• Janki T. Patel, PharmD

Soy: debunking myths and false science

Updated: Mar 7, 2020

Soy is one of the world’s most versatile crops, from use in consumption through diet and animal livestock, to the production of medications/supplements, bio-diesel and industrial use such as in plastic, solvents, rubber, and even replacement of petrochemicals. It has been around for thousands of years, with origins in southeast Asia. However, it has recently become a topic of conversation in relation to nutrition with contradicting ideas in circulation. So using science, let’s dive in!

About soy:

Soy, also known as Glycine max, is a part of the pea and legume family. It comes in many forms of food ranging from edamame, soy nuts, and soy milk, to fermented products such as natto, tempeh, soy sauce, tofu, and miso. More processed forms include oils and textured isolated soy proteins, commonly found in mock meat and protein powders/supplements.

Nutrition profile:

While focusing on a single macro or micronutrient follows a reductionist approach and is not ideal when looking at the bigger picture of a whole-food diet and the synergistic benefits from phytochemicals, it can be still be helpful to understand the nutrient density of this plant food.

  • Soy is a complete plant protein-- meaning it contains all nine essential amino acids that are needed, but not produced by the human body. Much like the popular grain, quinoa.

  • Low in saturated fat

  • High in fiber, vitamin C, iron, magnesium, phosphorus, potassium, folate, thiamine and other B vitamins

  • Per USDA, 100g of raw soybean sprouts contain 3.04 g of protein, 1.8 g of dietary fiber, 13 mg of calcium, 0.91 mg iron, 61 ug of folate, 149 mg of potassium. 100 g of Boiled, green soybeans with no added salt contains 12.35 g of protein, 4.2 g of dietary fiber, 145 mg of calcium, 2.5 mg iron, 61 ug of folate, 539 mg of potassium.

Myth versus Science:

The supposed bad guy here that’s rooted to these myths seems to be the hormone estrogen. It should be clarified that estrogen/estradiol/estrone are endogenous steroidal hormones produced by humans and other mammals, which is different than phytoestrogen, a biochemical found in soy products. Phytoestrogen can compete for estrogen receptors, but their potency and ability to bind and modulate receptors is much “weaker” than estrogen. To understand the pseudoscience behind claims of soy and estrogen, it is key to understand the molecular and functional complexity of the effects of human/mammalian estrogen which are positive in certain areas of the body (i.e. in the bone for prevention of osteoporosis) and negative in other areas of the body (i.e. in mammary glands which can increase the likelihood of developing breast cancer.) Selective estrogen receptor modulators (SERM) can provide both pro-estrogenic effects (i.e. breast cancer development) and anti-estrogenic effects (i.e. menopausal hot flashes) depending on its specific tissue selectivity and signaling. Phytoestrogens fall into the category of flavonoids, with one of the most potent being isoflavones (i.e. daidzein, genistein, and glycitin which comprise of 40%, 40%, and 5-10%, respectively, of soy isoflavone content with equol being a converted byproduct of daidzein by gut bacteria). Isoflavones bind to both types of estrogen receptors (ER)—ERα, found in breast and uterine tissue, and ERβ, which is cardioprotective and bone-stabilizing. Human/mammalian estrogen binds to both receptors equally, whereas isoflavones prefer binding to ERβ. For this reason, we see isoflavones exerting both selective SERM and tissue effects. Acknowledging this physiology allows us to apply the basic science of its' mechanism to navigate through some of the misinformation involving soy.

Myth: “Soy is feminizing for males.”

Science: There is one scientifically “weak” published case report of a 60-year-old male who reported bilateral gynecomastia (male breast enlargement) and erectile dysfunction with decreased libido while self-reporting consumption of 3 quarts of soy milk for a period of 6 months. This literature was reviewed and has not since been reciprocated and apart from this extreme case without further knowledge of this individual’s medical history, studies have shown that in relation to a single serving size, moderate to higher levels of soy consumption, have deemed to be safe and, contrarily, even beneficial. (See below for further details on the studied benefits.) Additionally, moderate to high intake of soy does not show harm. In fact, it has been commonly documented that Asian populations, who have a greater isoflavone intake, are observed to have much lower rates of prostate weight, prostate cancer, and breast cancer. Conversely, dairy and animal meat have high amounts of mammalian estrogen hormones for use in increasing the production and growth of livestock, with up to 80% of dietary intake of this hormone sourced from consumption of dairy and animal meat, per Estrogen Dominance Guidance reports. If one has concerns of gynecomastia or hormone imbalance, dairy and animal meat consumption should be eliminated from the diet as this estrogen is not a phytoestrogen and most certainly does not have the beneficial effects of isoflavones which have been shown to provide more benefit in the hormone balance of males and females.

Myth: “Soy causes or promotes breast, ovarian, and prostate cancer.”

Science: While there is a link between androgens and prostate cancer, there is misinformation regarding the role of soy and estrogen. This again should be broken down into the effects of mammalian estrogen versus phytoestrogens found in soy. Fertility and Sterility published a meta-analysis showing that both isoflavone supplementation and soy consumption do not affect male testosterone levels. Breast cancer, if ER+, depends on the estrogen hormone production for the growth of cancer cells. However, isoflavones present in soy products have an anti-estrogenic effect (SERM) at the breast tissue, meaning it blocks the promotion and growth of these cells, thus reducing the risk of breast cancer and improving breast cancer survival.

Myth: “It’s a genetically modified crop”.

Science: This sparks fair concern, as there are substantial amounts of genetically modified soy products and pesticides that are used for its’ production. However, if closely examined, majority of the genetically modified soy products are used for animal agriculture (i.e. feeding livestock such as pigs, cows, and chicken.) These animals are then, of course, consumed by humans which indirectly results in the consumption of these GM crops. On the other hand, non-GMO soy for direct human consumption, which has reduced levels glyphosate if also organic, is used by most major food producers. Another way to confirm this is to check the label of any soy products used for consumption to verify that it is made organically and/or without genetic modification.

The Benefits:

(Please reference the studies noted in the sources for full detail on the benefits listed below.)

  • Reduction of LDL cholesterol and risk of cardiovascular disease

  • Improved symptoms of Polycystic ovarian syndrome (PCOS)

  • Reduction of menopausal symptoms

  • Lower rates of breast and prostate cancers

  • Daily intakes of phytoestrogens (~ 45 mg) have been shown to stabilize hormone balance.

  • May shrink fibroids in women

  • Reduced risk of endometriosis

  • Reduced risk of osteoporosis (Fun fact: daidzein, a major isoflavone found in soy, is actually used in the active agent ipriflavone in several countries to prevent and treat osteoporosis, though from a therapeutic/guideline standpoint, it is not first line. It has also provided use in Padget’s disease, renal osteodystrophy, and paralysis associated from weakened bone density with stroke.)

In summation, phytoestrogens are still being researched in its nutritional role with diverse metabolic functions. I hope that the basis of this post backed by evidence provides enough clarity to those who may have heard conflicting jargon with soy consumption.

What questions do you have? Comment below!


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