The Health Benefits and Side Effects of Soy

Soy is exalted as a health food by some, with claims of taming hot flashes, warding off osteoporosis, and protecting against hormonal cancers like breast and prostate. At the same time, it is shunned by others for fear that it may cause breast cancer, thyroid problems, and dementia.

Whether reading a popular press article or a well-designed clinical study about soy, some debate remains. As a species within the legume family, nutrition scientists often label soy as a food with potential for significant health benefits. However, due to contrary research that suggests possible negative effects of soy in certain situations, there has been a hesitancy to wholeheartedly promote soy. Read on to know more about the details. 

What is Soy?

Soy is a refined product that originates from the soybean plant. Soybeans are part of the legume family, which includes other common foods like peas, dried beans and lentils. Soy is the refined, highly nutritional derivative of the soybean. 

A soybean is scientifically known as Glycine max. Soybeans are classified as an oil seed, which is a bit different from other legumes which are often known as pulses. Soybeans have become commonplace in many meal plans with the rising popularity of soy-based foods like tofu.

Evidence shows that it has been a part of traditional Asian diets for thousands of years. In fact, there’s evidence that soybeans were grown in China as early as 9,000 B.C. (1).

Soybeans are available in many forms: 

Whole Soy Products

Whole soy products are the least processed and include soybeans and edamame, which are immature, green soybeans. Soy milk and tofu are also made from whole soybeans (2).

While mature soybeans are rarely eaten whole in the Western diet, edamame is a favorite high-protein appetizer in Asian cuisines.

Soy milk is made by soaking and grinding whole soybeans, boiling them in water, and then filtering out the solids. People who cannot tolerate dairy or wish to avoid milk commonly use it as a milk alternative.

Tofu is made by coagulating soy milk and pressing the curds into blocks. It’s a common source of plant-based protein in vegetarian diets.

Fermented Soy

Fermented soy products are processed using traditional methods and include soy sauce, tempeh, miso, and natto (2).

Soy sauce is a liquid condiment made from:

  • Fermented soy
  • Roasted grains
  • Salt water
  • A type of mold

Tempeh is a fermented soy cake that originated in Indonesia. Though not as popular as tofu, it’s also commonly eaten as a source of protein in vegetarian diets.

Miso is a traditional Japanese seasoning paste made from:

  • Soybeans
  • Salt
  • A type of fungus
  • Soy-based processed foods

Soy is used to make several processed foods, including:

  • Vegetarian and vegan meat substitutes
  • Yogurts
  • Cheeses

Many packaged foods contain soy flours, texturized vegetable protein, and soybean oil.

Soy Supplements

Soy protein isolate is a highly processed derivative of soy made by grinding soybeans into flakes and extracting the oil.

The flakes are then mixed with alcohol or alkaline water, heated, and the resulting soy concentrate is spray-dried into a powder (3).

Soy protein isolate is available in many protein powders and also added to many processed foods, such as protein bars and shakes.

Other soy supplements include soy isoflavones, which are available in capsule form, and soy lecithin, which can be taken in capsules or as a powder.

Why is Soy Popular?

Soy is popular for a number of reasons. The high protein levels found in soy make it an ideal food for vegetarians who have a hard time getting protein. There’s a wide range of soy products that can be made from the soybean, which has allowed soy-based foods to expand to a huge market.

The protein content of soybeans is one of the main reasons soybeans are widely cultivated – they contain more protein per acre of land compared to any other crop. Soy is a green, low-lying plant that can grow up to 2 meters high. Soy protein has become universally reputed for its quality, containing all the essential amino acids that build up a complete protein, making this small bean globally significant.

Is Soy Healthy?

Soybeans contain high quality protein. A ¾ cup  of cooked soybeans has the same protein content as ½ cup of cooked, chicken, meat or fish. Soybeans are one of the very few vegetarian foods that contain all the amino acids that humans need for health.

Soy is higher in fat compared to other legumes, which are usually close to fat-free. However, the fat in soybeans is mainly healthy. It has polyunsaturated and monounsaturated fats, also known as the omega-3 fatty acids. All legumes, soybeans included, have no cholesterol.

Soybeans are excellent sources of minerals, particularly iron and calcium. Iron is necessary to carry oxygen through the bloodstream and towards tissue and muscle cells. Calcium is used to build and maintain strong teeth and bones. Soybeans also have a high concentration of isoflavones, which are a certain type of antioxidants that carry additional health benefits.

What Are the Possible Health Benefits of Consuming Soy?

