Beauty from within: A comprehensive review on interplay between gut health and skin

Prevalence

The gut microbiome is a major contributor to age-related health in all age groups. The gut microbiome relays environmental signals and indicates the risks of disease – these changes along with host age.^[4]

As individuals age, they undergo physical and intellectual changes, often referred to as “healthy aging.” This process is influenced by genetic, environmental, and lifestyle changes. The gut microbiome, which changes reciprocally with age, is influenced by factors such as physiological deterioration, lifestyle changes, medication, and diet.^[4]

The GM develops with the host from birth, crucial for long-term health by regulating immune system development, nutrient absorption, metabolism, and preventing pathogen colonization. Changes in its composition are linked to health issues such as obesity, asthma, metabolic syndromes, and chronic inflammatory diseases.^[5]

During childhood, we see less diversity in microbial species than in adults, but the intestinal microbiota develops and slowly changes, with decreases in the numbers of aerobes and facultative anaerobes such as Bifidobacterium and Clostridium genera, as well as concurrent increases in anaerobic species.^[6] Moreover, diet, sleep patterns, stress, alcohol and drug usage, and prescription antibiotic use all affect the GM.^[6]

Studies suggest that GM can have a long-term impact on brain stress circuitry during a small window of life, up to adolescence. This is particularly intriguing as dysregulation of stress-responsive brain circuitry is linked to common psychiatric illnesses in adolescence, suggesting that GM changes can affect these circuits.^[6]

Sex, beyond age, significantly divides the gut microbiome. Males and females differ anatomically, physiologically, and behaviorally, influenced by genetic, hormonal, and epigenetic factors across life stages. The gut microbiome helps maintain homeostasis and further distinguishes between the sexes.^[7]

The nutritional and energetic demands vary in males and females, which requires the maintenance of the different gut microbiomes through physiological and behavioral adaptations.^[7] Gender and sex hormones play a role in this after puberty. The bacteria-to-human cell ratio is higher in females than in males (1:3 in males and 2:2 in females).^[8] Tiny alterations in a single bacterial species within the gut microbiome can cause skin inflammation. These may lead to acne, alopecia areata, atopic dermatitis, and other skin diseases.^[9]

Pathophysiology

Immunological components that are present between the gut and the skin facilitate communication between the two organ systems.^[9] The microbiota and the immune system are vital for preserving health. Commensal bacteria in the gut and on the skin influence immune responses, maintaining barrier immunity and host homeostasis while reducing inflammation and microbial translocation.^[10] The mucosal firewall, consisting of gut epithelial cells, mucous layer, T-cells, immunoglobulin A (IgA), and dendritic cells, prevents commensal bacteria from entering gut-associated lymphoid tissues (GALTs), which act as a defense barrier between the host and the environment.^[11] GALT development is influenced by GM, including intestinal crypt cells, Peyer’s patches, isolated lymphoid follicles, appendix, and mesenteric lymph nodes, governed by the interaction between gut microbes and lymphoid tissue inducer cells.^[12] The complex relationship between the stomach and skin, which affects general homeostasis, is described by the gut-skin axis [Figure 1].^[13] The immune system is influenced by the GM, which controls both local and systemic inflammation.^[14] Microbial communities convert complex polysaccharides into vitamins and short-chain fatty acids (SCFAs), such as butyrate, to improve the intestinal barrier and preserve its integrity.^[15] Gut mucosal defense, led by GALT’s innate immune cells, stops microbial relocation. Pathogenic microbe translocation is restricted by AMPs, macrophages, and CD103+ CD11b+ DCs, which present commensal antigens for differentiation.^[16] Consequently, this preserves the integrity of the intestinal barrier. Controlling inflammatory responses to gut microorganisms is one of IgA’s functions.^[17] Lymphocytes that are specific to certain commensal bacteria gather in the lamina propria and Peyer’s patches, modifying the gut microbial profile in the direction of homeostasis.^[9] Further research is necessary to fully understand the relationship between skin health and the immunological responses triggered by the gut microbiome.^[18]

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Figure 1:

Overview of the gut-skin axis illustrated by Thangamuni A. and Mohan J.

