The Gut Microbiome and the Skin

The gut microbiome refers to the community of trillions of bacteria that live in the human gut and all of the molecules that they release.^[1] In an adult, there are approximately 1kg of bacteria found in the gut.^[2] The gut microbiome is thought to play a role in nutrition, metabolism, and the development of our immune system.^[3] Development of the gut microbiome is thought to start during pregnancy and continue until the age of two. After this age, the gut microbiome has matured to the point that it is similar to an adult’s gut microbiome.^[4]

What Is the Role of the Gut Microbiome?

In recent years, there has been extensive interest in the gut microbiome and its role in human health and disease. An imbalance in the gut microbiome, often referred to as “gut dysbiosis” has been linked to several diseases such as:

• Inflammatory bowel disease
• Irritable bowel syndrome
• Obesity
• Type 2 Diabetes
• Allergic diseases such as atopic dermatitis (eczema) and asthma^[3]

The role of the gut microbiome is emerging. The gut microbiome is thought to play a role in the metabolism of dietary components as the gut microbiota break down carbohydrates in food to release molecules such as short-chain fatty acids (SCFAs). SCFAs such as butyrate are important for human health, and they have many roles such as providing energy for cells and inducing cell death in colon cancer cells.^[5] Not only do the gut microbiota help break down carbohydrates, but they are also able to produce vitamins and amino acids. The lining of the gut is a site of interaction between the immune system and the environment. This interaction enables the development of the immune system due to the ongoing contact with the bacteria in the gut and the metabolites that they produce.^[3]

What Can Affect the Gut Microbiome?

We are only starting to understand the true nature of the gut microbiome. Many things can alter the gut microbiome such as:

• Diet: Having a high-fat diet changes the microbiome composition, leading to a reduction in Bacteroidetes bacteria and an increase in Firmicutes and Proteobacteria. A strict vegan or vegetarian diet can also change the gut microbiome.^[1]
• Age: During the aging process our gut microbiome undergoes gradual change. The “core microbiota” refers to the taxa that form the majority of the gut microbiome in most people. Frailty is associated with a reduced diversity in this core microbiota. Those who are aging also have an increase in Bacteroidetes.^[6]
• Oral Probiotics: These are living bacteria, which when ingested can benefit human health. They are thought to work by helping to restore the gut microbiome composition, reduce gut inflammation, and modulate the immune system.^[7]
• Prebiotics: These are non-digestible food components, which are able to help selectively stimulate the growth or activity of certain colonic bacteria.^[8]
• Antibiotic exposure: Antibiotics are able to cause changes in the gut bacterial communities that can persist for long periods of time. Patients treated for Helicobacter Pylori infection had changes in their gut microbiome that persisted for years after the course of antibiotics ended.^[9]
• Pregnancy: During pregnancy, the gut microbiome is altered. The gut microbiome changes dramatically between the first and third trimester, during which there is an increase in Proteobacteria and Actinobacteria.^[10]

How Is the Gut Microbiome Related to Skin Health?

Stokes and Pillsbury first hypothesized the link between the skin, brain, and gut in 1930. They felt that emotional states lead to changes in the gut microbiota, which then caused local and systemic inflammation.^[11]

Many patients who have gastrointestinal disorders can have skin problems associated with them.^[12] At the moment, studies are being conducted to see if there is a link between skin diseases and the gut microbiome. There is early evidence to suggest a potential link between an imbalance in the gut microbiome and skin disorders such as psoriasis,^[13] rosacea,^[14] and atopic dermatitis.^[15]

The mechanisms by which the gut and the skin communicate are currently unclear. However, there are many postulated mechanisms of communication:

1. Gut microbes may produce short-chain fatty acids (SCFA) or long chain saturated fatty acids (LCFA) based on the bacterial population in the gut. SCFAs such as butyrate, acetate, and proprionate may have anti-inflammatory effects,^[16] while LCFAs may activate more inflammation.^[17]
2. The intestinal epithelium has been shown to directly secrete inflammatory signals known as cytokines, which regulate inflammation.^[18]
3. The gut microbiome may directly affect the immune system.^[3]
4. Nutrition can lead to molecules that are able to go directly to the skin, such as beta carotenoids (vitamin A derivatives). Carotenoid consumption has been shown to protect the skin from becoming red with exposure to ultraviolet light.^[19]