Diet, Gut Microbiota, and Colorectal Cancer Prevention: A Review of Potential Mechanisms and Promising Targets for Future Research

Abstract

Diet plays an important role in the development of colorectal cancer. Emerging data have implicated the gut microbiota in colorectal cancer. Diet is a major determinant for the gut microbial structure and function. Therefore, it has been hypothesized that alterations in gut microbes and their metabolites may contribute to the influence of diet on the development of colorectal cancer. We review several major dietary factors that have been linked to gut microbiota and colorectal cancer, including major dietary patterns, fiber, red meat and sulfur, and obesity. Most of the epidemiologic evidence derives from cross-sectional or short-term, highly controlled feeding studies that are limited in size. Therefore, high-quality large-scale prospective studies with dietary data collected over the life course and comprehensive gut microbial composition and function assessed well prior to neoplastic occurrence are critically needed to identify microbiome-based interventions that may complement or optimize current diet-based strategies for colorectal cancer prevention and management.

Introduction

Colorectal cancer (CRC) is the third most commonly diagnosed cancer and the fourth leading cause of cancer death in the world.¹ Over the past few decades, numerous epidemiologic studies have identified a range of dietary factors that may potentially promote or prevent CRC.² Likewise, increasing evidence has implicated the gut microbiota in CRC development (Table 1).^3–14 Biological plausibility for this mechanism is supported by habitation of numerous gut microbes in the large intestine and the functional importance of the gut microbiota in maintenance of the gut barrier integrity and immune homeostasis, the disruptions of which are among the most important mechanisms in colorectal carcinogenesis.¹⁵ Given the critical role of diet in the configurations of gut microbial communities and production of bacterial metabolites, it has been proposed that diet may influence CRC risk through modulation of the gut microbial composition and metabolism that in turn shape the immune response during tumor development.

Table 1.

Summary of epidemiologic studies examining the association of the fecal microbiome with colorectal cancer

Author, Year	Country	Study Design	Sample Size	Main Findings Comparing Cases to Controls
Scanlan, 20083	Belgium	Case-control	20 cancers / 20 polyps / 20 controls	↑ diversity of the Clostridium leptum and C. coccoides subgroups
Sobhani, 20114	France	Case-control	60 cancers / 119 controls	↑ Bacteroides/Prevotella
Wang, 20125	China	Case-control	46 cancers / 56 controls	↑ Bacteroides fragilis and opportunistic pathogens; ↓ butyrate-producing bacteria
Ahn, 20136	USA	Case-control	47 cancers / 94 controls	↓ diversity; ↓ Clostridia; ↑ Fusobacterium, Porphyromonas;
Zackular, 20147	USA	Case-control	30 cancers / 30 adenomas / 30 controls	↑ Bacteroides fragilis, Fusobacterium, Porphyromonas; ↓ butyrate-producing bacteria
Zeller, 20148	France	Case-control	91 cancers / 42 adenomas / 358 controls	↑ Bacteroidetes, Fusobacteria and Proteobacteria; ↓ Actinobacteria and Firmicutes
Feng, 20159	Austria	Case-control	41 cancers / 42 adenomas / 55 controls	↑ B. dorei, B. vulgatus, E. coli, Fusobacterium; ↓ Lactobacillus and Bifidobacterium
Vogtmann, 201610	USA, France	Case-control	52 cancers / 52 controls	↑ Fusobacterium, Porphyromonas, Clostridia
Yu, 201711	Denmark, France, Austria	Case-control	74 cancers / 54 controls	↑ Peptostreptococcus stomatis, F. nucleatum, Parvimonas micra, Solobacterium moorei
Shah, 201712	Multiple countries	Pooled analysis of case-control studies	195 cancers / 79 adenomas / 235 controls	↑ Parvimonas micra ATCC 33270,Streptococcus anginosus, yet-to-becultured members of Proteobacteria
Liang, 201713	China	Case-control	203 cancers / 236 controls	↑ F. nucleatum, Clostridium hathewayi; ↓ B. clarus
Flemer, 201714	Ireland	Case-control	43 cancers / 37 controls	↓ Lachnospiraceae incertae sedis and Coprococcus

