L-carnitine, a natural amino acid found primarily in animal-based foods, plays a multifaceted role in cellular metabolism. It regulates mitochondrial activity through modulation of transcription factors and is a crucial cofactor in the β-oxidation of fatty acids, which facilitates the cellular energy production process (Iacobazzi et al., 2013). L-carnitine also impacts lipid metabolism by influencing various transcription factors related to lipolysis and adipogenesis, reducing lipid synthesis and deposition (Förster et al., 2021). Moreover, it exhibits antioxidant properties (Sahebnasagh et al., 2022), balancing hypertonicity-induced imbalances between oxygenase and antioxidant enzymes, reducing ROS production, inhibiting lipid peroxidation, preventing oxidative DNA damage, and suppressing the production of heme oxygenase-1 and cyclooxygenase-2. Commonly used as a dietary supplement, L-carnitine is acclaimed for its benefits in boosting metabolic energy, aiding in weight loss (Talenezhad et al., 2020), and improving cardiovascular (Nakajima et al., 2024) and cognitive functions (Zhao et al., 2023).

Lucius et al. (2023) demonstrated that L-carnitine, at concentrations of 1–3 mmol/L, effectively controls DED by inhibiting the activation of the transient receptor potential vanilloid subtype 1 (TRPV1) induced by hyperosmolarity. This occurs through the blockade of calcium ion (Ca²⁺) influx and capsaicin-induced cell volume contraction. Notably, they observed that 1 mmol/L of L-carnitine was more effective than 3 mmol/L, suggesting that higher concentrations may exert nonselective cytotoxic effects, leading to increased Ca²⁺ influx and less reduction in cell volumetric contraction (Lucius et al., 2023). Further investigations by López-Cano et al. (2021) on human corneal epithelial cells (CECs) under varying osmolarities (350–500 mOsm/L) indicated that L-carnitine (50 mmol/L) and taurine at an unspecified concentration exhibited higher inhibitory activity against Tumor Necrosis Factor α (TNF-α).

Recent research has uncovered dissimilarities between DED patients and controls by examining carnitine levels in the tears of DED patients, indicating a possible link between the onset of DED and insufficient carnitine in the tear film. Some experts suppose that L-carnitine can be a metabolic biomarker for ocular diseases (Theodoridis et al., 2022). Khanna et al. (2022) utilized metabolomics to validate the role of L-carnitine in the pathogenesis of DED. They hold that L-carnitine can prevent damage to the ocular surface by regulating tear film osmolarity. Studies by Pescosolido et al. (2009); Yamaga et al. (2021) further confirmed its effectiveness in alleviating symptoms in DED patients. Ma et al. (2021) formulated a levocarnitine thermosensitive in situ gel (LCTG) to evaluate its efficacy in treating DED in animal models. The study demonstrated that LCTG significantly increased tear secretion, promoted the repair of CECs, as well as downregulated the expression levels of matrix metalloproteinase-3 (MMP-3) and MMP-9. In addition, the authors reported minimal stimulation and highly effective treatment of DED by LCTG in animal models. They concluded that the overall therapeutic effect of LCTG was superior to that of conventional L-carnitine solution (Ma et al., 2021).

While L-carnitine is recognized for its energy-enhancing properties and potential to decrease appetite, prolonged use might lead to reduced nutrient intake, potentially resulting in malnutrition and decreased physical function.

Lactoferrin (LF) is a glycoprotein that mainly consists in mammalian milk and exhibits a wide range of biological functions. It plays a vital role in regulating iron metabolism by chelating iron, thereby inhibiting microbial growth through iron deficiency and interacting with lipopolysaccharide (LPS) to develop bactericidal effects (Zarzosa-Moreno et al., 2020). LF also enhances the host cell’s antiviral defenses, blocking viral replication (Ikeda et al., 2000). Additionally, it modulates inflammatory responses by reducing key cytokines such as TNF-α and interleukin 1β (IL-1β) (Asaad and Mostafa, 2022), and acts as a free radical scavenger, reducing oxidative stress and protecting cellular integrity (Ibuki et al., 2020). Due to its antibacterial, anti-infective, and immunomodulatory characteristics, LF has been incorporated into infant formula (Colombo et al., 2023) and utilized in adjunct anti-infective treatment (Jiang et al., 2014).

