A naturally-occurring ingredient of soy beans has been found to interfere with a part of the male reproductive system involved in sperm production.
Soy products are increasingly sold as substitutes for dairy-based food but there is evidence to suggest they contain natural chemicals that mimic the effect of female sex hormones. Soy contains genistein, known to interact with the "receptor" molecules on cells designed to respond to oestrogens.
A laboratory-based study by Ren-Shan Ge of the Wenzhou Medical College in China found that genistein can interfere with the production of vital enzymes involved in producing sperm.
"Following ingestion, soy isoflavones are known to reach the reproductive organs. Thus, excessive exposure to agents that exhibit oestrogenic activity may affect male reproductive tract developments and functions," the researchers say in the study published in the Asian Journal of Andrology.
"With regard to this concern, it has been estimated the genistein and daidzein can reach high concentrations in infants who consume large amounts of soy-based products," they say.
The concentrations of genistein used in the laboratory study are roughly equivalent to the levels that can build up in the human body following a diet rich in soy products.
However, Professor Ieuan Hughes of the University of Cambridge said that a comprehensive inquiry into the oestrogenic chemicals found in soy and other food has failed to find any adverse effects on male reproductive health.
"I suspect the genistein effect is of little relevance to male human health... there was no evidence that soy products had adverse effects on male reproductive health, either via testis function or any other mechanism such as androgen [male hormone] action," he said.