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What is the Effect of Soy on Your Health? A Comprehensive Review

Jennifer Hanes MS, RDN, LD

Soy seems to be a polarizing food in our society. But why?  

Some people are just offended by vegetarians and automatically reject meat alternatives. Many people have tried poorly prepared tofu and assume that it always tastes like that. Some just aren’t familiar with soy-based foods.  Others have heard that soy is terrible for us. 

From tofu and soy milk to tempeh and edamame, soy derives from soybeans, a legume that has been a dietary staple in many cultures for centuries.

Not only is soy a complete protein source, but it also contains unique compounds called isoflavones and phytoestrogens that have gained attention for their potential health benefits.

However, we also acknowledge the considerations and controversies surrounding soy consumption. Allergies, genetically modified organisms (GMOs), and concerns about thyroid function have raised questions about the safety and suitability of soy for everyone.

We address these concerns and provide evidence-based insights to help you make informed decisions about including soy in your diet.

What is Soy?

All soy-based foods come from the soybean, a legume native to East Asia. Soybeans have been eaten in Asia for longer than we’ve had written records. It appears to have come to North America in the 1700s.

Soy was primarily used to feed livestock until the 1920s, when research demonstrated nutritional adequacy for humans, then really surged in use during the Great Depression.

“First generation” soy products consist of edamame (baby soybeans), tofu, soymilk, soybean sprouts, and soynuts.  Fermented first-generation soy foods include miso, tempeh, natto, and soy sauce.

Developed more recently, second-generation soy foods include soy burgers and sausages, soy pasta, and soy yogurt.  There are also soy-based food additives, notably soy lecithin, monosodium glutamate, textured vegetable protein, and hydrolyzed soy protein.

When we are talking about the nutritional benefits of soy, we are generally speaking of first-generation foods.  Second generations foods are not necessarily bad but should be eaten less frequently. This is to moderate the amount of sodium and saturated fats that are included in second-generation soy foods. 

Soybeans are high in fiber, protein, omega-3 fats, antioxidants, and phytoestrogen. They are low in saturated fat and cholesterol free. They contain folate, copper, phosphorus, thiamine, and vitamin K, as well as other vitamins and minerals in small amounts.

The nutrition profile of individual soy foods will vary based on how it is prepared, so if you are looking to increase or reduce a particular nutrient, make sure you check the labels.

Rich Source of Plant-Based Protein

Protein is important for many aspects of our health. We all know that we need protein to maintain and grow our muscles. However, protein is also involved in pH balance, fluid balance, digestion, nutrient transport, and immune system function and is a structural component of all the cells in our body.

Soybeans, along with other legumes, have a higher proportion of protein compared to other plant-based foods.  Moreover, soy protein contains all of the amino acids in the amounts we need.  We can’t say this for most other plant sources of protein.

Heart Health

Soy has long been regarded as a heart-healthy food.  A meta-analysis performed in 1995 showed that when 50 grams of soy protein replaced animal-based protein study, participants’ total cholesterol levels dropped by almost 13%.   More recent studies have had much more modest results (as low as a 3% reduction in cholesterol).

However, no studies found soy to have a detrimental effect on cardiovascular health.   

In a 2012 study, soybean protein was found not to affect measured inflammatory markers or blood pressure when participants were supplemented with soy protein.

In contrast, in another (small) 2012 study, participants ate soy nuts daily and experienced a reduction in total cholesterol and blood pressure. And despite claims that soybean oil is inflammatory and causes poor health, the evidence shows no evidence of inflammation caused by soybean oil in multiple studies.

Because soy foods are low in saturated fat and calories and contain fiber, vitamins, minerals, unsaturated fat, and protein, they meet all the generalized requirements to be considered heart-healthy.

Diabetes

Soy isoflavones may improve insulin sensitivity, meaning that the cells respond more to insulin and can absorb and use more glucose for energy. 

A daily snack of soynuts has been shown to reduce an individual’s fasting blood glucose and serum insulin levels. Meaning that the patient’s diabetes improved.

