Adult male mice fed a soy-rich diet had sperm counts 25 per cent lower, and 21 per cent smaller litter sizes, than mice fed a soy-free diet, according to research published in Molecular and Cellular Endocrinology.
The male animals were generally considered to exhibit normal male behaviour and were fertile, report researchers from the University of Geneva (Switzerland), University Hospital, Lausanne (Switzerland), and the National Center for Toxicological Research in the US.
“This study extends our knowledge on the effects of soy and dietary phytoestrogens exposure on male reproductive system and provides a better understanding of the potential relationships between endocrine disruptors and reproductive abnormalities,” wrote the researchers, led by Serge Nef from the University of Geneva’s Department of Genetic Medicine and Development.
Of mice and men
The study was dismissed, however, by Michelle Braun, PhD, a member of soy supplier Solae's nutrition science group, who noted that scientific evidence from human studies shows no correlation between soy intake and changes in testosterone levels, therefore, suggesting no affect on male fertility (Fertil Steril 2009).
“In this study, animals in one of the experimental groups were fed a single source of protein (soy protein) for the duration of life, which is not comparable to typical human consumption,” said Dr Braun. “It is tenuous to attribute the differences observed between the two study groups to the phytoestrogen content of the diet.
“The health benefits of dietary soy protein go beyond the phytoestrogen content. There is an overwhelming amount of clinical evidence that indicates soyfoods, which are low in fat and a good source of high quality protein can make important contributions to an overall healthy diet,” she added.
How it was done
The latest findings follow previous research by researchers at the National Institute of Environmental Health Sciences (NIEHS) in the US that showed that female mice given genistein immediately after birth had irregular menstrual cycles, problems with ovulation, and problems with fertility as they reached adulthood.
For the new study, Nef and his co-workers fed male mice either a high soy-containing diet, or a soy-free diet from conception to adulthood. The researchers estimated that the soy-fed animals consumed between 22 and 26 milligrams of genistein per kg of body weight per day, and the same amount of daidzein.
At the end of the study, the soy-fed animals were considered fertile and exhibited normal male behaviour, but epididymal sperm counts were 25 per cent lower, and litter sizes were 21 per cent smaller.
Changes to genes involved in sperm mobility were also reported by the researchers.
Furthermoe, Nef and his co-workers report that “dietary soy decreased the size of the seminal vesicle but without affecting its proteolytic activity”. Seminal vesicles are glands involved in the production of a liquid that contributes to semen.
This is not the first study that has linked soy, and genistein in particular, to fertility problems.
In 2005 a team of UK scientists, headed by Dr Lynn Fraser, professor of reproductive biology at King's College London, revealed that genistein could damage human sperm.
But the majority of scientific studies so far have focused on the health benefits of soy, and there is a growing awareness amongst consumers that the product is high in fibre, protein and minerals yet low in saturated fat and free of cholesterol.
In 1999 the US Food and Drug Administration (FDA) approved an unqualified health claim linking consumption of soy foods to a reduced risk of coronary heart disease. According to a 2000 report in FDA Consumer, consumption of soy foods increased 20 per cent per year since 1995 and the approval of this claim led to surging interest.
The link between soy and heart health was also recently reinforced when a study published in November revealed that soy protein containing isoflavones could help reduce two strong indicators for coronary heart disease in postmenopausal women.
Other studies have also indicated that components found in soy could help reduce cholesterol, prevent high blood pressure, alleviate menopause symptoms and maintain bone density.
Source: Molecular and Cellular Endocrinology
Published online ahead of print, doi: 10.1016/j.mce.2010.02.011
“Potential detrimental effects of a phytoestrogen-rich diet on male fertility in mice”
Authors: C.R. Cederroth, C. Zimmermann, J-L. Beny, O. Schaad, C. Combepine, P. Descombes, D.R. Doerge, F.P. Pralong, J-D. Vassalli, S. Nef