Short-chain fatty acids (SCFAs) produced by dietary fibres act early on to lower the toxic build-up of phenols and ammonia and may explain the preventative role of resistant starch (dietary fibre), according to researchers reviewing its beneficial significance for CRC risk factors and gut microbiota diversity.
Butyrate (SCFA), for example, is a source of energy for colonocytes and has a biological effect on neoplastic cells, which inhibits proliferation of carcinogens.
“Butyrate inhibits migration and invasion of cancer cells by raising anti-metastasis genes expression and reducing the activation of pro-metastatic genes at 0.5 or higher mmol/L concentrations,” they say.
In tests, oral administration of bacteria that produces butyrate (Butyrivibrio fibrisolvens) increased levels in the colon and rectum and decreased the production of “aberrant crypt foci, an early colonic lesion”, they maintain.
Dietary fibre profile
The World Health Organisation recommends consumption of at least 25g of fibre per day, although regional advice for adults varies from 18-38g.
Dietary fibres are soluble or insoluble polysaccharides, resistant to digestive enzymes and are neither metabolised nor absorbed in the small intestine. Foods such as fruit, vegetables, legumes, and cereals are rich sources.
Consumption promotes bacterial cell growth, resulting in laxative effects and faster colon transit times. This reduces protein degradation and the accumulation of pathogenic chemicals.
“Dietary fibre improves viscosity, faecal bulking reduces proteolytic fermentation time and the amount of interaction between potential carcinogens and mucosal cells,” the researchers explain.
Other health benefits include improved mineral absorption, anticancer, lipid metabolism, anti-inflammatory and anticancer effects - mainly due to SCFAs from soluble fibre degradation in the colon.
The global incidence of CRC is expected to reach 3.2 million by 2040 due to excessive exposure to enabling factors, such as hereditary disease and lifestyle choices, the authors write.
Exact mortality rates are unclear since approximately 40% of rectal cancer deaths are falsely attributed to colon cancer instead of the rectal variant.
Main risk factors include excessive intake of red and processed meat, obesity, belly weight, and binge drinking, as well as age, race, and diabetes. Men are also more predisposed to CRC than women.
Western preference for ultra-processed foods, high in carbohydrates and fat, has exacerbated the problem and lead to widespread dietary fibre deficiency.
Poor diet also disrupts the balance of gut microbiome, which is a necessary arbiter for positive dietary fibre action, including appetite management, metabolic activities, and chronic inflammatory pathways, they say.
“Many features of our food are troubling and the evident lack of dietary fibre in our modern diet is particularly concerning. Our gut microbiota, similar to our bodies, did not evolve to embrace this nutritional maladaptive behaviour.”
They add that research suggests gut inflammation and CRC oncogenesis can be modulated by food, gut microbiota, and gut environment, and may “serve as adjustable variables in modifying CRC outcomes.”
In studies on the effect of diet on gut microbiota, the prevalence of the three major bacterial varieties (Bacteroides, Prevotella, and Ruminococcus) was considered a good indicator of the impact of diet on gut microbiota, they write.
A study of long-term dietary influences on gut health in industrialised and unindustrialised communities found increased Prevotella (non-Western) and Bacteriodes (Western) were associated with dietary fibre and mutton intake, respectively.
Prevotella is responsible for nurturing healthy microbiota and was generally more abundant among subjects in rural communities, whereas gut dysbiosis was more common among city dwellers introduced to a “Westernised way of life”, they say.
“Environmental lifestyle changes, rather than host genetics, affected gut microbiota diversity, and industrialisation results in a considerable loss of species, according to these findings.”
Furthermore, long-term dietary fibre deficiency was linked to an increase in mucin-degrading bacteria, such as Akkermansia muciniphila, which affects the mucus barrier.
“When the diet lacks dietary fibres, some gut bacteria adapt their metabolism to utilise mucin glycans by activating the gene expression of mucin-degrading enzyme.”
Despite advances in treating CRC, morbidity and mortality remains high and dietary interventions that correct gut imbalance and optimise dietary fibre activity “may possess a highly beneficial role in cancer prevention in general and CRC in particular”, the authors conclude.
Published online, November 2, 2022: http://doi.org/10.3390/dietetics1030017
‘A Mechanistic Overview on Impact of Dietary Fibres on Gut Microbiota and Its Association with Colon Cancer’
Authors: V. Biswas, et al.