Assessment Of Vitamin D Status

Before gross anatomical deformities are apparent in vitamin D-deficient children, bone density is lower than normal, and this can be detected by radiography. This preclinical condition is known as radiological rickets. At an earlier stage of deficiency, there is a marked elevation of plasma alkaline phosphatase released by osteoclastic activity; the reference range for alkaline phosphatase in children is 75 to 250 units per L. For many years, this stage of biochemical rickets was used as a means of detecting children with preclinical rickets.

Osteocalcin is induced in osteoblasts by calcitriol, and circulating osteocalcin can be used as an index of calcitriol action and metabolic bone disease. In rachitic children, the plasma concentration of osteocalcin is lower than in controls, and rises on therapy, remaining high until there is radiological evidence of cure. However, plasma osteocalcin can be undetectably low in normal subjects with adequate vitamin D status, so this does not provide a useful indication of deficiency (Greig et al., 1989).

The plasma concentration of calcidiol is the most sensitive and useful index of vitamin D status, and is correlated with elevated plasma parathyroid hormone and alkaline phosphatase activity (Table 3.4). As shown in Table 3.2, the reference range of plasma calcidiol is between 20 to 150 nmol per L, with a twofold seasonal variation in temperate regions. Concentrations below 20 nmol per L are considered to indicate impending deficiency, and osteomalacia is seen in adults when plasma calcidiol falls below 10 nmol per L. In children, clinical signs of rickets are seen when plasma calcidiol falls below 20 nmol per L. The plasma concentration of calcitriol does not give a useful indication of vitamin D status. The reference range is between 38 to 144 pmol per L and is maintained because of the stimulation of calcidiol 1-hydroxylation by parathyroid hormone secreted in response to falling concentrations of calcium (Holick, 1990).

Table 3.4 Plasma Concentrations of Calcidiol, Alkaline Phosphatase, Calcium, and Phosphate as Indices of Nutritional Status

Alkaline

Calcidiol

Phosphatase

Calcium

Phosphate

(nmol/L)

(units/L)

(mmol/L)

(mmol/L)

Infants

27-100

100-300

2.5

1.6-2.6

Children with rickets

<20

>390

2.0-2.25

1.0

Adults

20-87

57-100

2.5

1.0-1.4

Adults with osteomalacia

<10

300

2.25

0.6-1.0

Adults with osteoporosis

20-87

40

2.5-3

1.3-1.6

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