High salt intake (e.g., Chinese style salted fish) associated with nitrosamine formation and Epstein–Barr virus has been correlated with nasopharyngeal cancer, which is most common in Asian, Arctic, Middle East, and North African populations ^[1][2][8,88]. In contrast, increased consumption of high-fiber foods (fruits, vegetables), vitamin C, and folate has been associated with decreased risk of oral and pharyngeal cancers, although this evidence is merely suggestive as confounding environmental factors such as altered smoking and alcohol consumption patterns may influence disease outcomes also ^[1][2][8,88]. These correlative studies, which are often plagued by recall or selection biases, are not well supported by prospective studies; however, more recent mechanistic studies have begun to uncover how specific dietary factors found within certain fruits and vegetables influence tumorigenesis ^[1][8]. As well, protein breakdown creates a neutral-alkaline environment that can promote periodontal disease, which is a risk factor for oral cancer and oral cancer survival ^[3][4][70,89]. However, studies are limited, and the mechanisms are not well understood ^[3][70].

The oral microbiome has been referred to as the ‘oralome,’ an umbrella term encompassing the dynamic interactions between host cells of the oral cavity and microbial communities ^[5][90]. The oral microbiome therefore not only regulates interspecies interactions but also mediates the crosstalk between the microbial community and the oral cavity ^[5][90]. The balance between a healthy/homeostatic (i.e., eubiotic) and a diseased (i.e., dysbiotic) state depends on the interactions between microbial species within the oral cavity and between the host and the oralome itself. Head/neck cancer and oral cancers have been shown to have a distinct dysbiotic signature, but direct causality between the oralome and oral cancers remains limited ^[4][5][89,90]. The oral microbiome is predominantly composed of a bacterial biome.

Oral cancer is most associated with and thereby influenced by the oral and gut microbiota ^[6][69]. Induction of chronic inflammation by bacterial stimulation is one mechanism, which has been thought to influence pathogenesis via production of inflammatory mediators, causing mutagenesis and uncontrolled cell proliferation ^[7][91]. The latter process has been thought to be regulated through activation of the nuclear factor κB (NF-κB) signaling pathway and inhibition of apoptotic pathways ^[8][92]. The highest mortality oral cancer remains OSCC which is significantly influenced by the oral microbiota through carcinogenetic modulation of cell metabolism (i.e., regulating changing concentrations of nutrients and vitamins) ^[6][69]. This modulation can then promote cytokine production associated with different pathological conditions ^[6][69]. Porphyromonas gingivalis, Fusobacterium nucleatum, and Prevotella intermedia are the microbes most significantly associated with OSCC; P. gingivalis and F. nucleatum have been shown to promote tumor progression in mice, and in OSCC these bacteria increase toll-like receptor 2 (TLR2) and pro-inflammatory cytokines IL-6 and IL-8 production, potentially contributing to disease progression ^[4][89]. As well, P. gingivalis has been found to increase oral cancer cell invasion and proliferation, increasing myeloid-derived suppressor cells and chemokines (CCL2 and C-X-C motif) ^[4][89]. Interestingly, the abundance of F. periodonticum, Parvimonas micra, Streptococcus constellatus, Haemophilus influenza, and Filifactor alocis increases from stages one to four of OSCC ^[4][89]. Additionally, studies have shown that the Fusobacterium, Peptostrepococcus, and Prevotella genera increase in the periodontal tissues in patients with gingival squamous cell carcinoma ^[4][89].

The oral microbiome is an ideal biomarker for oral tumors, compared to other biomarkers, highlighting its possible role as an important immunotherapy agent ^[9][47]. Commensal bacteria have been shown to enhance the efficacy of immunotherapy with checkpoint inhibitors where tumor growth can be controlled through combined oral administration of Bifidobacterium and programmed cell death protein 1 ligand 1 (PD-L1)-specific antibody therapy ^[9][47]. Dietary changes over time, including introduction of dairy products, refined carbohydrates, vegetable oils, and alcohol, have been associated with, but not all causatively linked to, a decline in overall oral health and cancers ^[3][70]. Dietary factors have a significant influence on the gut microbiome, and this influence branches out to the oral microbiome, highlighting the critical role of the crosstalk between the human microbiome(s), diet, and disease ^[3][70].

