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Inhibitory Effect of Soybean Protein vs. Casein on Apparent Absorption of Magnesium in Rats Is Due to Greater Excretion of Endogenous Magnesium

Elizabeth J. Brink, Gerrit J. van den Berg^*, Roelof van der Meer¹, Hubert Th. Wolterbeek^*, Pieter R. Dekker and Anton C. Beynen^,

Department of Nutrition, Netherlands Institute for Dairy Research, P.O. Box 20, 6710 BA Ede ^* Interfaculty Reactor Institute, University of Technology, Delft Department of Laboratory Animal Science, State University, Utrecht Department of Human Nutrition, Agricultural University, Wageningen, The Netherlands

Apparent magnesium absorption is depressed in rats fed diets containing soybean protein or enriched with sodium phytate or phosphate in comparison with casein, whereas it is raised in rats fed lactose. However, the possibility that changes in apparent absorption are caused by changes in fecal excretion of endogenous magnesium cannot be excluded. We studied the effects of casein, soybean protein, sodium phytate, lactose and phosphate on apparent and true absorption of magnesium. True magnesium absorption was measured with the use of oral and intraperitoneal administration of tracer doses of ²⁸Mg. Fecal excretion of endogenous magnesium was calculated from apparent and true absorption. True magnesium absorption was not affected by either substitution of soybean protein for casein or by the addition of sodium phytate to a diet containing casein. Endogenous magnesium excretion in feces was significantly increased by soybean protein and sodium phytate. Thus the observed impairment of apparent magnesium absorption in rats fed soybean protein or sodium phytate is due to enhanced fecal excretion of endogenous magnesium. With the other dietary treatments, enhanced fecal excretion of endogenous magnesium was not associated with a discrepancy in the effects on apparent and true magnesium absorption. Dietary lactose vs. dextrose and supplemental phosphate both stimulated fecal excretion of endogenous magnesium, but lactose raised both true and apparent magnesium absorption, and phosphate depressed both true and apparent magnesium absorption.

KEY WORDS: • magnesium • protein • rats • lactose • phosphate

¹ To whom correspondence should be addressed.

Manuscript received 13 January 1992. Revision accepted 22 May 1992.

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