What exactly is a ‘Western diet’?
The review authors pointed out that while the diet is not clearly defined, in this case, it was characterized as being low in fiber, vitamins, minerals, and plant-derived molecules, such as antioxidants .
It also contains low amounts of unprocessed fruits and vegetables, whole grains, grass-fed animal products, fish, nuts, and seeds.
Instead, a Western diet, as described, features excessive amounts of saturated fat, refined grains, sugar, alcohol, processed and red meat, conventionally raised animal products, high-fat dairy products, and salt.
High amounts of ultra-processed foods and drinks are also characteristic of the Western diet, according to the review authors.
For the current research, they compared and contrasted this to a Mediterranean diet , which they stated as having lower levels of processed food and higher levels of fruits, vegetables, and plant-based proteins, among other nutrient-rich plant-based foods.
How diet impacts gut lining
The review authors highlighted the roles that certain bacteria play in the lining of the gut — for example, Akkermansia muciniphila and Faecalibacterium prausnitzii , two bacteria associated with greater lean muscle mass, both play a role in the maintenance of the gut lining.
Bacteroides vulgatus and Bacteroides dorei also play a role in maintaining the gut lining.
A high-fat diet, especially one high in saturated fat , has been shown to negatively effectTrusted Source the levels of Akkermansia muciniphila and Bacteroides species found in the gut.
A low-fiber diet appears to reduce the production of short-chain fatty acids , which are important for the production of mucus, as well as interfering with regulation of T-cells and some other immune functions.
How diet impacts the immune system
Clostridia clusters IV and XIVa and XVIII play a role in the regulation of T cells, a type of immune cell, in the gut.
Bifidobacteria also promote the production of anti-inflammatory cytokines, as well as aid in the maintenance of microvilli, which are responsible for absorption in the gut.
The review highlighted that a diet high in saturated fat is associated with lower amounts of Clostridiales and Bifidobacteria species in the gut.
Additives like artificial sweeteners are also associated with lower amounts of Bifidobacteria , noted the review authors.
How diet influences inflammatory bowel disease
Eating fast food more than once a week has been associated with an increased risk of IBD. According to the review, this poor dietary habit may increase the risk of ulcerative colitis by 43% and Crohn’s disease by 27%.
Similarly, the review authors note that high intakes of meat and fish have been shown to increase the risk of IBD, but not egg and dairy consumption.
Ultimately, the review suggests high intakes of red meat, ultra-processed foods, sugar, and saturated fat may be risk factors for IBD development, as well as increase the risk of irritable bowel syndrome (IBS) .
Diet’s impact on brain health
Studies in mouse models have shown that high-saturated-fat and high-sugar diets are associated with lower cognition.
Pre- and probiotics have shown some early potential to improve certain Parkinson’s disease symptoms, and there is some evidence that certain bacterial species occur in lower concentrations in people with major depressive disorderTrusted Source .
However, more robust research is needed to confirm these findings.
Obesity promoting or microbiome altering
The review authors analyzed research surrounding metabolic syndrome and type 2 diabetes with gut dysbiosis.
They propose that the dysregulation of the gut barrier increases inflammation, which can lead to chronic conditions and a tendency to gain fat, which is a key precursor to the development of type 2 diabetes.
While this review does not establish causation, the authors suggest the link between the Western diet and metabolic syndrome can be explained by both the direct impact of the diet’s poor nutrient qualities on systemic inflammation and obesity and also the resulting changes to the gut microbiota from consuming these foods, which indirectly contributes to these health issues.
Hasan Zaki, PhD , associate professor at UT Southwestern Medical School, who studies the molecular mechanisms of inflammatory disorders and was not involved in this research, told Medical News Today that the alteration of the microbiome could be a separate mechanism underpinning the development of some chronic diseases.
“Previously, it was considered that […] [in a] high fat-diet, […] fat and sugar are bad for our health, because they directly alter our body’s metabolism,“ he noted.
“[This type of diet] helps to increase our cholesterol level in the blood, and ultimately […] is bad for the health because cholesterol leads to heart disease and many other complications. And as a consequence, diabetes, and metabolic disorder [develop]. But there are a lot of studies showing that the diet not only alters metabolism but also it shifts the microbiome composition.”
– Hasan Zaki, PhD
The review was helpful to aid our understanding as while lots of links have been made between the microbiome and certain condition, “but we don’t know exactly what particular bacteria or what particular component or their metabolic products are responsible for,“ Zaki cautioned.
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