Assessing vitamin D in autumn and winter appropriately

In summer, a walk is often enough to provide the body with part of its vitamin D requirement. But as soon as the days become shorter, the situation changes. The sun is lower in the sky, our skin is covered more often and we spend more time indoors. As a result, the body's own vitamin D production can decline significantly in Germany in autumn and winter.

Vitamin D occupies a special position among vitamins. The body can produce it itself, but only when sufficient ultraviolet B radiation, or UVB radiation for short, reaches the skin. At the same time, only limited amounts of vitamin D can be obtained through diet.

This makes the colder months interesting: not everyone automatically develops a vitamin D deficiency in autumn or winter. An individual's vitamin D supply depends on several factors, including skin type, age, time spent outdoors, diet and existing vitamin D stores. However, seasonal fluctuations in vitamin D status in Germany are well documented.

The crucial question is therefore not whether one should generally take vitamin D in autumn. Far more important is: How can you sensibly assess your own vitamin D supply, and when might supplementation actually be useful?

Contents

1. The basics of vitamin D metabolism
2. When the sun becomes weaker in autumn
3. What remains stored in the body over winter
4. Who should pay particular attention to their vitamin D supply
5. When is vitamin D considered sufficient?
6. When a vitamin D test may be useful
7. Which foods provide vitamin D
8. What supplements actually achieve
9. Vitamin D3 and K2 compared
10. The right dosage and potential risks
11. Practical considerations in autumn
12. Vitamin D and healthy ageing
13. An overview of the key findings
14. References

The basics of vitamin D metabolism

Vitamin D is not a conventional vitamin in the strict biological sense. The body can produce it itself under suitable conditions. This is why the term prohormone is also often used for vitamin D.

The precursor substance 7-dehydrocholesterol is found in the skin, among other places. When UVB radiation reaches the skin, it initially produces previtamin D3, which subsequently converts into vitamin D3, or cholecalciferol.

However, the story does not end there.

Vitamin D3 is transported through the blood to the liver, where it is converted into 25-hydroxyvitamin D, or 25(OH)D for short. This form is the most important measurement when assessing vitamin D status in the blood.

In the kidneys and certain other tissues, this can subsequently become the biologically active form, 1,25-dihydroxyvitamin D. It acts through the vitamin D receptor and influences the activity of various genes.

Particularly well studied is the role of vitamin D in calcium and phosphate metabolism. It supports the absorption of calcium from the intestine and thus contributes to normal bone mineralisation.

In addition, the vitamin D receptor is distributed much more widely throughout the body. Vitamin D signalling pathways are therefore also found in muscle cells and cells of the immune system. This has given rise to the idea that vitamin D might be a kind of universal factor in health.

However, the scientific evidence here is considerably more nuanced.

Observational studies frequently find associations between low 25(OH)D levels and various diseases. However, this does not automatically mean that a low vitamin D status causes these diseases. People with chronic diseases, for example, often exercise less outdoors, eat differently or experience changes in their metabolism. All these factors may also be associated with a low vitamin D status.

This is precisely why randomised studies and current guidelines are crucial to the question of whether additional intake actually brings a health benefit.

When the sun becomes weaker in autumn

It is well known that vitamin D is linked to sunlight. Less well known is how strongly the conditions for vitamin D production depend on the season, time of day and geographical location.

The crucial factor is not simply whether the sun is shining. To synthesise vitamin D, the skin needs UVB radiation at suitable wavelengths.

The intensity with which this radiation reaches the Earth's surface varies considerably with the position of the sun. In summer, the sun is much higher in the sky in Germany. UVB radiation travels a shorter distance through the atmosphere and reaches the skin in greater quantities.

In autumn, the position of the sun changes. The days become shorter, the sun is lower in the sky and the available UVB radiation decreases.

This is a major reason why seasonal fluctuations affect the body's own vitamin D production in Germany.

The season is not the only deciding factor

How much vitamin D is actually produced in the skin depends on several factors:

• Skin pigmentation
• Age
• Season
• Time of day
• Latitude
• Cloud cover and atmospheric conditions
• Clothing that covers the skin
• Use of sun protection
• Time spent outdoors

Age also plays a role. As we get older, the skin's ability to produce vitamin D from its precursor declines.

This also means that no universal time specification can be given along the lines of: “X minutes of sunshine produce Y micrograms of vitamin D.”

Such a calculation would be too imprecise scientifically. The UVB exposure required depends on individual and environmental factors.

Nor should vitamin D supply be optimised by deliberately spending long periods in the sun without protection. UV radiation is also a relevant risk factor for skin damage.

What happens in Germany in autumn?

Data from the Robert Koch Institute show that vitamin D status in Germany is subject to seasonal fluctuations. Particularly relevant is the period when the body's own production through the skin declines because of reduced UVB radiation.