LF’s antioxidant properties on the ocular surface, where it chelates free iron and prevents the formation of harmful hydroxyl radicals, thus protecting CECs from oxidative stress (Pastori et al., 2015). A Randomized controlled trial (RCT) has demonstrated that LF supplementation enhances the integrity of the tear film, improves the morphology of conjunctival epithelial cells, and increases the lipid layer thickness, contributing to overall tear stability (Dogru et al., 2007). Moreover, as an antioxidant, it has demonstrated inhibitory effects on cytokine production, and it effectively attenuates the hyperinflammatory response of pathogens (Berthon et al., 2022). Additionally, LF can suppress excessive inflammation by inhibiting complement activation and reducing inflammatory mediators (Samuelsen et al., 2004).

The concentration of LF in the tears of patients with DED is low, and there is a belief that LF concentration can serve as a biomarker for diagnosing DED. This belief is supported by Ponzini et al. (2020)’s meta-analysis. Innovative diagnostic technologies, such as the use of nanophotonic metasurfaces (Ye et al., 2024) and fluorescence polarization-based aptasensors (Zhang et al., 2023b), have been developed to detect LF in tear fluid, offering new approaches for accurate DED diagnosis and classification.

LF’s therapeutic effects include mitigating CECs damage and promoting cellular repair, as demonstrated in animal models of DED (Pattamatta et al., 2013; Regueiro et al., 2023). Connell et al. (2021) also observed that LF can increase tear secretion, inhibit the expression of inflammatory factors, and induce a short-lasting effect on tear secretion by modulating the gut microbiota to stimulate the production of short-chain FAs in a mouse model of DED. However, it should be noted that in this experiment, vancomycin, a type of antibiotic, caused LF ineffective in treating DED by impacting the gut flora (Connell et al., 2021). With the growing research on the efficacy of LF in DED, many researchers have developed LF-loaded liposomes using nanomaterials. This addresses the poor aqueous stability of LF and its high excretion through the nasolacrimal duct, which can affect its therapeutic efficacy. López-Machado et al. (2021) have developed liposomes with LF that exhibit good anti-inflammatory properties, effectively alleviating discomfort without irritation.

According to the data, LF demonstrates minimal side effects, particularly in bovine LF. While LF is generally well-tolerated, some side effects such as temporary fecal loosening have been noted, particularly with oral administration, necessitating careful monitoring during extended use (Dogru et al., 2007). Long-term administration must be managed to avoid issues such as excessive iron absorption, which could lead to digestive disturbances and potentially interfere with the absorption of other vital nutrients.

Probiotics are live microorganisms that offer health benefits. These include the ability to suppress or eliminate harmful bacteria, balance the intestinal microecology, promote digestion and nutrient absorption, improve the intestinal environment, stimulate immune cell function, and strengthen the body’s immune response (da Silva et al., 2024). Probiotics exhibit antibacterial and anti-inflammatory properties and are commonly used in fermented dairy products, beverages, and other functional foods. They are also employed in managing intestinal disorders and as a supplemental therapy to enhance immune functions (Abouelela and Helmy, 2024).

Recent studies have advanced our understanding of the ocular surface microbiota, identifying key bacterial groups such as Firmicutes (such as Staphylococcus and Streptococcus), Actinomycetes (specifically Corynebacterium), and Proteobacteria (including Acinetobacter and Pseudomonas). These groups are crucial for pathogen defense and play significant roles in regulating the ocular immune response, thus maintaining ocular health (Li et al., 2020).

The Gut-Eye Axis concept suggests intestinal dysbiosis may significantly impact the onset and progression of eye diseases, including autoimmune DED. Patients with autoimmune DED often experience damage to ocular surface tissues such as the cornea, conjunctival goblet cells, and lacrimal glands. This damage is frequently associated with alterations in gut microbial diversity and abundance (Bai et al., 2023), which can trigger systemic inflammation and contribute to a range of ocular disorders, including age-related macular degeneration, uveitis, diabetic retinopathy, and glaucoma (Bai et al., 2023; Campagnoli et al., 2023).

According to Donabedian et al. (2022), autoimmune uveitis induced by gut flora may be attributed to four factors. Firstly, the gut flora regulates the levels of microbial metabolites, such as butyric acid and short-chain FAs, which have anti-inflammatory properties. Secondly, Imbalances caused by gut dysbiosis can disturb immune homeostasis, affecting various body organs. Thirdly, an imbalance between helper T-cells 17 (Th17) and regulatory T-cells (Treg) can lead to excessive IL-17 production, exacerbating inflammation. Lastly, microbial recognition in the gut can activate autoantigenic T cells in the uvea, contributing to autoimmune reactions (Donabedian et al., 2022). These four factors are closely associated with the ocular manifestations of systemic autoimmune disease, and it is highly probable that uveitis directly contributes to the development of DED. Schaefer et al. (2022) also discovered that the gut microbiota can influence the health of the ocular surface by impacting CD4⁺ FOXP3+ Tregs in the lymph nodes of the eye.

