mg (13.8 mmol). Daily magnesium intakes by pregnant women in the United States, where the study by Sibai and coworkers was completed, ranged from 158 to 259 mg (6.6 to 10.8 mmol) (Franz, 1987). Based on these baseline estimates of dietary magnesium intake and the amounts provided in the above trials, total magnesium intakes in the supplemented groups would have ranged from 420 to 625 mg (17.5 to 26 mmol). The results of magnesium supplementation trials indicate that the incidence of preeclampsia and intrauterine growth retardation was not affected by magnesium supplementation in two studies, the incidence of preterm delivery decreased in only one of three studies, and preterm labor was less frequent in one of two studies (see Table 6-5).

Accretion Rate during Pregnancy. The increase in body weight caused by lean tissue accretion during pregnancy is expected to result in a greater requirement for magnesium if there are no pregnancy-induced increases in intestinal absorption and renal reabsorption. Data are not available on accretion of magnesium in lean tissue during pregnancy, but this accretion can be estimated (see following section). Given that fat-free body mass contains about 470 mg (19.6 mmol) of magnesium/kg (Widdowson and Dickerson, 1964), it is possible to determine the amount necessary for accretion for an appropriate weight gain.

Inconsistent findings on the effect of magnesium supplementation on pregnancy outcome make it difficult to determine whether magnesium intakes greater than those recommended for non-pregnant women are beneficial. In addition, there are no data indicating that magnesium is conserved during pregnancy or intestinal absorption is increased. The gain in weight associated with pregnancy alone may result in a greater requirement for magnesium.

The EAR for pregnancy is set at an additional 35 mg (1.5 mmol)/day. This additional requirement is based on the following assumptions:

• Appropriate added lean body mass (LBM) is 6 to 9 kg with a midpoint of 7.5 kg (IOM, 1991).

• The magnesium content of 1 kg of LBM is 470 mg (19.6 mmol) (Widdowson and Dickerson, 1964).

• The adjustment factor for a bioavailability of 40 percent (Abrams et al., 1997) is 2.5.