Overall, the gut microbiome exhibits substantial inter-individual variation but high overall temporal stability within individuals.^16–21 Although gut bacterial abundance may respond rapidly to extreme changes in diet,²² predominant microbial community membership is primarily determined by long-term diet, and substantial inter-individual variation persists despite short-term dietary change.^{17, 23–26} Recent data suggest that such high inter-individual variability may to a large extent determine the differences in the metabolic response to dietary intervention,²⁷ highlighting the importance for microbiome-based personalized nutrition in disease prevention and treatment.²⁸

Herein, we review several major dietary factors that have been linked to gut microbiota and CRC, summarizing the most recent epidemiologic and experimental evidence, with a focus on potential immune mechanisms. Overall, most of the epidemiologic evidence derives from cross-sectional or short-term, highly controlled feeding studies that are limited in size. Thus, this review focuses on the dietary factors that have strong mechanistic support, including dietary pattern, fiber, red meat and sulfur, and omega-3 fatty acid. Given the close link between diet and obesity and the predominant role of obesity in CRC as well as the substantial data linking the gut microbiome to obesity, we also include obesity at the end of the review.

Dietary patterns

Convincing data indicate that a “Western dietary pattern”, characterized by high intake of red or processed meat, sweets and refined grains, is associated with higher risk of colorectal neoplasia; in contrast, diets that are rich in fruits, vegetables and whole grains (“prudent pattern diet”) are associated with lower risk of CRC.^{29, 30} Western diets are associated with gut dysbiosis (microbial imbalance or maladaption),^{31, 32} loss of gut barrier integrity,^{31, 32} increased levels of inflammatory proteins,^33–37 and dysregulated immune signatures.^38–40 A potential role of the gut microbiota in mediating the dietary associations with CRC risk is suggested by the dramatic difference of the gut microbial structures between populations consuming different diets. Rural Africans, whose diet is high in fiber and low in fat, have a strikingly different gut microbial composition than urban Europeans or African Americans consuming a Western diet, which parallels the lower CRC rates in Africa than Western countries.^{26, 41, 42} For example, the African gut microbiota is characterized by a predominance of Prevotella genus that are involved in starch, hemicellulose, and xylan degradation, whereas the American microbiota is predominated by Bacteroides genus with a higher abundance of potentially pathogenic proteobacteria, such as Escherichia and Acinetobacter.⁴² Fecal short-chain fatty acids (SCFAs) are higher in native Africans, whereas secondary bile acids are higher in African Americans. Notably, SCFAs (described in details below) and secondary bile acids have been suggested to mediate the anti- and pro-cancer effect of fiber and fat on CRC, respectively. Moreover, a crossover study indicates that switching African Americans to a high-fiber, low-fat diet for 2 weeks increases production of SCFAs, suppresses secondary bile acid synthesis, and reduces colonic mucosal inflammation and proliferation biomarkers of cancer risk.²⁶

Recently, we have shown that “prudent dietary pattern” was more strongly associated with lower risk of CRC subgroups enriched with tissue Fusobacterium nucleatum (F. nucleatum),⁴³ suggesting a potential role for intestinal microbiota in mediating the diet-CRC relationship. F.nucleatum is a core member of the human oral microbiome and localizes to CRC tissue through binding to a protein overexpressed in CRC.⁴⁴ Numerous studies have shown an enrichment of F.nucleatum in CRC tissue relative to normal adjacent colonic tissue and in stools from individuals with CRC compared to those without cancer.^{6, 45–52} High abundance of F.nucleatum in tumor tissue has also been associated with poor survival of CRC patients.⁵³ Experimental evidence supports that F.nucleatum may promote CRC development and worsen cancer survival by activating β-catenin pathway and potentiating tumoral immune evasion through recruitment of tumor-infiltrating myeloid cells and inhibition of natural killer (NK) cell function.^54–56 In support of the hypothesis that diet may influence CRC risk by modulating F.nucleatum abundance, a dietary intervention study noted a marked increase in stool F.nucleatum levels after individuals were switched to a low-fiber, high-fat diet.²⁶ Further studies are needed to identify the major dietary factors that influence F.nucleatum localization in the gut and elucidate the underlying mechanisms.

