Vitamin D deficiency: why the Swedish winter is a special case

Vitamin D deficiency: why the Swedish winter is a special case

On a clear morning in mid January, Stockholm can look almost Mediterranean. The light is hard, the snow throws it back off the ground, and after twenty minutes outdoors your face is stinging. Not one molecule of vitamin D forms in your skin while you stand there.

This has nothing to do with how bright the day is. It has to do with the angle at which the sunlight arrives, and in Sweden that angle is wrong for close to half the year.

The Nordic and Baltic countries lie between 54 and 71 degrees north, latitudes where, as NNR 2023 notes, solar radiation is insufficient for part of the year for vitamin D3 production in skin to occur. What follows is less flattering to the supplement industry than the headline suggests.

The short version

• Between October and March the sun in Sweden sits too low for UVB to reach the ground, so the skin makes no vitamin D however bright the day is. Every Swedish town lies further north than Edmonton, where the dead period was measured as October through March back in 1988.

• Insufficiency is not the same thing as deficiency. Around 40 per cent of adults dip below 50 nmol/L in winter, while frank deficiency under 25 to 30 nmol/L stays uncommon, and the large VITAL and D-Health trials showed no benefit from high doses in people who already have enough.

• Health Royals Peak Performance contains 25 µg vitamin D3 per daily dose, alongside 90 µg vitamin K2 as MenaQ7. Vitamin D contributes to the normal function of the immune system, to normal muscle function and to the maintenance of normal bones, and vitamin K contributes to the maintenance of normal bones. NNR 2023 recommends 10 µg a day for adults under 75.

Health Royals Peak Performance, supplement with vitamin D3, K2 as MenaQ7, folate as Quatrefolic and omega-3 from algal oil

Vitamin D at Health Royals

Peak Performance provides 25 µg vitamin D3 per daily dose, which is 1,000 IU, alongside 90 µg vitamin K2 as MenaQ7. Vitamin D contributes to the normal function of the immune system, to normal muscle function and to the maintenance of normal bones. NNR 2023 recommends 10 µg a day for adults under 75, and the tolerable upper intake level is 100 µg.

See Peak Performance

Why does the skin stop making vitamin D in Sweden between October and March?

Because the sun never climbs high enough for UVB light to reach the ground, and UVB is the only part of the spectrum that starts the reaction.

Vitamin D3 forms when ultraviolet B photons, roughly 290 to 315 nanometres, strike 7-dehydrocholesterol in the outer layers of the skin. Those are the shortest wavelengths that reach the surface at all, and the ozone layer absorbs them strongly. When the sun sits low, its light takes a long slanted path through the atmosphere and almost all the UVB is stripped out before it reaches you.

The measurements are old and uncontested. In 1988 Webb, Kline and Holick exposed vials of 7-dehydrocholesterol to natural sunlight through the year. In Boston, at 42.2 degrees north, no previtamin D3 formed from November through February. In Edmonton, at 52 degrees north, the dead period ran from October through March.

Sweden begins where Edmonton's problem gets worse. Smygehuk, the southernmost point, sits at about 55 degrees north, Stockholm at 59.3, Kiruna at 67.9. Every Swedish town is further from the equator than Edmonton, so the vitamin D winter here runs at least six months, longer in the north.

In summer, by contrast, the requirement is small. Livsmedelsverket estimates that a light-skinned person forms the equivalent of 5 to 10 micrograms of vitamin D by exposing face, arms and hands for about 6 to 8 minutes, two or three times a week during June and July. More pigmented skin needs roughly 10 to 15 minutes, because melanin competes for the same UVB photons.

Who in Sweden actually has vitamin D deficiency?

Fewer people than the word deficiency implies, and they cluster in groups you can name in advance.

The distinction that gets lost is between insufficiency and deficiency. Above 50 nmol/L of circulating 25-hydroxyvitamin D counts as sufficient. Below 25 to 30 nmol/L indicates deficiency.

Swedish figures, from the Swedish Food Agency's 2018 risk and benefit assessment, sit mostly in between. In Riksmaten 2010 to 2011 adults averaged 63.5 nmol/L: 65.7 in summer, 55.4 in winter, with 39.7 per cent below 50 nmol/L during the winter months. Among 206 children aged 10 to 12 sampled in March to May 2014, the mean was 52.9 nmol/L, 42 per cent were below 50 and 4.9 per cent below 30.

