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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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