Vitamin D

Vitamin D is a prohormone: the body turns it into the hormone calcitriol, which influences the activity of numerous genes via the vitamin D receptor – estimates range from a few hundred to around two thousand. Its role in the absorption of calcium and phosphate, bone metabolism, muscles and immune defence is well established; links with mood, sleep and blood sugar metabolism are being researched. Low values are widespread in Germany because from about October to March the sun is too low for the skin to produce enough vitamin D. The reference range is set out in your findings report.

How vitamin D acts in the body

The skin produces vitamin D3 under UV-B light; a smaller part comes from food. The liver converts it into 25-OH vitamin D – this is the storage value that is measured. Only the kidneys and many other tissues turn it into the active hormone calcitriol. Calcitriol binds to the vitamin D receptor in the cell nucleus and thereby controls genes. In the gut it increases the absorption of calcium and phosphate, in bone the incorporation and remodelling of minerals; it also acts on muscle cells and on immune cells, which use it, among other things, to make antimicrobial proteins such as cathelicidin.

Low vitamin D – what happens in the body?

With little vitamin D, the gut absorbs less calcium. To keep the calcium level in the blood stable, the parathyroid glands release more parathyroid hormone, which releases calcium from the bones. Over time bone is mineralised less well – in children as rickets, in adults as bone softening (osteomalacia) with bone and muscle pain – and bone density falls. Muscle strength declines, which makes falls more likely in older people, and immune cells produce fewer antimicrobial proteins.

High vitamin D – what happens in the body?

Very high levels arise almost only from high-dose intake over a long period. The gut then absorbs excessive calcium and the calcium level in the blood rises (hypercalcaemia): nausea, thirst, frequent urination, muscle weakness and heart rhythm disturbances may follow, and calcium can be deposited in the kidneys and blood vessels. Sunlight alone does not cause oversupply, because the skin limits production itself.