Infant Formulas. Commercial infant formulas that are cow milk-based are generally higher in magnesium concentration, 40 to 50 mg (1.7 to 2.1 mmol)/liter, than human milk. Soy-based formulas may have even higher concentrations of magnesium, 50 to 80 mg (2.1 to 3.3 mmol)/liter (Fomon and Nelson, 1993; Greer, 1989). In a large series of studies (>300 balances), Fomon and Nelson (1993) reported approximately 40 percent net absorption of magnesium in infants fed soy or cow milk-based formulas with a net retention of 10 mg (0.4 mmol)/day based on total intakes of 53 to 59 mg (2.2 to 2.5 mmol)/day of magnesium.
Higher absorption values of 57 to 71 percent of magnesium intake from standard cow milk-based infant formulas have been reported (Kobayashi et al., 1975; Moya et al., 1992). A dietary fractional absorption rate of 64 ± 4 percent was reported in three infants aged 4 to 10 months. Magnesium intakes of the infants exceeded 150 mg (6.3 mmol)/day in the latter study.
Direct assessment of an AI for magnesium for formula-fed infants is not possible due to the lack of data comparing magnesium absorption from human milk and from infant formulas. Based on the current U.S. practices of infant formula manufacturers, the increase of 20 percent above the level of magnesium intake of human milkfed infants allows for the possibility of a lower bioavailability of magnesium from formulas. This leads to an estimated intake of 35 mg (1.5 mmol)/day (human milk + 20 percent) for formula-fed infants. Similar absorption of magnesium from soy versus routine formulas (Fomon and Nelson, 1993) does not indicate a greater need for magnesium in soy formula-fed infants, but the practice of increasing magnesium intake in soy formulas relative to cow-milk formulas is widely followed by formula manufacturers.
A possible approach to determining human magnesium requirements is to estimate the intake required to achieve a level of retention associated with a beneficial outcome. Although magnesium retention during growth should be positive, the desirable extent of retention for magnesium is unknown. As discussed earlier, there are inadequate data upon which to attribute a specific benefit to