The Western Diet

Our world is characterised by a decrease in physical exercise and an increase in energy intake. The problem with these types of habits is their relation with health problems. More and more people are abusing fast food and highly processed products, making their diets unbalanced or unhealthy. An example is the typical Western diet. This is a diet high in animal protein, sugar, starch, and fat and low in fibre. Without a doubt, these eating habits are closely related to obesity and health problems. Here, you will learn that not all of those problems are due to the high excess of energy and fat intake itself, but also to changes produced in our microbiota as a consequence of the diet.

The Western diet acts as a selective force to allow the growth of certain types of bacterial genera while reducing others. As a result, increases in bacterial genera such as Bacteroides, Turicibacter, and Bilophila have been observed in these obese subjects while others, like Clostridium IV and Bifidobacterium, appear reduced in these obese subjects compared to lean. In the gut microbiota of obese subjects, there is an increase in compounds, namely lipopolysaccharides, or LPS, and trimethylamine N-oxide, or TMAO, and a decrease of short chain fatty acids. You may wonder, what is the problem with this? As we will explain next, both lipopolysaccharides and trimethylamine N-oxide are associated with detrimental health effects since they are related to a pro-inflammatory state.

Meanwhile, short chain fatty acids have proven beneficial effects.

What are these compounds and where do they come from? Lipopolysaccharides are formed by lipids and a polysaccharide preset at the outer layer of gramme negative bacteria. These molecules are fundamental for the bacteria to maintain the stability and integrity of their membranes. They give their ubiquitous presence in the bacterial world. Some of our cells present surface receptors to recognise lipopolysaccharides as a means to sense the bacteria colonising the gut. Trimethylamine N oxide is produced due to the fermentation of L-carnitine by gut bacteria. L-carnitine is an amino acid found mainly in animal protein, which is the primary source of protein in the Western diet.

Short chain fatty acids, as you probably remember, are some of the products that the gut microbiota produces when fermenting dietary fibres. Some short chain fatty acids are acetic, propionic, and butyric.

How do these molecules affect us? As you have learned, obesogenic diets induce changes in gut microbiota. This imbalanced ecosystem induces an inflammatory response by the interaction of lipopolysaccharides with receptors such as TLR-4 and CD-14, expressed by cells of the intestinal mucosa. The bacterial ecosystem created by unhealthy diets influences the ability of the enterocytes to stay together by reducing the expression of genes encoding for tight junction proteins, which are the glue of the unions. As a result, the gut barrier gets some breaches through which lipopolysaccharides can pass through and trigger a state of low grade inflammation, known as metabolic endotoxemia.

This state of metabolic endotoxemia is involved in some of the problems associated with the intake of unhealthy food, promotion of weight gain and adiposity, the elevation of inflammatory state and markers, and insulin resistance. Similarly, the compound trimethylamine N-oxide can increase gut permeability and gut barrier inflammation. Trimethylamine N-oxide can even make its way to the liver, where it might contribute to the onset of chronic hepatic disorders, like fatty liver disease.

An unhealthy diet implies a deprived intake of high fibre foods that also contribute in changing the microbiota. As a result, the gut microbiota receives less fibre to ferment and consequently produce less short chain fatty acids. Since there are fewer fibres, the number of bacteria able to eat or ferment fibre, such as Clostridium IV or Bifidobacterium, is not favoured and their numbers are reduced. Consequently, the person who follows a diet low in fibre cannot benefit from the metabolic effects that exert in our body the short chain fatty acids.

All in all, we have seen how the food we choose and, in general, the diet we follow not only affects the cells in our body, but also to the bacterial cells of our gut microbiota. Now, you have more information on the consequence an unhealthy diet has on you and your gut microbiota.