644 JOURNAL OF COSMETIC SCIENCE
7347, B. lactis CECT 8145 and L. casei CECT 9104 significantly decreased the SCORAD
index and reduced the use of topical steroids in patients with AD, although the exact
mechanism remains unclear.104
The largely hypothesized mechanism of action of these LBPs involves immune system
modulation. Preclinical trials using L. sakei WIKIM30 showed alleviation of AD symptoms,
due to the induction of Treg differentiation and altering the gut microbiota.105 Furthermore,
a study that utilized L. fermentum treatment in AD-induced mice demonstrated significant
reductions in serum IgE, tissue mast cells, eosinophils and T-helper 2 (Th2) related
cytokines.105 The treatment also led to increases in anti-inflammatory cytokines, such
as interleukin-10 (IL-10) and transforming growth factor-β (TGF-β), as well as changes
in amino acid levels, including methionine, phenylalanine, serine and tyrosine, and in
SCFAs such as acetate, butyrate and propionate.106 The proposed mechanism for these
Lactobacillus-containing LBPs is their ability to reduce T-helper 1 (Th1) and T-helper 17
(Th17) cytokines, in addition to Th2 cytokines, while simultaneously increasing IL-10 and
CD4+CD25+ regulatory T cells. This immune modulation is thought to influence the gut
microbiota and alleviate AD symptoms.106,107
In addition, supplementation with the LBP L. rhamnosus SP1 LSP1 has been shown to
be effective in treating adult acne.108 A prevalent hypothesis suggests that L. rhamnosus-
containing LBPs may enhance microbial diversity, increase SCFA production and normalize
gene expression related to insulin signaling in the skin.109 Additionally, an LBP containing
Bacillus species demonstrated improvement in acne lesions and reduced sebum excretion
rates.110 Notably, a blend of L. acidophilus, L. delbrueckii subspecies bulgaricus and B. bifidum
showed efficacy comparable to 100 mg of daily minocycline, a commonly used antibiotic
for acne treatment.111 Therefore, in addition to microbiome balance, the hypothesized
mode of action through which LBPs may treat acne is via immunomodulation and anti-
inflammatory effects. Specifically, LBPs are thought to generate regulatory dendritic cells
and CD4+ Foxp3+ T cells, leading to B and T helper cell hyporesponsiveness without
apoptosis, along with suppression of cytokine production.112
In addition, LBPs for acne may be effective through the reduction of oxidative stress. The
elevated local burden of lipid peroxidation associated with acne increases the demand for
antioxidants from the bloodstream. LBPs may address this issue by reducing systemic
oxidative stress and modulating inflammatory cytokine release within the skin.113,114
Specifically, a reduction in interleukin-1 alpha (IL-1α), observed under certain experimental
conditions, could offer therapeutic benefits for acne.115 Furthermore, similar to internal
antibiotics, LBPs have the potential to alter microbial communities in areas beyond the GI
tract, potentially enhancing their effectiveness in acne management.116
Research into the effects of LBPs in relation to rosacea is relatively limited. However, studies
have shown that oral supplementation with E. coli Nissle 1917, in combination with conventional
topical treatments such as tetracycline, steroids and retinoids, resulted in significantly greater
symptomatic improvement in rosacea patients compared to those who received only topical
treatments.117 Hence, E. coli Nissle 1917 is able to strengthen the intestinal epithelial barrier
and inhibit pathogenic bacterial growth by interacting with immunomodulatory and anti-
inflammatory mechanisms, suggesting its mode of action for rosacea treatment.117 Similarly, a
treatment regimen for rosacea that included a Bifidobacterium-containing LBP was associated
with a higher rate of clinical remission compared to conventional treatments alone.118 A case
of scalp rosacea was also effectively managed with a low dose of doxycycline combined with
an LBP containing B. breve BR03 and L. salivarius LS01.119

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,