Should Everyone Take Vitamin D in Winter?

Somewhere around October the sun stops being much use. It rises later, sits lower, and even on a bright cold afternoon your skin makes almost no vitamin D.

"Everyone" is a strong word, and official advice differs. UK guidance says everyone should consider a supplement through autumn and winter. German authorities aim it at people whose supply is likely to fall short. So the real question is which of those you are.

Why Winter Sunlight Stops Working

Vitamin D forms in the skin when UVB radiation reaches a cholesterol derivative below the surface. UVB only gets through when the sun is high, so above roughly 35 degrees of latitude production shuts down for months at a time.

How long the "vitamin D winter" lasts, by region (1):

• Germany, 47 to 55 degrees: four months, late October until mid-March
• Ireland, the UK, the Netherlands and Denmark, 50 to 56 degrees: five to six months
• Athens and Thessaloniki, 37 to 41 degrees: two months. In Crete, none at all

The rest of the year, sunlight supplies an estimated 80 to 90% of the body's vitamin D (2). That assumes you are outdoors while the sun is high, with arms or legs uncovered. For a lot of us, working hours are spent inside, sunscreen reduces the UVB that starts the reaction, and the day rarely gets planned around catching the sun.

How Common Is Low Vitamin D in Europe

The largest study of its kind measured blood vitamin D in 55,844 people across 18 European surveys (3). There are two marks to fall below. Under 30 nmol/L counts as deficient. Under 50 nmol/L is short of what bones need.

• Deficient: 13% across the year, rising to 17.7% in winter and dropping to 8.3% in summer
• Short of what bones need: 40.4%, or two in every five people

Most of the people below the bone line would never be told they were deficient.

More sun does not guarantee better status. Winter deficiency ran at 23.8% in Irish adults but 2.2% in Tromsø, at 69 degrees north, where cod liver oil and supplements push intakes far higher (1). What people eat closed that gap, not where they live.

Are You One of the People Who Needs It Most?

Five things move your level beyond the calendar.

Your age. Skin from people in their late seventies and eighties made less than half as much vitamin D precursor as skin from children given the same UV exposure (4,5). Low levels also track with dynapenia, the age-related loss of muscle strength (6).

Your skin tone. Melanin competes for UVB, so darker skin needs longer exposure for the same result. Deficiency ran several times higher in dark-skinned European subgroups than in white populations (3).

Your working hours. Indoor workers show lower status than people whose jobs put them outside, and shift workers lower still (7,8).

Your weight. Vitamin D distributes through body fat, so the same intake produces a lower blood concentration in a larger body (9).

Your gut. Conditions that impair fat absorption, such as Crohn's or coeliac disease, reduce how much you take up from food or a supplement (4).

How Much Actually Covers It

Vitamin D contributes to normal immune function, normal muscle function and the maintenance of normal bones. UK public health advice is 10 µg (400 IU) a day through autumn and winter (10). The German reference value is 20 µg (800 IU) a day for someone getting no sun at all (11), though BfR recommends supplements mainly for people whose supply is demonstrably low and unlikely to be met by diet or sunlight (11). EFSA sets the tolerable upper intake level at 100 µg (4,000 IU) a day (12).

So the practical range for a European winter runs from about 400 to 1,000 IU daily, depending on which guidance you follow. Choose D3 over D2, which raises and holds blood levels more effectively (13). Take it with a meal containing fat, which measurably increases absorption (14).

Closing the Gap

Food is the obvious first answer and it does not get far. Oily fish is the only common source of any size, and average intake across German surveys is 2 to 4 µg a day against a reference value of 20 µg (11). Which leaves a supplement doing the work.

Vitamin D3 1000 IU provides 25 µg per softgel, the straightforward winter option for most adults.

Vitamin D3 K2 (MK-7) provides 4,000 IU of vegan D3 with 75 µg of K2, a higher-dose option rather than an everyday one. It suits people supplementing on medical advice or who already know their levels are low. K2 contributes to normal blood clotting and the maintenance of normal bones; anyone on coumarin-type anticoagulants should speak to their doctor first.

If you have a diagnosed condition, take other medication or are pregnant, check with a doctor first.

So, Should Everyone?

