Parenteral magnesium load testing with 28Mg in weanling and young adult rats

Urinary excretion of an intravenous 26Mg dose as an indicator of marginal magnesium deficiency in adults

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.

A new in vitro blood load test using a magnesium stable isotope for assessment of magnesium status

Magnesium (Mg) status is currently assessed by various biochemical biomarkers, most of which, however, have some limitations. We developed an in vitro blood load test as a new Mg biomarker using a Mg stable isotope. This test is based on the hypothesis that cellular Mg uptake is increased in Mg deficiency. For this purpose, Wister male rats were fed either Mg-deficient or Mg-adequate diets for 1 mo and blood was sampled and incubated with the (25)Mg isotope (10 mg/L) for 2 h at 37 degrees C. Erythrocytes, lymphocytes and platelets were isolated and (25)Mg concentrations were determined by inductively coupled plasma/mass spectrometry. The feasibility of this approach was then tested on human blood. (25)Mg enrichments in erythrocytes, lymphocytes and platelets from Mg-deficient rats were greater than those from controls. (25)Mg enrichment was low in human erythrocytes (3%) compared with rat erythrocytes (38%), whereas high (25)Mg enrichments were obtained in human lymphocytes and platelets, suggesting that lymphocytes and platelets may be more appropriate cells than erythrocytes for examining Mg status in humans with this approach. More studies are required to validate the utilization of this test as a Mg status biomarker in humans.

The effect of variable magnesium intake on potential factors influencing endurance capacity

Rats fed a magnesium (Mg) deficient diet have a lower endurance capacity than rats fed Mg adequate diets. The current study evaluates the effects of marginal, moderate, and severe Mg deficiencies on physiological and biochemical changes that may contribute to the reduced endurance capacity of Mg deficient rats. Variable levels of dietary Mg (400, 200, 100, 50 micrograms/g) were fed for 23 d to 5-wk-old male Osborne-Mendel rats. Indirect blood pressure and heart rate were measured during dietary treatment. Forty-eight hours after an endurance test, rats were killed and sampled for plasma glucose, insulin, and triglyceride levels. Organ weights, mineral and trace element concentrations, and carcass composition were determined. Blood pressure was lower in rats fed 50 and 100 ppm Mg during the first half of the study than in controls (400 ppm Mg). There were no significant differences in blood pressure among groups at the end of the study. Heart rate was not affected by dietary Mg intake. Plasma insulin was lowered by decreasing dietary Mg; however, plasma glucose and triglyceride concentrations were not affected by dietary Mg intake. Rats fed 100 and 50 ppm Mg diets had significantly higher calcium concentrations in plasma and gastrocnemius muscle than controls. Dietary Mg variably affected tissue trace element (iron, zinc, copper, and manganese) concentrations but did not affect Mg concentrations in any organ studied. Body composition was significantly altered by dietary Mg intake. In conclusion, variable Mg intake differentially affects the parameters evaluated. Thus, the decreased endurance capacity of the Mg deficient rat is apparently not the result of a single biochemical lesion but is likely to be multifactorial.