A group of US researchers have found a potential link between high levels of soya in the diet and low sperm concentration. The findings were reported in the journal Human Reproduction.
Dr Jorge Chavarro led the study at the Department of Nutrition at Harvard School of Public Health, Boston, which looked at 99 men who had visited a fertility treatment clinic for fertility evaluation between 2000 and 2006. They were asked how often and how much soy-based food they had eaten in three months, including tofu or soy sausages, bacon and mince, soy milk, cheese and yoghurt.
The men were divided into four groups according to their intake of soy foods and, after adjusting for other factors such as age, BMI (body mass index), alcohol and caffeine intake and smoking, Chavarro found that men in the highest intake category had on average 41 million sperm per millilitre less than men who did not consume soy products. The 'normal' sperm concentration is between 80 and 120 million sperm per millilitre.
The study 'suggests that soy foods could have some deleterious effect on the reproductive system and especially on sperm production', said Chavarro. The correlation was also exacerbated in men who were overweight or obese.
The findings do not explain why soy foods have this effect on sperm concentration, but Chavarro speculates that it could be because of the levels of 'isoflavones'; plant-derived compounds with oestrogenic effects, something that may affect sperm production by interfering with hormonal signals. Obese men have higher levels of natural oestrogen in their body, further supporting the theory of a hormonal role.
In other parts of the world such as Asia, however, the normal intake of soya is much higher than in the study, and there is no evidence of higher levels of infertility in these areas. Chavarro said that this study is not sufficient to draw firm conclusions and further larger studies would be needed to determine whether soy intake has implications in infertility.