Soy could damage fertility in mice, new study

The new study, conducted by researchers at the National Institute of Environmental Health Sciences (NIEHS) in the US, together with the Syracuse University, adds to a mounting body of evidence linking genstein to reproductive problems.

Female mice that were injected with genistein during the first five days of their life were found to have reduced fertility, or to be totally infertile, according to the study, which was published in the January issue of Biology of Reproduction.

The latest findings follow previous research by the NIEHS that showed that mice given genistein immediately after birth had irregular menstrual cycles, problems with ovulation, and problems with fertility as they reached adulthood.

The scientists now say that these reproductive problems are partly a result of alterations to the ovaries during early development caused by the soy component.

The newly born mice that received high doses of genistein were found to have a high percentage of oocytes- or egg cells- remaining in clusters, which means these were less likely to be fertilized.

However, the study also points out that although newborn mice treated with genistein have multi-oocyte follicles (MOFs), and egg cells derived from these have a reduced fertilization rate, there is no proof that this is the case in humans.

In other words, it is still unclear if genistein has an effect on human egg cells.

But, according to Dr David Schwartz, NIEHS director, "although we are not entirely certain about how these animal studies on genistein translate to the human population, there is some reason to be cautious. More clinical studies are needed to determine how exposure during critical windows of development can impact human health."​

However, it is also worth noting that the levels of genistein used by the scientists were significantly higher than what might be found in an average soy-based infant formula.

Genistein is found in soy-based infant formulas as part of a blend of phytoestrogens, or soy components. According to one manufacturer of the product, average levels of genistein found in the phytoestrogen blend used in its line of infant formulas ranged from 8 to 18 percent.

And whereas the mice in the study were injected with 50mg/kg of genistein, an average level of phytoestrogen blend-only a proportion of which is genistein- found in an infant formula is 4mg/kg, based on guidance from the British Dietetic Association.

However, this is not the first study that has linked soy - and specifically the compound genistein- to fertility problems.

Last year a team of UK scientists, headed by Dr Lynn Fraser, professor of reproductive biology at King's College London, revealed that genistein could damage human sperm.

Animal studies on genistein have previously raised other concerns. A 2004 study on newborn piglets found that the compound inhibited intestinal cell growth.

And in 2003, research on rats showed that males whose mothers were fed genistein did not achieve full sexual development as adults.

But the majority of scientific studies so far have focused on the health benefits of soy, and there is a growing awareness amongst consumers that the product is high in fiber, protein and minerals yet low in saturated fat and free of cholesterol.

In 1999 the Food and Drug Administration (FDA) approved an unqualified health claim linking consumption of soy foods to a reduced risk of coronary heart disease. According to a 2000 report in FDA Consumer, consumption of soy foods increased 20 percent per year since 1995 and the approval of this claim led to surging interest.

The link between soy and heart health was also recently reinforced when a study published in November revealed that soy protein containing isoflavones could help reduce two strong indicators for coronary heart disease in postmenopausal women.

Other studies have also indicated that components found in soy could help reduce cholesterol, prevent high blood pressure, alleviate menopause symptoms and maintain bone density.