Mother's soya diet could harm unborn child

- Last updated on GMT

Related tags: Immune system, science, Pregnancy

Researchers are advising pregnant mothers not to eat soya after a
study found pinpointed it can damage the sexual development of
infant rats.

Eating soya during pregnancy could harm the sexual development of unborn infants, say researchers in a report this week.

In a study on rats, researchers in the US found that males whose mothers were fed a chemical found in soya did not achieve full sexual development as adults.

The animal study does not prove that soya has this effect on people, and no such effects have been observed in Asia where soya is a big part of many people's diets, noted the report on the study in New Scientist​ magazine. But the researchers say it is enough to spark concern and deserves further study.

"The urologists on this project are actually advising pregnant women to avoid soya,"​ Sabra Klein at the Johns Hopkins Bloomberg School of Public Health in Maryland, US, told the New Scientist​.

Other oestrogen-like chemicals found in cosmetics, plastics and birth control pills have been blamed for changing the gender of fish in polluted streams and even lowering sperm counts in people. But there have also been concerns about natural oestrogen mimics, or phytoestrogens, from the genistein in soya.

The report explains that large amounts of genistein are found in some baby formula milks and in the supplements designed as an alternative to hormone replacement therapy. Some studies have suggested that the genistein present can affect the immune system and the risk of cancer. And there has also been concern among some experts about the use of soy formula, with the UK government's scientific advisors just this week warning that soya-based formula milk should be only available on prescription.

In the new study, Klein's team fed pregnant female rats diets with amounts of genistein equivalent to that which could be found in a typical western or Asian diet. They found that male rats exposed to such levels in the womb grew up to have larger prostate glands and smaller testes, according to the report. Their sperm counts were normal and when placed with females they behaved as if they wanted to mate, but none was able to ejaculate, it continued.

Further, the effects were just as severe in males that did not eat genistein after weaning as it was in those that continued eating it. This suggests exposure in the womb and during breast feeding has the biggest impact, concludes the report.

Chris Kirk, who studies plant oestrogens at the UK's University of Birmingham, told the New Scientist: "These are serious questions that need answering."

The report rightly points out that there are no such dramatic effects in Asian men and vegetarian women. But one study has linked a vegetarian diet during pregnancy to an increased risk of hypospadias - a condition where the urethra emerges along the shaft of the penis rather than at the tip. Some researchers suspect this is due to the genistein in soya.

The report finds other problems with Klein's study: the group found that genistein-exposed male rats had a slightly larger thymus gland, an organ that produces immune cells, but this directly contradicts a previous study suggesting genistein shrinks the thymus (New Scientist, 25 May 2002). The levels of genistein and method of delivery were different in each study, said Klein, but she could not explain the discrepancy, according to the New Scientist​.

Also, in Klein's study moderate levels of genistein had a bigger effect than a huge dose. This presents a significant challenge for researchers trying to assess the effects of eating soya.

The team is to report the full findings in a forthcoming issue of Urology​.

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