Fiber

Numerous prospective studies have linked higher fiber intake to lower risk of CRC.² The most recent expert report from the World Cancer Research Fund and American Institute for Cancer Research in 2011 concludes that evidence that consumption of foods containing dietary fiber protects against CRC is convincing.⁵⁷ Besides systemic benefits for insulin sensitivity and metabolic regulation,⁵⁸ which have been implicated in colorectal carcinogenesis,^59–61 fiber possesses gut-specific activities, such as diluting fecal content, decreasing transit time, and increasing stool weight, thereby minimizing exposure to intestinal carcinogens.⁶²

Moreover, soluble fiber can be fermented by bacteria in the lumen of the colon into SCFAs, including butyrate, acetate, and propionate. Higher fiber intake has been shown to enrich butyrate-producing bacteria in the gut, such as Clostridium, Anaerostipes, Eubacterium, and Roseburia species, and increase production of SCFAs.^{26, 63} SCFAs have been suggested as the key metabolites linking the gut microbes to various health conditions, especially CRC. Butyrate is a major energy source for colonocytes and plays an important role in energy homeostasis in the colon tissue.⁶⁴ In cancer cells, however, butyrate is metabolized to a lesser extent due to the Warburg effect (the enhanced conversion of glucose to lactate by tumor cells even in the presence of normal levels of oxygen) and accumulates in the nucleus of cancerous colonocytes, whereby it functions as an inhibitor of histone deacetylase to epigenetically downregulate expression of numerous genes responsible for tumor growth (e.g., MYC, BAX, NRAS), angiogenesis (vascular endothelial growth factor family), migration (matrix metalloproteinase family, plasminogen-plasmin system), and chemoresistance (P-glycoprotein).⁶⁵ Studies using gnotobiotic (germ-free) mouse models have provided compelling data that dietary fiber protects against colorectal tumorigenesis in a microbiota- and butyrate-dependent manner via histone deacetylase inhibition.⁶⁶

In addition to suppression of histone deacetylase, butyrate can also function at the colonic epithelial cell surface as an agonist for certain G protein-coupled receptors (GPRs), such as GPR43⁶⁷ and GPR109a,^{68, 69} thereby inhibiting intestinal inflammation and possibly carcinogenesis. Recently, several studies have demonstrated the crucial role of SCFAs in intestinal immune homeostasis through modulation of regulatory T cells. As a T-cell subset with immunosuppressive functions, regulatory T cells plays a central role in the suppression of inflammatory and allergic responses by limiting proliferation of effector CD4⁺ T cells. Butyrate and propionate have been shown to induce extrathymic generation and functional differentiation of regulatory T cells and protect against colitis.^{68, 70–72} Possible mechanisms include histone deacetylase inhibition, enhancement of anti-inflammatory phenotype in colonic macrophages and dendritic cells via activation of GPR109a, and a T-cell intrinsic epigenetic upregulation of the Foxp3 gene, a prerequisite transcription factor for regulatory T cells. Moreover, butyrate may modulate the function of intestinal macrophages by histone deacetylase inhibition and downregulate lipopolysaccharide-induced proinflammatory mediators, thereby facilitating host tolerance to intestinal microbiota.⁷³

In agreement with these mechanistic data, resistant starch (a starch that resists digestion in the small intestine and undergoes bacterial fermentation in the large intestine to produce SCFAs) has been shown to have chemopreventive effects against colitis-associated CRC.⁷⁴ Moreover, preclinical studies have indicated the potential of butyrate and its analogs as chemotherapeutic agents in several tumor models,^{75, 76} including CRC.⁷⁷ Based on these reports, further translational studies are expected to provide more data about the clinical effectiveness of fiber or butyrate in CRC prevention and treatment.

Interestingly, the beneficial effect of butyrate may depend on the host genetic background. A recent study reported that butyrate fuels hyperproliferation of colon epithelial cells and induces CRC in APC^Min/+MSH2^−/− mice,⁷⁸ a model system of defective DNA mismatch repair which underlies the aggressive and rapid development of adenoma and CRC with microsatellite instability in hereditary nonpolyposis CRC (Lynch syndrome).⁷⁹ Future studies are needed to investigate whether these findings can be generalizable to human by assessing the fiber-CRC association according to microsatellite instability status.

