Vitamin D in Winter: Why Your Levels Stall

A lunchtime walk around the block in November does you good, but it does almost nothing for your vitamin D. From October to March the sun in Germany sits too low for your skin to make much vitamin D. If you reach for a supplement because of that, the number on your next blood test doesn't always climb. This article looks at where vitamin D in winter can get stuck, and how to read your result without guessing.

A national survey by the Robert Koch Institute (DEGS1, 2008 to 2011) shows how common low levels are. Among people examined in winter, one in four had a level below 30 nmol/l, which is about 12 ng/ml. In summer, half reached at least 50 nmol/l (20 ng/ml). People living further north, carrying more body weight or doing little sport tended to sit lower (Rabenberg et al., 2015).

Vitamin D in winter: why the sun can't deliver from October to March

Your skin makes vitamin D when UVB radiation hits a precursor molecule in the skin. For that, the sun has to stand high enough. When it's low, the light travels a longer path through the atmosphere and the UVB is lost on the way. At large solar zenith angles, meaning a low sun, there isn't enough UV radiation left for meaningful synthesis (Webb, 2006).

How long this “vitamin D winter” lasts depends on latitude. In a classic experiment, human skin exposed to sunlight on cloudless days made no previtamin D in Boston (42° N) from November to February, and none in Edmonton (52° N) from October to March (Webb et al., 1988). Berlin sits at about 52.5° N, and Germany as a whole between 47° and 55° N.

German authorities agree: according to the German Nutrition Society (DGE), the Federal Institute for Risk Assessment (BfR) and the Max Rubner-Institut (MRI), the sun in Germany isn't strong enough from October to March for adequate vitamin D production. The body can store vitamin D, mainly in fat and muscle tissue. These stores contribute to your supply in winter and refill from spring onwards (DGE, BfR and MRI, 2012).

A rule of thumb for this is the shadow rule: if your shadow is longer than you are tall, the sun is below roughly 45 degrees and usually too little UVB arrives for vitamin D production. The limit isn't sharp. Clouds, ozone and altitude shift it. The graphic shows how your midday shadow in Berlin changes through the year.

Even at the equinox, the midday shadow in Berlin is still longer than you. Only in the summer half of the year does the sun stand high enough around noon. Why even summer doesn't get everyone to a good level is covered in our article on vitamin D in summer.

From precursor to blood result: liver, kidney and magnesium

Whether vitamin D comes from your skin, your food or a supplement, it starts out as an inactive precursor. The liver converts it into 25-hydroxyvitamin D, or 25(OH)D for short, also called calcidiol. That's the storage form, and it's what a blood test measures. The kidney turns some of it into 1,25-dihydroxyvitamin D, also called calcitriol, the active hormone form, when the body needs it. How much gets made is regulated by the body itself.

The body goes to this trouble for a reason: vitamin D contributes to the maintenance of normal bones and to the normal function of the immune system (Regulation (EU) No 432/2012). That doesn't add up to protection from infections, and a high level doesn't guarantee anything.

Enzymes at both steps need magnesium as a cofactor. As far as current knowledge goes, all the enzymes that process vitamin D appear to depend on magnesium (Uwitonze and Razzaque, 2018). Whether that shows up in the result was tested in a randomised trial of 180 adults aged 40 to 85: magnesium raised 25(OH)D in people who started around 30 ng/ml and lowered it in people who started higher, at roughly 30 to 50 ng/ml. The authors read this as a sign that good magnesium status may matter (Dai et al., 2018).

That's a signal, not proof for everyday life. It was a single trial, and it shows that magnesium can shift the level. It doesn't follow that low magnesium sits behind every stalled vitamin D result.

Whether your magnesium status needs attention is something to clear up with your doctor. The liver is part of the route too. How everyday habits affect values such as GGT, ALAT and ASAT is covered in our article on liver values.

Why guessing your vitamin D is risky

Two people taking the same supplement in the same amount don't end up at the same level. In a study of 644 adults aged 60 to 84, dose and starting level together explained only 24 percent of the differences in response. Body weight, health status, local UV radiation and genetic variants added to it (Waterhouse et al., 2014). By feel alone, it's hard to tell what actually reaches you.

More isn't automatically better. Vitamin D is stored in the body. Toxicity is rare and usually follows very high doses over a long time, when blood calcium can climb too high (hypercalcaemia). The authors of one review warn that self-dosing above the established upper limits could make such cases more common (Marcinowska-Suchowierska et al., 2018). That's why you settle dose and duration with your doctor or a pharmacist.

Whether a test makes sense for you depends on your situation. The DGE advises measuring vitamin D status only when there's a reason to suspect a shortfall or in risk groups, such as people who rarely go outdoors, people with dark skin or older adults (DGE, BfR and MRI, 2012). International guidelines also advise against routine testing for everyone (Holick et al., 2011; Pludowski et al., 2022). If you take or are considering a supplement, a measured value gives you something concrete to discuss at your doctor's office.

Vitamin D level too low? How to read your result

The blood test measures 25(OH)D, in Germany usually in ng/ml and in some labs in nmol/l (1 ng/ml is about 2.5 nmol/l). Experts don't agree on the cut-offs. Internationally, there's broad consensus that levels below 25 to 30 nmol/l (10 to 12 ng/ml) should be prevented and treated. Many guidelines aim for at least 50 nmol/l (20 ng/ml) (Pilz et al., 2019). An expert panel from Central and Eastern Europe names 30 to 50 ng/ml as the treatment target (Pludowski et al., 2022). In studies, higher levels weren't consistently linked with more benefit (Ross et al., 2011).

In Germany, the DGE speaks of deficiency below 30 nmol/l (12 ng/ml) and names at least 50 nmol/l (20 ng/ml) as good supply for the bones. As an estimate for adequate intake it gives 20 µg (800 IU) a day. That's a population reference value for when the skin makes no vitamin D, not a dose for you. The DGE recommends supplements when an insufficient supply has been shown and can't be improved through food or sun, mainly in risk groups (DGE, BfR and MRI, 2012).

These are reference points, not a diagnosis. Your doctor can place your result in context.

Timing shapes the result too. A test towards the end of winter shows how far your level has fallen over the dark months: in the DEGS1 data, low levels were most common in winter and spring (Rabenberg et al., 2015). To see the course of the year, test a second time towards the end of summer.

Vitamin D in winter: frequently asked questions