From September onwards it gets dark earlier, the midday sun stands lower, and the question comes up again: should I take vitamin D now? Vitamin D is the only vitamin the body can produce itself — but only if the sunlight reaching the skin is intense enough. And that is exactly what is lacking in the Swiss winter half-year. Autumn is therefore the right time to clarify your own status instead of reaching for a high-dose capsule on suspicion in January. This article shows why the body's own production practically stops in winter, who is particularly at risk, how it is measured and what sensible supplementation looks like.
Why the body barely produces vitamin D in the winter half-year
Vitamin D is formed in the skin from cholesterol — provided the sunlight reaching the skin is intense enough. That is precisely the crux. Its formation requires UV-B radiation, and that only reaches the earth's surface when the sun stands high enough. At Switzerland's latitude, the sun stands so low in the winter half-year that most of the UV-B radiation is already filtered out in the atmosphere. From roughly October to March, the body's own production therefore runs on a very low flame, no matter how often you are outdoors.
On top of that come factors that slow things down in summer too: time of day, cloud cover, clothing, sun protection, darker skin types and age — the skin's ability to produce vitamin D declines over the years. Window glass does not help; UV-B does not pass through it. Anyone who wants to stay active outdoors in winter will find the arguments in the article Endurance training in winter — indoors or outdoors? and in the article on outdoor training. You should not count on vitamin D production between October and March, however.
The store: why autumn of all seasons counts
Vitamin D is one of the fat-soluble vitamins (A, D, E and K). This group is stored better by the body than the water-soluble vitamins and can only be absorbed in combination with fat. Vitamins do not have to be taken in daily anyway — a balanced intake across a week is enough.
For vitamin D, this storage capacity means: what you have built up over the summer is slowly used up over autumn and winter. That is why early autumn is the informative moment for a measurement — the value shows how well you came out of the sunny season. That fat is needed for absorption incidentally argues against reflexively avoiding fat; how this reflex arose is described in the article Where does the fear of fat come from?.
What the body needs vitamin D for
Vitamins themselves supply no energy, but they influence almost all processes in the body and are of great importance for metabolism. With vitamin D, three areas come to the fore:
- Bones and teeth. Vitamin D regulates calcium balance. Calcium in turn is a macromineral that serves to harden bones and teeth — without enough vitamin D, even the best calcium intake is of little use.
- Muscles. Non-specific muscle weakness is among the complaints reported early on when supply is poor.
- Immune system. Vitamin D is involved in regulating immune processes — which does not make it protection against colds. If you are ill, take a break, as the article Training with the flu explains.
For bone, incidentally, the most effective stimulus remains a mechanical one: loading. What strength training achieves for bone density is shown in the article Strength training for strong bones. Vitamin D is the building-materials manager, training is the order.
Who is particularly at risk
An increased risk of an inadequate micronutrient supply is carried by people with a one-sided diet, with impaired stomach or bowel function, and by groups with an increased requirement: children, older people and pregnant women. For older people the statement in nutrition training is particularly clear: that older people should supplement vitamin D during the winter months is part of the examinable material there. The reasons are the skin's reduced ability to produce it, less time outdoors and often a lower energy intake.
Why this weighs so heavily in old age is shown by the chain behind it: poorer supply, weaker bones, higher risk of falls. How it can be interrupted at several points is described in the articles on balance training as fall prevention and on training in old age.
A second group is also on the list: anyone eating a vegetarian or vegan diet is better off keeping an eye on vitamin D. In sports nutrition it counts among the critical nutrients of these dietary patterns — alongside omega-3 fatty acids, protein, iron, calcium, zinc, iodine, vitamin B2 and vitamin B12. What matters is planning good alternatives for the foods left out, as the article on vegetarian nutrition explains. On another of these nutrients, the article on omega-3 fatty acids provides the background.
