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I really love soy products: soy meat, tofu, soy sauce, and basically everything that’s possible. At the same time, some people assure me that soy is harmful to health, while others say that it’s beneficial. Help me understand what’s really going on.
Soy products are known for their controversial reputation. While they are packed with nutrients, particularly complete protein, they also contain components that could potentially pose health risks. Despite this, experts from international medical organizations opine that a reasonable intake of soy products is safe for individuals without soy allergies, and may even offer some health benefits.
What is soy?
Soy, scientifically known as Glycine max, is an annual herbaceous plant that belongs to the legume family and is native to East Asia.
A variety of soy products exist, which are used as food in both non-fermented and fermented forms. Traditional non-fermented soy products include soy milk and tofu, which are made from pre-soaked, boiled and strained soybeans.
Fermented soy products like miso and soy sauce are also available. They are prepared by adding certain types of fungi or bacterial strains to pre-soaked and boiled soybeans. The microorganisms absorb nutrients from the soy and excrete byproducts into it.
On the other hand, modern soy products like soy meat are made in a different way. Soy meat, also known as soy grits, is made by extracting most of the soybean oil from split and steamed beans, resulting in dry soy flakes. If the soy grits are treated with alkali or alcohol, heated, and then dried, a powder containing over 70% soy protein can be obtained.
Nutritional value of soy.
Soy is a highly nutritious food, with 100 grams of raw soybeans containing 30.2 grams of carbohydrates and 19.9 grams of fat. Additionally, soy provides 36.5 grams of protein, which contains all the essential amino acids that the human body cannot produce.
The American Academy of Family Physicians considers soy a good source of protein. However, scientists are still debating whether the amount of essential sulfur-containing amino acids (SCAA) in soy is enough to classify it as a complete protein. Although the lack of SCAA in soy consumption may be a concern, it is not a threat as there are other sulfur-containing amino acids in various other foods, including bread, cereals, meat, and fish. Furthermore, soy is also rich in soluble and insoluble fiber, as well as dietary minerals.
Soy is a rich source of fiber, minerals, and vitamins. For instance, 100 grams of soy provides:
- 15.7 mg of iron, which exceeds the daily requirement for adult men and nearly meets the requirement for women;
- 704 mg of phosphorus, which falls just 100 mg short of the daily adult requirement;
- 1800 mg of potassium, which covers more than half of the daily requirement for adults;
- 277 mg of calcium, which is more than 25% of the daily requirement for adults.
Although soy lacks vitamins B12 and D, it contains other important vitamins such as:
- 0.9 mg of vitamin B1, which is 60% of the adult daily requirement;
- 2 mg of vitamin B6, which meets 20% of the adult daily requirement.
- 6 mg of vitamin C, which is 6% of the adult daily requirement.
The benefits and harms of soy
Two unique components found in soy differentiate it from many other foods: antinutrients and isoflavones.
Antinutrients
Antinutrients, which are also known as anti-nutritional substances, are resistant to digestion and hinder the absorption of protein, vitamins, and minerals such as calcium, iron, phosphorus, and zinc. Soy has three types of antinutrients: tannins, trypsin inhibitors, and phytates. Nonetheless, this is only an issue if soy is eaten raw. Antinutrients reduce the nutritional value of soy feed for livestock.
However, soy is almost never consumed in its raw form. If the beans are soaked for a few hours and then cooked, such as boiling, frying, steaming, or microwaving, the antinutritional substances will be partially destroyed, and the digestibility of the protein will increase by over 60%.
Fermentation is also effective in reducing the concentration of antinutrients. Hence, soy milk, tofu, fermented products, protein supplements, and ready-made dishes made from soybeans and soy meat have such low antinutrient levels that they barely affect the nutritional value of these products.
Isoflavones
Isoflavones are compounds that can have a biological effect on the human body. For instance, studies in a laboratory setting have indicated that soy isoflavones may act as goitrogens, which can inhibit the chemical reactions that enable the thyroid gland to take in iodine. This suggests that consumption of soy products could potentially disrupt the production of thyroid hormones in those who consume them.
Despite this, a group of researchers who examined 14 clinical trials investigating the impact of soy products on thyroid function in both healthy individuals and those with hypothyroidism have concluded that either soy consumption has no effect on the thyroid or the effect is so weak that it does not have a significant impact on a person’s health.
Currently, there is no indication that products derived from processed soy pose a risk to either individuals who are healthy or those who have thyroid conditions. It is probable that they contain insufficient amounts of isoflavones to interfere with the thyroid gland’s function.
There has only been one documented case of an individual with hypothyroidism experiencing toxicity from isoflavones found in soy. This occurred in 2017 when a 72-year-old Japanese woman with thyroiditis was admitted to the hospital with symptoms of acute hypothyroidism after taking a daily dose of 9 grams of soy and cabbage extract for six months.
It is unclear how much isoflavones the supplement contained, but it was evidently much more than what is found in typical foods. Upon being hospitalized, the concentration of these substances in the woman’s bloodstream was very high, and her thyroid hormone levels were very low.
After stopping the supplement and starting to take levothyroxine for 3.5 months, the woman’s isoflavone levels in her blood decreased significantly, her hormone levels were restored, and she recovered.
Isoflavones can also function as phytoestrogens, which are female hormones derived from plants. Due to their chemical makeup, they have a similar structure to estradiol, a female hormone, and can bind to its receptors and activate them, although not as effectively as actual estradiol.
Estradiol plays a significant role in regulating the male and female reproductive systems, controlling fat metabolism, which is essential for the normal functioning of the cardiovascular system, and influencing the development of certain types of cancer. As a result, some researchers hold high expectations for the consumption of soy products, while others express concern.