Like most whole foods, soybeans have a number of beneficial health effects:

1. May Decrease Breast Cancer Risk

Cancer is one of the leading causes of death in modern society.

Eating soy products is linked to increased breast tissue in women, hypothetically increasing the risk of breast cancer (4, 5, 6).

However, most observational studies indicate that consumption of soy products may reduce breast cancer risk (7, 8).

Studies also indicate a protective effect against prostate cancer in men (9, 10, 11).

A number of soybean compounds, including isoflavones and lunasin may be responsible for the potential cancer-preventive effects (12, 13).

Exposure to isoflavones early in life may be particularly protective against breast cancer later in life (14, 15).

Keep in mind that this evidence is limited to observational studies, which indicate an association between soy consumption and cancer prevention but do not prove causation.

2. Lowers Cholesterol Level

Several studies suggest that soy may improve cholesterol levels, especially LDL (bad) cholesterol.

In an extensive review of 35 studies, researchers found that eating soy products reduced LDL (bad) cholesterol and total cholesterol while raising HDL (good) cholesterol.

These improvements were greater in people with high cholesterol levels (16).

However, the researchers observed that soy supplements didn’t have the same cholesterol-lowering effect as eating soy foods (16).

In another older review of 38 studies, researchers noted that an average soy intake of 47 grams per day was linked to a 9.3% decrease in total cholesterol and a 13% decrease in LDL (bad) cholesterol (17).

Fiber seems to play an important role in cholesterol-lowering effects of soy.

In one study, 121 adults with high cholesterol took 25 grams of soy protein with or without soy fiber for 8 weeks. The soy with fiber reduced LDL (bad) cholesterol more than twice as much as soy protein alone (18).

3. Tames Hot Flashes

Hormone replacement therapy has traditionally been used as an effective treatment for hot flashes and other unpleasant symptoms that accompany menopause, but its long-term use carries concerns of an increased risk of some diseases including breast cancer and stroke. Soy has been a popular alternative treatment but not clearly supported by research; in theory the potential estrogenic effects of soy isoflavones could help to tame hot flashes by giving an estrogen-like boost during a time of dwindling estrogen levels.

In Asian countries where soy is eaten daily, women have lower rates of menopausal symptoms, although research is conflicting as to whether soy is a primary contributor (19). Reports suggest that about 70–80% of U.S. women of menopausal and perimenopausal age experience hot flashes, in comparison with 10–20% of Asian women (20). Further, the average blood concentration of the isoflavone genistein in Asian women is about 12 times higher than that of U.S. women because of higher soy consumption.

A review of 43 randomized controlled trials have examined the effects of phytoestrogens on hot flashes and night sweats in perimenopausal and postmenopausal women. Four trials found that extracts of 30 mg. or greater of genistein consistently reduced the frequency of hot flashes. Other trials that used dietary soy or soy extracts suggested a reduced frequency and severity of hot flashes and night sweats when compared with placebo, but these trials were small with a possible strong placebo effect (19). No adverse effects were noted from the soy treatments when followed for up to two years. 

Another meta-analysis of 16 studies found that soy isoflavone supplements had a small and gradual effect in weakening menopausal hot flashes compared with estradiol, human estrogen. 

There are some results that aren’t conclusive though. This area needs further research as questions remain about a possible benefit of soy. 

4. Prevents Long-Term Kidney Disease

Taking soy protein by mouth seems to reduce protein in the urine in people with chronic kidney disease (CKD). It also seems to reduce levels of certain nutrients and waste products, such as phosphorus and creatinine. These molecules can build up in the blood of people with CKD. 

5. Reduces Blood Glucose Level

For more than a decade, this dreaded disease has been on the rise in the global population. Soybeans are proving to be an effective method of preventing and managing diabetes. Soybeans are able to increase insulin receptors in the body (20), thus assisting to manage the disease effectively or prevent it from happening in the first place.

Insulin is the hormone responsible for regulating blood sugar. Those who have diabetes often have decreased insulin sensitivity, meaning that the body’s naturally produced insulin doesn’t have enough effect. Increasing the sensitivity of insulin or the number of insulin receptors makes the body better able to fight diabetes.

6. Improves Metabolic Activity 

As mentioned earlier, soybeans are a great source of essential protein. An adequate supply of protein is required for your body’s metabolism to function properly. Your metabolism is responsible for converting food into energy and digesting the nutrients provided.

As long as being a crucial component of metabolic structure, proteins act as the building blocks of cells and blood vessels. This makes protein a crucial part of the human body’s development. Soy protein, complete with all essential amino acids, makes it a prime source for ensuring healthy development. 

