Vitamin D deficiency in winter: causes and solutions
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Time to read 11 min
Commuting in the dark in the morning, eight hours in the office, and back home in the dark in the evening: this is what a typical January day looks like for many working people. Even those who go for walks on the weekend usually only show their face and hands in the cold. In spring, the irritating question often follows as to why fatigue, muscle weakness, or vague symptoms do not disappear despite a seemingly "healthy lifestyle".
This does not necessarily have to be caused by a nutrient deficiency – but a vitamin D deficiency in winter is quite plausible in Germany. The crucial point is not just how much time someone spends outdoors. The sun's position, skin surface area, skin type, age, and how well the body's own stores were filled in summer are decisive.
This article explains why vitamin D production drops off seasonally, how to interpret blood levels, and when diet, testing, or supplementation actually makes sense.
Note: This article is for general health information and does not replace an individual medical diagnosis or treatment.
Table of Contents
How the body produces vitamin D – and why winter interferes Why some people develop particularly low levels Symptoms, threshold values, and typical misconceptions When medical clarification is particularly useful What practically helps with vitamin D deficiency in winter – without premature megadoses Conclusion FAQ Read more
In short: correctly interpreting vitamin D deficiency in winter
UV-B radiation is often too weak in Germany in winter for significant vitamin D production in the skin.
The relevant laboratory value is called 25-hydroxyvitamin D, or 25(OH)D for short.
Fatigue, susceptibility to infections, or low mood are non-specific and do not prove a deficiency.
People with darker skin, older age, or low outdoor activity have an increased risk.
Window glass lets visible light through but blocks most of the UV-B radiation required for vitamin D production.
A single low value must be assessed based on the season, symptoms, illnesses, and medications.
The DGE states 20 micrograms per day or 800 IU as a reference value in the absence of endogenous production – not as a blanket therapy dose.
High-dose preparations should not be taken without medical assessment.
How the body produces vitamin D – and why winter interferes
Vitamin D holds a special place among the vitamins: the body can produce it itself. When UV-B radiation hits uncovered skin, provitamin D3 is initially formed. The liver and kidneys then convert it into biologically usable forms.
The following points are particularly important for the production and assessment of vitamin D:
UV-B radiation: It is necessary for the skin to be able to produce vitamin D.
Liver and kidneys: They convert precursors into biologically usable forms.
25(OH)D: This blood value is predominantly used to assess vitamin D status because it better reflects supply over a longer period.
Season: The sun's position significantly influences how much UV-B radiation actually reaches the skin.
Between approximately October and March, the sun stands so low in Germany that the atmosphere filters out a considerable part of the UV-B radiation. A bright winter day is therefore by no means useless – daylight is important for the day-night rhythm, for example –, but the UV-B radiation may still be too low for sufficient vitamin D production.
Even behind a sunny office window, hardly any vitamin D is produced. Window glass lets visible light through, but blocks a large part of the UV-B radiation required for vitamin D production.
A large European analysis by Cashman and other research teams highlights the seasonal differences. Standardized data from 55,844 people from 14 European studies were analyzed. Values below 30 nmol/l were significantly more common in the winter half of the year than in the summer half.
The results can be boiled down to three key points for practice:
Vitamin D levels fluctuate seasonally.
Low levels occur more frequently in winter than in summer.
The timing of the blood sample should be considered when interpreting a 25(OH)D value.
The study thus shows one thing above all: a vitamin D blood value should not be viewed completely independently of the season, individual risk factors, and prior sun exposure.
Source: Cashman KD et al. Vitamin D deficiency in Europe: pandemic? American Journal of Clinical Nutrition, 2016.
https://pubmed.ncbi.nlm.nih.gov/26864360/
Why some people develop particularly low levels
A vitamin D deficiency in winter is rarely caused by a single factor. The combination of low UV-B radiation and individual circumstances is decisive. Older skin produces less vitamin D than young skin. Stronger pigmentation reduces vitamin D production under comparable UV-B conditions, so the same production may require longer or more intense exposure.
Additional factors include shift work, the need for care, consistently covering clothing, obesity, as well as certain diseases of the bowel, liver, or kidneys.
What German population data actually shows
A study by Rabenberg and other researchers based on the nationwide DEGS1 survey shows that low vitamin D levels are not an isolated phenomenon in Germany. 6,995 adults between 18 and 79 years oldwere examined. Around 30 percent were below 30 nmol/l; more than 60 percent did not reach 50 nmol/l.
Important for interpretation: Such population data describe the distribution of certain laboratory values. They do not mean that more than 60 percent of the population are automatically sick, have symptoms, or absolutely must be treated. This very distinction is crucial when making statements about a supposedly widespread "vitamin D deficiency".
