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. 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," says Sabra Klein at the Johns Hopkins Bloomberg School of Public Health in Maryland.
Synthetic oestrogen-like chemicals from 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. There are also concerns about natural oestrogen mimics, or phytoestrogens.
Soya contains a phytoestrogen called genistein. Large amounts of genistein are found in some baby formula milks and in the supplements that some women take as an alternative to hormone replacement therapy. While results have been mixed, some studies suggest it can affect the immune system and the risk of cancer. A British scientific advisory panel this week warned there is "clear evidence" of a potential risk from soya-based formula milk.
Now Klein's team has found another worrying effect. The researchers fed pregnant female rats genistein-laced diets equivalent to what Western and Asian people might eat. They found that male rats exposed to such levels in the womb grew up to have larger prostate glands and smaller testes. Their sperm counts were normal and when placed with females they behaved as if they wanted to mate, but none was able to ejaculate, the team will report in Urology.
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. "These are serious questions that need answering," says Chris Kirk, who studies plant oestrogens at the University of Birmingham in Britain.
There are no such dramatic effects in the sons of Asian 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.
Other aspects of Klein's study raise more questions. The group found that genistein-exposed male rats had a slightly larger thymus gland, an organ that produces immune cells (Molecular Medicine, vol 8, p 742). That directly contradicts a previous study suggesting genistein shrinks the thymus (New Scientist, 25 May 2002, p 9). The levels of genistein and method of delivery were different in each study, says Klein, but she can't explain the discrepancy.
Another complicating factor is that in Klein's study, moderate levels of genistein had a bigger effect than a huge dose. If this holds true in people it may prove impossible to tease out just what effects eating soya has. "People want this to be simple, but it isn't," says Kirk.
Author: Nicola Jones
New Scientist issue: 15th February 2003
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