Soy and Sperm Production: Sensational Story Lacks Science
By Mark Messina, PhD, Executive Director, Soy Nutrition Institute
Can eating tofu ruin your chances of becoming a father? A recent headline in the Daily Mail and other publications says it will. But the science behind these big headlines is pretty thin, as a little digging on my part found.
I couldn’t find the alleged study that served as the basis for the claims. As it turns out that’s because no such study has been published. Instead, the findings came from an abstract that was presented at a poster session at the October meeting of the American Society for Reproductive Medicine. This means that the study hasn’t yet been subjected to peer review. I was eventually able to obtain a copy of the abstract by contacting Professor Allan Pacey of Sheffield University in England, who was quoted in the news reports as cautioning against concluding too much from the study.
It was an extremely small epidemiologic study, involving just 25 men. All had proven fertility. According to 40-item questionnaire and an analysis of semen samples, there were associations between isoflavone intake and sperm abnormalities. But that’s nothing to get too concerned about. Epidemiologic studies show only associations, they aren’t able to establish cause and effect relationships.
While epidemiologic studies do make important contributions to the literature, typically meaningful findings only come from large cohort studies involving tens of thousands of individuals and many years of follow up. They also depend upon rigorous control of potentially confounding variables. This small analysis didn’t identify much in the way of lifestyle factors.
Since the study was conducted in Spain, where soy consumption is not the norm, it’s likely that those men who were eating soyfoods also had any number of other lifestyle differences with their non-soy-eating counterparts that might have affected the findings. On the other hand, the men in this study could have been fairly representative of the general population. In this case, even the soy consumers probably had very low isoflavone intakes — so low as to raise serious doubts about the biological plausibility of the findings. Unfortunately, the abstract didn’t provide this kind of important information on intake.
Importantly, the abstract stands in stark contrast to three clinical intervention studies which found no effects of even large amounts of soy isoflavones on sperm or semen parameters.1-3 These studies carry a lot more weight than a small epidemiologic study that hasn’t been published. Further reassurance about soy comes from a recent study that found soyfood intake by men was unrelated to clinical outcomes (e.g., fertilization rates, clinical pregnancy, live birth, etc.) among couples presenting at an infertility clinic.4
While the study from Spain makes for good clickbait, it doesn’t make for good science. The best evidence still shows that soyfoods are safe for men who are trying to conceive.