Source: Rabenberg M et al., Study on the vitamin D status of adults in Germany based on DEGS1.
https://pubmed.ncbi.nlm.nih.gov/25886941/
An often overlooked connection is body weight: vitamin D is fat-soluble and is partially distributed in the fatty tissue. Obesity can therefore lead to lower 25(OH)D concentrations in the blood.
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Complaints, threshold values and typical misconceptions
Mild deficiencies often go unnoticed. In contrast, pronounced or long-standing deficiencies can promote muscle weakness, bone pain and impaired bone mineralization. In adults, the resulting softening of the bones is called osteomalacia.
Fatigue, concentration problems, mood swings or frequent infections, on the other hand, are so non-specific that they can have numerous other causes – for example, sleep deprivation, iron deficiency, thyroid diseases or psychological stress. Such symptoms alone are therefore not sufficient to diagnose a vitamin D deficiency.
Why a low mood is not proof of vitamin D deficiency
Winter blues are often explained directly by low vitamin D levels. This is too simplistic: visible morning light influences the internal clock and melatonin regulation, even though it does not need to provide enough UVB radiation for significant vitamin D production. Light deficiency and low vitamin D levels can therefore occur simultaneously, but they are not the same mechanism.
Supplementation should therefore not automatically be understood as treatment for so-called winter depression. In cases of prolonged low mood, significant psychological symptoms or considerable restrictions in everyday life, medical or psychotherapeutic clarification should be sought.
Even in pharmaceutical advice – for example at the Mari Jane Pharmacy (https://mari-jane.de/) – the question "Which preparation?" should therefore not be the sole focus. Pharmacy teams can, among other things, help check already taken preparations for possible double dosing, explain dosage instructions clearly and select a suitable form of intake.
The confusion between micrograms and International Units, in particular, easily leads to misjudgments in everyday life: 20 micrograms correspond to 800 IU.
However, pharmaceutical advice does not replace a medical diagnosis. The assessment of complex laboratory results, the clarification of possible illnesses and treatment decisions in cases of relevant pre-existing conditions or pronounced symptoms belong in the hands of medical professionals.
When medical clarification is particularly useful
A vitamin D deficiency in winter should be medically clarified if there are symptoms or risk factors that go beyond a non-specific winter slump.
Medical clarification is particularly useful, for example, in cases of:
pronounced muscle weakness
persistent bone pain
repeated falls
bone fractures without adequate cause
chronic inflammatory bowel diseases
severe liver or kidney diseases
post-bariatric surgery
long-term intake of certain medications, such as some antiepileptics or glucocorticoids
These factors can affect bone and vitamin D metabolism and make the medical assessment of a possible deficiency more important.
Routine screenings without risk factors are not automatically useful. The laboratory value should always be assessed in connection with symptoms, pre-existing conditions, medications and the individual situation. Measurement methods and the reference ranges used can also differ.
The opposite is also important: a persistently significantly too high vitamin D intake can lead to elevated calcium levels in the blood. Possible symptoms of such oversupply can include:
nausea
intense thirst
frequent urination
weakness
in severe cases, also confusion or heart rhythm disturbances
In cases of severe symptoms such as heart rhythm disturbances, pronounced confusion, impaired consciousness or a significantly worsened general condition after taking high doses, medical help should be sought immediately.
In such a case, it is not enough to simply adjust the continued intake on your own.
What practically helps with vitamin D deficiency in winter – without hasty megadoses
The first step is a realistic assessment: How much time was spent outdoors regularly in summer? Are there risk factors or relevant symptoms? Are several dietary supplements or medications containing vitamin D already being taken?
For a personal assessment, the following points are particularly relevant:
Sun exposure: How regularly was the skin exposed to the sun in spring and summer?
Risk factors: Are there, for example, advanced age, low outdoor activity, or certain pre-existing conditions?
Symptoms: Are persistent muscle weakness, bone pain, or other noticeable symptoms occurring?
Preparations already taken: Do other dietary supplements or medications possibly also contain vitamin D?
If you are merely experiencing a non-specific winter slump, a blood test is not automatically necessary. In cases of increased risk or medically relevant symptoms, determining 25(OH)D can, on the other hand, help to assess supply more specifically.
Vitamin D can only be supplied in limited amounts through food. Foods that contain comparatively relevant amounts include:
fatty fish such as herring, salmon and mackerel
egg yolk
certain mushrooms
The German Nutrition Society states a reference value of 20 micrograms or 800 IU daily in cases of completely absent endogenous production. This value is an estimate for an adequate supply assuming no endogenous synthesis and is not an individual therapy dose.
