Use of Soy Protein-Based Formulas in Infant Feeding

Soy protein-based formulas have been available for almost 100 years. Since the first use of soy formula as a milk substitute for an infant unable to tolerate a cow milk protein-based formula, the formulation has changed to the current soy protein isolate. Despite very limited indications for its use, soy protein-based formulas in the United States may account for nearly 25% of the formula market. This report reviews the limited indications and contraindications of soy formulas. It will also review the potential harmful effects of soy protein-based formulas and the phytoestrogens contained in these formulas. T AMERICAN ACADEMY of Pediatrics (AAP) is committed to the use of human milk as the ideal source of nutrition for infant feeding. However, by 2 months of age, the majority of infants in North America are receiving at least some formula. Soy-based infant formulas have been available for almost 100 years.1 Despite limited indications, soy protein-based formula accounts for approximately 20% of the formula market in the United States. Because an infant formula provides a source of nutrition for an extended interval, its nutritional adequacy must be proven, and the indications for its use must be substantiated and well understood. This statement updates the 1998 AAP review of soy protein-based formulas and addresses the ongoing concern of phytoestrogens in soy formulas. COMPOSITION Isolated soy protein-based formulas currently on the market are all free of cow milk protein and lactose and provide 67 kcal/dL. All are iron-fortified and meet the vitamin, mineral, and electrolyte specifications addressed in the 2004 guidelines from the AAP for feeding term infants2 and established by the US Food and Drug Administration.3 The protein is a soy isolate supplemented with L-methionine, L-carnitine, and taurine to provide a protein content of 2.45 to 2.8 g per 100 kcal or 1.65 to 1.9 g/dL. The fat content of soy protein-based formulas is derived primarily from vegetable oils. The quantity of specific fats varies by manufacturer and is usually similar to those in the manufacturer’s corresponding cow milk-based formula. The fat content ranges from 5.02 to 5.46 g per 100 kcal or 3.4 to 3.6 g/dL. The oils used include soy, palm, sunflower, olein, safflower, and coconut. Docosahexaenoic and arachidonic acids now are added routinely. In formulas, carbohydrate sources are corn maltodextrin, corn syrup solids, and sucrose, with content ranging from 10.26 to 10.95 g per 100 kcal or 6.9 to 7.4 g/dL. Until 1980, mineral absorption from soy formulas was erratic because of poor stability of the suspensions and the presence of excessive soy phytates.4 Because soy protein isolate formulas still contain 1.5% phytates, and up to 30% of the total phosphorus is phytate bound, they contain 20% more calcium and phosphorus than cow milk-based formulas and maintain the ratio of calcium to available phosphorus of 1.1 to 2.0:1. With the current formulations, bone mineralization, serum concentrations of calcium and phosphorus, and alkaline phosphatase concentrations in term infants through 12 months of age are equivalent to those observed in infants fed cow milk-based formulas.5–7 Because soy phytates and fiber oligosaccharides also bind iron and zinc,9 all soy-based formulas are fortified with iron and zinc.8,9 Phytoestrogens in Soy Protein-Based Formulas Of the many heat-stable factors present in soy formulas, the phytoestrogens are of particular interest in human health. Phytoestrogens consist of several groups of nonsteroidal estrogens, including isoflavones. Isoflavones are commonly found in legumes, with the highest amount found in soybeans.1,10 Concerns raised in relation to phytoestrogens/isoflavones include their potential negative effects on sexual development and reproduction, neurobehavioral development, immune function, and thyroid function. On the other hand, epidemiologic studies have www.pediatrics.org/cgi/doi/10.1542/ peds.2008-0564 doi:10.1542/peds.2008-0564 All clinical reports from the American Academy of Pediatrics automatically expire 5 years after publication unless reaffirmed, revised, or retired at or before that time. The guidance in this report does not indicate an exclusive course of treatment or serve as a standard of medical care. Variations, taking into account individual circumstances, may be appropriate.