The Gut-Skin Axis: More than Skin Deep?
Our bodies are complex systems that are in a constant dance with one another. Recent research, largely beginning with the Human Microbiome Project, a US National Institute of Health initiative to improve understanding of the microbial ecosystems involved in human health and disease (launched in 2007), is showing that microbiome imbalances have been linked to metabolic diseases, neurodegenerative disorders, and even cancer.1 As the body of research has grown, it is becoming apparent that the human gut microbiome, or the bacterial community that resides within the intestines, has a direct relationship with our skin.
The Gut and its Microflora
Just as our skin is our barrier to the outside world, our gut acts as a barrier to potential pathogens that we ingest on a daily basis. We can think of our gut as an extension of our skin, but on the inside. Inside our gut, there is an entire ecosystem of bacteria, viruses, fungi and protozoa that have specialized functions that help keep our bodies running smoothly. This ecosystem, however, is constantly shifting as a result of our lifestyles, including our diet, exercise, sleep quality, stress levels, medication usage, and more. As these other parts of our life change, our internal microbiome changes with it, for better or for worse. Dysregulation of this system can lead to disruption of the natural barrier function of the gut, leading to what we call a “leaky gut,” meaning that harmful bacteria and toxins that are supposed to pass through the gut (and end up in the toilet) get absorbed into our circulation, causing an inflammatory response and chronic immune activation, which can quickly manifest itself as an inflammatory skin condition.2
The Role of Microbes in our Lives
We have evolved over the course of millions of years in tandem with these microscopic organisms and our bodily functions have come to rely on some of these species. They create chemical signals that communicate with our systems and can affect our bodily functions. Some of these functions are seen as beneficial: they can help strengthen our immune system, regulate cholesterol in the blood, reduce body mass index and reinforce the skin’s natural barrier function. These types of organisms that we find beneficial are called “commensal” as they live within us without harming us. Other types of bacteria, fungi and viruses that harm us, ones that cause us to flush our bowels or create inflammation are referred to as “pathogenic.”
One of the surprising findings of the new research on the human microbiome is that we actually have distinct microbial communities throughout our entire body, not just in the gut. We are inhabited by more microbes in and on our body than we can imagine: there are at least as many bacterial cells in the human body as human cells. They surround us and cover us.
Our bodies have vastly different microbial compositions in the different geographies of the body: the warm tropics of our underarms, groin and navel tend to have more bacterial presence, but with fewer microbial variety than drier parts of our bodies like our arms and hands. These communities affect how our brain, teeth, skin, heart, immune system and nearly every other body part and bodily function. As we progress through life, well-being is intimately tied to the microbes that surround us and our microbial presence can be influenced by changes in our diet and environment. The composition of commensal and pathogenic microbes on our skin affects its appearance and is a result of the microbial diversity in our gut and can be affected by topically applied products.
The Gut Brain
The gut could also be said to have a mind of its own. Our intestines are lined by a vast network of more than 100 million neurons that handle all of the automatic processes necessary to run the chemical processes of digestion to extract nutrients from food. You might expect that the gut takes orders from the brain, but the gut possesses a high degree of autonomy. The gut is connected to the brain by a pathway of fibers called the vagus nerve, a main component of the parasympathetic nervous system, which oversees a vast array of crucial bodily functions, including control of mood, immune response, digestion, and heart rate. It establishes one of the connections between the brain and the gastrointestinal tract and sends information about the state of the inner organs to the brain. The gut will actually function on its own even if the vagus nerve that connects the gut to the brain is severed. The gut actually sends signals to the brain to induce stress and anxiety when threats are perceived, giving literal meaning to the idea of a gut feeling.
Studies on Commensal Bacteria for the Skin
Lactobacillus LGG is just one of the hundreds of strains of bacteria that live within our intestinal tract, and this bacteria secretes a protein called p40, which acts as a guard against inflammatory damage to the skin’s surface.3 Another bacteria found in the gut, Escherichia coli, signals epithelial cells (or skin cells) to express β-defensin 2, which is a factor that prevents pathogens from entering epithelial cells that might later lead to an itchy infection or a painful inflammatory response.3 These are just two examples of the thousands of roles that these bacteria play to keep our skin happy and healthy.
One study found that S. Epidermidis is important to keep C. Acnes in check. Succinic acid, a fatty acid fermentation product of S. Epidermidis, inhibited C. Acnes growth.4 In another study, volunteers were fed 10 billion Lactobacillus plantarum daily for 12 weeks. The investigators found significant increases in skin water content in the face and hands; this hydration boosted the skin so that it appeared healthier with fewer wrinkles. The probiotic significantly decreased skin wrinkles, improved skin gloss, and enhanced skin elasticity and epidermal thickness.5 In yet another study, when probiotic Lactobacillus reuteri was fed to mice during the second half of their lives, researchers observed an increase in sebum production, shiny fur, and (as the authors put it) a general “glow of health” within seven days.6 Preliminary data shows that this probiotic may work by decreasing inflammation through an increase in an anti-inflammatory cytokine (IL-10).6
Our lifestyles directly affect our internal (and external) microbiomes, and these microbiomes in turn affect all other systems, including our skin.7 A healthy diet, supplementations with probiotics, reduction in stress, and good sleep are the most reliable and readily available ways to maintain a healthy lifestyle overall, including a happy internal ecosystem of beneficial bacteria.
1) Pharmacology & Therapeutics, February 2016, pages 52-62
2) Alcoholism: Clinical and Experimental Research, February 2020, pages 1-14
3) FEBS Letters, March 2014, pages 4195–4206
4) Journal of Dermatological Science, August 2015, pages 119-26
5) Journal of Microbiology and Biotechnology, December 2015, pages 2160-8
6) PLoS One, January 2013, pages 1-11
7) Frontiers in Cellular and Infection Microbiology, March 2020, pages 1–12