Vitamin D in Winter: Why the Sun Is Not Enough, and What to Do

Last updated: October 2026

Late November in Sofia. You leave for work at 7:30 while the street lights are still on, spend the day under office lighting and walk home after five in the dark. On the weekend there is sun, and you sit in it with a coat, a scarf and the feeling that it must be doing something. At this latitude, in these months, it is doing very little for your vitamin D: in a classic study at 42.2°N, winter sunlight produced no vitamin D precursor in the skin from November through February (Webb et al., 1988).

This guide explains why the body’s own vitamin D production stalls in winter, how common low levels are in Europe, how much you need and how much is too much, and where a daily oral dissolvable strip such as Smart Strips IMMUNE fits into a winter morning.

Why can’t you make vitamin D from winter sun?

Your skin makes vitamin D when ultraviolet B light from the sun reaches it. In winter the sun sits low in the sky, its rays travel a longer path through the atmosphere, and very little of that UVB arrives at ground level.

The study that measured this most clearly exposed human skin samples to sunlight on cloudless days. In Boston, at 42.2°N, sunlight produced no previtamin D3 from November through February. In Edmonton, at 52°N, that period ran from October through March. Further south, at 34°N and 18°N, sunlight still produced it in the middle of winter (Webb et al., 1988). Sofia lies at almost the same latitude as Boston, and most of Europe lies further north.

Public health bodies give the same message in plain language. The NHS states that between October and early March people in the UK do not make enough vitamin D from sunlight (NHS). The US Office of Dietary Supplements lists season, time of day, length of day and cloud cover among the factors that affect how much vitamin D the skin makes, and notes that older people and people with darker skin make less (NIH ODS).

How common is low vitamin D in Europe?

More common than most people assume. A large analysis that standardised blood measurements from 55,844 people across European population studies found that 13.0% had vitamin D levels below 30 nmol/L on average over the year. In samples taken in winter the share was 17.7%, against 8.3% in summer. Using a higher threshold of 50 nmol/L, which some guidelines use, the share was 40.4% (Cashman et al., 2016).

The pattern is the point. Levels rise in summer and fall in winter, and many people enter spring with less than they had in September.

Can you get enough vitamin D from food?

It is hard. As the US Office of Dietary Supplements puts it, "Few foods naturally contain vitamin D" (NIH ODS). The best natural sources are oily fish such as salmon, sardines, trout, herring and mackerel; smaller amounts come from red meat, egg yolks and fortified foods such as some spreads and breakfast cereals (NHS).

As for the target, the European Food Safety Authority has set an adequate intake of 15 µg a day for adults, based on conditions with minimal vitamin D production in the skin, which describes a European winter well (EFSA, 2016). The US recommended intake for adults aged 19 to 70 is the same, 15 µg or 600 IU a day (NIH ODS). The NHS advises adults in the UK to consider a daily supplement of 10 µg during autumn and winter (NHS).

A realistic winter week rarely reaches 15 µg a day from the plate alone, unless oily fish is on the menu several times a week.

How much vitamin D is too much?

Vitamin D is fat soluble and the body stores it, so more is not automatically better. EFSA has set a tolerable upper intake level of 100 µg a day for adults, including pregnant and breastfeeding women, and 50 µg a day for children aged 1 to 10 (EFSA, 2023). The NHS gives the same adult limit: do not take more than 100 µg (4,000 IU) a day (NHS).

The practical rule is to add up every source. If you already take a multivitamin, fish oil with added vitamin D or a separate vitamin D product, check the labels before adding another.

What does vitamin D do for the immune system?

In the EU, health claims for vitamins are only allowed in wording approved under Regulation (EU) No 432/2012. For vitamin D, one of them reads: "Vitamin D contributes to the normal function of the immune system." The same wording is approved for vitamin C: "Vitamin C contributes to the normal function of the immune system" (Regulation (EU) No 432/2012). These sentences describe a normal role in the body, and we use them exactly as written.

Where does IMMUNE fit in?

Picture the same November morning. Coffee, keys, a child who cannot find a second glove. The habits that last through winter are the ones that take no time and need no planning. This is the idea behind Smart Strips IMMUNE.

IMMUNE is an oral dissolvable strip you place on the tongue once a day, in the morning. It dissolves in 7 to 30 seconds, without water. It is free of sugar and gluten, sweetened with stevia, with a mixed berry flavour. Each strip contains:

Vitamin D3, 35 µg (1,400 IU). This is 700% of the nutrient reference value used on EU labels and about a third of EFSA’s adult upper level. Vitamin D contributes to the normal function of the immune system.

Vitamin C, 20 mg. This is 25% of the nutrient reference value. Vitamin C contributes to the normal function of the immune system.

Elderberry juice concentrate, 20 mg. The US National Center for Complementary and Integrative Health notes that only a small number of studies have evaluated elderberry (NCCIH). We make no claim for it.

IMMUNE is a habit. You will not feel anything after a strip, and that is normal for a daily basic. The dose is one strip a day, and it should not be combined with other vitamin D supplements without advice. Because vitamin D is fat soluble, taking the strip with or just before a breakfast that contains some fat is a sensible routine: in one study, a vitamin D3 dose taken with a meal containing fat produced a 32% higher peak in blood than the same dose with a fat-free meal (Dawson-Hughes et al., 2015).