Reproductive Health

Soy has few research studies that are specific to a specific reproductive diagnosis.  So we have to look to extrapolation at times, but also to take a step back from just a diagnosis to potential mediation of symptoms as well as side effects of any medications that may be used.

Polycystic Ovarian Syndrome

It is unclear if soy has any positive or negative effects on PCOS directly.   

However, it can help reduce some of the symptoms of PCOS, such as elevated cholesterol, inflammation, and insulin resistance.

Woman’s Fertility

Research suggests that soy has a mildly positive effect on a woman’s ability to conceive. However, studies have been very small.  More research is needed before a definitive answer is available.

Men’s Fertility

There isn’t quite as much research on men, but it appears that soy does not have an influence on fertility. A study looked back at the diets of men and concluded that the more soy a man ate, the more volume of semen he produced. However, the total number of sperm remained unchanged.

Isoflavones

Phytoestrogens are a class of compounds found in many foods that might have weak estrogenic effects when eaten. Phytoestrogens can further be divided into isoflavones (found in beans, but particularly in soybeans) and lignins (found in flaxseeds as well as some grains and fruits).

The isoflavones found in soy don’t increase the estrogen activity in your body, in either males or females.

A study in 2017 on 200 men sought to determine what happens to their hormones when they consumed soy. The men in this study had type 2 diabetes and low testosterone levels.

After eating a soy-based snack bar every day for three months, they had no decrease in testosterone levels or testosterone receptor activity, but their glucose control improved.

They also experienced a reduction in triglycerides,  CRP (a marker for inflammation), and blood pressure, all indicating an improvement in cardiovascular health.

In 2010, a meta-analysis of 32 separate studies showed no reduction in bioavailable testosterone, sex hormone-binding globulin, free testosterone, and free androgen index in men. Most of these studies had participants eat more than is typically consumed in Asian countries.

The phytoestrogens found in soy (genistein and daidzein) are more accurately described as estrogen receptor modulators.  They have not been found to have any of the effects you would expect to see with an estrogen supplement. 

A 2017 study showed improvements in hot flashes, sleep disturbances, irritability, anxiety, fatigue, mild depression, rheumatic complaints, and mild cardiac symptoms associated with menopause.

The study did not find improvements in menopausal symptoms of sexual dysfunction, bladder disturbances, or vaginal dryness.

Cancer Prevention

The most common fear of soy consumption is the idea that soy foods cause breast cancer due to their phytoestrogen content. 

In fact, the effect the phytoestrogen in soy has on estrogen activity in the body may be the reason that eating soy can actually decrease your risk of breast cancer. Soy consumption can also reduce the risk of breast cancer returning in survivors.

When discussing breast cancer in particular, studies have shown a positive or neutral relationship between soy consumption and risk.  Asian women appear to get a greater protective benefit from soy consumption than from other ethnicities.

Consumption starting in childhood also has greater protective effects.

The idea that soy may cause breast cancer comes from extrapolating data without monitoring outcomes.  The theory was that women with breast cancer have higher levels of estrogen, and soy contains phytoestrogen, so soy must increase cancer risk.

However, this idea fails to take phytoestrogen activity into account (largely that it displaces our own estrogen in function and does not raise our estrogen levels).

What about men?

Studies in men showed a decreased risk of prostate cancer as soy consumption increases.

Overall, studies done on cancer and consumption have been small and have produced mixed results. I couldn’t find a study that linked soy consumption to an increased risk of cancer.

Most likely, the mixed results probably stem from some other factor, such as the type of cancer or other lifestyle factors. In general, people that regularly eat soy also consume more vegetables and fewer processed carbohydrates.

Bone Health

Soy foods do appear to have a positive effect on bone health, particularly in postmenopausal women.  This effect seems to be the result of the isoflavone content in soy.

Digestive Health

Soy contains fiber, vitamins and minerals, and some phytochemicals that can improve gut health. 