There is an essential aspect to consider. For phytoestrogens to have a meaningful impact on health, it is necessary to consume a minimum of 40-70 mg of isoflavones daily for several months. However, it can be challenging for people who follow a typical Western diet to meet this requirement.
Fresh soybeans, on average, contain 155 mg of isoflavones per 100 grams. These substances dissolve in water like antinutrients, so their amount in soy decreases after soaking. They also degrade during heating and fermentation. As a result, 100 grams of miso soup, soy meat, and soy milk contain only 41, 34, and 11 mg of isoflavones, respectively.
Only those who reside in Asian countries or people of European descent who regularly consume soy dishes and consume at least two servings per day obtain approximately 50 mg of isoflavones per day throughout their lives on average. In contrast, the average Russian, European, or American who consumes soy sporadically receives no more than 2 mg of isoflavones per day. In such low quantities, phytoestrogens will not have any significant impact on health.
How soy products affect human health and the body
Soy and its derivatives have been extensively researched and there is a wealth of data on the effects of consuming soy products, protein supplements, and supplements containing pure soy isoflavones on human health. However, for the purpose of this article, I will specifically focus on studies that investigated the impact of soy foods, rather than just supplements. These experiments involved providing volunteers with sufficient amounts of soy products to elicit a biological response from phytoestrogens, and lasted anywhere from one month to one or two years.
The Effect of Soy on Cardiovascular Health
Research conducted on animals has indicated that soy products and supplements have the potential to decrease blood pressure and lower cholesterol levels, which can prevent the development of atherosclerotic plaques in blood vessels. Consequently, incorporating soy into one’s diet may lower the risk of atherosclerosis and related health issues, such as ischemic heart disease, heart attack, or stroke.
However, based on observations of 11 studies involving people who consumed soy products and supplements, it was concluded that soy did not have a significant impact on the risk of heart attack and ischemic disease.
Nevertheless, experts from the American Cardiology Association who evaluated 46 clinical trials reported that soy products slightly reduce the concentration of harmful cholesterol in the blood, whereas isoflavone supplements do not provide any benefits. As a result, they recommended that soy foods may still be beneficial for heart health, not due to the phytoestrogens, but because they are a healthy source of protein, unsaturated fats, and fiber. As such, it makes sense to replace some animal-derived products with soy-based alternatives.
The Effect of Soy on men’s health
Some scientists believed that the consumption of soy phytoestrogens could cause a drop in testosterone levels in men, potentially affecting their health and fertility. Conversely, others thought that soy phytoestrogens could reduce excess testosterone production and thereby lower the risk of prostate cancer.
However, studies investigating the effects of soy phytoestrogens on men have yielded conflicting results. A large study based on 15 clinical trials found that the consumption of soy protein supplements by adult athletes did not lead to a decrease in testosterone levels or a negative impact on sperm quality, after two months of supplementation.
In contrast, data from 30 observational studies showed that the testosterone levels of individuals in Asian countries who consumed traditional Asian food rich in fermented and non-fermented soy products from an early age were lower than those of Europeans and Americans. As a result, they had a 29-35% lower risk of prostate cancer than their Western counterparts in adulthood.
The evidence indicates that consuming customary amounts of soy products may have a protective effect against prostate cancer, but this effect appears to be dependent on starting consumption during childhood. Based on the available data, it is not possible to achieve the same benefit by taking soy supplements in adulthood.
When consumed in amounts typical for Asian populations, soy products do not appear to have negative effects on health. However, exceeding these amounts may lead to problems. For instance, a 60-year-old man who consumed an excessive amount of soy milk (three liters per day) for six months developed gynecomastia (breast enlargement) due to the influence of phytoestrogens.
Additionally, a 19-year-old vegan who mainly consumed soy food experienced a significant drop in testosterone levels, which led to the development of erectile dysfunction. This individual only recovered after giving up the soy diet for a year.
The Effect of Soy on Estrogen Levels in Women.
Similar to men, some researchers have expressed concerns that the consumption of soy phytoestrogens could lead to a decrease in estrogen production in women. Others hoped that phytoestrogens would bind to estradiol receptors instead of the actual hormone, thereby reducing the risk of breast cancer.
The findings from studies on phytoestrogens in women are nearly identical to those in men. According to data from 47 studies, the consumption of soy and isoflavone supplements has little effect on the levels of estrogens in adult women during perimenopause and menopause.
In addition, data from eight observational studies show that women in Asia who consume soy from childhood are less likely to develop breast cancer compared to their European and American counterparts.
However, some experts suggest that unlike men, where isoflavone supplements have no significant impact, such supplements may increase the likelihood of breast cancer in women with a high predisposition to this disease or accelerate its development in those already diagnosed. However, ordinary soy products, which contain a small amount of phytoestrogens, need not be avoided by such women.
How Much Soy Can You Safely Consume?
According to research, individuals who do not have soy allergies can consume up to 50 grams of soy protein per day safely, which is a substantial amount. A typical serving size of a soy product, such as a cup of soy milk, half a cup of boiled soybeans, 85 grams of tofu, or a soy-based protein bar, contains around 6-9 grams of soy protein.
However, consuming precisely 50 grams is not necessary to obtain the health benefits of soy. Consuming 15-25 grams of soy protein, or 2-4 servings of any soy products each day, is sufficient.
There is a limited amount of data available on the effects of soy products on pregnant and breastfeeding women and children. Although there is no direct reason to restrict soy product consumption for this group, some experts recommend that pregnant women consume no more than 25 mg of soy protein per day, as a precautionary measure.
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