Why the ‘Western’ diet is bad for the gut and how other diets compare

A typical Western diet bodes poorly for one's gut health, a new review affirms. Compared to other diets, a Western diet leads to a “marked reduction” in gut microbial diversity, according to a review published in Nature Reviews Microbiology, which synthesized existing research on the interplay between a person’s diet and their gut microbiome.“The loss of gut microbial diversity is concomitant with a Western diet,” the authors wrote. The gut microbiota plays an important role in regulating metabolism, and reduced diversity in the gut microbiome is linked to several metabolic disorders, the authors noted. Industrialized countries tend to adhere to a Western diet, while non-industrialized countries tend to adhere to diets with diverse, whole plant-based foods, the authors wrote. As a result, non-Westerners typically have a richer gut microbiota.“Many studies consistently reveal greater bacterial diversity and microbial richness in traditionally non-Western populations,” the authors wrote.The “enterosignatures,” or “microbial guilds,” of Western populations are dominated by members of the Bacteroides and Prevotella genera and the Firmicutes phylum, according to a study cited in the review that characterized 5,230 gut metagenomes. In contrast, the enterosignatures of non-Western populations consist predominantly of Prevotella species and Firmicutes, with minimal Bacteroides species.Bacteroides, along with the prevalence of species of Blautia and Ruminococcus (which are also associated with a Western diet), is linked to an increased risk of chronic inflammation and metabolic disorders such as inflammatory bowel disease, according to the review.A Western diet typically consists of “high-calorie content” and ultra-processed foods enriched with animal protein, saturated fats and simple sugars, with “inadequate” amounts of fibre, fruits and vegetables, the review noted.This diet is associated with a gut microbiome that produces fewer short-chain fatty acids (SCFAs), metabolites that “exhibit several health benefits” including protecting the intestinal lining from damage by increasing mucus secretion and decreasing luminal pH. SCFAs also have anti-inflammatory and immuno-modulatory effects, which contribute to overall gut health and reduces the risk of gastrointestinal diseases.Specific compounds from red meat can be detrimental to a person’s health, according to the review, because of how those compounds can interact with the gut microbiome. For example, choline and carnitine can be transformed by the gut microbiome into trimethylamine. The liver then converts that into Trimethylamine N-oxide (TMAO), which is associated with chronic diseases.Processed foods also contain many additives, preservatives and emulsifiers that can directly or indirectly interact with the gut microbiome, the authors wrote. For example, carrageenan, an additive found in many processed foods, can lead to inflammation of the gut and a disruption of the mucus layer, which cause changes in the gut microbiome.“The Western diet is overall linked to the surge of chronic inflammation leading to diet-related diseases, including obesity and many other non-communicable diseases,” the authors declared.The review also examined how other types of diets interact with the gut microbiome and, by extension, influence a person’s overall health.A caveat the authors noteed is that these are general effects and that everyone has a “personalized microbial response” to different diets.They also stated existing research overwhelmingly focuses on data from Europe and North America, with 70% of published human microbiome data coming from these regions, despite the fact they are home to less than 15% of the world’s total population.“Low-income countries are being overwhelmingly underrepresented and their associated diseases such as malnutrition receiving much less focus,” they wrote. Mediterranean dietThis diet is considered the “gold standard” in preventative medicine as it can help mitigate and manage multiple conditions including cardiovascular disease, Type 2 diabetes, inflammatory bowel disease, irritable bowel syndrome, cognitive decline and depression. Diet influences “the communication between the gut microbiome and the brain through various pathways,” the authors write. It consists of “a high intake of unprocessed, whole-plant foods, olive oil, dairy products, moderate consumption of poultry and fish, and low amounts of red meat.”It’s associated with an increased abundance of “health-promoting” Faecalibacterium prausnitzii and species of Roseburia. “These changes in the microbiome as a result of diet were linked to enhanced production of SCFAs and reduced production of metabolic by-products (such as ethanol, para-cresols and carbon dioxide).”High-fibre dietA high-fibre diet substantially increases the amount of “beneficial” species of Lactobacillus and Bifidobacterium in the gut microbiome.It’s also linked to an increase in SCFA production. When it reaches the intestines, dietary fibre undergoes fermentation by means of the gut microbiota, which results in the production of SCFAs such as acetate, propionate and butyrate. This activates G protein-coupled receptors (GPCRs) 41 and 43, which trigger the secretion of gut hormones including those that play crucial roles in regulating appetite.“The metabolism of dietary fibre by gut microbiota leads to a cascade of beneficial effects on human health, including improved insulin sensitivity and fatty acid oxidation, and reduced inflammation.”Plant-based dietPlant-based diets are enriched with polyphenols, which are secondary metabolites found in fruits, vegetables, cereals, wine, tea and coffee.Changes to the gut microbiome by polyphenols has been shown to support intestinal barrier integrity as well as homeostasis in the lungs and central nervous system.Small quantities of polyphenols (5% to 10%) are absorbed in the small intestine, while the majority of polyphenols (90% to 95%) interact with the microbiomes of the ileum and colon, “wherein they promote the abundance of Bifidobacterium, Akkermansia and Lactobacillus species, thereby providing substantial anti-inflammatory and anti-pathogentic properties, as well as cardiovascular protection.”High-protein dietA high-protein diet is generally considered to be a diet with protein intake of more than 1.5 grams per each kilogram of a person’s body weight. It’s typically used by athletes and prescribed for weight loss in people with obesity.Different complex proteins have varying levels of digestibility and amino acid compositions. There are a few species of bacteria involved in the process of proteolysis, or the breaking down of proteins, primarily Bacteroides, Bacillus, Clostridium, Phocaeicola, Propionibacterium, Fusobacterium, Lacticaseibacillus and Streptococcus. These bacteria are “enriched” in the gut microbiomes of people who adhere to a high-protein diet. Proteolytic bacteria use peptidases, proteases and endopeptidases—enzymes involved in breaking down protein—to release short peptides and free amino acids.Most amino acids are fermented into SCFAs. However, other amino acids are fermented into potentially inflammatory compounds, such as indoles and phenolic compounds, ammonia, amines, organic acids and hydrogen sulfide.Some of these compounds may be involved in diseases. For example, indoles can reach the liver and turn into indoxyl sulfate, which is a toxic metabolite that is “deleterious” for the kidneys, and is also involved in endothelial dysfunction.Hydrogen sulfide, for its part, is potentially “mutagenic” and may contribute to inflammation and increase the risk of colon cancer.Ketogenic dietThe ketogenic diet consists of a very low intake of carbohydrates, as well as a moderate intake of proteins and high-fat foods. This diet typically involves adhering to a 4:1 ratio of fat (measured in grams) to the combined total of protein and carbs.This dietary pattern replicates the metabolic responses while a person is fasting. This increases levels of ketone bodies, which are molecules derived from fatty acids that serve as an alternative energy source to glucose. The ketogenic diet has “long been used” for the treatment of epilepsy. Emerging research has also shown that the diet is beneficial for treating other disorders including Alzheimer’s, obesity and cancer.Preclinical research suggests that a ketogenic diet promotes a “substantial modification” to the composition of the gut microbiome, “most notably” characterized by increased levels of species of Akkermansia, Lactobacillus, Roseburia and Parabacteroides, and by substantially decreased levels of species of Turicibacter, Desulfovibrio, Escherichia and Shigella. “[O]wing to the subsequent decrease in beneficial gut microbiota and the promotion of pro-inflammatory and pathogenic gut bacteria, further investigation is needed to understand the long-term effects of the ketogenic diet on host health,” the authors wrote.

Why the ‘Western’ diet is bad for the gut and how other diets compare

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