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Failure of Balance Measurements to Predict Actual Retention of Magnesium and Calcium by Rats as Determined by Direct Carcass Analysis¹

Division of Biological Sciences, National Research Council of Canada, Ottawa, Canada K1A 0R6

Magnesium (Mg) and calcium (Ca) balances as well as determinations of atomic absorption analysis of total absolute contents of Mg and Ca of fat-free dry carcasses were done on gnotobiotic male Sprague-Dawley rats, kept at 28° or 6°, for 69, 240, or 517 days, and fed either a commercial stock diet or a semipurified diet containing a normal or a subnormal amount of Mg. The often-reported observation of negative balances when Mg intake is subnormal has been confirmed. Total Mg and Ca accumulation in the carcass estimated from balance values was totally inconsistent with the actual content of the carcass as determined by direct analysis. The large discrepancy between the actual analysis of the carcass and the estimated content of Mg and Ca in the carcass derived from balance data demonstrates clearly that balance of trace minerals cannot reflect chronic retentions or losses of these minerals. Possible explanation of this discrepancy are discussed.

KEY WORDS: • calcium • magnesium • carcass analysis

¹ NRCC no. 14861.

² Present address: The University of New England, Department of Physiology, Armidale N.S.W., Australia.

Manuscript received 4 February 1975.