Unequal distribution of a stable isotopic calcium tracer between casein and whey fractions of infant formulas, human milk and cow's milk.
Measurement of calcium absorption with tracers assumes a complete equilibration of tracer with milk calcium. In this study, the equilibration of tracer between the micellar casein and soluble fractions (primarily whey) of infant formulas, human milk and cow's milk was measured in vitro using milk samples enriched with 42Ca and analyzed by thermal ionization mass spectrometry. Incomplete equilibration of tracer occurred with the micellar casein fraction of all milks. The least equilibration with micellar casein was found with premature infant formula, for which the ratio of slopes of the equilibration lines (whey/casein) was 8.5/1. These differences may be due to Ca-casein binding in cow's milk-based formulas. The effects of the lack of tracer equilibration in vivo cannot be determined. However, unequal bioavailability of casein- vs. whey-bound Ca may exist.
Abrams
S A
SA
Laboratory of Theoretical and Physical Biology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892.
Vieira
N E
NE
Yergey
A L
AL
D000042:Absorption; D000818:Animals; D001682:Biological Availability; D002118:Calcium / Q000378:metabolism* / Q000493:pharmacokinetics; D002127:Calcium Isotopes; D002364:Caseins / Q000378:metabolism*; D006801:Humans; D007223:Infant; D007225:Infant Food; D008892:Milk / Q000378:metabolism*; D008894:Milk Proteins / Q000378:metabolism*; D008895:Milk, Human / Q000378:metabolism*; D011485:Protein Binding; D000067816:Whey Proteins
D002127:Calcium Isotopes; D002364:Caseins; D008894:Milk Proteins; D000067816:Whey Proteins; D002118:Calcium