In 525 BCE, at Pelusium on the eastern edge of the Nile Delta, the armies of Persia and Egypt clashed. The Persians won. Decades later, the bones of the fallen still lay on the battlefield, neatly separated: Persians here, Egyptians there. The traveler Herodotus went to investigate something the locals had told him.

The skulls of the Persians, he wrote, were so thin you could break them with a pebble. The Egyptians’ skulls, on the other hand, could withstand even a large stone. The locals explained: Egyptians shaved their heads from childhood and exposed them to the sun. Persians wore felt caps and lived in the shade.

That explanation doesn’t quite hold up for skulls. But the underlying idea was right: sun and bones are connected. The reason has a name—and it’s especially important for you right now, in October.

Vitamin D: the “vitamin” that isn’t really a vitamin

Almost all vitamins must be obtained from food. Not vitamin D. The body produces most of it itself, in the skin, when exposed to UV-B rays from the sun. Strictly speaking, it’s more a precursor to a hormone than a vitamin. It enables the gut to absorb calcium and deposit it in the bones. If you lack it for a long time, bones become soft. In children, this is called rickets; in adults, osteomalacia.

How closely this is linked to sunlight only became clear about a hundred years ago: In 1919, Berlin pediatrician Kurt Huldschinsky treated children with rickets using a mercury vapor lamp, and their bones healed. A few years later, the substance behind this was identified and given the letter D.

The Shadow Rule: What the Afternoon Prayer Has to Do with Vitamin D

Muslims have measured shadows for centuries. According to most schools of law, the time for the afternoon prayer (Asr) begins when the shadow of a stick is as long as the stick itself, plus its noon shadow. Some old mosques even had special sundials for this, such as the Umayyad Mosque in Damascus.

This same measurement tells you something about vitamin D. The necessary UV-B rays only reach you when the sun is high enough. The rule of thumb: If your shadow is longer than you are tall, your skin hardly produces any vitamin D. A shadow exactly as long as you are means the sun is at a 45-degree angle. Below that, the rays are so weakened by their slanted path through the atmosphere that almost nothing gets through. Model calculations set the cutoff roughly there, some a bit lower.

Try it out: The calculator shows you how long your shadow is at noon in your city today, and when it will be short enough again.

The shadow calculator is loading …

The result for Germany is sobering: From about October to March, even at midday the sun is too low. In Hamburg, this period lasts longer than in Munich. If you need vitamin D in winter, you rely on what your body stored in fat and muscle tissue over the summer—and on what you eat.

Persian miniature: winter bazaar with low sun and long shadows, fishmonger, eggs, mushrooms
In winter, the sun stays low and shadows grow long. That’s when diet matters: fatty fish, eggs, and mushrooms.

The Desert Mystery: Deficiency Despite Sun

Now it gets strange. You’d think that in sunny countries, vitamin D deficiency wouldn’t exist. The opposite is true. In Saudi Arabia, recent studies found about two-thirds to three-quarters of people had low levels, almost regardless of the season (Saudi expert consensus). In Germany, the proportion with true deficiency is about 15 percent of adults (Robert Koch Institute).

The reasons: In extreme heat, people avoid the midday sun and stay indoors in air-conditioned spaces. Skin is often covered. And darker skin needs more sun to produce the same amount, because its pigment blocks UV rays. Herodotus’s Persians with their felt caps would have understood: It’s not about how much sun shines, but how much actually reaches your skin.

This also applies to many people in Germany who come from these regions or dress in covering clothing. For them, winter is doubly challenging.

Are You in a Risk Group?

You often don’t notice a deficiency for a long time. Fatigue or muscle aches can have many causes. Only a blood test can confirm it. Whether it makes sense for you can be checked with this quick test:

The check is loading …

Do You Even Need a Supplement?

The German Nutrition Society recommends 20 micrograms (800 International Units) per day as a guideline—specifically for cases where the body produces none itself. For infants, the recommendation is 10 micrograms. If you spent a lot of time outdoors in summer and aren’t in a risk group, you can often get through the winter without supplements.

The sensible order is: check first, then supplement. If you’re in a risk group, talk to your doctor about a blood test. If a deficiency is confirmed, your doctor will determine the right dose.

Comparison: What You Really Need to Know

QuestionShort Answer
D3 or D2?D3. It raises blood levels much better with larger single doses. With daily use, the difference is small.
Drops or tablets?Both work equally well. Tablets are precisely dosed. Drops are good for people who have trouble swallowing, but count carefully.
When to take?With a meal. Vitamin D is fat-soluble.
Daily or weekly dose?Better daily and low. That’s the recommendation from the Federal Institute for Risk Assessment.
With vitamin K2?No proven added benefit. People taking coumarin-type blood thinners should not take K2 without medical advice.
How much is too much?For adults, 100 micrograms per day from all sources is the upper limit. For supplements, the BfR recommends no more than 20 micrograms per daily serving.

About D3 and D2: A review of several studies showed that D3 raises blood levels more, especially with larger, infrequent doses; with daily use, the advantage largely disappears (Tripkovic et al., 2012). Almost all supplements in Germany contain D3 anyway.

About K2: Combination products are often marketed with the claim that K2 directs calcium “into the bones instead of the arteries.” The German Consumer Advice Center considers this benefit unproven, and the current osteoporosis guidelines advise against K2 for treatment.

About high doses: Too much vitamin D increases calcium levels in the blood. Over time, this can lead to kidney stones and kidney damage. High-dose supplements with 50 or 100 micrograms per tablet should only be taken if prescribed by a doctor.

If, after consulting your doctor, you’re looking for a supplement, choose D3 with no more than 20 micrograms (800 IU) per day:

If you’re vegan: Traditional D3 is made from sheep’s wool. But there’s vegan D3 from lichen, and D2, which comes from mushrooms and yeast:

What Vitamin D Can and Can’t Do

Vitamin D was long hyped as a miracle cure for almost everything. Two very large studies have put that into perspective. In the American VITAL study with over 25,000 participants, it didn’t reduce cancer or heart attacks overall. A secondary analysis did find 22 percent fewer autoimmune diseases. In the Australian D-Health study, older people given monthly doses didn’t live longer than those without.

The takeaway: Vitamin D is no turbo booster. It mainly helps those who don’t have enough. If your levels are fine, more won’t help.

What You Can Do in Winter

  1. Eat fatty fish. Herring, salmon, and mackerel are the best natural sources. Once or twice a week is helpful.
  2. Eggs and mushrooms: Egg yolks contain some vitamin D, as do mushrooms—especially if they were grown in light.
  3. Make use of bright days in March and April. As soon as your shadow at midday is shorter than you are, every lunchtime walk is worthwhile.
  4. Stock up in summer. The Federal Office for Radiation Protection recommends exposing your face, hands, and arms to the sun two or three times a week, unprotected, for only half as long as it would take you to get sunburned. For fair skin, that’s often just 10 to 15 minutes at midday in high summer. Longer doesn’t mean more vitamin D—the skin only produces up to a certain limit, after which only the risk for your skin increases. And it doesn’t have to be the midday heat: In June in Germany, your shadow is already shorter than you are from about 10 a.m. onward.
  5. If you’re in a risk group, get your levels checked before taking high doses on your own.

And Ibn Sina?

Ibn Sina didn’t know about vitamin D. But for him, the air around us, with its seasons, was one of the six things that determine health. He classified winter as cold and damp, and advised adapting your lifestyle to the season: warming foods, more movement, less sluggishness. Whether this applies especially to you, according to his teachings, depends on your temperament.

→ Take the Ibn Sina temperament test: Are you more fire, air, water, or earth?

Further Reading

Sources