Comparison of two methods for the assessment of intra-erythrocyte magnesium and its determinants 49 3 Introduction Magnesium is the second most abundant intracellular cation and is essential for regulation of many functions within the cell, including modulation of ion channels, protein synthesis and energy metabolism 1. Of the total magnesium content in the human body, 55-65% is present in the skeleton, 34-44% in the intracellular space and only 0.3-1% in the circulation 2,3. We previously showed that lower plasma magnesium concentrations were associated with increased risk of type 2 diabetes mellitus (T2D ) in women 4. T2D is characterized by disturbed glucose and lipid metabolism 5. The effect of magnesium on insulin action could be one possible underlying mechanism by which magnesium affects diabetes risk, because the kinase function of the insulin receptors depends on the binding of two magnesium ions 6. Thus, low intracellular magnesium concentrations may lead to insulin resistance and decreased cellular glucose utilization 7. Unfortunately, plasma magnesium has been shown to be a poor predictor of intracellular magnesium 8. Several laboratory tests to assess cellular magnesium concentrations in the human body have been evaluated, of which measurements in leukocytes generally correlate well with other magnesium pools, including skeleton, vascular smooth muscle and cardiac muscle 9. However, concentrations of magnesium in leukocytes may vary according to pathological states and study population 10. An alternative approach to assess intracellular magnesium concentrations is the assessment of intra-erythrocyte magnesium (IEM), which can be done with direct (dIEM) 11,12 and indirect (iIEM) methods 13. Although the direct method has been suggested to be more accurate, this method requires labor-intensive sample preparation and high additional costs. Indirect methods, on the other hand, calculate IEM concentrations based on whole blood and plasma magnesium, while accounting for hematocrit. So far, only one study evaluated direct and indirect methods for the assessment of IEM, however, using samples of 10 subjects 14. In order to obtain a valid method comparison between dIEM and iIEM, we compared dIEM and iIEM in LifeLines, a large population-based cohort study in the northern part of the Netherlands. Furthermore, we studied correlations between dIEM, iIEM, plasma magnesium and 24-h urinary magnesium excretion and additionally investigated the determinants of these magnesium parameters.
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