So roughly four in ten Swedish adults dip under the sufficiency line in winter, while frank deficiency stays uncommon. The groups Livsmedelsverket singles out fit that picture: children under two, people who eat neither fish nor fortified foods, people who get very little sun including those in fully covering clothing, and adults over 75.

What symptoms does vitamin D deficiency cause, and can tiredness be one of them?

Severe, prolonged deficiency has recognisable effects on bone and muscle. Mild insufficiency has no reliable symptoms at all, which is why tiredness is a poor way to diagnose it.

Sustained deficiency causes rickets in growing children and osteomalacia in adults, with bone pain and weakness in the muscles closest to the trunk. These are clinical conditions with a clinical diagnosis, and they occur well below the 25 to 30 nmol/L mark, not at 45.

Between 30 and 50 nmol/L, the honest answer is that you would not know. No symptom separates a person at 42 nmol/L from one at 62. Winter fatigue in Sweden has a long list of causes far more common than low vitamin D, among them short daylight hours, disrupted sleep, iron deficiency, thyroid problems, low mood and sustained psychological stress. Attributing it to vitamin D without a blood test is a guess, and usually a wrong one.

How is vitamin D measured, and what does the blood value mean?

It is measured as 25-hydroxyvitamin D in serum, reported in nanomoles per litre, and the thresholds attached to it are set by expert committees rather than by biology.

25(OH)D is the main circulating form and the accepted status marker, because it combines what you ate with what your skin made over the preceding weeks.

EFSA set 50 nmol/L as the target concentration for all population groups when it derived dietary reference values in 2016, and NNR 2023 uses the same line for sufficiency.

Because the Swedish seasonal swing is large, the month of the test is part of the result. The same adult measured in September and again in March is describing two different states.

How much vitamin D do Livsmedelsverket and NNR 2023 recommend?

Ten micrograms a day for most adults, and 20 micrograms from the age of 75 or for anyone with little to no sun exposure.

NNR 2023 sets a recommended intake of 10 micrograms per day for women and men aged 19 to 74, with an average requirement of 7.5 micrograms, rising to 20 micrograms from 75 and for those with little or no sun. The tolerable upper intake level is 100 micrograms per day.

Livsmedelsverket turns that into supplement advice: 10 micrograms daily for all children under two, 10 for children and adults who eat neither fish nor fortified products, 10 for those with very limited sun exposure and 20 if they also avoid vitamin D rich foods, and 20 for adults over 75.

The authorities disagree. EFSA's adequate intake for adults is 15 micrograms per day, higher than the Nordic 10, because EFSA models a population with minimal skin synthesis while the Nordic figure assumes some summer sun.

Under EU law only claims on the authorised list may be made for a nutrient. For vitamin D these include that it contributes to the normal function of the immune system, and to the maintenance of normal bones and normal muscle function. We cover the physiology in our note on vitamin D3 for immunity, muscles and bones.

What did the large supplementation trials not show?

They did not show that extra vitamin D given to people who already have adequate levels reduces cancer, cardiovascular disease, fractures or death.

The VITAL trial randomised 25,871 adults in the United States to 2,000 IU, that is 50 micrograms, of vitamin D3 daily or placebo, with a median follow-up of 5.3 years. Invasive cancer occurred in 793 people on vitamin D and 824 on placebo, a hazard ratio of 0.96 with a confidence interval of 0.88 to 1.06. Major cardiovascular events occurred in 396 and 409, a hazard ratio of 0.97 with an interval of 0.85 to 1.12.

A later analysis of VITAL followed the same cohort for fractures. Across 1,991 incident fractures in 1,551 participants, supplemental vitamin D3 did not reduce total, non-vertebral or hip fractures, and the result did not change by age, sex or body mass index.

The D-Health trial gave 21,315 Australians aged 60 to 84 either 60,000 IU of vitamin D3 monthly or placebo for five years, with a median follow-up of 5.7 years. All-cause mortality was 5.3 per cent on vitamin D and 5.1 per cent on placebo, a hazard ratio of 1.04 with an interval of 0.93 to 1.18.

The caveat runs in both directions. Neither trial screened for deficiency at entry, so both were largely testing what happens when you add vitamin D to people who already have enough. They cannot tell you what correcting a genuine deficiency achieves. They do show that living in Sweden is not by itself a reason to take a large dose.

