 microgram Adequate Intake, and it is below the safety ceiling the NHS gives for adults, which is not more than 100 micrograms (4,000 IU) a day [4]. A 5,000 IU number on a label looks alarming until you notice the interval, and it would be genuinely too much if the same serving were taken daily. That is exactly why the pack says do not exceed the recommended intake. The same product page carries the rest of the label conditions: it is not intended for pregnant or breastfeeding women, and anyone taking anticoagulants should speak to their doctor first, because vitamin K interacts with that class of medicine.
Why K2 sits next to D3
Vitamin D increases how much calcium you absorb. Vitamin K contributes to the maintenance of normal bones and to normal blood clotting, and both of those are authorised claims under Regulation (EU) 432/2012 [7]. Pairing them is a formulation choice built on that complementary role rather than on any promise about where calcium ends up in the body, which is a much more speculative area than supplement marketing usually admits. We have written about that pairing separately in vitamin D3 and K2.
Form matters here as well. Vitamin D is fat soluble, which is why an oil base is a sensible carrier, and MK-7 is the long chain menaquinone form of K2. If you want the general principle behind that, see our piece on why form matters more than the dose.
What about winter immunity
Vitamin D contributes to the normal function of the immune system, and so does zinc, both under Regulation (EU) 432/2012 [7]. That is the honest scope of what either nutrient may be said to do. Neither prevents infection, and no supplement should be sold as if it did. If you are looking at winter nutrient status generally, our zinc picolinate page and the accompanying article on zinc forms cover that nutrient with the same discipline.
It is worth saying clearly that vitamin D research beyond bone and mineral metabolism remains unsettled. The skeletal actions are well established. The extraskeletal claims, the ones that fill headlines, are described by researchers themselves as carrying outstanding questions [12]. Buy vitamin D for what it is confirmed to do.
Should you get tested
A blood test for 25-hydroxyvitamin D is the only way to know your own status rather than the population average. The thresholds used above, below 30 nmol/L and below 50 nmol/L, come from public health definitions rather than from a personal diagnosis, and clinical guidelines differ on where to draw the line [13]. If you have reason to think you are at risk, or you are considering higher intakes than the label instruction, that is a conversation with your doctor, not with a website. A test taken at the end of winter tells you the most, because that is when your stores are at their lowest.
Frequently Asked Questions
Can I get enough vitamin D from winter sun if I stay out longer?
No. In the vitamin D winter the limiting factor is the UVB content of the light, not the duration of exposure. At Edmonton's latitude of 52 degrees north, which is close to Amsterdam and Berlin, no previtamin D3 was formed at all from October through March on cloudless days [1]. Extra hours outdoors multiply zero.
How much sun do I need in summer?
For white Caucasian adults in the UK, modelling combined with exposure studies puts it at about nine minutes of daily lunchtime sun from March to September, with forearms and lower legs exposed in June to August and hands and face in the cooler months [8]. Hands and face alone across the whole summer is not enough. Darker skin needs longer [10].
Do I need to supplement in summer too?
It depends on your actual exposure rather than the calendar. People who work indoors, cover their skin, use sunscreen consistently, have darker skin or are older may stay short even in July. European deficiency data still show 8.3 percent below 30 nmol/L during the summer months [11].
What is the maximum safe daily amount?
The NHS advises adults not to take more than 100 micrograms (4,000 IU) a day, noting that more could be harmful [4]. Higher intakes belong under medical supervision. Vitamin D is fat soluble and accumulates, which is exactly why the ceiling exists.
Is 5,000 IU per serving too much?
Not if the serving interval is respected. Our drops deliver 5,000 IU per 0.25 ml serving taken every three days, which averages about 1,667 IU or 41.7 micrograms per day, below the 100 microgram daily ceiling [4]. Taken every day instead, the same product would exceed that ceiling. Follow the label.
Does vitamin D from food make a difference?
It helps, but rarely enough on its own in northern Europe. Oily fish, egg yolk and fortified foods are the main sources, and typical intakes fall well below the 10 to 20 microgram range that European bodies advise for the months without synthesis [3][6].
The Bottom Line
Vitamin D is a seasonal nutrient with a hard cutoff. From roughly April to September, sensible midday sun on uncovered arms and legs does most of the work for most lighter-skinned people in northern Europe, and a few minutes is genuinely enough. From October to March the skin route is closed, and the 10 to 20 micrograms a day that EFSA, the NHS and the DGE converge on has to come from food or a supplement. Darker skin, older skin, covered skin and indoor lives shift that from a winter question to a year round one. Read the label for the serving interval, not just the number on the front, respect the 100 microgram ceiling, and if you want to know your own status rather than the average, test at the end of winter and take the result to your doctor.
Sources
- Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. Journal of Clinical Endocrinology and Metabolism, 1988.
- Engelsen O. The Relationship between Ultraviolet Radiation Exposure and Vitamin D Status. Nutrients, 2010.
- European Food Safety Authority. Vitamin D: EFSA sets dietary reference values. EFSA, 2016.
- NHS. Vitamin D. Vitamins and minerals, National Health Service, UK.
- Scientific Advisory Committee on Nutrition. SACN vitamin D and health report. GOV.UK, 2016.
- Deutsche Gesellschaft fuer Ernaehrung. Referenzwerte Vitamin D. DGE.
- Commission Regulation (EU) No 432/2012 establishing a list of permitted health claims made on foods. EUR-Lex.
- Webb AR, Kazantzidis A, Kift RC, et al. Meeting Vitamin D Requirements in White Caucasians at UK Latitudes: Providing a Choice. Nutrients, 2018.
- Kift R, Rhodes LE, Farrar MD, Webb AR. Is Sunlight Exposure Enough to Avoid Wintertime Vitamin D Deficiency in United Kingdom Population Groups? International Journal of Environmental Research and Public Health, 2018.
- Webb AR, Kazantzidis A, Kift RC, et al. Colour Counts: Sunlight and Skin Type as Drivers of Vitamin D Deficiency at UK Latitudes. Nutrients, 2018.
- Cashman KD, Dowling KG, Skrabakova Z, et al. Vitamin D deficiency in Europe: pandemic? American Journal of Clinical Nutrition, 2016.
- Bouillon R, Marcocci C, Carmeliet G, et al. Skeletal and Extraskeletal Actions of Vitamin D: Current Evidence and Outstanding Questions. Endocrine Reviews, 2019.
- Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology and Metabolism, 2011.
Food supplements are not a substitute for a varied, balanced diet and a healthy lifestyle. This article is general information, not medical advice. If you are pregnant, breastfeeding, taking medication such as anticoagulants, or managing a health condition, speak to your doctor before starting a supplement.
