Soybeans, hailed as a cornerstone in vegan diets, stand out as a vegan protein-rich food pivotal for gut health.
This article delves into the multifaceted benefits of soybeans, exploring how they enhance digestive wellness and contribute to a healthy gut microbiome.
We’ll examine the role of soy protein in digestion, the advantages of fermented soy products, and the impact of soy foods on reducing colorectal cancer risk.
As a versatile and nutritious plant-based protein source, soybeans offer a harmonious blend of health benefits, making them an essential component of any plant-based diet focused on gastrointestinal health.
Main findings
- Fermented soy milk shows more favorable changes in the gut microbiome compared to unfermented soy milk, likely due to probiotic effects.
- Soy proteins, at high levels of intake, may induce greater cytotoxic and genotoxic damage to intestinal tissue compared to dairy protein intake. However, adding fermentable fibers to the diet can mitigate these effects.
- There is either a lack of association or a small inverse association between soy/soy isoflavone intake and colorectal cancer risk.
- High intakes of soy isoflavones and phytoestrogens are associated with a small but significant reduction in colorectal cancer risk, particularly in Asian populations.
Keywords: soy; gastrointestinal; colorectal cancer; vegan protein-rich food; microbiome; soy protein
Welcome to Soy’s World
TABLE OF CONTENTS
The Science of Vegan Protein-Rich Foods: Soybeans, Gut Health, and More
Soybeans: A Nutrient Powerhouse
Soybeans (Glycine max) are a nutritional powerhouse, offering a rich source of plant-based protein.
Notably, they are the only plant protein containing all essential amino acids necessary for human health.
While most of the global soybean production goes into animal feeds, soy-based products such as soy milk, tofu, edamame, and tempeh have been gaining popularity in human diets.
In recent years, soy production has doubled, reflecting the growing demand for plant-based protein sources and global sustainability recommendations 1 2 3.
Soybeans stand out among beans and legumes for several nutritional reasons.
They boast twice the protein content of other common legumes and contain all nine essential amino acids.
Leucine, a vital amino acid for muscle protein synthesis and maintenance, is present in soybeans at levels comparable to fish and eggs 4 5 6.
Additionally, soybeans offer dietary fiber in the form of oligosaccharides and other non-starch polysaccharides, which promote intestinal fermentation.
High levels of polyunsaturated fatty acids, including essential omega-6 and omega-3 fatty acids, make soybeans a standout among legumes.
Furthermore, soybeans provide essential micronutrients like calcium, iron, and zinc, often lacking in plant-based diets.
The efficient absorption of calcium from soybeans makes them an ideal dairy alternative, leading to the availability of soy-based milk, cheese, and yogurt worldwide 7 8 9 10 11 12.
Soybeans and the Misconceptions About Isoflavones
Early investigations into soybeans and soy products primarily centered around their isoflavone content.
Due to their phytoestrogenic nature, concerns arose that high soy consumption might negatively impact hormonal function.
However, subsequent scientific research has debunked these fears.
A review concluded that soy isoflavones do not interfere with the endocrine system’s function 13.
Furthermore, they do not adversely affect thyroid function, hormones in men and women, or reproductive parameters.
There is limited evidence suggesting that soy isoflavones may reduce menopausal hot flashes and lower the risk of breast and prostate cancer 14 15 16 17.
However, it remains unclear whether these effects result from soy isoflavones themselves or from other nutritional components in soybeans, such as polyunsaturated fats, fiber, vitamins, and minerals 18.
Soy and Gut Health
The human gut plays a crucial role in overall health, and the composition of the gut microbiome influences gastrointestinal (GI) health.
Microbial dysbiosis, characterized by an imbalance favoring pro-inflammatory species, is linked to various diseases, including inflammatory bowel disease, obesity, and cardiovascular conditions.
Nutrients from soy products, like soy milk and textured soy proteins, resist digestion in the upper digestive tract and serve as substrates for microbes residing in the large intestine 19 20.
These nutrients include proteins, isoflavones, and fermentable non-starch polysaccharides like cellulose, hemicellulose, and pectin, as well as oligosaccharides like raffinose and stachyose.
The nutrient content and bioavailability in soy-based foods vary depending on the formulation and processing methods 21 22.
For instance, textured soy protein has a significantly higher oligosaccharide content than other soy foods, like tempeh, soy cheese, and milk 23 24 25.
Moreover, about 10% of dietary protein resists digestion in the small intestine and reaches the colon, where it becomes a nitrogen and energy source for specific bacteria, supporting their growth in the gut 26 27.
Fermented soy products, such as tofu, contain microbes that could function as probiotics if they escape digestion in the upper gut 28.
