They call it „the vitamin for bones," but that's the least it does. Vitamin D actually behaves more like a hormone than a vitamin - it sends signals and regulates functions throughout the whole body, doctors say. „Many cells have receptors ready to respond to it," explains Dr Ainara Ormazabal, an integrative doctor and nutritionist - which means its deficiency doesn't only hit the skeleton.

Besides the absorption of calcium and phosphorus, vitamin D plays a role in immunity, inflammation control, metabolism, and the nervous system. For people with autoimmune or inflammatory conditions, sufficient levels help toward a more balanced immune response - but only together with good nutrition, sleep, movement, and stress management. „The brain, the immune system, metabolism, and circadian rhythms are closely linked," the doctor says.

The biggest source isn't on the plate - 80 to 90 percent of vitamin D the body makes from the sun, and only 10 to 20 percent comes from food. That's why the advice is neither to hide from the sun nor to fry in it, but a „smarter relationship" with light: regular, moderate exposure without burns. From food, fatty fish (sardines, salmon, mackerel), egg yolk, liver, and full-fat dairy help - and since the vitamin is fat-soluble, you also need enough fat for it to be absorbed.

And here comes the warning the supplement industry doesn't advertise loudly: vitamin D is no miracle in a bottle. Taking high doses without measuring first can build up and cause a calcium imbalance in the blood. „It's not a miracle vitamin nor a guarantee of longevity on its own," Ormazabal says, „but maintaining sufficient levels is a basic tool for good health in the long run." In other words - measure, then supplement, not the other way around.