Background: Measurement of magnesium (Mg) status is problematic because tissue Mg deficiency can be present without low serum Mg concentrations.
Objective: To evaluate a modified version of the Mg retention test using stable isotopes for the assessment of Mg status in general, and the detection of marginal Mg deficiency in particular.
Design: A modified version of the Mg retention test using a small dose of (26)Mg was evaluated for assessment of Mg status in 22 healthy subjects. Muscle Mg concentration was used as reference for Mg status. A muscle biopsy was taken from the lateral portion of the quadriceps muscle from each subject. After 2 to 4 weeks, 11 mg of (26)Mg (as MgCl(2) in 14 ml water) were injected i.v. over a period of 10 min and all urine was collected for the following 24 h. Excretion of the isotopic label was expressed as percentage of the administered dose excreted in urine within 24 h.
Results: Mean +/- s.d. Mg concentration in muscle was 3.85 +/- 0.17 mmol/100 g fat-free dried solids. Mean +/- s.d. excretion of the injected dose within 24 h was 7.9 +/- 2.1%. No correlation was found between muscle Mg concentration and excretion of the isotopic label (r (2 ) = 0.061, P = 0.27).
Conclusions: In this study, urinary excretion of an intravenous Mg tracer was not influenced by muscle Mg concentration and its usefulness for the detection of marginal Mg deficiency could therefore not be demonstrated.
Sponsorship: Swiss Foundation for Nutrition Research and Swiss Federal Institute of Technology, Zurich, Switzerland.