Although many previous studies have shown that soya is good for humans by lowering cholesterol, boosting cognitive function and protecting against prostate cancer (just a few of its health benefits), its consumption may halve sperm concentrations, research published in the journal Human Reproduction reveals.

The study was carried out by Harvard School of Public Health and involved 99 men with fertility-related problems who visited the Massachusetts General Hospital Fertility Center with their partners between 2000 and 2006.

When looking at the men’s diet, the researchers found that men whose diet contained the most soy foods had the lowest sperm concentration. Usually, the “normal” sperm concentration is between 80 and 120 million per milliliter. Those men serving one soy portion every two days had a sperm concentration that was 41 million/mL less than that of men who ate no soy, Jorge Chavarro, MD, ScD, ScM, of Harvard School of Public’s nutrition department said.

Dr. Chavarro blamed isoflavones (daidzein, genistein and glycitein), some chemicals present in nearly all soy-based foods for the low sperm concentration. Isoflavones appear to have estrogenic effects (estrogen is known as a female hormone). Therefore, a diet rich in soy products could interfere with hormonal signaling involved in sperm production. The difference was even higher in overweight men, whose fat tissue produces more estrogen. They had a sperm concentration that was 50.45 million/mL less than that of men who ate no soy.

However, Dr. Chavarro said more study needs to be done in order to draw clear conclusions on soy consumption and sperm concentration.

"The clinical significance of these findings remains to be determined. Owing to the scarcity of human data in this area, it is very important that this issue is examined further, ideally in randomized trials," he said.

He further noted that traditional Asian diets high in soy haven’t been shown to harm fertility. However, men in the US and other Western countries are more predisposed to obesity, which might shift men’s sensitivity to phytoestrogens (estrogen-like plant compunds), but this theory has to be proved also. Obesity is not that common in Asian populations as it is among Western men, another reason to believe that obesity has to do with low sperm concentration and not the consumption of soy products.

The Soy Protein Association immediately refuted the findings of the study, pointing out that it was “very small and fails to acknowledge the large normal variation in sperm counts.” Moreover, Asian men are the best evidence ever that soy products do not harm fertility, the agency further said.

Dr. Allan Pacey, a senior lecturer in andrology from the University of Sheffield, supports the same theory as well.

"We will have to look at adult diet more closely, although the fact that such large parts of the world have soy food as a major part of their diet and don’t appear to suffer any greater infertility rates than those on western diets suggests that any effects is quite small."

The soy bean, Glycine max, is an annual plant native to East Asia. . Its seed is made up of 40 percent protein and around 20 percent oil. Since 1940, soy-based diets have risen steadily in the West and are currently included in many products, including biscuits, sweets, pasta, and bread. Being a cheap source of protein, it has gained significant popularity in Asia.