A practical way to enhance these benefits is through probiotic soy yogurt. Rich in live cultures, this yogurt supports a healthy gut microbiome, aligning with the benefits of soy’s dietary fibers and fermentable polysaccharides.
Soy and GI Health: Current Evidence
Although previous reviews have explored the effects of soy on the gut microbiota, they have not fully assessed its impact on critical parameters like microbial metabolites and GI health.
This review aims to fill that gap by evaluating the existing evidence regarding the effects of soy foods, including soy milk, soy protein, oligosaccharides, and isoflavones, on GI health.
A comprehensive approach that incorporates systematic reviews, umbrella reviews, randomized clinical trials, and animal feeding studies was used to summarize the findings 29.
Behind the Scenes
In a comprehensive study, a wide-ranging search in PubMed identified 1024 relevant publications on soy consumption and gut health.
These included human studies, reviews, meta-analyses, as well as animal and in vitro models.
After screening, 20 reviews/meta-analyses and 33 other publications were chosen for a detailed review.
The research delves into how soy protein, various soy products, and key soy components impact microbial composition, microbial metabolites, and a specific GI disease, colorectal cancer (CRC).
The Soy Milk Saga
Unveiling the Mysteries of Soy Milk and Gut Bacteria
Epidemiological evidence suggests that soy milk can influence the gut microbiome, particularly when comparing fermented and unfermented products.
A cross-sectional study 30 examined the effects of milk intake on the microbiome, revealing that higher consumption of milk and soy beverages was linked to increased levels of beneficial gut bacteria like Faecalibacterium, Bifidobacterium, and Parabacteroides.
However, it’s important to note that the study did not differentiate between milk types, making it likely that these effects were primarily attributed to dairy milk.
In three randomized controlled trials (RCTs) investigating unfermented soy milk’s impact on gut microbes, results were inconsistent 31.
One study observed an increase in Lactobacillus levels after a 2-week intervention 32, while another involving infants reported no changes after 4 weeks 33, and a study with overweight and obese men found decreased levels of Lactobacillus and Bifidobacterium after 3 months 34.
Conversely, the consumption of fermented soy milk was associated with increased levels of Bifidobacterium and Lactobacillus in two RCTs 35 36.
These findings align with studies in rodents, where fecal Bifidobacterium, Lactobacillus, and/or Bacteroides increased in animals fed fermented soy milk 37 38.
These results suggest that microbes present in fermented soy milk may have probiotic effects.
There’s limited evidence suggesting that nutrients from soy milk may have a prebiotic effect on the large bowel.
One study, using a cholesterol-induced colonic inflammation rat model, found that replacing casein with soy milk in the diet reversed diet-induced dysbiosis by altering the Firmicutes (Coprococcus, Lactobacillus, Blautia genera) to Bacteroidetes (Barnesiella genus) ratio 39.
Soy Milk: A Fighter Against Inflammation?
Two studies involving individuals with type-2 diabetes found that soy milk intake did not significantly impact C-reactive peptide and cytokine levels when compared to baseline measures 40 or cow’s milk consumption 41.
However, research using rat models of inflammatory bowel disease produced inconsistent findings, suggesting that soy milk might improve various indicators of intestinal cell damage and inflammation 42 43.
To delve deeper into this potential, a study is currently in progress.
It’s investigating whether soy milk consumption (250 mL/day for 4 weeks) by individuals with ulcerative colitis, compared to no dietary changes, can alleviate symptoms and reduce inflammation.
This investigation aims to shed light on how soy milk may influence gut microbiota and its role in addressing inflammatory bowel conditions 44.
The Power of Soy Proteins
How Soy Proteins Affect Our Gut Microbes
In three dietary intervention studies, rodents consuming soy protein concentrate or isolates exhibited higher microbial diversity, seen as a marker of gut health, and changes in specific microbial species compared to those on casein-based diets 45 46 47 48 49.
One study by An and colleagues found that rats on a soy protein diet had increased levels of Enterococcus and decreased levels of Lactobacilli and Ruminococcus when compared to those on diets with fish meal or milk casein 50.
Lactobacillus, known for its role in metabolic balance, may potentially reduce antigen transfer from gut bacteria to the host and alleviate inflammation related to metabolic syndrome 51 52 53 54.
Another study in rats, however, showed that soy protein-fed rats had a similar abundance of Lactobacillus compared to those fed casein and beef meat 55.
Meanwhile, when soy protein concentrate or isolate was added to a Western-style diet for golden Syrian hamsters, the most significant differences in microbial abundance were observed within the Bacteroidetes phylum, with lower relative abundances of Bacteroidaceae and Porphyromonadaceae compared to milk protein-fed groups 56.
