The rats were given a commercially available supplement in doses similar to those taken by women. The findings suggest a possible negative side effect for the supplements, which are becoming very popular among US women.

Isoflavones, the key compounds in soy, are estrogen-like molecules. Soy supplements are touted as a natural alternative to hormone replacement therapy and recent warnings of the risks of HRT may boost the numbers of women turning to the supplements.

The supplements do have other potential health benefits. Women in the Far East, whose regular diet includes relatively large amounts of soy, have much lower cancer rates than Western women.

But some studies highlighted possible downsides. Soy isoflavones have been show to interfere with estrogen function in rats, decreasing some of the brain molecules involved in social and sexual behaviour. And a specific isoflavone called genistein prevents the tumour-fighting drug tamoxifen from working in mice. The new study, showing that a soy isoflavone has negative effects on sexual behaviour in rats, joins these...