Vitamin D Deficiency: Why the Sun Alone Is Not Enough in Winter
Between October and March, the sun in Germany sits too low to produce vitamin D in your skin. No matter how long you stay outside. Here you will learn what that means for your level.
It is February. Outside it is bright, even sunny. You went for a walk, felt the light on your face. And yet you feel tired, heavy, as if behind frosted glass. Maybe you have heard that a bit of winter sun is enough for vitamin D. Physically, in our latitudes, that is unfortunately not true. And this one fact changes the whole way you look at vitamin D deficiency.
Hardly any topic is brushed off with half a sentence as often as vitamin D. Fifteen minutes of sun and you are fine. For Berlin's latitude, in winter, that is simply wrong. Instead of a blanket rule, I want to give you the logic behind it, so you can understand and interpret your own value.
Why the winter sun in Berlin is not enough
Strictly speaking, vitamin D is not a vitamin but a prohormone. Your body makes it itself when a very specific part of sunlight hits the skin: UVB radiation. In the skin it splits a precursor, from which, over several steps, vitamin D3 is formed. Without UVB, no production. That is how simple the first part is.
The second part is the decisive one. UVB reaches the ground only when the sun stands high enough in the sky. When it stands too low, the atmosphere absorbs almost all of the UVB, while the visible light passes through. This is why a winter day can be brilliantly bright and still deliver next to nothing for vitamin D production. It is not about brightness. It is about the position of the sun.
I know this pattern very well. Imagine a woman in her early forties, working in an office, who comes to me in February. Her opening line: "I am simply tired all the time, and I do not understand why. I did see a doctor, and everything was normal."
In our conversation, it stood out that vitamin D had never been measured in her previous check-ups. Only the targeted measurement of the 25-OH value made it visible that it was clearly in the deficient range. We interpreted the value, looked at her diet and daily routine, and planned the further support together.
Over the weeks, she described more stability in the afternoon. I cannot claim any causality from this, and I only document the temporal association. Tiredness is complex, and a single value rarely explains everything. Still, the lesson for me stays clear: what is never measured cannot be interpreted.
Just how much the position of the sun really counts was shown early and precisely by research. There is even a term for it: the vitamin D winter. It describes the months in which cutaneous production practically comes to a standstill.
A classic paper by Webb, Kline, and Holick examined in a skin model when sunlight can produce vitamin D3 at all. In Boston, at 42 degrees north, no previtamin D3 formed from November to February. In Edmonton, at 52 degrees north, this dead period ran from October to March.
What this means for you: Berlin sits at around 52 degrees north, so in the Edmonton category. For half the year, the sun here is barely usable for vitamin D production.
Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3. J Clin Endocrinol Metab. 1988;67(2):373-378. DOI: 10.1210/jcem-67-2-373A review summarizes the factors that govern vitamin D production. Its conclusion for mid and high latitudes: in deep winter, cutaneous synthesis is strongly inhibited, because UVB then barely reaches the ground. Diet and intake gain importance during this time.
What this means for you: In winter, it is not the time spent in the sun that limits you, but the amount of UVB that arrives at all. Staying outside longer changes little about this basic problem.
Engelsen O. The relationship between ultraviolet radiation exposure and vitamin D status. Nutrients. 2010;2(5):482-495. DOI: 10.3390/nu2050482The advice of 15 minutes of sun daily is sensible in summer and misleading in winter. It confuses two different situations. In summer the sun stands high, UVB arrives, and brief midday sun can contribute a lot. In winter the UVB is missing, no matter how long you show your skin. So good advice would need to ask about the season, not the clock.
And now you know why a bright winter day does not refill your vitamin D stores. It is not the light that decides, but the angle at which it arrives.
How many people really have too little
Many people with persistent tiredness think their case is the exception. With vitamin D in winter, the opposite is more often true. When the body's own production almost stops for half a year and diet supplies only a little, the level drops in very many people. The figures for Germany are clear.
In a large health survey, the Robert Koch Institute measured the vitamin D levels of nearly 7,000 adults. About 62 percent were below 50 nmol/l, and around 30 percent were even below 30 nmol/l. In winter, a quarter of people had values below 30 nmol/l, with a clear north-south gradient.
What this means for you: In Germany, a low winter value is not the rare special case, but rather the rule. With this pattern, you are in large company.
Rabenberg M et al. Vitamin D status among adults in Germany, results from DEGS1. BMC Public Health. 2015;15:641. DOI: 10.1186/s12889-015-2016-7A large European analysis standardized the measurements from eighteen population groups so they became comparable. On the yearly average, 13 percent were below 30 nmol/l. In the extended winter, October to March, it was 17.7 percent, in summer only 8.3 percent.
What this means for you: The deficiency is strongly seasonal. Exactly in the winter half of the year, when your body can produce nothing, the values across Europe slide downward.