To conveniently increase your intake of these beneficial proteins, consider incorporating soy protein powder into your daily diet. This powder is an easy way to boost your protein intake while enjoying the full range of amino acids that
Exploring the Role of Soy in Fermentation and Bile Acids
In animal studies, researchers have delved into how soy proteins affect microbial fermentation products, particularly short-chain fatty acids (SCFAs) and lipopolysaccharides (LPS).
SCFAs are essential for colonocytes and have a role in regulating T regulatory (Treg) cells while benefiting various organs, including the liver, adipose tissue, and the brain 57.
Soy protein diets in mice and rats have shown an increase in fecal SCFAs compared to casein-based diets 58 59.
Another study revealed that rats consuming a fiber-enriched diet with soy protein had higher caecal acetic acid levels 60.
However, soy protein’s impact on LPS, which can trigger inflammation, remains uncertain.
Some studies have suggested that rats consuming soy protein have higher levels of LPS-binding protein in the liver and serum compared to casein-fed rats 61 62, but more investigation is needed to clarify this.
Bile acids are crucial in the gastrointestinal tract, linking gut microbiota to hepatic and intestinal metabolism and affecting various aspects of gut health, including GI motility, intestinal permeability, and the risk of colorectal cancer 63.
Soy protein consumption appears to modulate bile acid metabolism.
In mice on a high-fat diet, soy protein isolate led to an enlarged caecal bile acid pool with an elevated secondary/primary bile acid ratio compared to casein-fed mice 64.
This diet also altered the gut microbiome, potentially influencing bile acid biotransformation.
However, research in rats with hepatic steatosis showed different results, with soy protein isolate reducing microbes associated with secondary bile acid production 65.
Soy Proteins: Friends or Foes of GI Health?
Numerous controlled studies in rats have explored the effects of soy protein-rich diets (>20% weight) compared to dairy proteins on gastrointestinal (GI) health.
The majority (5 out of 8) of these studies suggest that soy protein diets might have adverse effects on various GI measures.
For instance, one study revealed that rats fed soybean protein for 9 days experienced higher epithelial cell damage, increased colonic cell proliferation rates, and greater faecal water cytotoxicity compared to rats on casein-based diets during the same period 66.
Similarly, another study found that high dietary intakes of soy protein isolate (25%, but not 15% weight) increased the genotoxicity of the colonic environment, resulting in heightened colonic DNA damage 67.
Furthermore, in rat carcinogenesis models, soy proteins were associated with increased numbers of aberrant crypt foci (ACF), considered pre-cancerous lesions, or intestinal tumors.
Soy protein concentrate (low in isoflavones) increased ACF numbers, while high isoflavone soy products reduced total ACF compared to a starch control diet.
Interestingly, the timing of soy protein consumption also played a role, with increased colonic ACF numbers when soy protein was consumed before carcinogen exposure but reduced numbers after 6 weeks of consumption 68 69 70.
However, it’s essential to consider that these potential adverse effects of soy protein on GI health may be influenced by dietary fiber present in soy-based foods or other dietary sources.
Studies have shown that the addition of resistant starch or xylo-oligosaccharides to soy protein diets reduced genotoxic damage in the colon and decreased genotoxicity when soy protein was part of a fermented material 71 72.
Additionally, combining soy protein with raffinose increased caecum IgA concentration, potentially helping prevent pathogenic bacterial invasion in the large intestine 73.
Soy Oligosaccharides: Boosting Your Gut’s Microbial Activity
In the only human clinical study available 74, individuals consuming soy oligosaccharide fiber showed increased levels of faecal short-chain fatty acids (SCFA) and butyric acid compared to those on other fiber diets.
Importantly, there were no significant changes in bowel function indicators.
Animal and in vitro studies further support these findings.
A pig-feeding trial demonstrated that soybean oligosaccharides increased SCFA concentrations, boosted intestinal microbiota diversity, and promoted the growth of beneficial bacteria like Bifidobacterium sp., Faecalibacterium prausnitzii, and Roseburia 75.
It also reduced potentially harmful bacteria and decreased intestinal inflammation markers.
Additionally, an in vitro study showed that specific gut microbes, including Bifidobacteria, can utilize soybean oligosaccharides, while putrefactive bacteria like Escherichia coli and Clostridium perfringens cannot 76.
These findings suggest that soy oligosaccharides may hold promise for enhancing GI health, although further research is needed to fully understand their potential benefits.
Isoflavones Uncovered: Soy’s Secret Ingredient for a Healthy Gut
Isoflavones, naturally present in soy, possess antibacterial properties.
However, their effects on the gut microbiome vary among individuals, depending on their ability to metabolize specific isoflavones, particularly daidzein, which can be converted to equol in the intestine by certain gut microbes 77.
In postmenopausal women, three clinical trials explored how equol-producing capabilities influenced the response to soy consumption.
