1991). Hypomagnesemia (serum magnesium < 0.75 mmol/liter [1.8 mg/dl]) usually develops concurrently with moderate to severe magnesium depletion. However, in clinical studies in which concentrations of magnesium in blood cells, bone cells, or muscle cells are abnormally low (such as in diabetes mellitus, alcoholism, or malabsorption syndromes), serum magnesium values have been reported to be within the normal range (Abbott et al., 1994; Nadler et al., 1992; Rude and Olerich, 1996). These findings suggests that intracellular magnesium is a better guide to magnesium status in humans than is the concentration in serum (Reinhart, 1988; Ryzen et al. 1986). One recent longitudinal evaluation of 8,251 subjects who entered a study between 1971 and 1975 found 492 cardiovascular events when the subjects were reevaluated between 1982 and 1984 (Gartside and Glueck, 1995). Subjects with serum magnesium levels less than 0.81 mmol/liter (1.9 mg/dl) had greater risk of cardiovascular disease than did those with a serum magnesium level greater than 0.87 mmol/liter (2.1 mg/dl). Although both values were within what has been considered the normal range, 0.75 to 0.95 mmol/liter (1.8 to 2.3 mg/dl) (Elin, 1987), this variation in response questions the validity of serum magnesium levels as indicators of magnesium status (Gartside and Glueck, 1995). There are also reports that elderly subjects may have a decrease in magnesium status as determined by magnesium tolerance testing (see below) despite normal serum magnesium concentrations. Thus, serum magnesium concentration has not been validated as a reliable indicator of body magnesium status.

Recently, ion-specific electrodes have become available for determining ionized magnesium in the plasma. Early results suggest that this may be a better index of magnesium status than the total serum magnesium concentration; however, further evaluation is necessary. Few studies have been conducted under varying conditions to assess its validity (Altura et al., 1992; Mimouni, 1996).

The total magnesium contents of several tissues, including red blood cells, skeletal muscle, bone, and peripheral lymphocytes have been evaluated as indices of magnesium status. However, determination of intracellular magnesium concentration should be a more physiologically relevant measurement of magnesium status, as it is