Studies have also shown that imbalances in gut microbiota can amplify the inflammatory response to LPS, a component of gram-negative bacteria, aggravating DED (Bertani and Ruiz, 2018; Wang et al., 2019). Such disruptions have been associated with worsened DED symptoms, decreased corneal barrier function, and reduced epithelial cell density, indicating a negative correlation between microbial diversity and DED severity (Mendez et al., 2020).

Research by Qi et al. (2021) highlighted differences in gut microbiota between individuals with autoimmune disease-mediated DED and those with non-autoimmune DED. Findings included variations in bacterial populations like Corynebacterium, Streptococcus, and Prevotella, correlating with DED severity. Additionally, a separate study reported an elevated presence of Veillonella in individuals with DED compared to the gut microbiota of the Sjögren syndrome and healthy population. Meanwhile, the Subdoligranulum was significantly reduced in patients with DED (Moon et al., 2020a). Additionally, research has substantiated the significance of Prevotella in the modulation of tear secretion among individuals suffering from DED and demonstrated a positive correlation between the severity of DED and the presence of Prevotella (Zhang et al., 2023a).

Research has indicated that dietary choices, particularly high-fat diets, significantly influence the gut microbiota, exacerbating DED symptoms through mechanisms such as decreased tear secretion, increased oxidative stress, and heightened apoptosis (Wu et al., 2020). Furthermore, research by Zhang et al. (2022a) observed that a high-fat diet led to excessive growth of intestinal Deferribacterota in a mouse model of desiccation syndrome, closely correlating with increased severity of DED.

Recent research underscores the potential of optimizing gut microbiota to treat DED effectively. Tavakoli et al. (2022) highlighted that both probiotic and prebiotic therapies have shown promising results in treating DED. Specific strains such as Bifidobacterium lactis and Bifidobacterium bifidum have been documented to improve tear secretion and BUT (Chisari et al., 2017b). Additionally, Saccharomyces boulardii MUCL 53837 and Enterococcus faecium LMG S-28935 have been effective in alleviating subjective symptoms of DED (Chisari et al., 2017a). Further research has demonstrated the efficacy of a complex of five probiotics in enhancing tear production in both autoimmune and dry-stress DED mouse models, suggesting a broader therapeutic potential across different DED types (Moon et al., 2020b; Choi et al., 2020). Another study revealed that an oral formulation of Bifidobacterium and Lactobacillus plantarum not only increased tear secretion but also repaired CECs and reduced inflammatory markers like TNF-α and IL-1β (Yun et al., 2021).

In an RCT with 60 participants, a group receiving a probiotic preparation of Enterococcus faecalis and S. boulardii along with routine eye drops showed significant improvement in DED symptoms compared to the control group, which received only eye drops (Chisari et al., 2017a). Another RCT investigating a combination of Lactobacillus, Bifidobacterium, and Streptococcus with NutriKaneD confirmed its effectiveness in increasing tear secretion and reducing discomfort (Tavakoli et al., 2022). Building upon these studies, Lee et al. (2023) conducted a detailed investigation on the effects of Lactobacillus fermentum HY7302 in a benzalkonium chloride-induced mouse model of DED and found that it reduced corneal fluorescein scores, increased tear secretion, and repaired CECs. It also decreased oxidative stress and inflammatory cytokine production in the DED model. Concerning the utilization of probiotics, Heydari et al. (2023) evaluated the effectiveness and safety of administering Latilactobacillus sakei both orally and topically in DED patients. The study found that while both methods were safe, topical administration was more effective at reducing inflammation (inhibition of inflammatory factors such as IL-6, TNF-α, and Interferon-gamma) and improving tear stability (Heydari et al., 2023).

Aside from the orally administered probiotic preparations, there is a novel medical therapy called intestinal flora transplantation (FMT) that aims to reinstate the equilibrium of intestinal flora in patients through the transfer of fecal flora from healthy individuals to the patient’s intestines. This therapy has already proven effective in treating various conditions, including Clostridium difficile infections (Hui et al., 2019), Crohn’s disease (Fehily et al., 2021), irritable bowel syndrome (El-Salhy et al., 2021), and psychiatric disorders (Pascale et al., 2020). Initial studies have confirmed its safety and efficacy in a small cohort of patients with immune-mediated DED (Watane et al., 2022).