Cashman KD et al. Vitamin D deficiency in Europe: pandemic? Am J Clin Nutr. 2016;103(4):1033-1044. DOI: 10.3945/ajcn.115.120873Maybe you are now thinking: then people in the sunny south must be perfectly supplied. Interestingly, that is not consistently true either. Lifestyle, skin protection, and indoor life all play into it.
A systematic overview of 107 studies with more than 600,000 people found a frequent vitamin D deficiency in southern Europe too, despite plenty of sun. Indoor life, clothing, and sun protection reduce production, even where UVB would be available.
What this means for you: Sun is a possibility, not a guarantee. A good status comes from the position of the sun, from behavior, and, in winter, from intake.
Manios Y et al. A systematic review of vitamin D status in southern European countries. Eur J Nutr. 2017;57(6):2001-2036. DOI: 10.1007/s00394-017-1564-2And now you know why your value in February is probably lower than in August. That is not a personal weakness, that is geography and season.
The key point: a reference range is not the same as a target value
Here lies the second thread. Many people are told their vitamin D is within the reference range and still do not feel well. The reason is a subtle but important distinction. A laboratory threshold marks the point at which a deficiency is certain. It does not say when your body is optimally supplied. These are two different questions.
For orientation, a look at the numbers. Vitamin D is given in two units, nmol/l and ng/ml. A value in ng/ml times 2.5 gives roughly the value in nmol/l.
| 25-OH vitamin D | in ng/ml | How it is usually interpreted |
|---|---|---|
| below 30 nmol/l | below 12 | Severe deficiency. Here the supply is clearly too low. |
| 30 to 50 nmol/l | 12 to 20 | Insufficient, a gray area. Not yet sufficient for the bones. |
| from 50 nmol/l | from 20 | Defined as sufficient for bone health, broad professional consensus. |
| from 75 nmol/l | from 30 | Discussed by part of the professional community as a functional target range. A clinical interpretation, not a fixed number. |
The Endocrine Society guideline defines a deficiency as a value below 20 ng/ml and an insufficiency between 21 and 29 ng/ml. It expressly recommends measuring 25-OH vitamin D with a reliable laboratory test in people at risk before treating.
What this means for you: The reference range is a safety line against deficiency, not a promise of optimal well-being. This is why it is worth looking at the specific number.
Holick MF et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911-1930. DOI: 10.1210/jc.2011-0385Where exactly the best target value lies is, honestly, not yet conclusively settled. Part of the professional community considers higher values favorable, another part stays cautious. Here it is important to separate: what is solidly proven and what remains clinical judgment.
A review on the immunological role of vitamin D describes that many experts aim for at least 30 ng/ml, and in part for 40 to 60 ng/ml. At the same time, it stresses a large individual spread, because people respond genetically differently to the same intake.
What this means for you: There is no single correct value for everyone. A functional target range is a guide that should be interpreted individually.
Charoenngam N, Holick MF. Immunologic effects of vitamin D on human health and disease. Nutrients. 2020;12(7):2097. DOI: 10.3390/nu12072097Proven by consensus is: below 50 nmol/l is too little. Discussed clinically, but not yet proven with hard endpoints, is a functional target range from about 75 nmol/l. I describe this as guidance, not as a proven target mark. This is exactly the distinction I want to make transparent for you.
By the way, this pattern is not limited to vitamin D. With functional iron deficiency despite normal values or with functional thyroid underactivity, the same question comes up: is a value within the reference range enough for you to feel truly resilient?
And now you know why within the reference range and well supplied do not have to be the same thing.
Why the blood test comes before supplementation
In winter, many people simply reach for a supplement, often at a randomly chosen dose, without ever knowing their baseline value. I understand the impulse. But without measuring, you are in the dark. You know neither how low your stores really are, nor whether and how much intake would make sense.
There are three good reasons to measure first. First, the baseline value varies widely, from severe deficiency to just about sufficient. Second, people respond genetically differently to the same amount, as the immunology review above shows. Third, vitamin D is fat-soluble and can accumulate, which is why very high, uncontrolled doses over a long time are not harmless.
Supplementing without measuring is like filling up without knowing the fuel gauge. Maybe the tank is nearly empty, maybe it is already half full. The 25-OH value is your fuel gauge. It turns a guessing game into a decision. This is exactly why the guideline names measurement as the first step in people at risk.
And now you know why the first sensible action is not swallowing a capsule, but a single blood draw.
What a good status has to do with the immune system and mood
Vitamin D is far from being only a bone topic. Seen through the KPNI lens, it is a signaling molecule that reaches into several systems. Best proven is the connection to the immune system, because immune cells carry the receptor for the active form of vitamin D.
