Some popular fitness and health-related sources tend to claim that the link between soy and estrogen is real. On top of that, they say that this connection has a harmful effect on men’s health. Let’s see where the truth is!
Soy and Estrogen Connection
Soybeans and products based on it can hardly be called popular types of food among men. However, protein soy isolates are included in sports nutrition, and soy concentrates are common in the food industry as fillers for various meat and sausage products, as well as meat surrogates.
Vegetarians are also active consumers of soy. They use it as a source of protein. The protein quality of soybeans is one of the most attractive reasons for the interest in soy and soy foods among vegetarians .
Recently, soy and high-protein products derived from it have been criticized for allegedly having unpleasant properties to influence men’s health, which is due to the content of phytoestrogens, isoflavones with a molecular structure similar to endogenous estrogens.
Knowing how many traditional food products are subjected to completely unreasonable demonization, I would like to understand the objectivity of the claims made to soy and estrogen.
Are Soy Isoflavones to Blame?
Isoflavones belong to the functional class of non-steroidal phytochemicals called phytoestrogens (which also include lignans and coumestans), which have a chemical structure and functions similar to endogenous estrogens in animals [2, 3].
The main isoflavones found in soybeans are genistein, daidzein, and glycitein. The isoflavone content of soy products varies between brands and products.
For example, in Canada, the isoflavones in soybeans range from 360 to 2241 mg/kg.
Isoflavones in soybeans from Romania are ranged from 210 to 1340 mg/kg, 1176-3309 mg/kg in the US, and 525-986 mg/kg in India.
Soy temperature and moisture are critical for the differential synthesis of soy isoflavones during plant growth, with the highest concentrations of isoflavones occurring at low temperatures and high soil moisture, mainly depending on the first one.
Interaction of Isoflavones With Estrogen Receptors
Isoflavones have the ability to interact with estrogen receptors (ER) due to their structural similarity to 17β-estradiol.
However, isoflavones have low estrogenic activity compared to estradiol .
Indeed, the affinity of soy isoflavones for ERa and ERß was found to be 1/1000 and 1/3 times, respectively .
Interacting with ERa, isoflavones exhibit an estrogenic effect, and with ERß, an antiestrogenic one. Genistein has a higher affinity for estrogen receptors than daidzein [5, 6, 7].
It has been suggested that soy isoflavones may act as selective regulators of tissue estrogenic activity and as a selective modulator of estrogen receptors, having mechanisms other than direct interaction with the receptor [8, 9, 10, 11].
Phytoestrogens produce estrogenic, antiestrogenic, or no activity depending on tissue [12, 13]. T
he estradiol receptors ERa and ERβ demonstrate different, often directly opposite effects in men and women; therefore, phytoestrogens do not show their activity as classical estrogen agonists .
Concerns about the adverse effects of soy consumption were originally based on animal data. Nowadays, it is well known about the difficulties of extrapolating the data of such studies to humans in everyday use .
In mouse models, for example, intrauterine exposure to estradiol was incomparable with that of a human due to a more than a millionth difference in concentration .
Monkeys, just like rodents, have a completely different metabolism of isoflavones [17-23].
Initially, a link between soy and estrogen/male fertility was postulated in 1940 after the recognition of endocrine disruption in sheep caused by red clover isoflavones, leading to a syndrome of infertility called “clover disease” .
It is also worth adding that the usual levels of consumption of soy products in various countries, including in Asia, even among people who adhere to vegetarianism, often do not lead to the intake of isoflavones in the body in quantities that could cause problems.
Unfortunately, this also reduces the beneficial effects of isoflavones. To obtain the beneficial effects of soy isoflavones, their intake must be at least 60-100 mg per day , which is currently difficult to achieve in Western countries.
The highest consumption of isoflavones is observed in the countries of East and South Asia (20-50 mg/day). In the USA, it is 0.15-3 mg, and in European countries, it is 0.49-1 mg [35, 36].
In general, the low content of bioactives in modern second-generation soy products and moderate amounts in traditional ones provide moderate health benefits with very limited risk of potential adverse health effects .
Effect of Soy Phytoestrogens on Sex Hormones
In humans, the disturbance of the sex hormone system and the problems of infertility associated with soy foods are in stark contrast to those seen in large populations of soy-consuming countries.
For example, reproductive health disorders, including feminizing effects, erectile dysfunction, and decreased libido, associated with a very high intake of isoflavones, have been reported.
Although the available data have found only minor harmful effects or no health effects at all [25, 26].
In clinical studies with high exposure to isoflavones in men, no estrogen-related disorders have been reported .
Conversely, sperm count, and motility were positively associated with isoflavone intake  or no correlation between isoflavone intake and sex hormones or sperm quality .
Testosterone levels were assessed in a meta-analysis of 32 clinical reports without a convincing interaction between consumption of soy or soy isoflavones and free testosterone concentration .
Most of the information about the interaction of soy with sex hormones has come from in vitro and in vivo studies , but not human studies.
In a recent paper on the hormonal effects of soy products, the authors concluded: “Clinical data show that neither soy foods nor soy protein, nor isoflavones affect circulating reproductive hormone levels in men or women.
However, some studies show that soy protein, but not isoflavones, affects insulin-like growth factor I (IGF-1). ” 
Regarding the effect on IGF-1, it should be clarified that it manifests itself in a small increase in circulating hormone in response to consumption servings containing more than 25 g of soy protein.
Similar effects are observed after consuming other types of protein. That is, in this regard, soy protein is just as good as others.
Conclusions on the Effects of Soy and Estrogen on Men’s Health
- Currently, there is no evidence of the negative effects of soy-based products on men’s health, and the available evidence (soy and estrogen link) comes from animal studies that are not applicable to humans;
- In those who regularly consume soy products, the typical intake of isoflavones, which are phytoestrogens, is low and often insufficient even to produce positive health effects, let alone the perceived risk of negative effects.
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