It can be hard for vegetarians or vegans to get enough proteins, so soybeans are a perfect addition to the vegetarian diet to replace the proteins usually supplied from red meat, dairy products, chicken, eggs, and fish. Vegetarians who don’t eat a lot of soy may find their metabolic process isn’t as effective when compared to those who regularly consume high protein animal foods.

7. Stops Diarrhea

Feeding infants formula supplemented with soy fiber, alone or together with rehydration solution, seems to reduce the duration of diarrhea compared to cow’s milk formula or rehydration solution alone. However, in some studies formula supplemented with soy was no more beneficial than cow’s milk formula. In adults, early evidence suggests that taking soy fiber does not decrease the incidence of diarrhea.

8. Protective Against Prostate Cancer

The incidence of prostate cancer is highest in Western countries and lowest in Asian countries, where soy foods are a regular part of the daily diet. In addition, observational studies have found an increased risk of prostate cancer in Chinese and Japanese men who move to Western countries and adopt a Western diet, but not in those who continue eating a traditional diet (21). Soy isoflavones, specifically genistein and daidzein, are found to collect in prostate tissue and may act as weak estrogens and exert a protective effect against the development of prostate cancer (22). 

9. Improves Memory and Cognitive Function 

Menopause has been linked with changes in mood and memory impairment. Long‐term low levels of estrogen in women can reduce the number of estrogen receptors in the brain that are necessary for specific cognitive functions like memory and learning (23). The soy isoflavone, daidzein, has been hypothesized to reduce decline in cognitive function or disease processes related to cognition and behavior. Thus, research has raised the possibility that eating soy foods might help prevent age-related memory loss or decline in thinking skills (24). 

One large study in men found a detrimental effect on cognitive function. In a prospective cohort study of more than 3,700 Japanese-American men living in Hawaii, the highest intakes of tofu, eaten almost daily, at midlife ages were significantly associated with greater cognitive impairment and brain atrophy in late life compared with men with the lowest tofu intakes (25). However, the actual number of men eating very high amounts of tofu was small, and past dietary information was collected by relying on the participants’ memory, some of whom may have already experienced cognitive decline. Because of this, the researchers stated that the findings were too preliminary and more research is needed before making final recommendations (26). 

10. Promotes Bone Health 

Osteoporosis is characterized by reduced bone density and an increased risk of fractures, especially in older women.

Consumption of soy products may reduce the risk of osteoporosis in women who have undergone menopause (27).

These beneficial effects seem to be caused by isoflavones (28, 29, 30).

Are There Any Side Effects?

1. Affects Thyroid Function 

Soy may interfere with thyroid hormone medication used to treat hypothyroidism. In one randomized double-blinded trial, 60 patients with a mild form of hypothyroidism called subclinical hypothyroidism were given low or high-dose phytoestrogen supplements, which consists of 30 grams of soy protein, the amount that might be obtained from a vegetarian diet (31). 

Risk of developing clinical hypothyroidism was increased in the higher phytoestrogen group and there was no effect in the lower phytoestrogen group. The authors suggested that female vegetarian patients with subclinical hypothyroidism may need more careful monitoring of thyroid function. 

2. Soy Allergy

Food allergy is a common condition caused by a harmful immune reaction to certain components in foods.

Soy allergy is triggered by soy proteins, glycinin and conglycinin, found in most soy products (32).

Even though soybeans are one of the most common allergenic foods, soy allergy is relatively uncommon in both children and adults (33, 34).

3. Contains GMOs

Most soy contains GMOs. More than 90% of the soy produced in the United States is genetically modified (35).

There’s much debate over the safety of genetically modified organisms (GMOs). More long-term scientific studies are necessary to determine their effects in humans and in what quantity they’re safe (36).

Additionally, most genetically modified soy products withstand the pesticide glyphosate, which is controversial.

Certain GMO soy products have been found to contain glyphosate residues and have a poorer nutritional profile compared to organic soybeans (37).

Therefore, to avoid GMOs and exposure to glyphosate, stick with organic soy.

Key Takeaway 

Soy is a unique food that is widely studied for its estrogenic and anti-estrogenic effects on the body. Studies may seem to present conflicting conclusions about soy, but this is largely due to the wide variation in how soy is studied. 

Results of recent population studies suggest that soy has either a beneficial or neutral effect on various health conditions. Soy is a nutrient-dense source of protein that can safely be consumed several times a week, and is likely to provide health benefits especially when eaten as an alternative to red and processed meat.








































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