lo et al., 1997). After 3 months of supplementation of 350 mg (14.5 mmol)/day of magnesium, the mean retention of 37 percent did not change significantly. Thus, the sensitivity of this method in normal subjects is not yet validated and so cannot be accepted as the primary indicator for assessing adequacy at this time.
One of the problems in using the magnesium tolerance test is that it requires normal renal handling of magnesium. Urinary magnesium loss (related to conditions such as diabetes or drug or alcohol use) may yield an inappropriate negative test. Moreover, impaired renal function may result in a false positive test (Martin, 1990). Age may also be a confounding variable, since older subjects (73 ± 6 years) have been reported to retain significantly more magnesium than younger subjects (33 ± 10 years), despite a comparable mean daily dietary magnesium intake of 5.1 mg (0.2 mmol)/kg of body weight (Gullestad et al., 1994). Supplements of 225 mg (9.4 mmol)/day of magnesium as magnesium lactate-citrate for 30 days to the elderly subjects turned the test results toward normal. Martin (1990) studied 30 elderly females (mean age of 82 years, range 72 to 93 years) who were stated to have “lower than recommended dietary magnesium intakes. ” Subjects with serum magnesium less than 0.59 ± 0.07 mmol/liter (1.4 ± 0.2 mg/dl) retained a higher percentage of the magnesium load (61 ± 12 percent) than subjects with mean serum magnesium levels of 0.72 ± 0.02 mmol/liter (1.7 ± 0.05 mg/dl) whose retention was 43 ± 16 percent. Both of these levels of retention, however, are high compared to that seen in younger age groups. Thus the influence of renal function in this test cannot be ignored. Of significant concern, even if this test is validated in future studies as a primary indicator of magnesium status, is the invasive procedure (intravenous administration) used.
As discussed in the previous section, epidemiologic studies have suggested that individuals or groups ingesting hard water that contains magnesium, consuming a diet higher in magnesium, or using magnesium supplements, have decreased morbidity from cardiovascular disease or less hypertension (Altura et al., 1990; Ascherio et al., 1992; Hammer and Heyden, 1980; Joffres et al., 1987; Leoni et al., 1985; Luoma et al., 1983; Ma et al., 1995; McCarron, 1983; Nadler et al., 1993; Neri and Johansen, 1978; Neri et al., 1985; Rubenowitz et al., 1996; Witteman et al., 1989). Low magnesium intake has also been linked to osteoporosis (Sojka and Weaver, 1995). Because of the difficulty of conclusively establishing that the lack of dietary magnesium