Vitamin D is needed in the development of strong, healthy bones. Its chief function is to maintain the right concentration of calcium and phosphorus within the body. This is particularly important in babies and young children whose bones are developing rapidly.

When vitamin D is absent from the diet, serious bone deformities occur. This is the condition known as rickets. The bones become soft and easily bent. Under the constant pull of the muscles, the joints lose their shape and are longer able to function as they should. In young infants there may be a marked softening of the bones of the skull. The head may be flattened on one side. The chest is also deformed. A complete description of rickets will be found on page 31.

Premature infants are very prone to have rickets. They must therefore be given adequate amounts of vitamin D. then danger is even greater in darker-skinned children who live in cooler climates. Because of the slanting rays of the sun, rickets occurs more frequently during the winter months. Clouds also prevent the ultraviolet light from coming through. This is sometimes a real problem in growing children. Today the widespread use of irradiated milk, fortified with vitamin D, has greatly lowered the incidence of this disease. Breast-fed babies are more likely to develop rickets, particularly when the mother is not on a well-balanced diet.

Best sources of vitamin D are egg yolk, cod-liver oil, and irradiated vitamin D milk. The human body produces much of its own vitamin D in the summer under the influence of sunlight. The natural oils in the skin are changed, and large quantities of vitamin D are produced. Too much of the commercially prepared vitamin D may make a person ill because of overdosage.