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. Dr. 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 3 ounces of a beverage per day -- about one 8 ounce 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 that "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. Dr. 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."