Can soya bean consumption harm sperm?

Scientists in China have been studying genistein, a naturally-occurring ingredient of soya beans that they claim could interfere with the production of enzymes involved in sperm production.

The researchers, who published their work in the Asian Journal of Andrology (a Nature journal) said:

“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.”

With soy increasingly an ingredient in food, particularly as a substitute to dairy, the SMC wrapped up reaction from scientists on the research.

Ian Shaw, Professor of Toxicology, Department of Chemistry, University of Canterbury comments:

“There are many chemicals that interfere with the synthesis and breakdown of steroid hormones and that mimic their activity in hormone receptors. The effects of these chemicals are at least additive and therefore it is meaningless to single out a particular compound (e.g. genistein) or group of compounds (e.g. soy phytoestrogens) for scrutiny without putting our exposure to them in the context of the myriad other chemicals with similar effects (e.g. bisphenol-A from plastics) – this is very naive!

“Unfortunately regulators seem only to be able to produce safe exposure limits (e.g. Acceptable Daily Intake – ADI) for individual compounds which is why this philosophy is still followed. It is the sum of the exposures that leads to a biological effect not an individual exposure to an individual compound.”

From our colleagues at the Science Media Centre in London:

Professor Ieuan Hughes, Chair of the Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment, University of Cambridge, comments:

“I suspect the genistein effect is of little relevance to male human health. Testis microsomes, rather than whole cell assays, were used and the concentrations of genistein used in the in vitro experiments would not replicate what the fetal male, newborn male, pubertal male or the young adult male would be exposed to. I refer you to the comprehensive FSA report published a few years ago on phytoestrogens and human health and 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 action.”

Dr Paul Kroon, Project Leader (Polyphenols and Health), Institute of Food Research, comments:

“The researchers used data from test-tube experiments to show that a particular form of genistein that is typical of soy foods inhibited a particular enzyme involved in testosterone synthesis.

“Isoflavones are in different forms in soy compared to blood. In soy, they are linked to sugars (glucose). In blood, >95% is attached to a different sugar (glucuronic acid) or sulphate and these blood forms are now labelled for excretion via the liver (bile) or kidney (urine). The other 5% is present as genistein, which is the compound used by the researchers who authored the new publication.

“Human intervention trials where subjects have been fed soy foods give rise to concentrations of total genistein in blood of around 1000 nmoles per liter. Only about 5% of this is present as genistein, therefore a concentration of about 20 nmoles per liter is achievable. Therefore, it is conceivable that consumption of genistein may result in inhibition of the particular enzyme involved in testosterone synthesis in the testes of men.

“However, I am not aware of there being much evidence, if any, that soy consumption by men alters testosterone-related functions. Nevertheless, there is some data indicating this may occur in animals – this may be because high doses were used.

“Inhibition of this particular enzyme would only result in reduced testosterone synthesis in humans if this particular step was the rate-limiting one in testosterone synthesis – I don’t know if this is the case.”

*Soy isoflavones inhibit production of testosterone, Ren-shan Ge et al., to be published in the Asian Journal of Andrology, 0900 UK time Monday 10 May 2010