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Unequal Distribution of a Stable Isotopic Calcium Tracer Between Casein and Whey Fractions of Infant Formulas, Human Milk and Cow's Milk

Steven A. Abrams^*,, Nancy E. Vieira and Alfred L. Yergey^*

^* Laboratory of Theoretical and Physical Biology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892 Children's National Medical Center, Washington, DC 20010

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.

KEY WORDS: • calcium • human milk • cow's milk • infant formulas • isotopic tracer

Manuscript received 8 February 1990. Revision accepted 6 June 1990.