A comprehensive review by Italian researchers has shed light on how the Western diet may contribute to chronic health conditions through its impact on the microbiome. Published in the journal Best Practice & Research Clinical Gastroenterology, the study examines how dietary habits influence microbial populations in the body and the subsequent risk of diseases such as inflammatory bowel disease (IBD) and Alzheimer’s disease.

The review delves into the roles specific bacteria play in gut health and how different diets, particularly the Western diet, affect these microbial populations. The Western diet is typically characterized by low intake of fiber, vitamins, minerals, and antioxidants, with minimal consumption of unprocessed fruits and vegetables, whole grains, grass-fed animal products, fish, nuts, and seeds. Instead, it is rich in saturated fats, refined grains, sugars, alcohol, processed and red meats, conventionally raised animal products, high-fat dairy products, and salt. Ultra-processed foods and drinks are also prevalent in the Western diet.

In contrast, the Mediterranean diet, which the researchers used for comparison, features lower levels of processed foods and higher levels of fruits, vegetables, and plant-based proteins, along with other nutrient-rich plant-based foods.

The review highlights the importance of certain bacteria in maintaining the gut lining. Akkermansia muciniphila and Faecalibacterium prausnitzii, both linked to greater lean muscle mass, are crucial for gut lining maintenance. Similarly, Bacteroides vulgatus and Bacteroides dorei play significant roles in this function. However, a high-fat diet, especially one rich in saturated fats, negatively impacts the levels of Akkermansia muciniphila and Bacteroides species. A low-fiber diet reduces the production of short-chain fatty acids, which are essential for mucus production and the regulation of T-cells and other immune functions.

Certain bacterial clusters, such as Clostridia clusters IV, XIVa, and XVIII, are vital for regulating T-cells in the gut. Bifidobacteria help produce anti-inflammatory cytokines and maintain microvilli responsible for absorption. However, diets high in saturated fats are associated with lower levels of Clostridiales and Bifidobacteria species. Artificial sweeteners, another common component of the Western diet, also reduce Bifidobacteria levels.

The review highlights a significant association between fast food consumption and the risk of IBD. Eating fast food more than once a week increases the risk of ulcerative colitis by 43% and Crohn’s disease by 27%. High intakes of meat and fish also correlate with an increased risk of IBD, though egg and dairy consumption do not. The review suggests that high consumption of red meat, ultra-processed foods, sugar, and saturated fats may be risk factors for developing IBD and irritable bowel syndrome (IBS).

Studies in mouse models indicate that diets high in saturated fats and sugars are linked to lower cognitive functions. There is early evidence suggesting that pre- and probiotics might improve certain symptoms of Parkinson’s disease, and that specific bacterial species are found in lower concentrations in individuals with major depressive disorder. However, more extensive research is needed to confirm these findings.

This review underscores the profound impact of diet on the microbiome and its subsequent effects on health. The findings highlight the need for dietary interventions to prevent and manage chronic conditions linked to dysregulation of the microbiome.