Not everyone. The people likely to fall short are anyone over 70, indoors all day, darker-skinned, carrying extra weight, or rarely outside. That is a long list, and it still undercounts: two in five Europeans sit below the level guidelines aim for, and winter makes it worse. Unless you can build your day around the hours when the sun is high, a supplement is the dependable way to close that gap, and at 400 to 1,000 IU a day it sits well inside the safety limit.

Rule of thumb: start when the clocks go back, stop when they go forward, a month longer in Scandinavia and a month or two shorter in the south. If you think you need more, get tested.

Read More

• What is the Optimal Dosage of Vitamins D3+K2
• Why Do You Need D3+K2 Vitamins?

Literature Sources

1. O'Neill CM, Kazantzidis A, Ryan MJ, Barber N, Sempos CT, Durazo-Arvizu RA, et al. Seasonal changes in vitamin D-effective UVB availability in Europe and associations with population serum 25-hydroxyvitamin D. Nutrients. 2016;8(9):533. https://doi.org/10.3390/nu8090533
2. Bundesinstitut für Risikobewertung. Einnahme hoher Einzeldosen Vitamin D über Nahrungsergänzungsmittel im Abstand von Tagen oder Wochen birgt gesundheitliche Risiken. Stellungnahme 031/2025, 3 September 2025. https://doi.org/10.17590/20250903-104032-0
3. Cashman KD, Dowling KG, Škrabáková Z, González-Gross M, Valtueña J, De Henauw S, et al. Vitamin D deficiency in Europe: pandemic? American Journal of Clinical Nutrition. 2016;103(4):1033-1044. https://doi.org/10.3945/ajcn.115.120873
4. Holick MF. Vitamin D deficiency. New England Journal of Medicine. 2007;357(3):266-281. https://doi.org/10.1056/NEJMra070553
5. MacLaughlin J, Holick MF. Aging decreases the capacity of human skin to produce vitamin D3. Journal of Clinical Investigation. 1985;76(4):1536-1538. https://doi.org/10.1172/JCI112134
6. Delinocente MLB, Luiz MM, de Oliveira DC, de Souza AF, Ramírez PC, Máximo RO, et al. Are serum 25-hydroxyvitamin D deficiency and insufficiency risk factors for the incidence of dynapenia? Calcified Tissue International. 2022;111(3):264-274. https://doi.org/10.1007/s00223-022-01021-8
7. Sowah D, Fan X, Dennett L, Hagtvedt R, Straube S. Vitamin D levels and deficiency with different occupations: a systematic review. BMC Public Health. 2017;17:519. https://doi.org/10.1186/s12889-017-4436-z
8. Martelli M, Salvio G, Santarelli L, Bracci M. Shift work and serum vitamin D levels: a systematic review and meta-analysis. International Journal of Environmental Research and Public Health. 2022;19(15):8919. https://doi.org/10.3390/ijerph19158919
9. Drincic AT, Armas LAG, Van Diest EE, Heaney RP. Volumetric dilution, rather than sequestration, best explains the low vitamin D status of obesity. Obesity. 2012;20(7):1444-1448. https://doi.org/10.1038/oby.2011.404
10. Scientific Advisory Committee on Nutrition. Vitamin D and Health. London: SACN; 2016. https://www.gov.uk/government/publications/sacn-vitamin-d-and-health-report
11. Bundesinstitut für Risikobewertung. Ausgewählte Fragen und Antworten zu Vitamin D. BfR FAQ. https://www.bfr.bund.de/cm/343/ausgewaehlte-fragen-und-antworten-zu-vitamin-d.pdf
12. EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA). Scientific opinion on the tolerable upper intake level for vitamin D, including the derivation of a conversion factor for calcidiol monohydrate. EFSA Journal. 2023;21(8):e08145. https://doi.org/10.2903/j.efsa.2023.8145
13. Tripkovic L, Lambert H, Hart K, Smith CP, Bucca G, Penson S, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. American Journal of Clinical Nutrition. 2012;95(6):1357-1364. https://doi.org/10.3945/ajcn.111.031070
14. Dawson-Hughes B, Harris SS, Lichtenstein AH, Dolnikowski G, Palermo NJ, Rasmussen H. Dietary fat increases vitamin D-3 absorption. Journal of the Academy of Nutrition and Dietetics. 2015;115(2):225-230. https://doi.org/10.1016/j.jand.2014.09.014