Scientists found that even modest consumption of soy products, such as meat and dairy substitutes and bean curd, can have a significant impact on sperm count.
Men who ate an average of half a serving of soy food a day had lower concentrations of sperm than those who did not.
Low sperm count is known to make it harder for a man to conceive.
Soy compounds called isoflavones, which mimic the female sex hormone oestrogen, are thought to be behind the effect.
Animal studies have linked high consumption of isoflavones with infertility, but until now, there has been little evidence of their impact on human reproduction.
Sperm count ranges between 80 and 120 million sperm per milliliter (million/ml) of semen for normal healthy men.
However, researchers in the U.S. found that among the men they studied, those with the highest soy intake produced much less sperm. On average their counts were 41 million/ml lower than those of men who did not consume soy products.
The scientists, led by Dr Jorge Chavarro, from the Harvard School of Public Health in Boston, questioned 99 men seeking help for fertility problems about their consumption of 15 soy-based foods.
Each man was asked about his diet in the previous three months. The foods included tofu, tempeh, soy sausages, bacon, burgers and mince, soy milk, cheese, yogurt and ice cream, and other soy products such as roasted nuts, drinks, powders and energy bars.
The scientists varied serving sizes to take account of the fact that different foods contained different levels of isoflavones. A standard serving of tofu was said to be 115g, and for soy milk, one 240ml cup.
Factors such as age, body mass index (BMI) which relates weight and height, smoking, alcohol and caffeine intake, and the length of time since last producing semen, were adjusted for.
"Men in the highest intake group had a mean soy food intake of half a serving per day: in terms of their isoflavone content, that is comparable to having one cup of soy milk or one serving of tofu, tempeh or soy burgers every other day," said Dr Chavarro.
"It is important to highlight that the figure of half a serving a day is the average intake for men in the highest intake group. Some men in this group had intakes of soy foods as high as nearly four servings a day."
The association between soy consumption and sperm count was stronger in men who were overweight or obese, according to their BMI.
This applied to 72% of the group, which reflected the general proportion of overweight and obese men in the U.S. population.
The effect was also most pronounced in men with higher sperm counts.
"The implication is that men who have normal or high sperm counts may be more susceptible to soy foods than men with low sperm counts," said Chavarro.
Soy appeared to have no effect on sperm activity or shape, which can also influence fertility.
Overweight men may be extra affected because their bodies produce more estrogen than slimmer individuals, the researchers believe.
Writing in the journal Human Reproduction, they concluded, "We found an inverse association between the consumption of soy foods and sperm concentration which was more pronounced at the higher end of the sperm concentration distribution and among overweight or obese men.
"The clinical significance of these findings remains to be determined. Owing to the scarcity of human data in this area, it is very important that this issue is examined further, ideally in randomized trials."
In their paper, the scientists addressed the question of why Asian men, who eat large amounts of soy, still appear to be perfectly fertile.
Previous studies had shown Asian men to have slightly smaller testicles and lower sperm counts than non-Asian men, although it was not known whether the findings were statistically significant.
One post-mortem study found that Asian men had lighter testicles than Caucasian and Hispanic men. They also had fewer immature sperm in their testes.
"Although it is true that Asian men consume five to 10 times more phytoestrogens (plant versions of estrogen) than men in our study, there may be other factors that could make Western men more susceptible to phytoestrogens," the scientists wrote.
"One possibility is that excess body weight modifies the relation between phytoestrogen intake and semen quality as our data suggests. While increasing at alarming rates, the prevalence of overweight and obesity is still much lower in Asia than in the USA."
It is possible that higher levels of oestrogen from body fat made reproductive organs more sensitive to oestrogen-like chemicals in the environment, said the researchers.
Soyfoods Association Response
Not surprisingly, the Soyfoods Association of North America attempted to discredit the study:
Headlines claiming “soy products lower sperm count” do not tell the whole story. The small-scale, preliminary study that Dr. Jorge Chavarro, published online in Human Reproduction, is based on recollected intake of soyfoods and not on specific diets containing soyfoods. “This study is confounded by many issues, thus I feel the results should be viewed with a great deal of caution,” warned Dr. Tammy Hedlund, a researcher in prostate cancer prevention from the University of Colorado Health Sciences Center, Department of Pathology. Chavarro found that “soyfood and soy isoflavone intakes were unrelated to total sperm count, ejaculate volume, sperm motility, or sperm morphology” which are the important measures of sperm quality and male fertility. The study also did not determine directly what other foods, medications, supplements, existing medical conditions, sexual activities or environmental factors may have directly affected the drop in sperm count. The study also classified high intakes of soyfoods as less than 3oz of a beverage per day—about one 8oz glass of a beverage every three days. This is not considered a high intake of any food under most circumstances.
Dr. Larry Ross, past president of the American Urology Association, noted, “Like most epidemiological reports, this study is retrospective and is therefore inherently subject to a variety of biases. The total study population was small (100 subjects), and little is known or reported about other dietary, stress factors, medications or lifestyle issues that might also affect sperm counts in these men. It is well known that of all semen parameters, variation in count (#s of sperm/cc seminal fluid) is highly variable from day to day and seasonally in all men. This is most clear when one considers the "range" of "normal" sperm count accepted in our field (20-200,000 million.)”
Chavarro found that the men with the highest soyfood intake produced more ejaculate fluid volume with equivalent amounts of sperm count as those with lower intakes, neither the volume or the number of sperm was significantly different, however this larger volume lead to the lower sperm concentrations (millions per millimeter) in the higher intake individuals. This watering-down effect of sperm concentration should not be mistakenly associated with a decrease in fertility.
Obesity may be the explanation for the negative findings of this study. Chavarro found high soyfood intakes are associated with lower sperm concentration, but “the association was more pronounced among overweight and obese men than among lean men.” Men with high levels of body fat are likely to produce more estrogen than their slimmer counterparts.
Chavarro’s study conflicts with the large body of U.S. government and National Institute of Health-sponsored human and primate research, in which controlled amounts of isoflavones from soy were fed and no effect on quantity, quality or motility of sperm were observed. Upon hearing of Chavarro’s findings, Dr. Stephen Barnes, a pharmacologist at the University of Alabama at Birmingham, noted, “This study is the first to find this correlation. The research on soy in men has not found a negative impact on male hormones but rather has suggested a preventive effect in prostate cancer.”
From the August 4, 2008, Prepared Foods e-Flash