Can Too Much Soy Protein Affect Fertility?

There have been a number of recent studies that have investigated the possible effects of soy protein on fertility. Nutritionists have been praising soy protein for years because it provides a low-fat source of protein that is a viable alternative to the consumption of meat products. Vegetarians and vegans rely heavily on soy products for day-to-day protein sustenance. But, are there possible side effects involved with increasing your consumption of soy protein?

Animal Studies and Background

Fertility implications involving diets high in soy protein have been observed in animal populations since the 1940s. At that time, sheep farmers in Australia found that when their sheep grazed on clover that contained a high level of isoflavones, an organic compound that is found in high levels in soy as well, a high percentage of them became infertile. Isoflavones act as phytoestrogens, which can mimic and affect estrogen levels in the body. In the case of the sheep, it was thought that the isoflavones adversely affected the hormone levels leading to infertility. Similar observations have been made with other animals as well.

Scientists found that a group of California quail that was forced to feed on desert plants during a food shortage experienced a lower fertility rate. Similar studies have been carried out on rats, cattle and other birds and all have led to similar results: isoflavones adversely affect fertility.

Effects in Humans

In humans, there have unfortunately been only a handful of studies that have delved into the effects of soy proteins and phytoestrogens on humans. For women, a study conducted in the UK involved a group of premenopausal women who consumed 60 grams of soy protein per day for 30 days. The study found that the women experience significant effects. Although none of the women in the study failed to ovulate during the 30-day period, levels of luteinizing hormone and follicle-stimulating hormone, both of which are integral in reproduction processes, were reduced as much as 50%. Though the study only lasted for one month, these side effects lasted for three months following, making the likelihood of longterm complications as a result of these changes in hormone levels possible.

For men, there have been many studies that have investigated the effect of estrogen exposure during key periods of male reproductive development. They have found that if males are exposed to estrogen during childhood, normal reproductive processes can be adversely affected and interrupted. However, studies have shown that such adverse effects are limited to childhood exposure and do not carry through to adult males.

Possible Benefits and Future Research

Researchers have also found that phytoestrogens may have beneficial attributes, especially in the treatment of prostate, breast, bowel cancers, as well as menopausal symptoms and heart disease. Therefore, it is extremely important that researchers find out the extent to which such compounds adversely affect reproductive health in humans. Though animal studies have shown that the consumption of soy and other foods high in isoflavones can lead to reproductive complications, further human testing is needed as the possible health benefits certainly do warrant further investigation.


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