Source: German Nutrition Society (DGE), Reference Values for Vitamin D.
https://www.dge.de/wissenschaft/referenzwerte/vitamin-d/
Large intervention studies also show why vitamin D should not be understood as a universal protective agent. In the VITAL study, 25,871 predominantly healthy adults received vitamin D3 or a placebo. Supplementation did not significantly lower the investigated overall numbers of invasive cancers and severe cardiovascular events.
For practice, this means: The targeted treatment of a confirmed deficiency is something different from the assumption that high doses of vitamin D fundamentally make every healthy person even healthier. Therefore, the following are particularly important:
an actual individual need
an adequate dosage
consideration of preparations already taken
in cases of increased risk, a medical or pharmaceutical assessment
Source: Manson JE et al. Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. New England Journal of Medicine, 2019.
https://pubmed.ncbi.nlm.nih.gov/30415629/
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Conclusion
A vitamin D deficiency in winter is biologically well explainable in Germany: The low sun position limits the UVB radiation available for endogenous vitamin D production, while modern work and lifestyle habits can additionally reduce skin exposure.
However, personal risk does not depend solely on the season. Other factors include:
Skin type
Age
Body weight
Pre-existing conditions
Medications taken
the vitamin D reserves built up in summer
Non-specific fatigue alone justifies neither self-diagnosis nor high-dose preparations. In cases of risk factors or persistent symptoms, a medically correctly assessed 25(OH)D value can provide more clarity.
For a meaningful vitamin D supply, three points are particularly important:
the best possible basic supply through everyday life and nutrition
a needs-based supplementation, when it makes sense
avoiding uncontrolled megadoses
Megadoses without medical supervision offer no proven general additional benefit and can cause health problems in the event of an overdose.
The goal is therefore not the highest possible laboratory value, but a needs-based, safe and individually appropriate vitamin D supply.
Sources & further information
Cashman KD et al. Vitamin D deficiency in Europe: pandemic? American Journal of Clinical Nutrition, 2016.
https://pubmed.ncbi.nlm.nih.gov/26864360/Rabenberg M et al. Study on the vitamin D status of adults in Germany based on the DEGS1 survey.
https://pubmed.ncbi.nlm.nih.gov/25886941/Manson JE et al. Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. New England Journal of Medicine, 2019.
https://pubmed.ncbi.nlm.nih.gov/30415629/German Nutrition Society (DGE): Reference values for vitamin D.
https://www.dge.de/wissenschaft/referenzwerte/vitamin-d/Robert Koch Institute (RKI): Information and population data on vitamin D status in Germany.
Editorial note: The content was created based on the current state of the sources provided. It is intended for general information purposes and does not replace an individual medical diagnosis or medical treatment. If you have questions about preparations, forms of intake, or possible double dosing, a pharmacy can assist; the diagnosis and treatment of complex medical causes are in the hands of medical professionals.
Silas Kühl - Author at Canasups
Author at Canasups and has been working centrally in the development of the brand and Grünschwert GmbH for several years. He is intensively involved in the connections between consumption behavior, sleep, regeneration, and functional supplements in the context of modern cannabis culture.
Through his many years of experience in the company as well as his continuous engagement with scientific studies, user experiences, and market trends, he has a sound understanding of the challenges that many consumers face in everyday life. His content combines current research with practical insights and easy-to-understand explanations.
His focus is on presenting complex topics transparently, with nuance, and without exaggeration. Education, personal responsibility, and mindful use are at the center.
FAQ
When is it considered a vitamin D deficiency?
A 25(OH)D value below 30 nmol/l or 12 ng/ml is often classified as inadequate supply. Values between 30 and 50 nmol/l are assessed differently depending on the definition used and the individual situation. Laboratory values, symptoms, and risk factors should therefore be considered together.
Can you produce vitamin D in winter through sunlight?
In Germany, UV-B radiation between approximately October and March is often insufficient for significant endogenous vitamin D production. Spending time outdoors still makes sense, for example for exercise and the day-night rhythm. Virtually no relevant vitamin D production takes place through the skin behind window glass.
What symptoms does a vitamin D deficiency cause in winter?
A mild deficiency often causes no clear symptoms. In cases of pronounced deficiency, muscle weakness, diffuse bone pain, and osteomalacia in the long term can occur. Fatigue or low mood alone are not reliable diagnostic indicators.
Should everyone take vitamin D in winter?
No, a blanket supplementation at the same dosage is not necessary for everyone. Age, diet, sun exposure, skin type, illnesses, and medications can influence the individual situation. In cases of increased risk or uncertainty, the dosage should be assessed by a medical professional or pharmacist.
When should vitamin D levels be measured?
A measurement can make sense in cases of relevant risk factors, bone diseases, noticeable muscle weakness, or suspected malabsorption. A baseline value can also be helpful for medical assessment before starting long-term high-dose intake. For healthy people without symptoms or special risk factors, a routine test is not generally recommended
