By: Laura Huuskonen, PhD and Arthur C. Ouwehand, PhD – Health R&D, International Flavors & Fragrances
Human skin has three main layers: the epidermis, the dermis, and the subcutaneous fat (hypodermis) (Figure 1). The epidermis is the outermost layer that protects the body from water loss, pathogens, and ultraviolet (UV) radiation. The dermis is the middle layer that contains blood vessels, nerves, glands, hair follicles, and various structural proteins. The subcutaneous fat is the third and the deepest layer of the skin that stores energy, insulates the body, and connects the skin to muscles and bones. Collagen type I is the most abundant protein in the dermis, accounting for 80% of the skin’s dry weight (Haydont et al., 2019). It forms a triple helix structure that gives the skin tensile strength and elasticity. Collagen synthesis and degradation are tightly regulated by various factors, such as hormones, cytokines, growth factors, and matrix metalloproteases (MMPs).
Figure 1. Overview of skin structure. (Created with BioRender)
During the lifetime of a human, the skin gets exposed to multitude of factors, such as pollution, tobacco smoke, UV radiation, extreme temperatures, poor nutrition, stress, and sleep deprivation. All these factors can contribute to the skin aging with numerous molecular mechanisms (Krutmann et al., 2017).
Natural aging and photoaging are both result of accumulation of years, but they have different features and mechanisms (Rittié and Fisher, 2015). Natural aging affects all layers of the skin, resulting in thinning, dryness, wrinkles, and loss of elasticity, whereas photoaging affects mainly only the top two layers of the skin. It is caused by chronic exposure to UV radiation, which induces oxidative stress, inflammation, and MMP activation in the skin, leading to hyperpigmentation, elastosis, and collagen degradation. On the other hand, UV radiation can stimulate melanin production for protection, and is a significant and important stimulator of vitamin D synthesis in the skin.However, as skin ages, it may synthesize less vitamin D, affecting bone health, immune functions and energy metabolism, if its intake is not supplemented accordingly (Chalcraft et al., 2020, Janoušek et al., 2022). As overall, collagen content and quality decline with age, leading to fragmentation, disorganization, and loss of mechanical tension, promoting skin wrinkling (Reilly and Lozano, 2021). Changes in skin appearance due to aging can additionally cause issues with self-esteem and mental health having consequences on the human well-being (Gupta & Gilchrest, 2005).
Weather and seasons can influence skin aging by affecting the hydration, pigmentation, and antioxidant capacity of the skin. Dry and cold air can increase transepidermal water loss and skin dryness, especially in areas that are less covered. On the other hand, humid and hot air can increase sebum production and acne breakouts. The changes in season and climate can also affect the skin microbiota composition and function (Seo et al., 2023).
The gut is connected to many organs, so too with the skin through the so-called gut-skin axis which describes the bidirectional connection between the gastrointestinal (GI) tract and the skin (O’Neill et al., 2016). The gut microbiota and its metabolites can affect the skin through the blood circulation, the nervous system, or the immune system(Mahmud et al., 2022). Dysbiosis of the gut microbiota, or other disturbances in the gut function, can impair the intestinal barrier, increase the translocation of microbial components, and induce systemic low-grade inflammation, which can translate to as impairment in the skin structure and function. Aging, lifestyle, and diet can all alter the diversity and metabolic activity of microbes in the gut. The link between the gut and skin has been studied primarily in relation to GI and dermatological diseases and conditions, such as inflammatory bowel disease, celiac disease, atopic dermatitis, psoriasis, and acne. Even though much is learned already, there is still much to discover about the interaction mechanisms in the gut-skin axis.
As live microorganisms, probiotics can modulate the gut microbiota, produce beneficial metabolites and enzymes, inhibit pathogens, and modulate immune function (Plaza-Diaz et al., 2019). Inactive microbes and/or their components and products that also have health benefits are proposed to be called postbiotics (Salminen et al., 2021), although the definition what postbiotics are and are not may still be refined (Aguilar-Toalá et al., 2021). Lactobacilli and bifidobacteria are the most commonly used probiotics that have been widely studied for their effects on the gut health and the immune system. Through the gut-skin axis, by supporting the gut barrier integrity, immune functions, and stress response, oral consumption of these organisms, or their components as postbiotics, may contribute to support skin integrity and health. Among other effects, a decrease in plasma zonulin, representing decrease in intestinal permeability, and changes in microbiota composition were noticed after probiotic consumption in healthy participants (Nam et al., 2020). Gut microbiota can produce harmful metabolites that affect the skin integrity, such as phenols. Circulating serum phenol was found to be decreased after successive intake of fermented milk with probiotics and prebiotics (Kano et al., 2013). Prebiotics are substances that selectively feed host microorganisms and thereby create health benefits (Gibson et al., 2017). Further, probiotics, especially together with prebiotics, increased the serum short-chain fatty acid levels besides benefits on the skin in UV-radiated mice (Kim et al., 2021). Guéniche et al., (2006) reported that the administration of probiotics to mice that were challenged by UV radiation increased serum IL-10 levels and lowered the number of immune cells in the skin. There are also indications, that pro- and postbiotic treatments improve sleep and cortisol levels (Haarhuis et al., 2022). Chronic stress induced overexpression of stress hormones can negatively affect skin collagen homeostasis and dermal fibroblast proliferation (Chae et al., 2021, Le et al., 2022, Kahan et al., 2009). Furthermore, benefits for the aging skin can be provided in different ways: by preventing UV-induced oxidative stress, by modulating signaling pathways in skin cells resulting in stimulating collagen synthesis and decreasing it’s degradation, by enhancing skin barrier function, and by affecting immune functions in the skin (reviewed in Huuskonen, 2023).
Skin aging isn’t just about wrinkles; it has far-reaching effects on the entire body. During aging, skin cells move to a phase of senescence, where they are no longer able to divide, but they still form and release molecules. Because skin covers the entire body, the molecules released from these aged cells could eventually result in inflammatory protein levels that significantly affect the aging of the other parts of the body through circulation via the bloodstream (Franco et al., 2022).Maintaining skin health through proper skincare, sun protection, balanced lifestyle, and supporting gut health, contributes to overall well-being.
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