One study involving short-term soy milk and soy germ intake revealed that the gut microbiota changes depended on equol metabolism.
Strong equol producers exhibited reduced levels of unfavorable Clostridium coccoides-Eubacterium rectale bacteria but increased levels of gut-inflammation-associated sulphate-reducing bacteria 78.
Another one-week study found that soy bar consumption increased Bifidobacterium abundance, particularly in equol producers 79.
A two-month study with gelified soy extract showed that isoflavones stimulated various beneficial microbes, with the extent of stimulation depending on equol excretion.
Notably, the bifidogenic effect was more prolonged in equol producers 80.
For a targeted approach to enjoying the benefits of isoflavones, consider trying isoflavone supplements. These supplements offer a concentrated and controlled way to integrate these compounds into your diet.
Soy and Colon Health: Can Soy Foods Reduce Cancer Risk?
Colorectal cancer (CRC) ranks among the most prevalent cancers worldwide, with its risk closely linked to diet.
While some dietary elements like whole grains, dairy, and fiber-rich foods have been shown to lower CRC risk, the impact of soy remains less clear, although there have been hints of its potential benefits in CRC prevention 81.
An umbrella review of CRC prevention highlighted the mixed evidence on soy and CRC risk.
Three meta-analyses of observational studies were reviewed, with two reporting a slight reduction in CRC risk associated with soy consumption 82 83, while one found no significant link 84.
Overall, the certainty of evidence regarding soy’s impact on CRC risk was deemed very low, although it might offer a modest reduction in colon cancer risk 85.
The inconsistency in soy’s protective association with CRC could be attributed to various factors leading to study variability.
These factors include imprecise measurement of soy consumption and phytoestrogen intake, limited diversity in soy products studied, reliance on questionnaire-based retrospective studies prone to recall bias, and differences in soy composition between Asian and Western diets.
Interestingly, the inverse correlation between soy intake and GI cancer seemed stronger in countries like China and Japan, where soy consumption is more diverse and higher overall 86.
Moreover, several meta-analyses delved into the relationship between specific soy components, particularly isoflavones or phytoestrogens, and CRC risk.
Some found that soy isoflavone consumption was associated with a reduced CRC risk, particularly in Asian populations 87.
Others reported an inverse association between phytoestrogen intake and CRC risk, although this was more pronounced in case-control studies 88.
These findings suggested that soy’s impact on CRC risk might vary depending on its components and population.
However, it’s crucial to note that these studies exhibit heterogeneity, and further research is necessary to establish definitive connections.
Additionally, a systematic review examining the effect of total dietary flavonoid intake on colorectal adenoma and CRC incidence yielded inconsistent results, partly due to the challenges in measuring flavonoid intake across diverse foods and ethnic subgroups 89.
Laboratory studies using intestinal cancer cell lines have explored the mechanisms through which soy and its nutritional constituents, including isoflavones, phytosterols, and phenolic acids, exert anticancer effects.
These studies have suggested that soy components can inhibit cell growth, affect signal transduction pathways, and induce apoptosis in cancer cells.
However, the antitumor and antiangiogenic effects of soy isoflavones on CRC remain unproven, and further research is needed 90 91 92 93 94 95 96 97.
Unanswered Questions: Research Gaps in Soy and Gut Health
The current body of research on how soy impacts gastrointestinal (GI) health is limited.
To gain clearer insights, large, well-designed clinical trials are needed, considering the significant variability in microbiome composition among individuals and populations.
While existing trials have mainly focused on soy milk, there’s a pressing need for research on textured soy protein foods, which are increasingly popular.
To address concerns about soy protein and colorectal cancer risk, trials examining microbial metabolites and carcinogenesis biomarkers in intestinal biopsies are essential.
Some evidence suggests that soy oligosaccharides and isoflavones may promote beneficial bacteria and short-chain fatty acid production, but a personalized nutrition approach is required to understand individual responses and potential broader health benefits.
Key Takeaways: Concluding Thoughts on Soy and Gastrointestinal Health
This review delves into the effects of soy-based foods and bioactive components on gastrointestinal (GI) health.
Over 40 epidemiological studies have examined the link between soy/isoflavone intake and colorectal cancer (CRC) risk, with results indicating either no association or a slight inverse link.
Among soy foods, fermented soy milk, known for its probiotic effects, appears to have a more consistent impact on the gut microbiome compared to regular soy milk.
Moreover, individuals capable of metabolizing equol experience more consistent GI changes following soy product consumption.
Animal feeding trials show that while soy protein intake can increase beneficial short-chain fatty acids, high levels (>25% weight) may cause intestinal damage.
Future research should explore the effects of soy protein isolates and textured soy proteins on GI health, particularly in individuals with equol-metabolizing microbiomes.
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