Vitamin D

What it does

Vitamin D's central job is to control calcium and phosphate in the body, and through them the health of bones and teeth. It increases the absorption of calcium from food in the gut and helps the kidneys hold on to it, keeping blood levels in the narrow range the body needs. Without enough vitamin D the body cannot absorb calcium efficiently, so bones fail to harden properly — causing rickets in children and soft, painful bones (osteomalacia) in adults.

Vitamin D also supports muscle function and the immune system. Adequate levels help maintain muscle strength, which matters for balance and reducing falls in older people, and vitamin D receptors are found on immune cells, where it helps regulate the body's defensive responses. Its influence therefore reaches well beyond the skeleton.

Uniquely among vitamins, most of our vitamin D is made in the skin from sunlight rather than obtained from food, which is why deficiency is common in places with long, dark winters and in people who cover up or spend little time outdoors. Dietary sources — oily fish, egg yolk and fortified foods — provide relatively little, so many health bodies recommend a supplement during autumn and winter. Because it is fat-soluble and stored, very high supplement doses over time can cause harm by raising calcium too much.