Colorectal cancer (CRC) is the second leading cause of cancer death in Europe. The geographical variation of incidence demonstrates how environmental factors, chiefly dietary habits, play a major role in this disease. Convincing evidence suggests that risk of CRC is increased by red meat and processed meat consumption (classified, as regard cancer hazard as 2A and 1, respectively, WHO) and decreased by foods containing dietary fibers. As regarding other food groups, like fish, although epidemiological studies suggest a reduction of CRC risk associated with its consumption, the evidence for this link is considered limited; similarly, for non-starchy vegetables and fruits, although there is suggestive evidence of a protective effect, this is considered limited, and thus less convincing than that for red and processed meat.

Several hypotheses are proposed to explain the positive association between red, processed meat and CRC: meat-based diets contain mutagens-carcinogens formed during cooking, but also lipid peroxidation and N-nitroso compounds whose formation is catalyzed by heme in the colon. Accordingly, recent experimental and epidemiological studies on the E3N cohort and on the randomized placebo-controlled trial SUVIMAX carried out by our group, demonstrated the central role of heme iron in the positive association between meat and CRC. According to these studies, this effect is largely explained by the ability of heme iron to catalyze peroxidation and induce high luminal polyunsaturated fatty acids (PUFAs) peroxidation, forming cytotoxic and genotoxic alkenals which will in turn induce positive selection of precancerous cells mutated for Apc, the key gene in CRC. On this basis, it was also demonstrated that antioxidants, in particular tocopherol, modulate the risk of cancer associated with red meat in experimental animals and humans by controlling heme iron-induced peroxidation. Recent data from the investigators demonstrated that microbiota is involved in the heme-induced peroxidation and it has been reported that the gut microbiota is required for heme-induced epithelial hyperproliferation and hyperplasia because of the capacity to reduce mucus barrier function. Notwithstanding these reports, the role of gut microbiome in determining cancer risk associated with red and processed meat is not clear.

Microbial fermentation of plant-based foods, associated with a low CRC risk increases intestinal Short Chain Fatty Acids (SCFAs), among which butyrate, endowed with antineoplastic activity through its inhibition of histone deacetylase and promotion of apoptosis and microbial activated phytochemicals, such as polyphenols with anti-inflammatory and antioxidant activities. Accordingly, it is also known that the process of fermenting fibers to SCFAs needs intestinal bacteria because germ-free mice produce almost no SCFAs. These data clearly indicate that, at least part of the effect of the diet on colon carcinogenesis is mediated by the intestinal microbiome. Accordingly, it is well know that diet shapes gut microbiome composition, as described in a human rural population study and emerging evidences implicate an involvement of the gut microbiota in CRC. Microorganisms and their metabolites have been proposed to promote carcinogenesis by several mechanisms, including induction of inflammatory signaling pathways, genetic mutations, and epigenetic dysregulation. Recently, a review conducted on 31 studies (human and animal models) has shown that certain bacterial groups are increased (eg. Fusobacterium spp, Alistipes, Staphylococcaceae, Akkermansia spp. and Methanobacteriales), while others (eg. Bifidobacterium, Lactobacillus, Faecalibacterium spp) are consistently diminished in CRC, with consequent increase of potentially carcinogenic metabolites (nitrogen compounds, bile acids) and decrease of SCFAs (eg. butyrate). However, it is still not clear whether dysbiosis (imbalanced microbiota) is the cause or consequence of CRC. Tumorigenesis can indeed produce inflammation, ulceration and necrosis of the mucosa, by changing the microenvironment and growth conditions for different microorganisms, thus it is difficult to understand what comes first. And furthermore, how dietary risk is mediated by the interaction with the gut microbiome? How can the modulation of the microbiome promote or prevent the development of a microenvironment containing pro-inflammatory and carcinogenic metabolites that favour the neoplastic initiation process? Cohort studies with subjects consuming various types of diets (i.e omnivores, vegetarians, vegans) suggest that diet alters the intestinal microbiome as well as the cytotoxic and genotoxic activities of the luminal colonic content. In a study on ten volunteers following for 5 days a strictly vegetarian diet and then moving to a strictly carnivorous diet, it was demonstrated that the intestinal bacteria react very quickly (24-48 hours). In particular, bacterial species capable of digesting complex carbohydrates prevailed during the vegetarian diet period while during the animal proteins based diet, specific bacterial species were selected, such as Bilophila wadsworthia, which are able to metabolize proteins and produce toxic compounds like secondary bile acids (BA, promoters of carcinogenesis) and with great pro-inflammatory potential. Feeding a high-fibre low-fat African-style diet to African Americans at high risk of colon cancer and, vice versa, feeding a high-fat, low-fiber western style diet to rural Africans at low risk of cancer, causes variation in microbiome and in parameters associated with CRC risk. Preliminary data in humans also demonstrate that when an animal-based diet rich in fat and simple sugars is introduced into a traditional African diet, composed of cereals, legumes and vegetables, the gut microbiome shifts, leading to progressive loss of biochemical functions associated with SCFAs production, suggesting indeed that faecal microbiome may act as determinant of colon cancer risk related to the diet. Regarding heme iron, the investigators recently demonstrated that after a short term exposure (14 days), the gut microbiota of heme-fed rats was enriched with Enterobacteriaceae and B. fragilis whereas Roseburia spp. and Lactobacillus spp. were underrepresented. Notably, Enterobacteriaceae expansion is also found in Inflammatory Bowel Disease (IBD) patients at high risk for CRC and can induce inflammation in the host gut epithelium. Moreover, B. fragilis has been associated with inflammation-induced colon cancer. It has also been shown that Lactobacilli can inhibit iron-induced lipoperoxidation. Remarkably, the gut microbiota modulations observed after heme iron intake present similarities with that observed comparing colorectal cancer patients and healthy volunteers. However, although the association between gut microbiota and CRC is conceptually interesting, these outcomes do not help to explain the mechanism behind the modulations of the gut microbiota by the diet and the consequent impact on CRC risk. In addition, while many studies have been conducted for bacteria, fungi have been virtually unexplored by metagenomic studies, yet they are emerging as key players involved in autoimmune or inflammatory disorders, as recently demonstrated. Their relative under-representation in number compared to bacteria, results in their underestimation, since fungal DNA often cannot be purified using standard approaches. Preliminary results revealed a higher frequency of S. cerevisiae in IBD, a group pathologies associated with increased risk of CRC.