Validity of the parenteral magnesium load test for mature mammals

Because of the difficulty in accurately assessing the Mg status of a hospitalized patient, the parenteral Mg retention test may be a valuable diagnostic tool. In former studies in animal models, this test has reliably identified adult rats fed two extremes of dietary Mg, but we found no tests of intermediate levels of Mg in adult animals. As a means of assessing the validity of the parenteral Mg retention tests in adult rats, the present study was conducted to learn (1) the relationship between Mg in plasma and bone in adult mammals that had optimal nutrition at the onset of the experiment, and (2) the relationship between Mg retention and Mg in plasma and bone in those animals. Animals were fed five levels of dietary Mg from 0 to 150 mg/100 g purified diet. Parenteral Mg retention tests were conducted after 2 weeks of dietary treatment, and 18 h after the completion of the tests, plasma and bone were analyzed for Mg. We found that plasma and femur levels of Mg varied linearly under conditions of Mg deficit, indicated by a high retention of the Mg load. As the deficit diminished, both plasma and femur levels approached a limit and the slope of the curve tends to zero. Mg retention and plasma or femur Mg level was approximated by a negative exponential curve. Stated another way, the logarithmic values of Mg retention decreased approximately linearly with increase in bone or plasma Mg. It was concluded that the parenteral Mg retention test is a valid test to evaluate Mg deficiency and to identify Mg sufficiency in adult mammals.