What’s the science say about soy?

Soy has been a food surrounded with controversial claims over the past several decades and has now become one of the most well-researched foods of all time. There is a lot of misleading information about soy and health. Some of these misunderstandings are just that – misunderstandings of the data as a whole and what influenced the outcomes. Others may actually be dubious in nature due to lobbyists, industries in competition with soy, or confirmation biases. Let’s delve into the research and break things down simply.

So what exactly IS soy?

Soy is classified as a legume, which is technically a type of dry fruit contained within a pod. Other examples of legumes include peas, lentils, beans, and even peanuts. Unprocessed or minimally processed soy has similar  benefits as any other whole plant food in that it contains fiber, loads of micronutrients and various other phytonutrients.

On average, one ninety gram serving of whole soybeans, cooked from dry, contains about 155 calories made up of a whopping 16.4 grams of protein, 7.5 grams of carbohydrates with 5.4 grams coming from fiber, and 8.1 grams of fat, most of which in the form of poly and monounsaturated fats. This same serving size also contains about 4.6 milligrams of iron or fifty-eight percent of the RDA and is also very high in magnesium, copper, manganese, phosphorus, potassium, vitamin B2, vitamin B6, folate, vitamin K and so on.

It’s pretty much indisputable that higher fiber diets are better for human health. A series of systematic reviews and meta-analyses concluded that individuals consuming higher fiber diets have a fifteen to thirty percent decrease in all-cause and cardiovascular related mortality, stroke, heart disease, type 2 diabetes and colorectal cancer than those consuming low amounts of fiber. The higher fiber groups also benefited from lower total cholesterol, lower bodyweight and reduced risk of hypertension. Risk reduction was noticeable when daily fiber intake was above twenty-five grams and seems to be even more protective at levels above that (Reynolds, 2019). 

Soy is particularly high in protein and lower in fat and carbohydrates when compared to other legumes. Plant protein has been found to be more cardioprotective than animal based protein in several studies. In a study that analyzed 81,337 men and women’s diets, the individuals that consumed more plant protein and little or no animal based protein suffered fewer cardiovascular related deaths or illnesses over a course of a 9.4 year period (Tharrey, 2018).

There are a few mechanisms that have been researched that explain why animal based protein in particular seems to be associated with cardiovascular illnesses.

1.) TMAO, or trimethylamine N-oxide, is an organic compound produced from choline, lecithin, and L-carnitine rich foods by certain gut microbiota. These substances are most commonly found in animal based foods. Ingestion of these foods over an extended period of time seem to foster the gut microbiota that produces TMAO, since long time vegans do not seem to experience this conversion. Increased levels of TMAO seems to impair cholesterol transport, which leads to increased atherosclerosis, which is a buildup of plaque in arterial walls (Koeth, 2013).

2.) Heme iron, found almost exclusively in meat products, seems to be directly associated with incidence of coronary heart disease. A meta analysis of prospective studies concluded that heme iron specifically was found to cause a thirty-one percent increase in coronary heart disease when compared to low heme iron intake (Yang, 2014).

3.) Saturated fat intake and dietary cholesterol are both associated with higher levels of LDL cholesterol which is linked to heart disease as well, but that’s a whole article in itself (Hopkins, 1992)(Clarke, 1997).

Organic, conventional, GMO?

Genetically modified soy was first introduced to the markets in 1998 by Monsanto. Genetic modification involves genetic engineering techniques to actually introduce new DNA. Roundup-ready soybeans, produced by Monsanto, are modified soybeans that are resistant to the pesticide roundup, which Monsanto also produces.

The health effects of consuming genetically modified foods is very controversial. One study demonstrated that organic soybeans had the healthiest nutritional profile when compared to conventional and genetically modified, containing more protein and zinc and less omega-6 fatty acids and saturated fat. The genetically modified soy contained residue of the pesticide Roundup which wasn’t found with the conventional or the organic soy (Bohn, 2014) . But how harmful is Roundup, really?

