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, 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.