Does vitamin D work without a capsule?

The research on vitamin D delivery formats is encouraging. In a winter crossover trial in 22 healthy adults, vitamin D3 given as an oral spray raised blood levels as effectively as the same dose in a capsule, and the authors concluded that the spray is "an equally effective alternative to capsule supplementation" (Todd et al., 2016). That study tested a spray rather than a strip, and it shows that vitamin D does not need to arrive in a capsule to do its job.

An oral dissolvable strip also starts in the mouth. The lining under the tongue is only 100 to 200 µm thick (Hua, 2019), and part of a strip’s dose is absorbed through the lining of the mouth, while the rest is swallowed with saliva. For the strip format as a whole, compared with a swallowed tablet, Smart Strips works with three figures: up to 95% bioavailability, up to 4x faster absorption and up to 30% higher effectiveness. The strips are made with QuickRelease™ technology (patent-pending).

There is a winter advantage that has nothing to do with chemistry. The sachet is light and compact. It travels in a coat pocket, a ski bag or a carry-on, and works in a hotel room without a glass of water. Our guide to taking supplements without water covers travel in more detail, and Oral strips vs pills, capsules and gummies compares the formats.

A winter morning in three minutes

• Daylight first. Ten minutes outside in the morning helps the body clock, even when the sun is too low for vitamin D.
• Breakfast with some fat. Eggs, yoghurt, a slice of bread with butter.
• One strip. IMMUNE on the tongue while the coffee brews. If you take NMN as part of a longer-term routine, mornings suit it too; our daily ritual guide shows how the range fits into one day.

Who should talk to a doctor first?

Speak to your doctor before taking vitamin D supplements if you are pregnant or breastfeeding, if you take medication, or if you have kidney disease, a condition affecting calcium levels or any other medical condition. If you want to know your own vitamin D status, a blood test arranged by your doctor is the only reliable way to find out.

IMMUNE is for adults. Children have a lower upper level than adults (EFSA, 2023), and their vitamin D needs should be discussed with a paediatrician. Food supplements are not a substitute for a varied, balanced diet and a healthy lifestyle. Consult your doctor if you are pregnant, nursing, taking medication or have a medical condition.

FAQ

Can you get vitamin D from the sun in winter?

At Sofia’s latitude, very little. At 42.2°N, sunlight on cloudless days produced no vitamin D precursor in the skin from November through February (Webb et al., 1988).

How much vitamin D do adults need per day?

EFSA’s adequate intake for adults is 15 µg a day (EFSA, 2016), the same as the US recommended intake of 600 IU (NIH ODS).

What is the upper limit for vitamin D?

100 µg (4,000 IU) a day for adults, according to EFSA (2023). For children aged 1 to 10 it is 50 µg.

How much vitamin D is in IMMUNE?

35 µg of vitamin D3 (1,400 IU) per strip, together with 20 mg of vitamin C and 20 mg of elderberry juice concentrate. Take one strip a day.

When is the best time to take IMMUNE?

In the morning, ideally with or just before a breakfast that contains some fat, since vitamin D is fat soluble.

Where are Smart Strips made?

Made in the USA, in an FDA registered, cGMP certified facility. Each box contains 30 strips, enough for a month of daily use.

From November to February the sun will not do this part for you. One strip with breakfast will. Discover IMMUNE.

Sources

1. 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. J Clin Endocrinol Metab. 1988;67(2):373-378. DOI 10.1210/jcem-67-2-373
2. NHS. Vitamin D. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
3. NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
4. Cashman KD, Dowling KG, Škrabáková Z, et al. Vitamin D deficiency in Europe: pandemic? Am J Clin Nutr. 2016;103(4):1033-1044. DOI 10.3945/ajcn.115.120873
5. EFSA NDA Panel. Dietary reference values for vitamin D. EFSA Journal. 2016;14(10):e04547. DOI 10.2903/j.efsa.2016.4547
6. EFSA NDA Panel. 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):8145. DOI 10.2903/j.efsa.2023.8145
7. Commission Regulation (EU) No 432/2012 establishing a list of permitted health claims made on foods, Annex (vitamin C and vitamin D entries). https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:02012R0432-20210517
8. National Center for Complementary and Integrative Health. Elderberry. https://www.nccih.nih.gov/health/elderberry
9. Dawson-Hughes B, Harris SS, Lichtenstein AH, Dolnikowski G, Palermo NJ, Rasmussen H. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet. 2015;115(2):225-230. DOI 10.1016/j.jand.2014.09.014
10. Todd JJ, McSorley EM, Pourshahidi LK, Madigan SM, Laird E, Healy M, Magee PJ. Vitamin D3 supplementation in healthy adults: a comparison between capsule and oral spray solution as a method of delivery in a wintertime, randomised, open-label, cross-over study. Br J Nutr. 2016;116(8):1402-1408. DOI 10.1017/S0007114516003470
11. Hua S. Advances in nanoparticulate drug delivery approaches for sublingual and buccal administration. Front Pharmacol. 2019;10:1328. DOI 10.3389/fphar.2019.01328