Vitamin D metabolism in rachitic pre-term infants

Calcium and phosphorus metabolism in the premature infant

During the last trimester of pregnancy, there is a sixfold increase in fetal calcium and phosphorus accumulation. Unsupplemented human breast milk may not provide sufficient calcium and phosphorus for the rapidly growing preterm infant to match the accumulation that should have taken place in utero and to permit normal bone mineralization. Rickets of prematurity may present clinically between the 6th and 12th postnatal week. The clinical diagnosis may be confirmed using simple biochemical tests. Inadequate mineral substrate intake, particularly of phosphorus, is the most common cause, although a delay in the maturation of the renal enzyme, 1-alpha hydroxylase, with low plasma concentrations of 1,25-dihydroxyvitamin D, may also occur. The biochemical response to treatment can be determined by documenting a fall in plasma alkaline phosphatase activity and a rise in plasma phosphate concentration and urinary phosphate excretion.

Plasma alkaline phosphatase activity and its relation to rickets in pre-term infants

Sequential measurements of plasma alkaline phosphatase activity were made in 84 pre-term infants delivered before 38 weeks gestation. In 67% of infants, without evidence of rickets but in whom three or more measurements were made and the peak activity was less than 10 times the adult reference range, the activity rose to a peak and declined to previous levels, or lower. The time taken for these changes varied greatly. The range of values was inspected at each week of age, and in all but six cases the level did not exceed 10 times the upper limit of the adult reference range. Three infants had radiological evidence of rickets, and of the six cases in whom higher levels were found, only one had radiological evidence of rickets; the remainder did not. It is recommended that in the latter, the high plasma alkaline phosphatase activity should be regarded as evidence of subclinical bone disease.