The picture beforehand was more optimistic. A Cochrane review pooled 56 trials and 95,286 participants in 2014, reporting that vitamin D3 might reduce mortality mainly in elderly people in institutional care, while flagging high dropout rates. VITAL and D-Health were the trials it called for.

Respiratory infection is the one area where a signal survives, and it is small. A meta-analysis of 43 trials and 48,488 participants found that 61.3 per cent of people given vitamin D reported at least one acute respiratory infection, against 62.3 per cent on placebo, an odds ratio of 0.92 with an interval of 0.86 to 0.99. That is about one percentage point. EU food law does not permit describing a food or supplement as preventing infection, and on these numbers the description would not be honest anyway. The scoping review behind NNR 2023 put it plainly: the totality of evidence does not support that intake beyond current recommendations brings additional health benefits.

Is it possible to get enough vitamin D from food in Sweden?

In principle yes, but it takes deliberate effort, because the Swedish diet draws vitamin D from a very short list of foods.

The list is fatty fish such as salmon, herring and mackerel, egg yolk, and fortified products, mainly milk, fermented milk, plant drinks and fat spreads. Livsmedelsverket's advice is conditional for that reason: it applies to people who eat neither fish nor fortified products. Reaching 10 micrograms a day from food alone means eating fish regularly and choosing fortified products, which many households do not do consistently.

What is reasonable to do during a Swedish winter?

Match the dose to the recommendation rather than to the season's reputation, and test only when there is a reason to.

  • If you are in one of the named groups, follow the 10 or 20 microgram advice. If not, the case is weaker than the marketing suggests.
  • A blood test is worth doing for a clinical question, not routine curiosity. Interpret it with a doctor, and remember that the month changes what it means.
  • Stay well under the upper intake level of 100 micrograms per day. More is not better.
  • Do not expect a supplement to fix winter tiredness. If tiredness is the real problem, sleep, iron status and thyroid function are better places to look. Our wellbeing page covers the wider picture.

One honest note on combinations. Vitamin K2 is often paired with D3 because the two work together in bone tissue. Vitamin K contributes to the maintenance of normal bones under the EU list, but the evidence that adding K2 to D3 improves outcomes in healthy, replete adults is limited rather than settled. We set out what is and is not known in our piece on vitamin K2.

For readers who want both in one daily formula, Peak Performance contains vitamin D3 at 25 micrograms, which is 1,000 IU and 500 per cent of the EU reference intake, with MenaQ7 vitamin K2 at 90 micrograms. That figure is more than the 10 micrograms NNR 2023 recommends for adults under 75, and a quarter of the upper intake level. It is not, on current evidence, better than 10 micrograms for someone already above 50 nmol/L.

Further reading

Frequently asked questions

Does everyone in Sweden need a vitamin D supplement in winter?
No. Livsmedelsverket's advice applies to defined risk groups: children under two, people who eat neither fish nor fortified products, people who get very little sun, and adults over 75. For everyone else the case is weaker than the marketing suggests.

How much vitamin D should I take?
NNR 2023 sets 10 micrograms a day for adults aged 19 to 74, and 20 micrograms from 75 or with little sun exposure. The tolerable upper intake level is 100 micrograms a day. EFSA arrives at 15 micrograms, because it models a population with minimal skin synthesis.

Can my vitamin D be low without me noticing?
Yes. Between 30 and 50 nmol/L there are no reliable symptoms, and nothing separates a person at 42 nmol/L from one at 62. Severe, prolonged deficiency causes rickets in children and osteomalacia in adults, but those are clinical conditions well below that level.

Is winter tiredness caused by vitamin D deficiency?
Rarely. Short daylight hours, disrupted sleep, iron deficiency, thyroid problems and low mood are all more common causes. Attributing tiredness to vitamin D without a blood test is a guess, and usually a wrong one.

Is 25 micrograms too much?
It is more than the 10 micrograms NNR 2023 recommends for adults under 75, and a quarter of the upper intake level of 100 micrograms. On current evidence it is not better than 10 micrograms for someone already above 50 nmol/L. If you take several products containing vitamin D, add them up.

Can I get enough vitamin D from food?
In principle yes, but the list is short: fatty fish such as salmon, herring and mackerel, egg yolk, and fortified products such as milk, fermented milk, plant drinks and fat spreads. Reaching 10 micrograms a day from food alone takes regular fish and deliberate choices in the shop.

Sources