Esophageal cancers are common in areas of Africa, China, and Iran ^[10][93]. Green tea is thought to be anti-tumorigenic; however, scalding hot drinks above 65 °C in temperature, such as coffee and tea, have been associated with increasing risk of esophageal cancers, potentially due to thermal injury ^{[11][12][13][14]}[94,95,96,97]. Squamous cell carcinoma is the more common form of esophageal cancer globally, and both smoking and alcohol consumption have been noted as risk factors, with limited evidence supporting the direct effects of micronutrients or dietary factors ^[1][15][16][8,71,73]. However, some suggest that polycyclic aromatic hydrocarbons from high-temperature foods might increase risk and dietary folate, which can be produced by microbes, and might reduce risk of esophageal squamous cell carcinoma ^[15][17][71,72].

Damage of the esophagus caused by acid reflux (via diet or gastroesophageal reflux disease) complicating Barrett’s esophagus (precursor to esophageal adenocarcinoma) can increase risk of esophageal adenocarcinomas ^[16][18][19][73,98,99]. The esophageal microbiome in esophageal adenocarcinoma is dominated by lactic acid producing Lactobacillus which can acidify the esophageal microenvironment which can be further exacerbated by the hydrogen peroxide produced by these microbes ^[16][73]. Furthermore, the functions of the esophageal microbiota are altered in esophageal adenocarcinoma, including an upregulation of cell replication and metabolism along with a decrease in fatty acid biosynthesis and D-alanine and nitrogen pathways ^[16][73]. Interestingly, while H. pylori is best known for its significant role in stomach cancers, there is also an increase in the abundance of this pathogen in esophageal tumor tissues ^[16][73]. Meanwhile human papillomavirus and Epstein–Barr virus are reported to increase the risk of developing esophageal squamous cell carcinoma ^[20][100]. Furthermore, increased Porphyromonas gingivalis, and Fusobacterium, along with reduced Streptococcus, have been identified in esophageal tumor tissues ^[16][73]. Both the oral and intestinal microbiota have also been linked to esophageal cancers; increased Neisseria and Streptococcus pneumoniae were uncovered in esophageal adenocarcinoma, while increased P. gingivalis, Actinomyces, and Atopobium were indicative of high risk of esophageal squamous cell carcinoma ^[21][22][23][101,102,103]. Alterations of the microbiome in models of esophageal cancers and precancerous lesions have been associated with TLR and NLR inflammatory pathways, demonstrating these pro-inflammatory pathways along with pro-inflammatory cytokines are increased in esophageal malignancy, possibly via interactions with the resident microbiota ^[16][23][73,103].

Interestingly, a high-fat diet in a mouse model of Barrett’s esophagus induced tumors faster than the control diet, likely related to altered microbiota and an increase in neutrophils and cytokines in the esophagus ^[24][74]. Importantly, body size had no effect on tumor growth, suggesting diet rather than obesity influences cancer risk ^[24][74]. Associations have been identified between diets that are high in red and processed meats and low in fruits, vegetables (leafy greens especially), and cruciferous vegetables with an increased risk of esophageal cancer ^[17][72]. As well, N-nitroso from processed foods is associated with increased risk, influencing cell cycle progression (increased cyclinE 1 and cyclinD 1) and epidermal growth factors (increased transform growth factor α � and epidermal growth factor receptor) ^[15][71]. Further inverse associations identified include dietary fiber, vitamin E, vitamin C, and β�-carotene intake and esophageal cancer ^[17][72]. Studies have shown that fiber intake can change the composition of the esophageal microbiome, increasing Firmicutes and decreasing gram-negative bacteria, with limited detection of SCFA-producing bacteria regardless of fiber intake ^[25][75]. Disease progression specifically has been worsened by sugar, which increases pro-inflammatory cytokines and causes dysbiosis ^[15][25][71,75].