A large meta-analysis evaluated the individual data from 25 randomized trials. Vitamin D intake slightly lowered the overall risk of acute respiratory infections. The protection was clearest in people with a very low baseline value below 25 nmol/l and with daily or weekly dosing.
What this means for you: Especially those who are deep in deficiency in winter may benefit most from a good status. The effect is real, but moderate.
Martineau AR et al. Vitamin D supplementation to prevent acute respiratory tract infections: meta-analysis of individual participant data. BMJ. 2017;356:i6583. DOI: 10.1136/bmj.i6583An updated meta-analysis of 46 randomized trials confirmed a small protective effect against respiratory infections. It showed most readily with daily, moderate doses over a limited period.
What this means for you: Vitamin D is one building block of your defenses, not a shield. It can support the foundation, but it does not replace sleep, movement, and nutrition.
Jolliffe DA et al. Vitamin D supplementation to prevent acute respiratory infections: updated meta-analysis. Lancet Diabetes Endocrinol. 2021;9(5):276-292. DOI: 10.1016/S2213-8587(21)00051-6And mood, especially in the dark winter? Here I have to honestly separate two levels, because the evidence is contradictory. Otherwise I would promise you more than the science supports.
A meta-analysis of observational studies found lower vitamin D levels in people with depression. In the cohort studies, the risk of later depression was clearly elevated in the lowest-value group.
What this means for you: There is a connection. Whether low vitamin D helps cause the low mood or rather accompanies it, a study like this cannot settle.
Anglin RES et al. Vitamin D deficiency and depression in adults: systematic review and meta-analysis. Br J Psychiatry. 2013;202:100-107. DOI: 10.1192/bjp.bp.111.106666A very large randomized trial tested whether daily vitamin D over a good five years can prevent depression. The result: no difference between vitamin D and placebo, neither in frequency nor in mood scores.
What this means for you: Vitamin D is not an antidepressant. A good status belongs to the winter foundation, but it is not a treatment for depression. Anyone suffering from genuine low mood deserves real help.
Okereke OI et al. Effect of long-term vitamin D3 supplementation vs placebo on risk of depression (VITAL-DEP). JAMA. 2020;324(5):471-480. DOI: 10.1001/jama.2020.10224It is tempting to look to vitamin D for the simple explanation of every winter heaviness. A more honest view is different: a deficiency can contribute to tiredness and susceptibility to infection, and it is easy to check. For mood, it is a possible player, not a proven cause. This modesty protects you from false hopes and from overlooking other reasons.
And now you know why I am cautiously optimistic about the immune system and deliberately reserved about mood.
Three levers for winter
Finally, the practical part. No recipes with dose and sequence, because that belongs in individual consultation. But three clear directions you can take with you right away.
What makes sense to do now
- Have the value measured. A single 25-OH vitamin D value, ideally in the winter half of the year, shows you your starting point. Without this number, everything stays guesswork.
- Interpret the number instead of just reading it. Do not only ask whether the value is within the reference range, but where in the range. This interpretation belongs in medical hands, together with your symptoms and your history.
- Think of intake in winter, of the sun in summer. From October to March, your body produces almost nothing. Whether and how much you then take in is decided by the measured value, not by gut feeling.
In my work at ViveCura in Berlin, I almost never look at vitamin D in isolation. It sits at the intersection of energy and exhaustion, immune status, and hormonal balance. A low value is rarely the only answer, but often a building block that was overlooked for a long time.
The sun alone is not enough in winter, because in Berlin it sits too low for half the year. The reference range is often not enough, because it rules out a deficiency but promises no optimum. And the capsule does not come first, the measured value does. Measure first, then interpret, then act.
Frequently asked questions
Why is the sun in winter not enough for vitamin D?
Vitamin D3 is produced in the skin only under UVB radiation, and UVB reaches the ground only above a certain solar elevation. In Germany, the sun sits too low from October to March, so almost no vitamin D is produced. At Berlin's latitude of around 52 degrees north, this vitamin D winter lasts about half a year. How long you stay outside changes little, because the amount of UVB itself is too small.
At what level is it called a vitamin D deficiency?
A severe deficiency is usually below 30 nmol/l, that is below 12 ng/ml. The range of 30 to 50 nmol/l, which is 12 to 20 ng/ml, counts as insufficient. From 50 nmol/l, that is 20 ng/ml, supply is defined as sufficient for the bones. Many experts consider a higher functional range sensible, but that is a guide and not a fixed number for everyone.
What is a functional target value for vitamin D?
The laboratory reference range marks the point at which a deficiency is certain. A functional target value describes the range in which many people, by experience, tend to do better. Part of the professional community names 75 nmol/l and above here, that is 30 ng/ml upward. This is not yet conclusively proven scientifically and is understood as clinical guidance, not as a proven target mark.
Should you have vitamin D measured before you take it?
