Submitted to: International Journal of Toxicology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 9, 2001
Publication Date: N/A
Interpretive Summary: The American Academy of Pediatrics recommends breastfeeding over formula feeding, and recommends milk-based formula over formula made with soy proteins. However, in cases where milk-based formula cannot be fed to infants, it recommends soy formula. There are approximately one million infants per year fed soy formula in the United States with excellent success. However, there are critics to the use of soy formulas because of the high levels of estrogen-like compounds (phytoestrogens) found in soy. We have conducted a study of several generations of rats to determine if there are any adverse effects of soy intake on development. We have found that rats grow and develop normally and except for very minor differences that have not been found in human, there are essentially no adverse effects of continuous intake of high levels of soy protein over several generations. This study adds support to the safety and efficacy of feeding soy infant formula.
Technical Abstract: Dietary factors other than the traditional nutrients are found in the so-called functional foods. They are becoming increasingly recognized as potentially important for maintaining good health. Soybeans are rich in such factors thought to help prevent certain chronic diseases. Soy protein isolate (SPI) is one of the three major proteins used in infant formulas sold in the United States, with casein (CAS) and whey (WPH) proteins being the others. We have been studying the health effects of these proteins. Safety concerns have developed over the consumption of soy-based infant formula, partly because of the high circulating levels of the total isoflavones (phytoestrogens) during "critical periods of infant development." There is a paucity of data on developmental, physiological, neurophysiological, behavioral, metabolic, or molecular effects of soy phytochemicals in humans, especially during pregnancy and infancy. We have studied the effects of CAS, SPI, and WPH in short-term, long-term, and multigenerational studies in rats. Aside from minor differences in body weight gain profiles, CAS-, SPI-, or WPH-fed rats did not differ in development, organ weights, in vitro hepatic metabolism of testosterone (T), or reproductive performance. However, some endocrine-related functions differed between rats fed these proteins. We found that SPI accelerated puberty in female rats (P<.05) and WPH delayed purberty in males and females, as compared with CAS (p<.05). Gender differences were also found in gonadectomy-induced steroid responses. Male rats had normal serum T levels, but female rats fed SPI and reduced serum 17 beta-estradiol concentrations and a blunted 17 beta-estradiol response to ovariectomy, as compared to rats fed CAS or WHP (P<.05). Female rats fed SPI or WHP or treated with genistein had reduced incidence of chemically-induced mammary cancers (P<.05) compared to CAS controls, with WHP reducing tumor incidence by as much as 50%, findings that replicate previous results from our laboratory. Together, these results suggest gender-specific differences in development and certain endocrine responses among rats fed diets composed of a single protein source such as those used in infant formulas. Whether similar developmental effects occur in human infants is unknown, but unlikely because (1) most infants do not consume such diets throughout life as these rats did, and (2) no such effects have been reported in millions of American infants fed infant formula containing these proteins. The long-term health consequence implications of early diet exposure to SPI and WPH, such as reduced breast cancer incidence, are likely to be very positive.