Stomach cancers are most commonly diagnosed in Eastern Asia and comprise the fifth most common cancer worldwide ^[10][93]. Similar to oral cancers, a diet high in salted foods (e.g., salt preserved fish) may increase risk directly or via nitrates, nitrites, and N-nitroso compounds commonly contained in preserved high-salt foods, smoked foods, and as food additives ^[26][27][28][76,104,105]. As well, these nitrates and nitrites can mix with heme irons, amines, and amides from other foods to produce N-nitroso compounds ^[26][76]. For example, consuming large amounts of pickled foods is thought to increase stomach cancer risk via fungal species commonly found in these foods that produce N-nitroso compounds and also inhibit prostaglandin E synthesis, which protects the mucosa ^[26][29][76,106]. As well, a strong correlation has been identified between increased alcohol intake and risk of gastric cancer ^[26][76]. The metabolism of alcohol produces reactive oxygen species (ROS), which can block blood vessels and promote inflammation and injury, and acetaldehyde, which binds to and inhibits DNA synthesis in gastric glands ^[26][76]. Again, some correlative evidence supports the benefits of a diet high in fruits, vegetables, and vitamin C in reducing cancer risk, although mechanisms have yet to be uncovered ^[30][107]. However, it has been suggested that vitamin C can act as an enzyme cofactor and ROS scavenger, decreasing oxidative damage ^[26][76]. Studies show mixed results about the effects of vitamin E on gastric cancer risk; however, one study demonstrated that vitamin E succinate (a vitamin E derivative and by-product of fiber fermentation by microbiota) could induce autophagy in gastric cancer cell lines ^[26][76]. Interestingly, supplementation with combinations of α tocopherol, β carotene, vitamin C, and selenium in clinical studies resulted in a regression of precancerous lesions and significant reduction in stomach cancer mortality ^[31][32][33][108,109,110]. Further, it has been suggested that carotenoids drive a shift from a Th-1 response to a Th-1/2 response, reducing inflammation ^[26][76]. As well, green tea consumption in non-smoking women has been shown to reduce risk of stomach cancer, potentially linked to the polyphenols in tea ^[34][111]. Helicobacter pylori infection directly causes stomach cancer by inducing chronic inflammation and causing DNA damage by converting nitrogen compounds into N-nitroso in gastric fluids, explaining why salted foods might promote tumorigenesis ^[35][36][37][77,78,112]. Interestingly, studies of the gastric microbiome have been limited due to difficulty culturing most microorganisms residing in the stomach; however, microbial diversity is thought to be significantly lower in patients with gastric cancer ^[28][105]. In particular, studies have highlighted an increase in acid-producing microbiota genera including Lactobacillus and Lactococcus, along with higher pro-inflammatory and pathobiont microbes such as Fusobacterium, Veillonella, Leptotrichia, Haemophilus, and Campylobacter in gastric cancer patients ^[28][105]. Recent advances in sequencing technologies have allowed for the detection of these microbes that make up the gastric microbiome, which have helped improve our understanding of microbes involved in gastric cancers ^[28][105].

The gut microbiome is the most well studied of the microbiomes to date, demonstrating direct links to development of colorectal cancer which is the third most common cancer globally ^[10][38][93,113]. A wealth of evidence further supports obesity, alcohol, and smoking as risk factors of colorectal cancer, while consumption of processed meat and unprocessed red meat have been significantly linked to carcinogenesis ^{[1][39][40][41][42][43][44][45]}[8,114,115,116,117,118,119,120]. Use of nitrates and nitrites in meat preservatives may expose the gut to mutagenic N-nitroso compounds, while heme iron contained in red meats may also increase N-nitroso and resulting cytotoxicity and gut damage ^[1][8]. Interestingly, exposure of certain meats to high heat during cooking can increase mutagenic heterocyclic amines and polycyclic aromatic hydrocarbons, while lactic acid bacteria (primarily Lactobacillus helveticus and Streptococcus thermophilus and less so Lactobacillus kefir and Lactobacillus plantarum) in the gut are capable of binding to these chemicals and reducing their mutagenic potential ^[1][43][46][8,118,121]. Calcium, including in milk products which are associated with moderate reduction in colorectal cancer, may also bind secondary bile acids and heme, reducing their tumorigenic potential ^[47][48][49][122,123,124].

Consumption of greater than 10 g of total dietary fibers (found in fruits, vegetables, and grains) a day is associated with reduced risk of colorectal cancer ^[50][51][125,126]. Interestingly, humans do not digest dietary fibers; they require gut microbes to ferment them into by-products such as SCFAs, which display beneficial and anti-inflammatory effects in the gut and systemically reduce risk of colorectal cancer ^[52][53][54][127,128,129]. However, recent evidence suggests there is a pro-inflammatory impact of specific dietary fibers in settings where the gut microbiota is altered, suggesting the microbiota are key to mediating these diet-associated benefits ^[55][79]. The opportunities to utilize microbe-altering therapies to manipulate production of SCFA levels in the intestinal tract for the treatment and prevention of cancers, including colorectal cancer could be considered ^[54][56][57][129,130,131]. This also highlights the implications of dysbiosis (altered microbiota composition) in development and progression of colorectal cancer, as commensal microbes that are key for fiber fermentation (Bifidobacterium, Faecalibacterium, and Blautia) are commonly reduced in colorectal cancer patients ^[58][59][80,81]. Significant associative changes between the gut microbiome and host factors have been highlighted in a recent study of healthy controls, irritable bowel syndrome, inflammatory bowel disease, and colorectal cancer patients, demonstrating increased Parvimonas, Bacteroides fragilis, Peptostreptococcaceae, and Streptococcus spp. associated with Syndecan-1, DNA replication, and cell cycle pathways ^[60][132]. Significantly greater details of the highlighted similarities and differences in associations between microbes and host genes in these GI disorders are discussed in the referenced manuscript ^[60][132].