Fiber, in particular, is known to prevent or improve diarrhea and constipation, reduce the risk of colon cancer, and improve the overall number and diversity of the gut microbiome.

Brain Health

Some evidence suggests that soy can slow cognitive decline in some neurological disorders, such as Alzheimer’s and Parkinson’s diseases.

This seems to be due to at least 2 different reasons. One is those good old isoflavones we mentioned several times before. However, another indirect (and potentially more potent) cause may be at play. 

Soy’s positive effect on gut health can then lead it to influence brain health.  The “good” bugs in the gut produce a number of compounds that can affect a wide range of brain functions. One of these compounds is butyrate, which has been increasingly linked to improved brain function in recent studies.

There are some other studies that did not find a statistically significant difference in brain function, as well as one poorly designed but large study that showed negative effects. 

My takeaway is that soy likely does contribute to slowing cognitive decline (memory, reasoning, depressed mood, and agitation). However, it is not a magic bullet that will prevent Alzheimer’s by itself.

Additionally, some of the reasons that soy seems to help are not just the soy itself. Other beans/legumes have the same compounds and effects in the gut and brain. Essentially, soy fits but shouldn’t be the only source of fiber and other nutrients in a well-balanced diet.

 

Kidney Health

Soy is relatively low in oxalates, which can contribute to the formation of kidney stones, though tempeh and soy protein isolate may have more. Additionally, phytates in soy may reduce the likelihood of oxalates causing problems. 

For most people, soy will not cause kidney stones. However, some people are more sensitive to the effects of oxalates. The answer is less clear.  It appears that if the high oxalate foods are avoided (spinach, rhubarb, chocolate, etc), then soy shouldn’t cause a problem.

iodThis really should be considered on a case-by-case basis.

Sleep

Soy contains a good amount of magnesium, known to help improve the quality of sleep both in terms of initially falling asleep and waking up easier.

Of course, choosing magnesium-containing foods is only part of the sleep equation. Other factors of sleep hygiene must also be considered, such as limiting caffeine, following a nighttime routine, and addressing your sleep environment.

Thyroid Function

A couple of large studies showed that soy consumption has no effect on the thyroid functioning of a healthy adult. In other words, eating soy will not cause hypothyroidism or other thyroid conditions in a person that doesn’t already have a thyroid problem.

Another study indicated that soy consumption could cause someone to need a higher dose of medication if they are already being treated for hypothyroidism.  It appears that this effect is reduced if the individual ensures that they are taking in enough iodine.

This is easy to do in the United States with iodized salt. Other vegetarian sources of iodine include:

  • Seaweed (kelp, nori, kombu, wakame)
  • Greek yogurt
  • Milk
  • Eggs
  • Cheddar cheese

It is thought that soy may interfere with the absorption of the medication used to treat hypothyroidism.  While it isn’t quite clear exactly the impact of soy on the absorption of thyroid meds, if you’re concerned, try to avoid eating soy within 4 hours of taking your medication.

Considerations and Controversies

Soy has such a polarizing presence on social media and other web-based sources of information. This often arises from misunderstanding the literature, applying personal anecdotes to the entire population, or some other bias.

Generally, when you see two extremes like this (all bad or all good), you’ll find the actual science (truth) somewhere in the middle.  What we’ve seen so far is that soy has a neutral to positive effect on various systems in our bodies when considering the general population.

However, some individuals do have legitimate reasons to stay away from them. Furthermore, some worry about GMOs’ environmental impact or are fine with first-generation soy products but are concerned with second-generation products, particularly food additives.

Allergies and Sensitivities

Soy is one of the top 8 (now 9) allergens in the US. Symptoms of a food allergy can range from mild to severe, but an individual with a food allergy should take every step to avoid their allergen.   

Additionally, just like any other food, individuals may have a non-allergy sensitivity to soy.  This is sometimes attributed to the individual’s inability to break down various fibers or other compounds in the food. 

While generally not as serious as an allergy, individuals with food sensitivity should also aim to avoid that food wherever possible.

