Vitamin D deficiency is characterized by aberrations in the mineralization of the bone. In children, the deficiency results in rickets (see also Chapter 2), in which the cartilage fails to mature and mineralize normally. Rickets is characterized by widening at the end of the long bones, rachitic rosary, deformations in the skeleton, including craniotabes and deformities of the lower limbs, known as bowed legs and knocked knees. In adults, the deficiency of vitamin D leads to osteomalacia in which the newly deposited bone matrix fails to mineralize adequately, and there are wide unmineralized bone matrix (osteoid) seams.

Vitamin D–dependent rickets type I (VDDR I) is an autosomal recessive trait that results in abnormally low calcitriol levels but normal serum 25OHD levels. The mutation in VDDR I affects the 1α-hydroxylase enzyme and leads to impaired intestinal calcium absorption and the resulting rickets (Fraser et al., 1973). VDDR I manifests in the first year after birth and is treated with calcitriol. Supplemental calcium and phosphate are usually not needed. The second disorder is vitamin D–dependent rickets type II (VDDR II), which results in hypocalcemia, tetany, convulsions, alopecia, and rickets. VDDR II is also an autosomal recessive trait, resulting from a mutation in the Vdr gene, which can appear in the second year after birth or go unrecognized until adulthood.

Overall, vitamin D’s role at different life stages is less clearly age-related than that of calcium, and also less well understood, with numerous gaps in basic information. Although some aspects of vitamin D nutrition and physiology have been found to differ with life stage, most of the functions of vitamin D are quite consistent across life stages from infancy and childhood, to adolescence, adulthood, and old age. For all life stages highlighted below, specific studies and conclusions are detailed in Chapter 4.

Healthy skeletal development in infancy requires adequate intakes of vitamin D as well as calcium. Inadequate vitamin D intake during periods of growth leads to development of vitamin D deficiency rickets, which when it occurs in North American populations typically manifests around 20 months of age (DeLucia et al., 2003). If rickets is diagnosed early, vitamin D therapy can cure it, but not if skeletal deformities are severe and growth plates have started to mature in puberty (DeLuca, 1979a). Infants at risk for developing rickets include those who are exclusively breast-fed, because vitamin D and 25OHD are normally present at low levels in breast milk (Bachrach et al., 1979; Ward et al., 2007). Health Canada currently