The effects are not the same for everyone. Those who regularly spend time outdoors in summer can build up larger vitamin D stores than someone who spends most of their time indoors.

Skin type, age and lifestyle also play a role.

This is why autumn does not begin with the same starting point for everyone.

What remains stored in the body over winter

Vitamin D is not completely excreted again immediately after it is produced. Some of it is stored in fatty tissue and muscles. These stores can help to continue supplying the body with vitamin D even when its own production is temporarily lower.

This explains why a deficiency does not necessarily develop immediately when the days become shorter.

Rather, it is a process involving several factors. Someone who had a good vitamin D supply in summer may start winter differently from someone whose vitamin D status was already low beforehand.

Diet can also contribute, although its contribution in Germany is comparatively limited.

The German Nutrition Society points out that a diet consisting of usual foods contributes only a relatively small part of the vitamin D supply. Vitamin D is found primarily in oily fish such as salmon, herring and mackerel. Egg yolks, fortified margarine and some edible mushrooms can also contain vitamin D.

For its vitamin D supply, the body therefore depends to a particularly large extent on its own production through UVB radiation.

Who should pay particular attention to their vitamin D supply

Not everyone in Germany has a vitamin D deficiency in winter. However, there are groups of people in whom an inadequate supply is more likely.

These include, in particular, people who rarely spend time outdoors. This may be the case, for example, with restricted mobility, certain chronic diseases or a daily routine spent predominantly indoors.

Older people are also at greater risk. Firstly, the skin's vitamin D production declines with age. Secondly, older people may, on average, spend less time outdoors.

Other factors may also play a role.

Little sunshine

People who rarely get daylight outdoors have fewer opportunities for the body's own vitamin D synthesis. What matters here is not how bright it appears through a window. Window glass blocks a large proportion of UVB radiation, so sunlight behind a pane of glass cannot be equated with direct UVB exposure.

Clothing that covers the skin

People who cover large areas of their skin for cultural, occupational or personal reasons consequently have less skin surface on which vitamin D can be produced.

Darker skin

Melanin absorbs UV radiation and thereby reduces the production of vitamin D3 in the skin. People with more heavily pigmented skin may therefore need longer UVB exposure under comparable conditions to achieve the same vitamin D synthesis.

This does not mean that a particular skin colour automatically equates to a vitamin D deficiency. What matters is a person's actual living conditions and individual vitamin D supply.

Older age

As we get older, the skin's ability to synthesise vitamin D declines. Changes in lifestyle and mobility may also occur.

Certain diseases and medicines

Diseases that affect nutrient absorption in the intestine, as well as certain medicines, can also impair the vitamin D supply.

Here, the issue is no longer general seasonal supply, but individual medical situations. In such cases, a medical assessment may be useful.

When is vitamin D considered sufficient?

At this point, things become more complicated than they first appear.

The measurement normally taken is 25-hydroxyvitamin D, or 25(OH)D for short. This level is regarded as an established marker of vitamin D supply and is much better suited to assessing vitamin D status than the active form, 1,25-dihydroxyvitamin D.

Blood levels are often expressed in nanomoles per litre or nanograms per millilitre.

The conversion is:

1 ng/ml corresponds to 2.5 nmol/l.

However, what is considered “optimal” depends on the particular question and guideline.

For a long time, relatively fixed thresholds were used, such as 20 ng/ml or 30 ng/ml. However, current evidence shows that such cut-off values cannot be justified equally for every health question.

This is precisely where the Endocrine Society's current 2024 guideline comes in.

It concludes that, for healthy adults without a specific medical indication, there is insufficient evidence to set a particular 25(OH)D level as a universal target for the prevention of various diseases.

This provides important scientific context.

A laboratory value may therefore appear low without this automatically meaning that high-dose supplementation is necessary. Conversely, targeted diagnostic testing and treatment may be useful for certain diseases or combinations of risk factors.

Why laboratory thresholds are not automatically treatment thresholds

A laboratory report often contains reference ranges. Put simply, these describe which values are observed in a particular population or how a laboratory classifies a measurement range.

This is not the same as a clinical target range.

With vitamin D, there is the additional consideration that the significance of a particular 25(OH)D level may depend on the health question involved.

The question “Do I have enough vitamin D?” is therefore less clear-cut medically than many online guides suggest.

For healthy adults, the Endocrine Society's 2024 guideline explicitly recommends no routine screening of 25(OH)D levels in the absence of a corresponding medical indication.

This is particularly relevant because vitamin D tests are now readily available.

A test may be useful if there is a specific reason to check the vitamin D supply. However, current evidence does not support blanket testing of the entire population.

When a vitamin D test may be useful

A vitamin D test may be useful particularly when there is a specific medical reason to check vitamin D status. These reasons may include certain diseases, impaired nutrient absorption or an increased risk of an inadequate supply.

