3.4 The impact of diet

As you learnt previously, the gut microbiota digest nutrients from the diet of the human host for their own growth and survival, as well as providing the human host with important nutrients. Undigested fibre and protein results in the preferential growth of bacteria that can use these substances, and this in turn increases the production of SCFAs by these bacterial groups. SCFAs are not only important in the health of the gut itself but also throughout the body of the human host (Zhang, 2022).

As humans typically eat a diverse diet, having good bacterial diversity is also important to be able to digest and metabolise the wide range of nutrients required for good health. Dietary fibre in particular is known to be essential for maintaining a good diversity of bacteria in the gut. However, Westernised diets often contain low amounts of fibre and protein, with high levels of fat, sugar and ultra processed foods (UPFs). In contrast, diets such as the Mediterranean diet and Japanese diets, which are higher in fresh ingredients, particularly fruit and vegetables, are known to be beneficial to the health of the gut microbiome and promoting good bacterial diversity.

Not only does high fibre maintain the diversity of the bacteria within the gut microbiome, but it also helps to reduce colonisation by pathogenic bacteria and improve the mucus barrier of the gut wall. Diets which are low in fibre increase the risk of a number of health conditions, including obesity, heart disease, some cancers, and diabetes.

Another important aspect of diets high in fresh food, particularly fruit and vegetables, are higher levels of micronutrients (vitamins and minerals). This helps to increase the proportion of Firmicutes (and so SCFA-producing bacteria). For example:

• Vitamins A, B6, B12, D and E – increases Firmicutes compared to Bacteroidetes, which helps SCFA-producing bacteria thrive.
• Calcium with phosphorous increases SCFA-producing bacteria.
• Vitamin C has antioxidant properties to protect against damage to cells by some metabolites.

Both high sugar levels and UPFs found in Westernised diets can change the gut microbiome diversity and are associated with inflammation of the gut wall (Shi, 2019).

Microbes and additives in the UPFs have been found to lead to dysbiosis of the microbiome. This results in an increase in pathogenic bacteria, reduced structural integrity of the gut wall with increased permeability, inflammation of the gut wall and altered signalling via the microbiota-gut-brain axis. This has been linked to several immune conditions, such as Crohn’s disease, ulcerative colitis and rheumatoid arthritis.

Introduction of beneficial bacteria or their products, such as SCFAs, can help to reverse these effects. However, prolonged consumption of a Westernised-type diet or UPFs can lead to permanent changes in the microbiome, gut wall and wider responses to the microbiome throughout the body. This is at least partly due to epigenetic changes of the human host which, as you learnt earlier, can also be passed down through the generations (Zinocker, 2018).