Relative importance of carbohydrate and protein sources in the differential effects of soy-based vs casein-based formulas on bone minerals in rats.

Studies reported here examine the extent to which differences in the source of protein (soy vs casein) and of carbohydrate (absence or presence of lactose) may be responsible for differential effects of soy-based and casein-based infant formulas on bone minerals. Growth and bone minerals were measured in rats fed casein-based or soy-based diets with or without lactose. Analysis of variance indicated that presence of lactose in the diets increased calcium, phosphorus, magnesium, and zinc concentrations in the vertebrae (P less than 0.02) and also increased magnesium (P less than 0.01) and zinc (P less than 0.05) in femur and tibia-fibula. In contrast, the source of protein had an effect only on bone magnesium concentration, with the soy protein resulting in lower magnesium retention in all bones studied (P less than 0.05). These data suggest that differences in lactose content rather than in the source of protein may be mainly responsible for the differential effects of milk-based and soy-based infant formulas on bone minerals.