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Acne vulgaris

Acne vulgaris is a common facial skin condition that significantly impacts appearance and self-confidence, characterized by comedones, papules, pustules, and nodules. Its development involves excessive sebum production, disturbances in the skin microbiome, and activation of immune responses by Propionibacterium acnes bacteria. Studies suggest acne patients often show changes in GM, such as decreased Firmicutes and increased Bacteroidetes, influenced by dietary factors like high glycemic index foods. However, conflicting findings regarding Firmicutes levels before acne treatments highlight the complex relationship between gut microbes and acne pathogenesis.^[23]

Rosacea

Rosacea, a common chronic inflammatory condition primarily affecting the central face, presents with symptoms such as flushing, persistent redness, papules, pustules, telangiectasia, and sensations of burning or stinging, along with dryness, rough texture, facial swelling, phymatous changes, and eye irritation.^[24]

It is associated with various gastrointestinal conditions, influenced by genetics, microbiota, and immune responses. Genetic predisposition may link rosacea with gastrointestinal diseases. Changes in microbiota and immune function play key roles in this relationship, impacting both skin and systemic health. Dysbiosis and small intestinal bacterial overgrowth (SIBO) disrupt immune responses by altering SCFAs and intestinal microbes, potentially affecting skin microbiota and immune responses.^[24]

While evidence is still inconclusive, treatments targeting SIBO and Helicobacter pylori have shown promise in managing rosacea effectively in some studies. Recognizing gastrointestinal issues in rosacea patients suggests new treatment approaches, given the limitations of conventional therapies prone to frequent relapses.^[24]

Hair disorders

Human skin and hair follicles harbor diverse microbial communities that are essential for regulating immune responses and maintaining skin health. These microbes interact closely with immune cells, influencing the balance within the skin and its inflammatory responses.^[25] Changes in the microbiome of hair follicles have been associated with various inflammatory skin conditions, indicating their potential role in the development and progression of these diseases. While the hair follicle infundibulum is a site of active immune interaction, the bulb and bulge regions are considered immune-privileged areas critical for hair growth. Disruption of this immune privilege can lead to inflammatory hair disorders such as alopecia areata and cicatricial alopecia, which are characterized by inflammation and damage to the follicles.^[25]

Aging

A recent focus on the GM, aging, and longevity highlights its crucial role in how we age. Factors influencing GM diversity include biological factors, habits, environment, and age itself. Differences in GM composition between centenarians and younger older adults suggest GM is a promising target for strategies that slow down aging.^[26]

Studies have looked into interventions such as probiotics, prebiotics, symbiotics, physical activity, and the Mediterranean diet to change GM and potentially delay aging and its problems.^[26] These interventions improve colon health, immune function, lipid profile, cognition, muscle health, and reduce inflammation, oxidative stress, and harmful bacteria. The microbiota’s ability to slow aging is mainly due to their anti-inflammatory and antioxidant effects. More research is needed to develop effective ways to promote healthy aging and longer life through managing GM.^[26]

Atopic dermatitis (AD)

The AD is a persistent rash that often initiates the atopic march. Current treatments focus on managing symptoms rather than curing the condition due to its chronic nature. The appearance of AD varies depending on disease stage, patient age, skin color, and infections, presenting as blisters (acute), thickened skin (chronic), or a combination (subacute). Infants typically experience lesions on the face, neck, and limbs’ extensor surfaces, while children and adults commonly have them on flexor surfaces.^[23]

