645 Bidirectional Gut-Skin Axis
NUTRICOSMETIC APPLICATIONS
Nutricosmetics are a diverse range of wellness products designed to improve the cosmetic
health of skin, nails and hair through nutritional supplementation.120 Often marketed as
promoting “beauty from within” or as “beauty pills,” these products incorporate nutraceuticals
or food-derived substances that support cosmetic appearance and concerns.121 This category
includes probiotic products, as well as their complementary prebiotics and postbiotics, which
are used to enhance overall health, and other supplements, such as collagen peptides and
omega-3 fatty acids. Other examples beyond the scope of this review include polysaccharides
(e.g., hyaluronic acid), minerals (e.g., zinc, selenium), vitamins (e.g., vitamin E, vitamin
C), secondary metabolites (e.g., carotenoids, astaxanthin, coenzyme Q10) and other plant
extracts (e.g., aloe vera, horsetail).120 Unlike LBPs, which are intended for therapeutic disease
prevention or treatment, nutricosmetics generally enhance general health and well-being.
As previously stated, prebiotics are non-digestible food ingredients that selectively
stimulate the growth and activity of microorganisms, primarily focusing on promoting
beneficial bacteria.122 These prebiotics may be oral or topical products that serve to nourish
probiotics, which are live microorganisms known for their ability to positively influence
the host organism by modulating the immune response and competing with pathogenic
bacteria.123 The definition of probiotics has evolved in conjunction with advancements in
research, although the term originally comprises the Greek root word meaning “for life,”
coined by Parker in 1974.34 Postbiotics are utilized to enhance the effects of probiotics they
consist of non-viable, active by-products derived from probiotic lysates, including cellular
components and metabolites that exhibit potent immunomodulatory properties. Probiotics
and postbiotics may also be administered orally or topically.97
The use of probiotics to address various cosmetic skin conditions has shown promise.124 While
most research in this field focuses on topical applications, this review will concentrate on oral
supplementation within the context of the gut-skin axis, targeting cosmetic concerns. One
area of interest is skin brightening and whitening, targeting issues like ephelides (freckles)
and melasma, wherein melanin is overproduced.125 In a preclinical study, B. adolescentis
culture filtrate was shown to reduce melanin levels in B16F10 murine melanoma cells by
inhibiting tyrosinase activity, a key enzyme in melanogenesis.126 Similarly, Lactobacillus
species, like L. helveticus NS8, have demonstrated the ability to decrease melanin levels and
improve skin pigmentation in B16F10 murine melanoma cells.127 Rhodobacter sphaeroides
(Lysogen™) has also shown an ability to reduce melanin synthesis in B16F10 murine
melanoma cells and zebrafish, in a dose-dependent manner.128
Additionally, research has demonstrated that probiotics may cause alterations in the gut
microbiome that provide a photoprotective effect by modulating the immune response to
UV radiation.129 This effect was investigated in a study where participants consumed the
oral probiotic L. johnsonii daily for 2 months. Following this period, sample sites on their
skin were exposed to short-term UV radiation to assess the impact on skin homeostasis.
The results showed that epidermal cells in the probiotic group recovered significantly faster,
with immune function improving or fully returning by day 4, compared to day 10 in
the placebo group.130 Further research exploring the antioxidant and anti-inflammatory
properties of nicotinamide mononucleotide (NMN) and lactic acid bacteria, specifically L.
fermentum TKSN041, demonstrated its ability to mitigate skin damage from UV radiation,
potentially offering health benefits in treating age-associated skin conditions.131
Furthermore, other strains of Lactobacillus have also demonstrated anti-photoaging effects. Oral
probiotic L. plantarum HY7714 demonstrated a reduction in moisture loss and wrinkle depth,

646 JOURNAL OF COSMETIC SCIENCE
as well as the ability to increase skin hydration and elasticity.132 Tyndallised L. acidophilus IDCC
3302 restored collagen expression and decreased inflammation caused by UV exposure by
suppressing the mitogen-activated protein kinase (MAPK) signaling pathway.133 Additionally,
L. sakei lipoteichoic acid (sLTA) can inhibit the phosphorylation of MAPK, reducing matrix
metalloproteinase-1 (MMP-1) synthesis and protecting against photoaging.134
Although the literature specifically addressing the role of oral prebiotic supplementation
in skin health is limited, their complementary actions with probiotics support their use.
Moreover, prebiotic supplementation has demonstrated effective persistence throughout the
gut.135 The most widely used prebiotics include oligosaccharides (glycans), fructans (inulin-
type), sugar alcohols and complex polysaccharides (e.g., β-glucans and cellulose).136 Fructo-
oligosaccharides have been shown to reduce skin inflammation in conditions such as eczema
(also known as allergic contact dermatitis (ACD)) in mice, by increasing the abundance of B.
pseudolongum.137 Furthermore, prebiotic konjac glucomannan hydrolysates (GMH) have been
shown to inhibit Artemisia vulgaris and C. acnes by promoting the proliferation of Lactobacilli
and other microorganisms.138 In addition, heat-killed cells of L. lactis strain H61 improved skin
elasticity.139 Lastly, an extracellular metabolite from B. coagulans MTCC 5856 demonstrated
the ability to restore and improve skin barrier integrity, attributed to its antioxidant activity
and its ability to inhibit enzymes that degrade the extracellular matrix.140
Two dietary supplements widely advertised as nutricosmetic supplements to enhance overall
skin health are collagen and omega-3 fatty acids. Hydrolyzed collagen (HC) functions
as an antioxidant, with oral consumption increasing the abundance of collagen-derived
peptides in the bloodstream, ultimately improving skin elasticity, moisture and reducing
transepidermal water loss.141,142 HC has been shown to successfully diminish signs of aging,
including improvements in wrinkles and fine lines.143,144 Through oral consumption, HC
promotes fibroblast growth and collagen production in the dermis by decreasing matrix
metalloproteinase expression, which is responsible for collagen breakdown.141 A study on
a nutricosmetic blend including HC demonstrated improved structure and stratification
of consumers’ epidermal layers.145 It is worth noting that in addition to HC, this product
also contained ingredients such as hyaluronic acid, N-acetylglucosamine, borage oil,
various vitamins, minerals, antioxidants and additional bioactive ingredients. While
topical applications of HC have been explored, research suggests that oral supplementation
provides greater benefits to skin health.141 The effects of HC on the gut microbiome are
well-documented, suggesting that changes induced by HC supplementation may influence
subsequent effects on the skin.146–148
Similarly, oral supplementation with omega-3 fatty acids in the form of fish oil has been
extensively studied for its benefits to overall skin health for the past 50 years.149 Fish
oil is rich in PUFAs, particularly omega-3 fatty acids like docosahexaenoic acid (DHA)
and eicosapentaenoic acid (EPA).149 Supplementation with fish oil/omega-3 has been
associated with improvements in skin conditions like photoaging, skin cancer, dermatitis
and cutaneous wounds, as well as changes to the gut microbiome, suggesting a potential
mechanism of influence.150–161
EMERGING EVIDENCE OF THE SKIN-GUT AXIS AND ITS IMPLICATIONS FOR
HUMAN HEALTH
The expanding applications of the gut-skin axis reveal emerging evidence supporting the
potentially bidirectional nature of this communication pathway. While it is well-established
that the gut microbiome influences various organs such as the lungs, heart, brain and skin,