At the turn of the 20th century more than 80% of children living in the industrialised cities of the Western hemisphere had rickets.1 2 Rickets became extremely rare in the United Kingdom, Europe, and United States after it was realised that exposure to ultraviolet light was the major source of vitamin D, and after the fortification of milk and other foods with vitamin D.2 At least a billion people worldwide are estimated to be vitamin D deficient, mainly because of inadequate exposure to sunlight and inadequate fortification of food with vitamin D.1 3 4 5

Skin pigmentation absorbs ultraviolet light, thereby reducing vitamin D production; this can be a problem for certain racial groups who now live in the Northern hemisphere. Human breast milk contains very little vitamin D and women with vitamin D deficiency provide no vitamin D for their infant. Such infants will be at high risk of developing rickets if they are exclusively breast fed. Rickets is the most overt sign of severe vitamin D deficiency in Europe—around 1-5% of children with vitamin D deficiency have skeletal signs of rickets and probably 10-25% of adults with vitamin D deficiency have symptomatic osteomalacia.

Vitamin D deficiency causes secondary hyperparathyroidism and increases …