Several factors have been directly linked to development and progression of liver cancers, particularly consumption of alcohol which induces liver inflammation associated with cirrhosis (including systemic inflammation) and alcoholic hepatitis ^[1][61][8,133]. Furthermore, alcohol can alter the epithelial barrier of the gut allowing microbes and microbial metabolites (including toxins) to be taken up more readily and translocate to the liver where they are capable of inducing inflammation and subsequently fibrosis and cirrhosis ^[62][63][134,135].

Another disease called non-alcoholic fatty liver disease (NAFLD) has the potential to progress into non-alcoholic steatohepatitis (NASH), which causes inflammation and damage due to excess fat stored by liver cells, cirrhosis, or hepatocellular carcinoma (HCC) ^[64][136]. Recent studies have shown data about the role of the gut microbiome in the etiology of NAFLD ^[64][65][66][82,136,137]. NAFLD is caused by the accumulation of triglycerides (TG) hepatocytes formed from the esterification of fatty acids in the liver ^[67][138]. During gut dysbiosis, gut permeability increases, allowing the increased absorption of fatty acids and translocation of bacteria and inflammatory cytokine and leading to worsened inflammation. SCFA producers and fiber fermenters such as Faecalibacterium prausnitzii and Akkermansia muciniphila are reduced in NAFLD ^[65][82]. This lowered production of SCFA could in turn increase gut permeability. Patients with NAFLD also have increased population of Escherichia coli that in turn leads to an increased production of ethanol in anaerobic conditions, stimulating the NF-kB pathway that leads to inflammation ^[65][68][82,83].

Furthermore, the fungal species Aspergillus, which is commonly found in foods such as dried fruits, nuts, and grains when stored incorrectly (hot and humid), produces a mutagenic toxin, aflatoxin ^[1][8]. This is particularly considered a risk factor for individuals with active HBV and HCV infections, which are the primary microbial cause of liver cancers ^[1][8]. Meanwhile, some studies suggest consumption of coffee with the bioactive compounds found in coffee beans may reduce risk of liver cancers ^{[1][69][70][71]}[8,139,140,141]. Consumption of caffeine increases Bifidobacterium species as well as the expression of Aquaporin 8 in the colon, which reduces the risk of cirrhosis and HCC as well as improves barrier integrity ^[72][84]. Similarly, green tea extracts have been shown to lead to improved liver enzymes, to reduced body fat, and to an increase in barrier function ^[73][142]. The catechins in green tea extract are poorly absorbed and therefore degraded by gut microbes such as Bifidobacterium, Lactobacillus, and Ruminococcus, producing SCFA ^[74][143].

On the other hand, a study found that certain fermentable fibers such as inulin induced cholestasis and HCC in dysbiotic mouse models ^[75][85]. After receiving a diet rich in soluble fermentable fibers, the HB mice had lower abundance of Tenericutes and increased abundance of Proteobacteria, which have been indicated in hepatocarcinogenesis in humans ^[75][85]. In addition, there was an increase in Clostridia, which are a fiber-fermenting species that also increased the amount of butyrate and secondary bile acids that further aggravated the disease and created a tumor-promoting environment when in a large amount ^[75][85].

