How can soy foods impact upon fertility?
The last few decades have seen a rise in the use of soy foods and soy-derivative products. Soy-based ingredients are cheap to produce, and can be added to a wide number of manufactured food products such as sausages, luncheon meats, protein shakes and bread.
Soy foods such as soy milk, soy yoghurt, soy cheese, soy ice-cream, tofu and infant soy formulas contain isoflavones, which are a class of phytoestrogens. Soy foods can be a major source of phytoestrogens in the diet.
Plant-based phytoestrogens are naturally occurring compounds that are similar in chemical structure and therefore mimic the sex hormone oestrogen in the body. This can have a potentially negative effect on male fertility.
Another potentially negative effect of consuming unfermented soy foods (such as the food examples listed above) is that they contain high levels of phytic acid, a substance which may block the absorption of minerals such as calcium, zinc, magnesium and iron.
The research
Research published in the journal Human Reproduction suggested there may be a negative effect on semen quality from soy-based phyoestrogens.
The 3 month study found that men who ate the most soy foods had less sperm compared to men who did not consume soy products.
The association between phyoeostrogens and a lower sperm count was particularly marked in men who were overweight. According to the researchers, it’s possible that excess body weight modifies the relation between phytoestrogen intake and semen quality.
The foods analysed in this study included tofu, tempeh, soy sausages, bacon, burgers, mince and other meat substitutes, soy milk, soy cheese, soy yoghurt, soy ice cream, and other soy products such as roasted soy nuts, soy drinks, soy powders and energy and bars containing soy protein.
An additional study conducted on 12 healthy men found that supplementation with a soy protein powder over 4 weeks resulted in a small but statistically significant decrease in serum testosterone levels. The testosterone levels increased again within 2 weeks after the powder supplementation was discontinued.
Study limitations
Of the two studies mentioned above, one is on a small sample group (12 men), while the other mentions previous research on humans that contradicts their findings.
The researchers also state that a lot of the evidence establishing a link between soy and male infertility comes from animal studies.
This suggests that more detailed research is needed to help develop a better understanding of the connection between male fertility, soy foods (and the phytoestrogens they contain) and even body fat. In the mean time, the following tips may help you to minimise the impact that soy based foods have on your fertility.
References available on request