The study, carried out by Harvard School of Public Health, looked at the diets of 99 men who had fertility-related problems. When the sperm concentration of men eating the most soy was compared with those eating the least, there was a significant difference.
The researchers found 41 million fewer sperm per milliliter of semen after just one portion every two days. The "normal" sperm concentration for a man is between 80 and 120 million per milliliter.
The presence of isoflavones and estrogen in soy can have a potentially negative affect on reproduction, including decreased fertility. These chemicals can have similar effects to the human hormone oestrogen, the BBC reported.
The effect was more prominent in overweight or obese men indicating that higher levels of body fat can also lead to increased oestrogen production in men. However, soy does not seem to affect fertility among Asian populations probably because obesity is not as common in Asian populations as it is among Western men.
Soy-based diets have risen steadily in the West since the 1940s and are now included in many products, including biscuits, sweets, pasta and bread. Being a cheap source of protein, it has gained significant popularity in Asia.