One study concluded that endocrine and toxic effects can be seen in mammals at doses smaller than those found with agricultural use and the effect increases with dose and exposure time (Richard, 2005). Roundup also appears to be significantly more toxic than its individual component chemicals, when analyzed individually. One study observed the difference with Roundup appearing to be up to 1,000 times more toxic than its active constituents (Mesnage, 2014).

Based on this research, long term ingestion of genetically modified foods most likely isn’t the best decision if the consumer is primarily concerned about health. Not all conventional foods are going to be genetically modified, but there’s a high chance that conventional soy is since so much of it is genetically modified. On the other hand, the use of genetic engineering is prohibited for organic soy and it’s very easy to find organic soy products such as tofu and tempeh in stores.

Soy isoflavones and phytoestrogens

The term phytoestrogen is basically ubiquitous with soy foods, which is interesting because there are many food products on the market containing more phytoestrogens or estrogenic compounds than soy. The hops found in beer is one example for high phytoestrogen content and dairy milk contains high amounts of mammalian estrogen, on the other hand (Maruyama, 2010). Even animal-based food products can contain trace amounts of phytoestrogens due to their diet. Many are fed high phytoestrogen foods such as GMO soy which can be detected in the meat and secretions and are re-consumed by us later on (Jargin, 2014).

So, what exactly are phytoestrogens and how do they act in the human body though?

Different types of phytogen molecular structures

Phytoestrogens are naturally occurring compounds in plants that are similar in chemical structure to mammalian estrogen molecules but behave a bit differently in the body. How phytoestrogens behave in the human body is very complicated, although science is learning more about it every year. A growing amount of research seems to demonstrate beneficial effects in the human body, contrary to some research seem in animal studies. It’s important to base findings off of studies conducted on humans when they’re available due to obvious differences in biochemistry and in the case of soy, there’s more than enough available.

There have been two different types of estrogen receptors identified in the human body known as ER-alpha and ER-beta receptors. Both receptors are involved in complex, physiological processes; some of which have beneficial reactions in the body and some harmful. Both receptors are responsible for sexual maturation and have cardioprotective effects when functioning healthily. The alpha subtype may specifically promote skeletal and metabolic health whereas the beta subtype has more of an effect on the central nervous and immune system. ER-alpha activation may promote cell proliferation in breast and uterine tissue whereas ER-beta seems to counteract this (Paterni, 2014).

The phytoestrogens found in soy have been found to preferentially bind to the ER-beta receptor and essentially act as a natural SERM (Selective Estrogen Receptor Modulator) in this regard (Mueller, 2004). This explains the mechanism for how soy appears effective for reduction and protection from hormonal related cancers, such as prostate, breast and uterine cancers. Besides binding to estrogen receptors, phytoestrogens also may be able to influence the amount of endogenous hormones by binding or inactivating certain enzymes and depressing or increasing the production of SHBG (sex hormone-binding globulin).

Soy and Men’s Health

Prostate cancer is the most diagnosed cancer for men in the United States and the second most lethal cancer. Death is not usually from prostate cancer itself, but from spreading cancerous cells to another area of the body.

A 2004 study demonstrated that activation of ER-beta resulted in a tumor-suppressing effect in prostate cancer, specifically (Cheng, 2004). Another 2014 study demonstrated that the genistein found in soy (a type of phytoestrogen) at moderate dietary levels was able to inhibit prostate cancer cells from spreading (Pavese, 2014). Additionally, a 2004 study on diet was conducted on patients with advanced prostate cancer. One group ate a bread high in soy grits every day, another ate a bread high in soy grits and linseed, and another ate a regular wheat bread. The first two are high in phytoestrogens while the last is low. The high phytoestrogen diet group showed a favorable influence on PSA (prostate-specific antigen) levels, which is a biomarker used to identify potentially malignant cancer cells in the prostate. These results demonstrated that men following a diet high in dietary soy seem to have a reduced risk of prostate cancer development and progression (Dalais, 2004). Finally, a 2009 systematic review and meta-analysis of five cohort studies and eight case-control studies concluded that consumption of certain soy-based foods (specifically higher in the phytoestrogens genistein and daidzein) could lower the risk of prostate cancer (Hwang, 2009).