How it is measured
What is determined in the blood is 25-hydroxyvitamin D, 25(OH)D for short — the storage form, which reflects the supply status of the last few weeks. The active form is not suitable for this because it is short-lived and tightly regulated. The test is a simple blood draw at the doctor's practice; the interpretation belongs there too, because what follows from the value depends on your history, medication and accompanying conditions.
Why measure at all instead of simply taking something? Because the early signs of an inadequate supply are entirely non-specific: tiredness, poor concentration, dizziness. They fit dozens of causes — from too little sleep and too little food to iron deficiency or a thyroid disorder. Anyone who attributes them to vitamin D without checking may be treating the wrong thing. The winter mood dip, too, has more than one explanation; the part that exercise plays in it is shown in the article Exercise and mental health.
Food sources are limited
Unlike with most vitamins, the requirement can hardly be covered through food alone. Appreciable amounts are supplied by:
- Oily sea fish such as salmon, herring and mackerel
- Egg yolk
- Edible mushrooms, especially those exposed to daylight
- Fortified products such as some margarines and plant drinks
Vitamins react sensitively to water, heat and light, which is why shopping, storage and preparation play a role. Realistically, though, this remains true: with vitamin D, diet is a contribution, not the solution.
Supplementation in sensible measure
In principle it is assumed that a healthy and varied diet supplies enough vitamins and minerals, as long as energy requirements are also met. Vitamin D is the well-known exception to this rule, because its main source is not the plate but the sun. At the same time there are indications that vitamin supplements can lead to lower disease risks even with a balanced diet.
That is not a licence for high self-chosen doses, however. For vitamins and minerals there is a safe range between the minimum needed and the maximum tolerable dose — the "Upper Level". If it is exceeded, negative health consequences are possible, and with supplements in particular this does happen. Hypervitaminosis is especially relevant with fat-soluble vitamins because of their storage capacity.
Hence the pragmatic route: have it measured in autumn, have the result interpreted by a doctor and have any dosage set there — instead of adopting capsule strengths from the internet. With existing illnesses, during pregnancy or when taking medication, a medical assessment is mandatory in any case.
Course tip from update Akademie: Do you want to answer micronutrient questions like these on a sound professional basis instead of reacting to advertising promises? The Ernährungscoach programme at update Akademie covers vitamins, minerals, reference values and the limits of supplementation — including the question of when a situation belongs in medical hands.
Frequently asked questions about vitamin D in autumn
Why does the body barely produce vitamin D in winter?
Because vitamin D is only formed in the skin from cholesterol when the sunlight is intense enough. In the Swiss winter half-year the sun stands so low that hardly any UV-B radiation reaches the skin — the body's own production practically comes to a standstill.
When is the best time for a measurement?
Early autumn, because the value then shows what level of stores you are coming out of the sunny season with. What is determined is 25-hydroxyvitamin D in the blood; the interpretation belongs at the doctor's practice.
Can I cover my requirement through diet?
Only to a small extent. Appreciable amounts are supplied by oily sea fish, egg yolk, mushrooms and fortified products. As vitamin D is fat-soluble, absorption also requires fat in the meal.
Who should be particularly attentive in winter?
The risk groups for an inadequate micronutrient supply include children, older people and pregnant women. For older people, supplementation during the winter months is considered sensible. With a vegetarian and vegan diet, too, vitamin D is one of the critical nutrients.
Can you overdose on vitamin D?
Yes. Above the so-called Upper Level, negative health consequences are possible, and because fat-soluble vitamins are stored, an excessive supply is particularly relevant here. The dosage therefore belongs in the hands of a doctor and should not be chosen on your own.
Conclusion
Vitamin D is a special case in the Swiss winter half-year: the most important source disappears for around half a year, and diet only partly makes up for it. That is why autumn is the right moment to have your status determined — especially for older people, for children, during pregnancy and with a purely plant-based diet. What comes next is decided by the measured value together with your doctor, not by the package leaflet. And regardless of the laboratory value, the strongest stimulus for strong bones remains what you can do yourself: train regularly and keep moving.