GMO Concerns

Genetically Modified Organisms cause a lot of fear in a lot of people. Some of that this just a general lack of knowledge of what that means.

A GMO is an organism that has been genetically engineered for various purposes. This is different from selective breeding, such as what we did to make various dog breeds or our impact on corn. 

Genetically engineering a crop generally happens to make them more pest or environmentally resistant. But on occasion, nutrition factors have also been considered.

In the US, soy, sugar beet, canola, corn, cotton, alfalfa, papaya, summer squash (not common), pink pineapple, and a few varieties of apples and potatoes are the only GMO crops allowable.

GMO foods are regulated much more tightly than traditionally grown crops, so they are actually MORE studied and evaluated for safety than other foods.  It can take up to 16 years of research, development, and safety evaluation from the time a crop is developed to the time it reaches the shelf.

No one can say that about the quinoa you had for dinner recently.

This discussion obviously has way more to consider than the scope of this article.  If you are concerned about eating GMO foods, look for a USDA-certified organic product.  GMOs and foods that contain them cannot get Organic certified, even if they are otherwise grown organically.

Anti-Nutrients

I’ve discussed the concept of anti-nutrients before. Basically, an anti-nutrient is a compound in your food that inhibits the absorption of other nutrients.

For the vast majority of us, this has zero impact on our health. In fact, some anti-nutrients have positive benefits despite reducing the absorption of other nutrients. These are called phytochemicals and should absolutely be included in your diet

Those with gout and a tendency towards kidney stones may have to pay more attention to these compounds.

The only time I see people assert that anti-nutrients must be avoided, they are peddling some bullshit diet book to scare you and then take your money.

Soy Lecithin

Soy lecithin is a food additive often used as an emulsifier or to keep the fat and non-fat compounds of the food together.   

This ingredient is used in such small amounts that there is virtually no chance for it to cause negative health effects.  Additionally, some people take soy lecithin supplements to help manage their cholesterol.

Even though soy lecithin derives from soy, the allergenic compounds are removed. This means those with a soy allergy don’t always have to avoid it. 

Soy lecithin is often used in highly processed food. We should be limiting our intake of this ingredient as part of a well-balanced diet anyway.

Soy Protein Isolate

Soy protein isolate is simply the protein in soy isolated from the rest of the soybean. This is similar to whey protein powder.  This is often used as a good vegan alternative to weight protein shakes.

Soy is preferential to other plant sources of protein supplements because it contains all amino acids in high amounts naturally. Whey protein is generally considered the best for protein supplementation for muscle growth, but soy is a pretty close second. 

Additionally, soy protein isolate has a slightly more nutty, bitter taste and is a bit more grainy than whey, which has almost no taste and a more creamy texture.

Moderation and Variety

Remember that even with the positive attributes of soy, variety is always an important factor in a well-balanced diet, vegetarian or otherwise.   Use soy as a great source of protein, fiber, vitamins, and minerals, but also use other foods as well.   

There is no one food that is going to give you everything you need to be healthy.

Conclusion

Eating soy fits well into a healthy diet because of its low saturated fat and high fiber and protein content. It’s really easy to fall into the trap of believing that one food is a “superfood” or is utterly terrible for you. 

In fact, that is the premise of most fad diets currently out there. This extreme view of foods can lead to disordered eating patterns and fear of specific foods that can lead to nutritional deficiencies and poor health. 

Think of tofu, edamame, tempeh, and soy nuts as sources of protein and fiber that fit into everyone’s diet, vegetarian or not.

In your journey to optimal well-being, soy can be a valuable ally, contributing to a wholesome and nourishing diet. Armed with the knowledge and understanding of soy’s potential benefits and considerations, you can make informed choices that align with your health goals.

For more individualized recommendations, I recommend speaking with a dietitian that is familiar with your medical history as well as vegetarian diets.

For yummy, soy-based comfort food, try my Tempeh Shepherd’s Pie!

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