What started as the domestication of a bean species in ancient China has blossomed into a massive, worldwide industry. Soy production in 2017 (in the U.S. alone) was valued at almost 41 billion dollars. Soy has become common in animal feed, baby formula, and 60% of processed foods. It is hailed as a nutrient-dense protein source by many, whether it’s in tofu, tempeh, sauce, oil, milk, nut, or bean form.
Interestingly, soy also has the ability to influence our sex hormones.
Soybeans are a phytoestrogen, meaning they are plant-derived compounds that can weakly mimic estrogen in the human body. In soy’s case, it is compounds called isoflavones that appear to be responsible for its estrogenic effects. Estrogen, found in people of all genders, is a hormone that plays important roles in puberty (especially in women), childbearing, and bone health. Estrogen’s touch extends through many different body systems, affecting your heart, cholesterol and blood sugar levels, memory, and emotional balance.
Estrogen is one of three main sex hormones, with the other two being progesterone (females only) and testosterone (males have higher amounts).
Soy’s estrogen-esque qualities mean consuming it can have beneficial, negative, or non-existent effects on your body and its systems. Can it serve as part of estrogen replacement therapy? Or, does it affect your system in ways similar to BPA, DDT, and a host of other synthetic nasties?
Potential Benefits of Consuming Soy Products
1. May alleviate symptoms of menopause
Soy consumption may have a beneficial effect on managing menopausal symptoms like hot flashes, but the evidence is weak. Hot flashes are far less common in Asian women, and the high level of soy compounds in their blood (12x that of women in the U.S.) may be partly responsible. The North American Menopause Society describes the evidence as being too insufficient to say for sure if soy or isoflavone supplements work or not.
Because any observed side effects have been mild, there’s probably no harm in trying it out, especially if used in conjunction with lifestyle-related strategies like regular exercise.
2. May lower your risk of…
- In one study of Japanese immigrants, higher soy consumption was associated with higher bone density. However, when multiple studies were taken together in a meta-analysis, no significant association with bone density was found, further suggesting that ethnicity play an important role on the effects of soy.
- Heart disease:
- While it was soy’s supposed connection to improved cardiovascular health that boosted its popularity in the West (largely thanks to the Food & Drug Association), the FDA has since reevaluated its statements. In 2017, the FDA officially revoked it’s previous statement on the association between soy protein and heart disease on account of new research that suggested the effect was much smaller than originally thought. If you’re at risk for heart disease though, replacing your meat (especially red meat) with soy on occasion is likely beneficial.
- Breast and/or prostate cancer:
- Genistein, one of the isoflavones found in soy, has been found by multiple labs to have anti-tumor effects, which means it may have therapeutic potential for prostate and breast cancer. However, increased exposure to estrogen is associated with increased breast cancer risk, so it’s a complex relationship. It’s further complicated by the fact that breast cancer tumors can either be hormone positive or hormone negative, which likely changes soy’s effects.
3. May improve depression symptoms
Multiple studies have shown soy or isoflavone consumption to be associated with improved depression symptoms, though others have not (review here).
4. Source of plant-based nutritional goods
Soy is high in polyunsaturated fats, fiber, B vitamins, magnesium, and potassium, and has some potentialantioxidant activity (from genistein). Soy is a complete protein, meaning it has all nine essential amino acids humans can’t make on their own. Consuming soy in moderation can be a good way to get quality protein into a restricted diet.
It’s also important to note that not everyone can digest all of soy’s components: it requires a gut microbiome that only a 1/3-1/2 of people have. Diadzein, another isoflavone in soy, can’t be broken down effectively by most people (though vegetarians and individuals of Asian descent are more likely to be able to).
Risks of Consuming Soy Products
Because the isoflavones in soy bind to estrogen receptors in the human body, it meets the Environmental Protection Agency’s definition as a compound that can “alter the structure or function(s) of the endocrine system and cause adverse effects”. This means it may be responsible for a whole host of negative effects on hormones, regardless of gender. Any endocrine disrupting behavior is likely to have the strongest effects on developing bodies, whether it be a fetus or a teenager. These effects however, may not show up for years to come.
1. Disrupted menstrual cycle and possible fertility issues
In three cases reported by the SUNY Downstate Medical Center, women experienced reproductive issues that were resolved when their soy consumption was reduced to zero. Soy consumption has also been shown to lengthen cycles and decrease levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which play important roles in ovulation.
2. Developmental influences
Estrogen is an important signaler in both male and female development, and as such phytoestrogens pose a danger to healthy development.
Soy Formula: Many countries are far more wary of it than the U.S. tends to be, with some countries even requiring a prescription for soy-based formula. One study found that while soy formula (vs. dairy formula) had no major effects on male or female reproductive health later in life, females fed soy as infants experienced longer periods and greater menstrual discomfort than those fed dairy milk. Another study found soy formula consumption was associated with increased allergies and asthma for females later in life.
Because infants are small and their diet is extraordinarily limited, feeding them exclusively soy formula is powerful. It results in very high concentrations of phytoestrogens in their bodies. Given that soy exposure in young animals has interfered with development of the brain and reproductive tract, and resulted in decreased fertility and increased cancer risk, soy formula should be used with caution in human infants (review here).
3. Mood & behavior changes
In female rats, one study found soy intake to be associated with decreased sex behavior. Soy consumption in male animals has been associated with increased aggressiveness (in monkeys, for example). Given its phytoestrogen status, depression, anxiety, ADD/ADHD, and even seizures (in mice) can be negatively influenced by soy consumption (book).
In one study of 60 patients with subclinical hypothyroidism (aka, a minor/early case of underactive thyroid), individuals with high soy protein intake experienced a 3x increase in their risk of full hypothyroidism. This suggests that soy intake may worsen the condition of an already underactive thyroid.
5. GMO & Roundup exposure
90% of soy worldwide is genetically modified to allow it to withstand being sprayed with Roundup, a toxic herbicide. Steer clear of GMO soy products to minimize your potential exposure to the associated toxins.
6. Reduction in male fertility
Soy food intake was associated with lower sperm concentration in this study of 99 subfertile men. The men who ate the most soy exhibited an average of 41 million sperm/ml less than men who consumed no soy. However, other studies (like this one) have also suggested that soy consumption does not affect semen quality. At least, in fertile men.
In the end, whether you should consume soy or not depends primarily on you. Your gender, age, ethnicity, and even your gut microbiome can determine whether it is a helpful or harmful addition to your diet. Given the conflicting research, it is probably a good idea for most people to limit soy intake. Whole soy foods (like edamame) a few times a week should be fine. There’s also probably no harm in eliminating it completely, as long as you’ve got other quality protein sources in your diet.