Yes, that is the most sensible path. Professional guidelines recommend measuring the 25-OH vitamin D level in people at risk as the first diagnostic step. That way you know where you stand instead of dosing blindly. The need varies individually, and the baseline value helps decide whether and how much intake makes sense. The interpretation belongs in medical hands.
Does a vitamin D deficiency have anything to do with the immune system?
Vitamin D is a signaling molecule for immune cells. In a large meta-analysis, supplementation was able to slightly lower the risk of acute respiratory infections, most clearly in people with a very low baseline value. The effect is real, but moderate. Vitamin D is therefore one building block of your defenses and not a shield that reliably prevents infections.
Can vitamin D improve mood in winter?
Honesty matters here. Observational studies show a connection between low vitamin D and depressed mood. A large, high-quality trial, however, found no benefit of taking vitamin D for preventing depression. A connection is therefore not the same as a cause. Vitamin D can be part of a good winter foundation, but it is not a mood remedy.
Are 15 minutes of sun enough in winter?
In summer, brief midday sun on uncovered skin can contribute a lot to production. In winter it cannot, because UVB radiation at our latitudes is then too weak. The limiting factor is not time, but the solar elevation. This is why the blanket advice of a few minutes of sun in winter is misleading.
Can you cover vitamin D through diet?
Only to a small extent. Oily sea fish, eggs, and a few other foods supply some vitamin D, but they rarely cover the need in winter on their own. This is why, with a confirmed deficiency in the winter half of the year, intake is often considered. How much makes sense depends on the measured value and should be discussed individually.
Can too much vitamin D be harmful?
Yes. Vitamin D is fat-soluble and can accumulate. Very high, uncontrolled doses over a long time can strain the calcium balance. This is exactly why the measured value comes first, alongside interpretation by a medical professional. More is not automatically better.
Read more on the ViveCura Blog
Sources
- Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3. J Clin Endocrinol Metab. 1988;67(2):373-378. DOI: 10.1210/jcem-67-2-373 [In vitro, skin model]
- Engelsen O. The relationship between ultraviolet radiation exposure and vitamin D status. Nutrients. 2010;2(5):482-495. DOI: 10.3390/nu2050482 [Review]
- Rabenberg M, Scheidt-Nave C, Busch MA et al. Vitamin D status among adults in Germany, results from the German Health Interview and Examination Survey for Adults (DEGS1). BMC Public Health. 2015;15:641. DOI: 10.1186/s12889-015-2016-7 [Real-world, RKI population survey, n=6,995]
- Cashman KD, Dowling KG, Skrabakova Z et al. Vitamin D deficiency in Europe: pandemic? Am J Clin Nutr. 2016;103(4):1033-1044. DOI: 10.3945/ajcn.115.120873 [Real-world, standardized, n=55,844]
- Manios Y, Moschonis G, Lambrinou CP et al. A systematic review of vitamin D status in southern European countries. Eur J Nutr. 2017;57(6):2001-2036. DOI: 10.1007/s00394-017-1564-2 [Systematic review]
- Holick MF, Binkley NC, Bischoff-Ferrari HA et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911-1930. DOI: 10.1210/jc.2011-0385 [Consensus guideline, Endocrine Society]
- Charoenngam N, Holick MF. Immunologic effects of vitamin D on human health and disease. Nutrients. 2020;12(7):2097. DOI: 10.3390/nu12072097 [Review, immunology]
- Martineau AR, Jolliffe DA, Hooper RL et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583. DOI: 10.1136/bmj.i6583 [Meta-analysis, k=25, n=11,321]
- Jolliffe DA, Camargo CA, Sluyter JD et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data. Lancet Diabetes Endocrinol. 2021;9(5):276-292. DOI: 10.1016/S2213-8587(21)00051-6 [Meta-analysis, k=46, n=75,541]
- Anglin RES, Samaan Z, Walter SD, McDonald SD. Vitamin D deficiency and depression in adults: systematic review and meta-analysis. Br J Psychiatry. 2013;202:100-107. DOI: 10.1192/bjp.bp.111.106666 [Meta-analysis, observational, n=31,424]
- Okereke OI, Reynolds CF, Mischoulon D et al. Effect of long-term vitamin D3 supplementation vs placebo on risk of depression or clinically relevant depressive symptoms (VITAL-DEP). JAMA. 2020;324(5):471-480. DOI: 10.1001/jama.2020.10224 [RCT, n=18,353]
This article is for information and does not replace a conversation with a doctor. The thresholds and target values mentioned are orientations from guidelines, surveys, and studies, not an automatic rule. The functional target range from about 75 nmol/l rests in part on expert consensus and clinical observation and is not yet conclusively proven scientifically. I make this interpretation transparent so you can decide for yourself how strongly to weigh the statements. Whether and how much vitamin D makes sense for you depends on your measured value and your individual situation, and should be interpreted medically.