Red meat and sulfur

There is convincing evidence that red and processed meats are associated with increased risk of CRC.⁸⁰ Recently, the International Agency for Research on Cancer has classified processed meat as a carcinogen to humans. Mechanisms underlying the pro-cancer effects of red or processed meats include heme iron, N-nitroso compounds, or heterocyclic amines,^{81, 82} and hydrogen sulfide production.⁸³ Hydrogen sulfide has been implicated in inflammatory disorders associated with risk of CRC, such as ulcerative colitis,^84–86 and directly with CRC.^87–93 In the colon, excess chronic hydrogen sulfide exposure is associated with key drivers of carcinogenesis, including impaired colonocyte nutrition, DNA damage, epithelial hyperproliferation, inflammation, and alterations in immune cell populations and function.^{89, 94–97} Hydrogen sulfide is also emerging as a modulator of T cell survival and proliferation; cysteine intake and hydrogen sulfide production influence gut T cell responses.⁹⁸ Hydrogen sulfide-high environments may favor regulatory T cells that in turn suppress the activation and proliferation of effector T cells, leading to impaired anti-tumor immunity.

Gut luminal hydrogen sulfide production appears to be fundamentally dependent on the action of sulfur-reducing bacteria, which metabolize dietary sulfur.⁸³ Dietary sulfur in turn modifies the abundance of sulfur-reducing bacteria in the colon.^{89, 90} Meat is a rich source of sulfur-containing amino acids such as cysteine and methionine, and processed meat typically contains inorganic sulfur (sulfate and sulfite) routinely used as a preservative.⁸³ Thus, the consistent association between meat, particularly processed meat, and CRC may at least in part be due to the influence of meat on the abundance of sulfur-reducing bacteria. The sulfur content of foods alone is likely not the only determinant of the abundance of sulfur-reducing bacteria or hydrogen sulfide production. Macronutrients such as specific fats consumed with sulfur-containing amino acids might modulate this association.⁹⁹ Furthermore, meat-based sources of sulfur are distinct from vegetable-based sulfur, particularly glucosinolates abundant in cruciferous vegetables. A core of gut microbes distinct from sulfur-reducing bacteria appears to hydrolyze the sulfur-containing glucosinolates into isothiocyanates, which, in contrast with hydrogen sulfide, are associated with cancer preventative properties.^{2, 100}

As a member of sulfur-reducing bacteria, F.nucleatum has been implicated in CRC development (see section for Dietary Patterns). Besides its immunomodulatory effects, F.nucleatum may also promote genotoxicity by its ability to convert cysteine to hydrogen sulfide.¹⁰¹ Limited data have also shown an association between other sulfur-reducing bacteria and CRC. In two case-control studies, the stool or luminal microbiota in colon cancer patients was enriched with bacteria producing hydrogen sulfide, such as Porphyromonas, or bacteria from the Prevotellaceae family.^{6, 48} However, the retrospective design makes these studies unable to dissect whether sulfur-reducing bacteria is a cause or effect of colorectal carcinogenesis. Further prospective studies are needed to examine sulfur-reducing bacteria in relation to CRC risk and better understand how diet may influence CRC by altering the abundance and function of sulfur-reducing bacteria.

Omega-3 Fatty acid

Marine omega-3 polyunsaturated fatty acid, including eicosapentaenoic acid, docosahexaenoic acid and docosapentaenoic acid, possess potent anti-inflammatory activity and may protect against CRC.^102–105 Fish oil, a rich source of omega-3 fatty acid, is the most popular natural product used by U.S. adults.¹⁰⁶ Substantial data support the beneficial effect of omega-3 fatty acid on CRC prevention and treatment.¹⁰³ In randomized controlled trials, omega-3 fatty acid supplement reduces the number and size of polyps in patients with familial adenomatous polyposis and improves survival of CRC patients with liver metastasis.^{105, 107} The anticancer effect of omega-3 fatty acid may be related to its multifaceted anti-inflammatory activity mediated by alterations in lipid raft structure and changes in fatty acid composition of cell membranes. These changes modify downstream metabolite production, including a decrease in inflammatory eicosanoids (e.g., prostaglandin E₂), and an increase in pro-resolving lipid mediators (e.g., resolvin and lipoxin).^108–113 Our recent study showed that omega-3 fatty acid was primarily associated with lower risk of CRC subsets infiltrated with high density of FOXP3⁺ T cells, and might protect against CRC by downregulation of the immunosuppressive activity of regulatory T cells.¹¹⁴ These findings suggest a potential interaction of omega-3 fatty acid with tumor immunity in prevention of CRC.

