Newbold, RR, EP Banks, B Bullock, and WN Jefferson 2001. Uterine adenocarcinoma in mice treated neonatally with genistein. Cancer Research 61: 4325-4328.
Newbold et al. report that when neonatal mice are exposed to genisteina phytoestrogen present in soylater in life they develop uterine cancer of the same form caused by diethylstilbestrol (DES). The levels of genistein used in these experiments are comparable to those found in infant formula based on soy.
This study makes two profoundly important points:
A key question that should be resolved as rapidly as possible is whether differences in the way that humans digest, metabolize and chemically partition genistein once ingested, compared to mice, make human infants less susceptible to this effect than the mice used in these experiments. Until this question is resolved, parents would be well-advised to minimize infant exposure to soy formula. It is plausible that cultural differences in exposure to soy would have led to systematic differences among populations of people in their sensitivity to genistein.
What did they do?
Newbold et al. exposed newborn female mice (days 1-5) to genistein by injection. Another group of newborn females was exposed to diethylstilbestrol, with the exposure level adjusted compared to genistein so that DES and genistein exposed pups received equal exposures in terms of estrogenic potency. The DES group was used as a positive control in the experiment. Another control group was exposed only to corn oil.
Some pups were sacrificed at the age of 5 days to allow examination of body and uterine weights. At the age of 18-months old the exposed mice were then examined to identify cases of uterine adenocarcinoma.
What did they find?
Newbold et al. report several significant findings:
What does this mean?
From the authors: "Of particular significance in this study is the incidence of uterine adenocarcinoma after neonatal exposure to genistein. This compound is readily available to many infants during the first year of life as a component of soy-based formula... The amount of genistein used in our study is slightly higher than the amount consumed by infants, but it is certainly within one order of magnitude of the level of human exposure" (approximately 27 mg of genistein per day for infants feeding on formula vs. 50 mg/day in the experiment).
Newbold et al. also briefly summarize in the paper a variety of other impacts resulting in mice from genistein exposure, including reductions in fertility during adulthood following exposure as newborn.
"The findings of the present study raise concerns over the amount of phytoestrogens in soy-based infant formulas and other soy-based products that are fed to young children. Additional studies are needed to determine the potential effects in humans exposed to high quantities of phytoestrogens during critical stages of neonatal or early development."