Okay, well Soy looks like it has potential to be protective against prostate cancer, but what about testosterone levels? Everyone has heard that soy lowers testosterone levels in men, right? This idea stems from the fact that soy contains higher levels of phytoestrogens than many other plant foods, but as we discussed above, soy phytoestrogens behave differently in the body than mammalian estrogens and acts as a natural SERM, in a way. A 2010 meta-analysis looked at nine clinical studies and determined that there’s no evidence that isoflavones effect circulating estrogen levels in men (Messina, 2010). Another 2010 meta-analysis reviewing thirty-two reports concluded that soy protein and/or isoflavones had no effect on testosterone, free testosterone, SHBG, or FAI (free-androgen index) (Hamilton-Reeves, 2010).

Soy and Women’s Health

There’s a wide array of studies that have been conducted on soy and hormonal related issues in women. An overwhelming amount of evidence points to beneficial effects for breast cancer, especially when consumed often starting at an early age.

Researchers have noticed that Asian women have lower rates of breast cancer, but after migrating to a western society, breast cancer risk goes back up to match. A 2009 study looked at soy intake in multiple races in Asia and the United states and found that soy consumption through all stages of life seems to decrease rates of breast cancer, with the biggest reductions in individuals who consumed soy regularly from childhood onward (Korde, 2009). Meta-analyses of 8 studies concerning Asian women populations showed a markedly decreased risk of breast cancer with increased soy food intake as well. There appeared to be about a sixteen percent reduction in risk per ten milligrams of isoflavones consumed per day. Age was also a factor observed here, with earlier exposure to soy isoflavones showing an even more decreased risk of breast cancer rather than later exposure in life (Wu, 2008).

Another study followed 5,042 breast cancer survivors over an average time period of 3.9 years. There were 444 deaths and 534 recurrences during this period. Soy food intake showed about a four percent difference on average for both mortality and recurrence between the high soy food group and low soy food group, which is small but statistically significant (Shu, 2010). An additional meta-analysis of forty-six prospective studies conducted in 2015 also demonstrated that higher soy food intake seemed to reduce the risk of breast cancer, especially when compared to red meat and processed meat, which seemed to be risk factors (Wu, 2016).

Intake of soy foods also seems to decrease frequency of hot flashes in menopausal women without side effects, according to a 2015 meta-analysis that reviewed fifteen randomized control trials with thirty to 252 subjects (Chen, 2015).

Soy isoflavones also may improve bone mineral density in women by up to fifty four percent, although most of these benefits are likely to be seen in postmenopausal women with above seventy-five milligrams of isoflavones daily (Wei, 2012). Post-menopausal women drinking two glasses of soy milk a day containing seventy-six milligrams of isoflavones have been shown to completely eliminate their risk of bone loss in the lumbar spine over a two year period in one trial. The women consuming none or less lost an average of 4.2%, demonstrating  a significant difference (Lydeking-Olsen, 2004). A reduced risk of fracture has also been observed in post-menopausal women. Over an average time of 4.5 years, 1,770 fractures were recorded in 24,403 post-menopausal women. After the study adjusted for age, socioeconomic status, major factors of osteoporosis, other dietary factors and so on, the study concluded that soy food intake seems to reduce risk of fracture in post-menopausal women, especially in the early years after menopause (Zhang, 2005). Some studies in western nations yielded inconsistent results regarding bone mineral density, but this may be because of using soy protein supplements and processed soy rather than whole, unprocessed soy foods, which were used for the other studies that found positive results (Reinwald, 2010). For the most part, it’s usually better to favor the least processed foods as possible for optimal health benefits.

General Health

The phytoestrogens in soy are largely responsible for the differences in health benefits between the two sexes, but several shared benefits have been observed besides what we’ve already discussed so far.

