HAILED as the 鈥渘atural鈥� and healthy alternative to hormone replacement therapy, plant oestrogens are also said to help prevent cancer, strokes and bone loss following the menopause. But they could also pose a health risk, as evidence emerges that they act like 鈥済ender-bending鈥� pollutants that may increase the risk of cancer.
Many foods, including soya, olives and onions, are rich in plant oestrogens, and extracts rich in these substances are marketed as a tonic for menopause symptoms. Some studies in rats have hinted that they protect women against breast cancer, an idea bolstered by the fact that Japanese women, who eat a lot of soya, have one-sixth the Western incidence of breast cancer.
But some experts remain unconvinced of these benefits. 鈥淭he evidence that plant oestrogens protect against cancer is very circumstantial,鈥� says Chris Kirk at the University of Birmingham. Genetic differences make populations difficult to compare, he points out. And people in Japan eat much less fat, which is known to contribute to breast cancer.
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In fact, far from keeping cancer at bay, Kirk has found evidence that plant oestrogens may actually encourage tumours to grow. He fears they may act in a similar way to pollutants such as PCBs, insecticides and alkylphenols, which have been widely linked to hormone disruption and reproductive problems in animals.
One effect of these pollutants is to block an enzyme involved in oestrogen metabolism called sulphotransferase. In breast cancer cells its activity is abnormally low, but injecting DNA that encodes the enzyme reduces the rate at which cancer cells divide. Kirk says this means these pollutants may contribute to the growth of breast tumours.
This week, Kirk told a meeting of the Society for Experimental Biology in Swansea that plant oestrogens behave in exactly the same way. In experiments, his team has shown that some plant oestrogens reduce the activity of sulphotransferase in platelets and liver cells.
Richard Sharpe at the Medical Research Council鈥檚 Human Reproductive Sciences Unit in Edinburgh agrees that this is cause for concern. He fears plant oestrogens could also cause abnormalities during a fetus鈥檚 development and says women should avoid consuming large amounts while pregnant. 鈥淲e鈥檝e got to get away from this idea that 鈥榥atural鈥� is necessarily good,鈥� he says.
But Herman Adlercreutz of the University of Helsinki wants to see more evidence before we blame plant oestrogens. 鈥淢ost of this is speculation,鈥� he says.