Absorption and endogenous faecal excretion of calcium by low birthweight infants on feeds with varying contents of calcium and phosphate.

Abstract

SUMMARY Low birthweight infants aged 4-41 days were given from birth one of three experimental milk formulae varying widely in content of calcium and phosphate. Ca and P in feed, urine, and faeces were measured between carmine markers corresponding to a metabolic period of 48 hours. Calcium enriched in 46Ca to provide a marker for the dietary Ca was added to one feed and 46Ca measured in urine and faeces. True absorption of Ca and endogenous excretion into the bowel could then be inferred. True absorption of Ca was proportional to Ca intake and independent of P intake. Endogenous faecal excretion seemed to be independent of both Ca and P intakes, and varied widely between different infants in the range 4-150 mg/day. Urine Ca was low and retention was essentially the difference between true absorption and endogenous faecal excretion. Retention of Ca tended to be much greater on a high Ca intake, but the variability in retention between infants on a given intake was large, paralleling the variability in endogenous faecal excretion.The variability in plasma Ca concentrations in newborn infants may in part be a consequence of wide individual variability in endogenous faecal excretion. The 46Ca marker technique provides a means of investigating the factors determining this variability.

Absorption of Calcium in Premature Infants as Measured With a Stable Isotope 46Ca Extrinsic Tag

“…The validity of this assumption appears to hold for zinc (l3,20) and iron (16) but may not hold for selenium (8). As previously demonstrated by Barltrop et al (7), our data indicate that the extrinsic tag approach can be utilized to examine the absorption of calcium in growing premature infants. Although a stool collection period of about 50 h following the administration of the stable isotope solution appeared to be sufficient for a complete collection of the unabsorbed isotope and determination of fractional 46Ca absorption, stools were pooled from the time that the stable isotope solution was given until the stool marker administered 72 h later was passed.…”

“…In contrast, the stable isotope tracer method permits an accurate assessment of true absorption (25,26). Barltrop et al (7) have utilized this method to study calcium absorption and endogenous fecal excretion of calcium in formula-fed premature infants, and found that calcium absorption averaged 36% and remained constant while calcium intake rose from about 200-500 mg/dl (about 100-250 mg/kg/day). We (1 1) have previously demonstrated the suitability of this methodology in a study of dietary zinc absorption in the premature infants described herein.…”

“…Barltrop et al (7) reported mean endogenous fecal calcium excretion to be 70-89 mg/day (approximately 35-45 mg/kg/day), to be largely independent of calcium intake, and to be characteristic of an individual infant, but highly variable among infants (ranging from 4-1 50 mg/day). Endogenous fecal calcium secretion has been shown to be relatively constant for a large adult population; it is independent of diet and disease and averages 130 mg/day (17).…”

“…This methodology is based on the use of naturally occurring, nonradioactive isotopes as tracers (25,26). This method has been previously utilized by Barltrop et al (7) to study calcium balance in premature infants fed one of three experimental formulas that varied widely in content of calcium and phosphorus, and by Ehrenkranz et al (1 1) to investigate zinc absorption in the premature infants described herein. In addition, this methodology has been extensively employed in human adults for assessment of the dietary availability of calcium (21), zinc (23,27), copper (36), iron (22,23), and selenium (1 9, 24).…”

Zinc and Copper Nutritional Studies in Very Low Birth Weight Infants: Comparison of Stable Isotopic Extrinsic Tag and Chemical Balance Methods1

“…We (4,5) and others (6)(7)(8)(9) have recently demonstrated that the extrinsic tag approach with stable isotope methods can be successfully used to study absorption of dietary minerals by growing infants. This methodology is based upon the use of naturally occurring, nonradioactive isotopes as tracers (10,11).…”