and Nelson (1993) and Koo and Tsang (1997) used data from cadavers to estimate a net accretion of 10 mg (0.4 mmol)/day during the second year of life. However, further information is needed before this approach may be uniformly used to estimate magnesium needs throughout childhood.

The total magnesium content of an infant weighing 3.5 kg (7.7 lb) is approximately 220 mg (9.2 mmol)/kg or 760 mg (32 mmol). Magnesium as a percentage of fat-free body mass increases during gestation, but at birth the percentage is much less than that of an adult (Widdowson and Dickerson, 1964). The content of magnesium in an adult man is estimated to total 27.4 g (1,141.7 mmol), or about 390 mg (16.2 mmol)/kg (Widdowson and Dickerson, 1964).

In order to accumulate approximately 26.6 g (1,108.3 mmol) over the 20 years of growth from infancy to adulthood, an average daily accretion during this period of about 3.6 mg (0.2 mmol) would be necessary. However, the growth rate is not linear with age. It has been suggested that an adequate accretion rate (positive balance) for girls 10 to 12 years of age and weighing about 40 kg (88 lb) is 8.5 mg (0.3 mmol)/day (Andon et al., 1996). For older children who are heavier and experiencing greater growth in lean and bony tissue, a positive balance in the range of 10 mg (0.4 mmol)/day would be appropriate.

Magnesium Tolerance Test

The magnesium tolerance test, which is based on the renal excretion of a parenterally administered magnesium load, has been used for many years. It is considered by some to be an accurate means of assessing magnesium status in adults, but not in infants and children (Gullestad et al., 1992; Ryzen et al., 1985). However, the sensitivity of this method in detecting magnesium depletion may be different between subjects with and without hypomagnesemia. In 15 hypomagnesemic subjects, 85 ± 3 percent of a parenterally administered magnesium load was retained, compared to only 14 ± 4 percent in 23 normal controls (Ryzen et al., 1985). In a group of 24 chronic alcoholics at risk of magnesium deficiency, retention of 51 ± 5 percent was also significantly greater than the control group. While the magnesium tolerance test has been shown in this and other studies (Cohen and Laor, 1990; Costello et al., 1997; Gullestad et al., 1992) to detect magnesium depletion in both hypomagnesemic and normomagnesemic subjects at risk of magnesium depletion, the test was not sensitive to detect treatment effects of magnesium supplementation in otherwise healthy subjects (Costel-

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