Measurement of true calcium absorption in premature infants using intravenous 46Ca and oral 44Ca.
We have developed a method for measuring true fractional calcium absorption (alpha) in premature infants using two stable isotopes of calcium and tested it in seven studies in seven infants (birth weight 1543 +/- 65 g, gestation 32.8 +/- 7 wk). A total of 7.5 micrograms/kg 46Ca was given as a single intravenous bolus. Immediately thereafter 1.25 mg/kg of 44Ca was given in a single gavage feeding of standard infant formula (Enfamil). A metabolic isolette was used to obtain 4-h collections of urine for 24 h total. 46Ca and 44Ca were measured in urine by thermal ionization mass spectroscopy and expressed as the ratio to naturally occurring 48Ca. The differences in the 46Ca/48Ca and 44Ca/48Ca ratios from natural levels (delta % excess 46Ca and delta % excess 44Ca) were calculated. Percent absorption (alpha) equals a constant times cumulative delta % excess 44Ca/delta % excess 46Ca. The calculation of alpha is independent of urine volume or concentration. The delta % excess 46Ca, showed the expected multiexponential decline as a function of time, and delta % excess 44Ca usually peaked during a 4- to 8-h urine collection. Calculations of alpha using increasingly long sampling times showed that a plateau had been reached by 12 h. alpha values calculated after 16-24 h in the seven infants at 2 wk of age were 41, 48, 45, 46, 25, 55, and 51%. Repeat studies at 3 wk of age were 46, 60, and 54%. These values are somewhat higher than net percent calcium absorption values reported for standard formula and thus appear very appropriate.(ABSTRACT TRUNCATED AT 250 WORDS)
Hillman
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Edward E. Malinckrodt Department of Pediatrics, Washington University School of Medicine.
Tack
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Covell
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Vieira
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Yergey
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D000042:Absorption; D000284:Administration, Oral; D002127:Calcium Isotopes / Q000008:administration & dosage / Q000378:metabolism*; D006801:Humans; D007231:Infant, Newborn; D007234:Infant, Premature / Q000378:metabolism*; D007275:Injections, Intravenous; D007408:Intestinal Absorption
D002127:Calcium Isotopes