6. Pancreatic Cancers

Pancreatic cancer remains one of the most lethal cancers globally, with surgery as the only potentially curative option for intervention ^[76][144]. Several factors associated with obesity increase risk of pancreatic cancer, including diabetes (heightened insulin), suggesting a role for diet ^[1][77][8,145]. Recent research has begun to uncover these links as high-fat diets significantly increase pancreatic metastases and activate receptors involved in driving progression of precancerous pancreatic lesions to pancreatic cancer ^[78][79][80][146,147,148]. A high-fat diet readily leads to obesity and elicits changes in the gut microbiome and microbial metabolites ^[81][149]. In addition, it might lead to the translocation of intestinal microbes as well as detrimental metabolites into the blood stream, which then make their way to the pancreas ^[82][150]. This dysbiosis has been associated with tumorigenesis and more aggressive pancreatic cancer ^[76][144]. For instance, Helicobacter pylori, which has been associated with increased risk of pancreatic cancer, can bind to epithelial cells in the stomach using the adhesin HopQ and carcinoembryonic antigen-related cell adhesion molecules (CEACAM) and inject its virulence factor CagA into the epithelial cells. This then activates a signaling pathway called the Wnt/β-catenin pathway, which is involved in various cellular functions such as proliferation ^[83][151]. Moreover, although inconsistent, some studies have also associated the increased intake of red meat and processed meat containing carcinogenic nitrites and N-nitroso compounds (NOCs) with the risk of developing pancreatic cancer due to their ability to form DNA adducts, inducing mutations ^[84][85][152,153]. On the other hand, increased fruit and vegetable intake have been shown to reduce the risk of pancreatic cancer ^[86][87][154,155]. Black raspberries, for instance, have been found to inhibit inflammation, cell transformation, and tumor-specific gene expression as well as increase tumor-infiltrating CD8+ T cells in pancreatic ductal adenocarcinoma ^[88][156]. Similarly, another study saw a down-regulation in the miRNA gene responsible for the development of inflammation, metabolic disease, carcinoma, invasion, and metastasis after introducing resistant starch diet in xenograft mice models ^[89][157]. This suggests that microbe-altering therapies such as prebiotics, probiotics (e.g., Faecalibacterium prausnitzii and Lactobacillus casei), and fecal microbiota transplant could offer potential to improve pancreatic cancer outcomes by reducing severity and improving treatment response ^[76][82][90][144,150,158].

The second most common cancer globally is breast cancer, while prostate cancer is the fourth most common global cancer ^[1][10][8,93]. Interestingly, while hormonal factors including estrogen, testosterone, and progesterone are key determinants of risk, the intestinal microbiome has been identified as a major regulator of circulating estrogen, as a producer of testosterone, and as a source for increased risk of breast and prostate cancer ^{[91][92][93][94][95]}[159,160,161,162,163]. Obesity has been linked to breast cancer risk, likely through increased circulating estrogens which are produced in adipose tissues, and to the aggressiveness of prostate cancer, although the evidence remains controversial, and prospective observational studies have been null ^{[1][96][97][98][99]}[8,164,165,166,167]. Adipose tissue in obese individuals may also secrete high levels of plasminogen activator inhibitor-1 (PAI-1), which inhibits enzymes involved in remodeling tissue and degrading blood clots and has been associated with increased risk of breast cancer ^[100][168]. Interestingly, the microbes of the gut are able to translocate to the skin and in turn to the breast tissues altering the breast microbiome which has been shown to impact breast malignancies ^[101][61]. As mentioned earlier, the micronutrition queuine, which is produced by microbes, is increased in breast cancers, and modifications of queuine can impact tight junction pathways leading to increased migration, invasion, and metastases of breast cancer ^[101][61]. Major changes in the breast microbiome in breast cancers include decreased Anaerococcus, Caulobacter, Streptococcus, Propionibacterium, and Staphylococcus; these changes were associated with increased oncogenic immune potential ^[102][169]. Similarly, in prostate cancer, various dietary factors which have been profoundly linked to prostate malignancies can impact the gut, urinary, and prostate microbiomes, demonstrating a decrease in microbes and microbial metabolites that play a significant role in regulating anti-cancer immune surveillance ^[103][104][67,68].

Dietary studies suggest a benefit of vegetable intake, dietary fibers, and soya isoflavins in relation to breast and prostate cancer, although the evidence remains largely inconclusive ^{[105][106][107][108][109]}[170,171,172,173,174]. These foods increase the production of SCFAs by beneficial bacteria such as Bifodobacterium and F. prausnitzii, which modulate anti-inflammatory and anti-cancer responses ^[110][111][86,175]. As well, total meat and processed meat intake have been associated with an increased risk of breast and prostate cancer ^[112][176]. This could be caused by the fat in meat increasing estrogen production and the meat components (heme iron, heterocyclic amines, polycyclic aromatic hydrocarbons, and N-nitroso compounds) causing DNA damage ^[112][176]. Further correlative evidence suggested beneficial effects of tomato lycopene, β carotene, vitamin D, vitamin E, and selenium in prostate cancer; however, the data remain inconclusive, requiring further investigations ^{[1][113][114][115][116]}[8,177,178,179,180].