Although AD primarily affects the skin, its pathology involves complex immunological changes affecting organs such as the gut and lungs. Dysbiosis of the microbiota is linked to AD, with evidence suggesting that GM play a crucial role in regulating immune responses and influencing allergic diseases. Reduced microbial diversity early in life may hinder immune development, favoring Th2-driven allergic responses. Increased levels of certain gut bacteria, such as Bacteroides spp., are associated with food allergies and other atopic conditions, potentially triggering inflammation and altering gut permeability.^[23]

Vitiligo

Researchers investigated the skin and GM in vitiligo patients compared to healthy controls. Results showed reduced microbial diversity in the stools of vitiligo patients, with significant differences in microbiota composition between lesional and non-lesional skin areas.^[27] Lesional skin biopsies reveal depletion of protective bacteria and enrichment of pathogenic species, with elevated mitochondrial deoxyribonucleic acid levels and immune markers in the blood, suggesting a link between skin dysbiosis, mitochondrial damage, and immune dysfunction in vitiligo, highlighting microbiota as potential targets for therapeutic interventions.^[27]

Ill effects of having an unhealthy gut environment and how it affects general health and skin in particular

There are many adverse physical effects of having an unhealthy gut. These include digestive symptoms such as gas, bloating, constipation, diarrhea, and heartburn. Increased infections such as the common cold, as well as sleep disturbances and fatigue, are also seen in a person with an unhealthy gut.^[28]

The gut microbiome plays an important role in a wide array of skin disorders. It is an essential immune system regulator. Dysbiosis in the gut is associated with an altered immune response, allowing the development of many skin diseases such as AD, psoriasis, acne vulgaris, dandruff, and even skin cancer.^[13]

Gastrointestinal disorders can present with dermatological skin findings. Inflammatory bowel disease (IBD) is linked to skin manifestations such as pyoderma gangrenosum, and erythema nodosum. Celiac disease is associated with skin manifestations such as dermatitis herpetiformis, alopecia, vitiligo, and oral mucosal lesions. Psoriasis is more commonly found in patients with Crohn’s disease than in healthy people.^[29]

Not only does an unhealthy gut affect physical well-being but also the mental state of a person. High stress, low mood, and anxiety can be observed due to this.^[30]

Research in humans has also shown a link between the bacteria in the gut and obesity. When there is less variety in the types of bacteria found in fecal samples from overweight or obese individuals, it is connected to increased body fat, cholesterol problems, difficulties in controlling blood sugar levels, and higher levels of mild inflammation.^[31]

Prebiotics

Prebiotics are fermented components that enhance GM composition and activity for the host. They have low-dosage action, no side effects, and remain in the intestines. Common prebiotics include oligosaccharides (such as glycans), fructans (inulin-type), sugar alcohols, and complex polysaccharides. They aid in the treatment of disorders such as acute contact dermatitis (ACD), acne, and photoaging by stimulating probiotic development.^[32]

Probiotics

Specific probiotic bacteria have been demonstrated to prevent ACD and moderate its symptoms. They appear to do so by altering several biological processes not only in ACD but also in a variety of other skin diseases (e.g., acne, psoriasis, photoaging, and wounds).^[32]

The skincare sector has observed a rise in topical products containing Lactobacillus bulgaricus to treat acne and seborrhea. However, other concerns persist, such as manufacturers using antiseptics to restrict microbial growth, which may impair the survival of probiotic strains and modify the microbiota of receptors because external goods cannot be made under sterile conditions.^[33]

Postbiotics

“Postbiotic” refers to the “preparation of inanimate microorganisms and/or their components that confers a health benefit on the host.”^[35] Typically, the forms could be a diverse range of cellular structures and metabolites, including teichoic acids, exopolysaccharides, peptidoglycan, bacteriocins, and so on. Postbiotics’ efficacy is driven by three underlying mechanisms: Pathogen protection, epithelial barrier enhancement, and inflammatory and immune response control.^[35] They are currently used not only in the fermented food sector but also as a promising therapy method for a variety of health diseases, including gastrointestinal disorders such as bloating and diarrhea.^[35]