Soy appears to have beneficial effects in regards to several different types of cancers. A systematic review and meta-analysis which included eight case control studies and two prospective cohort studies found a protective effect against endometrial cancer risk (Zhang, 2015). There’s also evidence to suggest that high intake of several different types of soy foods protects against gastric cancer risk as well. Interestingly enough, regular miso soup consumption specifically (one to five cups per day) was actually found to slightly increase the risk in men, most likely due to the high amount of sodium (Weng, 2017). Soy foods and isoflavones also appear to play a protective role in colorectal cancers in both men and women. The highest isoflavone intakes resulted in the most benefits and was more prominent among post-menopausal women. High intake of soy paste actually appeared to result in a higher risk of colorectal cancer in men, however (Shin, 2015).

Fermented soy foods also seem to reduce inflammatory related biomarkers. A cross-sectional study that included 1,426 Japanese workers, both male and female, observed a marked decrease in inflammatory markers in groups consuming higher amounts of fermented soy foods including miso and soy sauce (Yang, 2018). This likely ties in with some of the benefits seen in inflammatory related diseases.

One study comprised of 403 men and 373 women aged sixty and eighty-one years old examined the risk of cognitive impairment between soy consumers and non-consumers. A three day dietary record was observed for all participants at baseline. Cognitive function was analyzed using a mental state exam. The women in particular seemed to experience slightly improved results on the mental exam, indicating a potential decreased risk of cognitive impairment (Nakamoto, 2018).

Soy for athletes

Besides the benefits that we’ve discussed already, soy can be an effective food for building muscle in a well-planned diet.

Minimally processed or whole soy products such as edamame, whole soybeans, tempeh, natto, and so on are high in micronutrients, fiber, protein, antioxidants, low in saturated fat, and has zero cholesterol. These are all beneficial to the average person as well as an athlete. Some research shows that diets particularly high in dietary antioxidants may help improve recovery processes by combating oxidative stress, which is very important for an athlete (Pingitore, 2015).

Soy protein ingestion, from food or supplements, has been demonstrated to build muscle effectively. A 2018 meta-analysis compared multiple types of animal based protein powders against soy protein powder. The study found equivalent gains in strength and muscle between all groups and no particular protein powder supplementation seemed to excel (Messina, 2018).

For athletes looking to reap the health benefits of soy, supplementing soy protein powder or soy foods for animal-based foods doesn’t appear to alter muscle or strength gains.

For more information on optimal protein needs for athletes, read more here.


Soy and the phytoestrogens in soy have been under fire for decades but it appears that a lot of the claims are unsubstantiated.

Whole soy foods contain high amounts of fiber, protein, antioxidants, low amounts of saturated fat, no cholesterol, and high levels of vitamins and minerals such as iron, magnesium, copper, manganese, phosphorus, potassium, vitamin B2, vitamin B6, folate, vitamin K, and so on.

The phytoestrogens in soy also appear to have many beneficial effects in both men and women due to the way they interact with estrogen receptors. They preferentially bond to beta-type estrogen receptors which seems to be responsible for the beneficial effects without the negatives. In men, they do not affect testosterone levels or other sex hormones and seem to have a protective effect against prostate cancer, which is the most diagnosed cancer for men in the United States. In women, there is a good deal of evidence to support protective effects against breast cancer. Soy intake also appears very beneficial in postmenopausal women by reducing frequency of hot flashes, protecting against bone loss and decreasing risk of fracture, and possibly increasing cognition slightly.

Including soy in a balanced diet may also decrease risk of gastric cancer, endometrial cancer, colorectal cancer, and reduce inflammatory markers.

Individuals looking to benefit from soy should opt for the whole food source or a minimally processed version, such as edamame or tempeh. Tempeh and natto may be particularly beneficial as a fermented food as well. Soy is a great source of protein and nutrients for an active person, and is completely suitable as a replacement for animal products.