The measurement usually taken is 25-hydroxyvitamin D, or 25(OH)D. This level gives a better indication of vitamin D supply than the biologically active form, 1,25-dihydroxyvitamin D.

For healthy adults without particular risk factors, however, a routine test is not generally necessary. The Endocrine Society does not recommend general screening of vitamin D status in healthy adults without a specific indication.

This is also relevant because a single blood value should not be considered in isolation. Season, time spent outdoors, skin type, diet and individual health factors may influence the level.

When checking may be particularly relevant

Testing may be considered, for example, if there is already a known vitamin D deficiency or if factors are present that make an inadequate supply more likely.

These may include certain diseases that impair the absorption or processing of vitamin D. Medicines can also influence vitamin D metabolism.

For people at increased risk, a medical assessment may therefore be more useful than indiscriminate self-testing.

What a blood value cannot tell you

A vitamin D test is not a general health check.

A good 25(OH)D level does not automatically mean that all other factors affecting bones, muscles or the immune system are optimal. Conversely, a low level does not automatically mean that a vitamin D supplement needs to be taken at the highest possible dose.

What always matters is the medical context.

Which foods provide vitamin D

Unlike many other vitamins, vitamin D has only a limited dietary source. The body can produce vitamin D itself under suitable conditions, while comparatively few foods contain significant amounts.

Among the most important natural sources are oily sea fish. These include salmon, herring and mackerel, for example.

Egg yolks also contain vitamin D, but in much smaller amounts. Depending on the product, fortified foods such as certain margarines may also contribute to the supply.

In mushrooms, vitamin D content depends heavily on the species and conditions. Mushrooms exposed to UV light may contain significantly more vitamin D than ordinary edible mushrooms.

Is diet sufficient in winter?

There is no universal answer to this. However, the German Nutrition Society points out that, under usual conditions, only a relatively small proportion of the vitamin D supply in Germany comes from diet.

This is mainly because vitamin D occurs in relevant quantities in only a few foods.

Regularly eating oily fish can contribute to the supply. However, diet does not automatically replace the body's own production through UVB radiation.

Autumn and winter in particular therefore make it clear why sunshine and diet should be considered together.

Putting vitamin D in foods into perspective

Here too, more is not automatically better.

Although an individual food can contribute to vitamin D supply, it cannot reliably determine whether a person has an adequate supply.

What matters is the overall situation over a longer period.

What supplements actually achieve

When UVB radiation decreases in autumn and winter, taking vitamin D as a precaution seems an obvious idea. However, this does not automatically mean that every healthy adult needs a supplement.

The crucial question is rather who actually benefits from supplementation and at what dose.

Vitamin D performs important functions in calcium and bone metabolism. A pronounced deficiency can impair bone mineralisation. In children, a severe deficiency can cause rickets; in adults, osteomalacia may develop.

In such situations, treating a vitamin D deficiency has clear medical significance.

The situation is different for people who already have an adequate supply.

What large studies show

One of the best-known investigations is the VITAL study. In this randomised, placebo-controlled study, more than 25,000 adults received either 2,000 International Units of vitamin D3 per day or a placebo for around five years.

In the overall group, vitamin D supplementation did not significantly reduce invasive cancers or major cardiovascular events.

This does not mean that vitamin D has no health benefits in general. Rather, the study investigated whether daily supplementation could prevent certain diseases in a large group of predominantly healthy adults.

The result argues against the idea that blanket intake of higher vitamin D doses automatically provides a broad additional health benefit.

Vitamin D and the immune system

Vitamin D receptors are also found in cells of the immune system. In addition, vitamin D influences various processes of gene regulation that are relevant to immune function.

This provides a biological basis for the question of whether vitamin D might protect against infections.

However, the clinical results are more nuanced.

Systematic investigations into the prevention of acute respiratory infections have shown differing results. Possible benefits have sometimes been observed, particularly in people with a low baseline supply. However, this does not support the conclusion that vitamin D supplements provide general protection against infections.

Here too, baseline status and the individual situation are therefore what matters.

Vitamin D3 and K2 compared

Vitamin D3 and vitamin K2 are frequently offered together in food supplements. This is based on a biological link between the two vitamins and calcium metabolism.

Vitamin D influences, among other things, the production of certain proteins involved in calcium balance. Vitamin K, in turn, is needed for certain vitamin K-dependent proteins to fulfil their biological function.

These proteins include osteocalcin and matrix Gla protein.

This link gives rise to the frequently used idea that vitamin K2 ensures that calcium absorbed through vitamin D is specifically directed into the bones and does not accumulate in the blood vessels.

However, this simplified account falls short.

The body's calcium balance is regulated through a complex interaction between various hormones, organs and nutrients. The development of vascular calcification also depends on numerous factors.