Dietary fat composition is a major driver of the gut microbial community structure.^115–120 Compared to other types of fat, omega-3 fatty acid have been associated with higher intestinal microbiota diversity and omega-3 fatty acid-rich diet ameliorates the gut dysbiosis induced by omega-6 polyunsaturated fatty acid or antibiotics.^{117, 118, 121, 122} Animal studies indicate that omega-3 fatty acid supplements increase the abundance of anti-inflammatory bacteria, such as lactic acid-producing bacteria (mainly Lactobacillus and Bifidobacteria), and decrease the abundance of immunosuppressive and pro-inflammatory bacteria, such as F. nucleatum, lipopolysaccharide-producing bacteria (e.g., Escherichia coli) and Akkermansia.^{115–120, 122}

Some species from Lactobacillus and Bifidobacteria genera support the host immunoprotective system,^{123, 124} promote antitumor immunity, and facilitate cancer immunotherapy.^125–127 Anaerobic gut bacteria, including some species of Lactobacillus, have been implicated in the saturation of polyunsaturated fatty acid, a detoxifying mechanism that transforms bacterial growth-inhibiting polyunsaturated fatty acid into less toxic fatty acid, such as hydroxyl fatty acid.^128–134 These microbial metabolites may help preserve intestinal barrier integrity, reduce oxidative stress, and lower inflammation.^{135, 136} Given that Lactobacillus is selectively enriched by omega-3 fatty acid, there may exist a reciprocal mechanism by which gut microbes adapt to host dietary change with functional consequences for host health. Moreover, a cross-feeding effect has been noted between human Bifidobacterium, which produces lactate and acetate, and the butyrate-producing species, such as Eubacterium rectale, which convert lactate to butyrate.^137–139 Butyrate, a short-chain fatty acid, has potent anti-inflammatory¹⁴⁰ and potential anti-CRC properties.^{66, 141} (see section for Fiber) On the other hand, higher serum levels of lipopolysaccharide antibodies have been associated with increased CRC risk in men,¹⁴² and higher abundance of F. nucleatum has been linked to higher CRC risk and shorter survival.^{6, 45, 46, 49, 51, 53} Taken together, these findings support the hypothesis that omega-3 fatty acid may preserve colonic immune homeostasis and suppress CRC through modulation of the gut microbiota.

Several potential pathways may contribute to the microbe-modifying effect of omega-3 fatty acid. A recent study showed that high omega-3 fatty acid might alter the production of microbiota regulators in colonic tissue.¹⁴³ Omega-3 fatty acid metabolite resolvin stimulates host epithelial expression of a transmissible factor, intestinal alkaline phosphatase,¹⁴⁴ whose lipopolysaccharide-detoxifying activity leads to decreased abundance of lipopolysaccharide-producing and/or pro-inflammatory bacterial groups and increased abundance of lipopolysaccharide-suppressing and/or anti-inflammatory bacteria.¹⁴³ Moreover, luminal unabsorbed omega-3 fatty acid may alter the gut environmental conditions and changes in immune response due to omega-3 fatty acid may in turn confer selective pressure on the gut microbial community.^145–147 Given the sparse data, further investigations are needed to better understand the interaction network between omega-3 fatty acid, gut microbiota, and the immune system. This may lead to novel prevention strategies based upon dietary modification, manipulation of microbial ecology, or development of microbiome and immune profiling as a biomarker of chemopreventive efficacy.

Obesity

Since the 1970–1980s, the prevalence of obesity has markedly increased worldwide.¹⁴⁸ The obesity epidemic is believed to be largely driven by global westernization characterized by overconsumption of easily accessible and energy-dense food, and a sedentary lifestyle.^{149, 150} Obesity is an established risk factor for CRC and several other cancers.¹⁵¹ Possible mechanisms include increased insulin levels and bioavailability of insulin-like growth factor 1, altered secretion of adipokines and inflammatory cytokines, and changes in sex hormone levels.^{152, 153}

