Vitamin D

Vitamin D is less a vitamin than a hormone the body manufactures for itself. Ultraviolet B light striking the skin begins the process; the liver and then the kidney complete it, which is why long-standing renalRenalRenal simply means relating to the kidneys. It appears throughout medical language — renal function, renal failure, renal stones — where the everyday word would be kidney.Read the full definition disease causes deficiency regardless of sunlight. Its central job is enabling calcium absorption from the gut — without it, calcium in the diet is largely wasted.

Why deficiency is common in sunny climates

Sunlight is necessary but not sufficient. Clothing that covers the skin, staying indoors during the middle of the day, sunscreen, air pollution, darker skin pigmentation and older age all reduce production substantially — and in much of South Asia deficiency is widespread despite abundant sun. Very few foods contain useful amounts naturally, so diet rarely compensates.

Deficiency is often silent, or presents vaguely as aching bones, muscle weakness and fatigue. Severe deficiency causes rickets in children and osteomalacia in adults — soft, painful bones — and lesser deficiency contributes to osteoporosisOsteoporosisOsteoporosis is loss of bone density that leaves bones fragile enough to break from a minor fall or even a cough. It causes no symptoms until a fracture happens, which is what makes it dangerous.Read the full definition and to falls in older people through muscle weakness.