The biological link between vitamin D and vitamin K therefore does not automatically mean that healthy people should generally supplement both vitamins together.

There is insufficiently robust evidence for a blanket recommendation of K2 in addition to vitamin D.

When caution is needed with vitamin K

Vitamin K can interact with certain medicines. Particularly relevant are vitamin K antagonists such as warfarin or phenprocoumon.

People taking these medicines should not change their vitamin K intake independently, but should discuss this with a doctor.

The right dosage and potential risks

With vitamin D, a principle applies that can easily be overlooked with food supplements: a higher dose does not automatically mean a greater benefit.

Vitamin D is fat-soluble and can be stored in the body. Persistently very high intake through food supplements can therefore become problematic.

An excessive supply can lead to hypercalcaemia, meaning an excessively high calcium level in the blood.

Possible symptoms include nausea, vomiting, weakness, excessive thirst and frequent urination. A pronounced and prolonged excessive supply may also damage the kidneys and other organs.

Vitamin D poisoning through normal sunlight exposure or a usual diet, however, is not the typical problem. What matters above all is excessive supplementation over the long term.

4,000 International Units is not an intake recommendation

The European Food Safety Authority, EFSA, gives a tolerable upper intake level for adults of 100 micrograms of vitamin D per day, corresponding to 4,000 International Units.

This is an upper safe intake level, not a recommendation to take 4,000 International Units every day.

This is an important distinction.

People already using several food supplements should also consider their total intake. Vitamin D may, for example, be present simultaneously in a multivitamin preparation, a vitamin D-only product and a combination preparation.

The individual amounts may seem moderate at first glance. Together, however, they can add up to a much higher daily intake.

Practical considerations in autumn

Seasonally reduced UVB radiation does not automatically mean that everyone needs to supplement vitamin D in autumn.

It is more useful to consider your own situation step by step.

1. Consider your daily time outdoors

Those who regularly spend time outdoors have a different starting point from someone who spends most of the day indoors. Vitamin D supply should not be optimised at the expense of sensible sun protection.

2. Take personal risk factors into account

Age, skin pigmentation, mobility, diet, certain diseases and medicines can influence vitamin D supply.

3. Do not automatically regard a test as compulsory

For healthy adults without a specific medical question, the Endocrine Society does not recommend routine screening.

4. Consider total vitamin D intake

As well as the body's own production, foods and food supplements form part of the overall picture.

5. Take targeted action in the event of an actual deficiency

If a deficiency has been identified or a particular risk situation exists, targeted supplementation may be useful. The appropriate dose depends on the individual situation.

The aim should therefore not be to keep vitamin D levels as high as possible in autumn. It is more useful to ensure an adequate supply and avoid unnecessarily high doses.

Vitamin D and healthy ageing

Vitamin D is frequently discussed in connection with healthy ageing and longevity. It is important here to distinguish between different levels of scientific evidence.

There is no question that vitamin D is important for calcium and phosphate metabolism. Vitamin D also plays a role in the normal function of muscles and bones. A pronounced deficiency can therefore have health consequences.

Less clear, however, is whether additional vitamin D intake beyond an adequate supply influences ageing itself.

Observational studies frequently show that people with low vitamin D levels have various chronic diseases more often. However, this does not automatically mean that a low vitamin D level causes these diseases.

Part of the association may arise because people with health problems spend less time outdoors, exercise less or eat differently.

Between biological plausibility and clinical benefit

Vitamin D influences numerous biological processes. This makes it scientifically interesting, but alone says nothing about the effect of additional intake on health.

What matters is clinical studies that investigate whether an intervention actually changes relevant health outcomes.

So far, there is no convincing basis for assuming that healthy adults can generally live longer or substantially slow ageing by consuming as much vitamin D as possible.

For healthy ageing, the focus is therefore less on maximum vitamin D levels than on an appropriate supply and avoiding a pronounced deficiency.

An overview of the key findings

Vitamin D supply changes in autumn because UVB radiation in Germany decreases. However, this does not mean that everyone automatically develops a vitamin D deficiency.

How well the supply is maintained throughout the colder months depends, among other things, on individual vitamin D stores, time spent outdoors, age, skin pigmentation, diet and health factors.

The most important laboratory parameter is 25-hydroxyvitamin D. However, a single blood value should always be considered in the context of the individual situation.

For healthy adults without particular risk factors, current evidence does not recommend routine vitamin D screening or blanket intake of high doses.

The situation may be different with a proven deficiency or certain risk factors. Targeted diagnostic testing and supplementation may then be medically useful.

The most important practical question in autumn is therefore not: “What is the maximum amount of vitamin D I can take?”

But rather: What is my personal vitamin D supply like, and is there a specific reason to change it?

References

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