Emerging evidence suggests a bidirectional relationship between obesity and the gut microbiota. On the one hand, obese individuals are more likely to demonstrate dysbiosis than lean individuals. Specifically, a decrease in the phylum Bacteroidetes and an increase in Firmicutes associated with obesity was observed in some^154–156 but not all¹⁵⁷ studies. Moreover, the relative abundance of Bacteroidetes increases as obese individuals lose weight on either a fat- or a carbohydrate-restricted low-calorie diet and the increase in Bacteroidetes is significantly correlated to weight loss.¹⁵⁴ On the other hand, these microbial changes are likely not a mere consequence of obesity, because the obese phenotype can be transmitted by transplantation of the obesity-associated gut microbiota in mice. When colonized with a conventional mouse microbiota, gnotobiotic (germ -free) mice that are normally lean and resistant to diet-induced obesity accumulate more adipose tissue mass and develop insulin resistance despite an associated decrease in food consumption.^{158, 159} Similarly, the gut microbiota transplanted from mice with diet-induced obesity to germ-free recipients promotes greater fat deposition than transplants from lean donors.¹⁶⁰ It has been hypothesized that antibiotic use in early life, a critical window for metabolic development, increases risk of childhood obesity by disrupting the composition and metabolic activity of the gut microbiota that can exert long-lasting effects on body weight in adulthood.^{161, 162} Interestingly, antibiotic use, especially during early life, has been linked to increased risk of CRC and colorectal adenoma in a few studies.^163–166 While this association needs to be confirmed by further studies, it remains unclear whether increased adiposity plays any mediating role in this association.

Mechanistic data suggest that the gut microbiota may influence energy homeostasis and obesity pathogenesis through several pathways, including peripheral control of energy harvest, central regulation of food intake via the gut-brain neural communication, and inflammation and impaired gut barrier through activation of pattern-recognition receptors.^167–169 Taken together, these data support that while the gut microbial profile may change due to changes in body weight accompanied by systemic metabolic alterations, the composition of the gut microbiota can also predispose to the development of obesity. However, because most evidence is from animal or small human studies with short-term intervention, it remains to be characterized how the community structure and function of the gut microbiota varies with host adiposity over a long-term period, which is more relevant to cancer development.¹⁷⁰

Given the link between obesity and gut microbiota and the role of the gut microbiota in cancer development, it has been proposed that changes in the gut microbiota may contribute to obesity-associated carcinogenesis. Indeed, studies in liver cancer have suggested that increased enterohepatic circulation of the obesity-induced gram-positive gut microbial metabolite deoxycholic acid facilitates hepatocellular carcinoma development by inducing cellular senescence and the senescence-associated secretory phenotype in the tumor microenvironment.^{171, 172} Besides deoxycholic acid, another gut microbial component, lipoteichoic acid, may also contribute to obesity-induced liver cancer by enhancing senescence-associated secretory phenotype and upregulating the expression of prostaglandin-endoperoxide synthase 2.¹⁷³ As a critical enzyme in inflammation, prostaglandin-endoperoxide synthase 2 mediates production of prostaglandin E₂, which governs tumor-mediated immune dysfunction and contributes to a shift in the tumor microenvironment from anti-tumor responses to immunosuppressive responses.¹⁷⁴ Given the potential role of prostaglandin E₂¹⁷⁵ and secondary bile acid¹⁷⁶ in promoting CRC, further studies are needed to investigate whether microbial imbalance-induced metabolic change also mediates obesity-related tumor promotion in the colon.

Conclusion

CRC is one of the cancers that are most closely associated with diet. Human intestinal tract is colonized by ~100 trillion microbes, the vast majority of which resides in the large intestine and is integral to host genomic stability, immune homeostasis, and metabolism. A growing body of evidence indicates a complex interrelation between diet, gut microbiota and CRC. However, most of the evidence derives from cross-sectional or short-term, highly controlled feeding studies that are limited in size. Given the multistage process and long latency of colorectal carcinogenesis, high-quality prospective studies with dietary data collected over the life course and gut microbial composition and function assessed well prior to neoplastic occurrence are critically needed. To make these studies possible, further investments are needed for stool collection in the existing, preferably younger epidemiologic cohort, standardization of the microbiome study pipeline, and development of novel user-friendly statistical tools to link the high-dimensional omics data to longitudinal epidemiologic data (including diet). These investigations will provide essential data to identify microbiome-based interventions that may complement or optimize the current diet-based strategies for CRC prevention.