Vitamin D deficiency
Your skin makes vitamin D using sunlight – but in the German winter that isn't enough. According to the Robert Koch Institute (RKI), around one in three adults has too little of it in their blood.
Ausführlich erklärt
Vitamin D is actually the precursor of a hormone: your skin produces it itself with the help of UVB sunlight, and it ensures that calcium is built into bones and teeth. Muscles and the immune system need it too. The catch: from October to March, the sun is too low in the sky in Germany, so the skin makes hardly any vitamin D and the body lives off its summer reserves stored in fat tissue.
According to the RKI, around 30% of adults have a blood level (25-OH vitamin D) below 30 nmol/l, which is considered deficient; a good 30% more are not optimally supplied. Things look hardly any better among children and teenagers. Food doesn't provide much: only a few foods such as oily sea fish, egg yolk and some mushrooms contain significant amounts of vitamin D, so on average we only take in 2 to 4 micrograms a day this way. For people whose skin produces none of its own, the German Nutrition Society (DGE) gives an estimated value of 20 micrograms (800 IU).
A deficiency goes unnoticed for a long time and then tends to show up in unspecific ways: tiredness, muscle weakness, bone pain, frequent infections and, in severe cases in children, rickets, with soft, deformed bones. That's why all babies in Germany are advised to receive vitamin D daily as a tablet or drops during their first year of life – depending on the month of birth, until the second early summer of their life; the paediatrician will discuss this at check-ups. It's one of the oldest and most successful preventive measures.
What you can do: from March to October, it's enough to expose your face, hands and parts of your arms and legs to the sun without sunscreen two to three times a week – for about half as long as it would take you to burn; in high summer, that's often just a few minutes. After that: protect your skin – sunburn is never necessary. In winter, for risk groups – people who rarely go outdoors, older people, people with darker skin, people with severe obesity – it's worth talking to your GP; a supplement of 20 micrograms a day may then make sense.
More isn't better: 100 micrograms (4,000 IU) a day is considered the safe upper limit; taking more over long periods can raise calcium levels dangerously. Without medical advice, experts recommend no more than 20 micrograms a day from supplements. And one more misconception: vitamin D doesn't help you lose weight. The reverse is true, though – people with a lot of fat tissue often have lower levels, because the vitamin is distributed throughout it.