Can one really measure magnesium deficiency using the short- term magnesium loading test?

Abstract

Objective. To compare a 1-h-version of a magnesium-loading- test (MLT) designed for outpatients in healthy controls with the 8-h standard; to establish the test in patients after renal transplantation prone to develop magnesium (Mg) deficiency; to correlate femur Mg-concentration and percentage retention of the given load. Design. Comparison of mean values from healthy controls with respective from the literature; a prospective, randomized, controlled 4-month study; an intra-individual correlation of Mg-serum values and loading-test data with femur- Mg concentrations. Setting. One centre study in a medical university; outpatients from the transplant unit; inpatients from the orthopedic unit. Subjects. Twenty-four healthy controls aged 36.7 ± 7.4 years; 34 patients after renal transplantation (46.5 ± 14.3 years); 41 patients with hip replacement therapy (63.9 ± 18.6 years). Intervention. Baseline Mg values were measured by atomic absorption spectroscopy (AAS) in serum and urine. An intravenous Mg load with 0.1 mmol Mg-aspartate hydrochloride per kilogram bodyweight was given during 1 h. In 24 h-urine, the amount of excreted Mg was measured by AAS and the percentage retention of the given load calculated according to the formula: 1 - [Mg 24 h-urine/Mg test dose] x 100. Femur Mg was measured by AAS in a peace of the femur neck. Patients after renal transplantation were randomized after the first Mg load to either obtain daily 5 mmol Mg-aspartate hydrochloride per kilogram bodyweight, or placebo. Four months later a second loading- procedure was performed. Main outcome measure. Serum Mg, percentage retention of the given Mg load (%Ret) and femur Mg concentration. Results. Mean serum Mg values were within the normal range. In controls. %Ret was -18 ± 21 and not different from the literature. In the first MET after renal transplantation, %Ret was 47 ± 43. In patients under Mg medication it decreased significantly to 16 ± 26, but was 58 ± 27 in the placebo group. Femur Mg concentration was 62.6 ± 20.9 mmol kg-1 dry substance and the corresponding %Ret was 14 ± 28 with r =0.7093. Conclusion. The short-term version of the MLT is as good as the standard and was easily applied in outpatients. The indication from the good correlation between bone-Mg and %Ret and a marked decrease in %Ret in patients after Mg medication was that one can really measure magnesium deficiency.