While probiotics are generally safe, they can cause adverse reactions, including bacterial and fungal sepsis, immune system hyperstimulation, microbial resistance, and gastrointestinal issues, especially in vulnerable populations like premature infants and immunocompromised individuals (Sotoudegan et al., 2019). It is critical to monitor patients closely when using probiotics in clinical settings. Furthermore, challenges remain regarding the colonization capacity of probiotic strains and the standardization of probiotic preparations (Donabedian et al., 2022).

Coenzyme Q10, also known as ubiquinone, is a lipid-soluble compound found predominantly in mitochondria and cell membranes. It is critical for cellular functions (Hargreaves et al., 2020), particularly in energy production through adenosine triphosphate synthesis (Aussel et al., 2014), acting as an antioxidant to combat free radicals (Bentinger et al., 2007), and inhibiting lipid peroxidation to protect cell membranes from oxidative damage (Zhang et al., 2013). Clinically, CoQ10 is utilized in the management of heart failure and myocarditis, owing to its ability to preserve the morphology and structure of mitochondria in ischemic cardiomyocytes (Alarcón-Vieco et al., 2023). Moreover, it is also used in the treatment of antioxidants (Samimi et al., 2024) and maintenance of muscle function (Talebi et al., 2024).

A causal relationship exists between oxidative stress and mitochondrial dysfunction, as the copious amount of free radicals produced by oxidative stress can cause damage to mitochondrial complex I. Conversely, suppressing mitochondrial complex I results in an escalation in the generation of free radicals. It is, therefore, imperative to safeguard the function of mitochondria and prevent any potential dysfunction to effectively treat diseases such as DED (López-Lluch, 2021). CoQ10 also inhibits apoptosis by specifically safeguarding mitochondrial DNA and inhibiting mitochondrial depolarization. In cases of oxidative stress, it scavenges oxygen free radicals and keeps mitochondrial DNA deletion mutations in check, maintaining the whole respiratory chain. Additionally, CoQ10 binds to the mitochondrial permeability transition pore, preventing mitochondrial depolarization and cytochrome C release to lower cysteine protease-3 activation and reduce apoptosis (Bentinger et al., 2007; Hseu et al., 2018; Fatima et al., 2021).

Research indicates that CoQ10 significantly improves lacrimal gland function and reduces inflammation due to its antioxidant properties. Studies using animal models also have demonstrated its effectiveness in mitigating mitochondrial damage linked to oxidative stress in the lacrimal gland (Uchino et al., 2012). Clinical trials using combinations of CoQ10 with cross-linked hyaluronic acid (XL-HA) have shown enhanced treatment outcomes in DED patients, attributed to CoQ10’s antioxidant effects and HA’s inherent water retention and viscoelasticity properties, which are essential for repairing ocular tissues (Postorino et al., 2018; Posarelli et al., 2019). In another study, researchers concluded that including CoQ10 in treatment reduced all cytokine levels and a notable elevation in total antioxidant status levels. The efficacy of CoQ10 in enhancing protection against oxidative damage and safeguarding the lacrimal gland was established through histopathological and tissue cytokine level analyses (Yakin et al., 2017). Serrano-Morales et al. (2022) conducted a prospective study using a double-blind approach to assess the effectiveness of a combination supplement containing XL-HA, CoQ10, and vitamin E in women experiencing menopausal DED symptoms while undergoing antidepressant treatment. The study’s results indicated that patients who received the intervention experienced improvements in DED symptoms, as well as increased tear film stability (measured by the BUT test) and higher tear production (measured by the Schirmer test). It is worth noting that HA exhibits favorable lubricating properties, particularly when cross-linked, as it forms a liquid matrix on the ocular surface. Moreover, CoQ10 and vitamin E in this combination supplement act as antioxidants, improving ocular repair (Serrano-Morales et al., 2022). Additionally, Tredici et al. (2020) demonstrated that applying this complex can protect the ocular surface of professional swimmers frequently exposed to chlorinated water for extended periods.

When evaluating potential drug interactions, it is imperative to consider the impact of CoQ10 on warfarin metabolism. Due to its structural similarities with vitamin K, CoQ10 may selectively interact with cytochrome P450 enzymes, reducing response to warfarin (Sharma et al., 2016). This could introduce challenges for patients with lifelong anticoagulation needs, such as those with heart failure and atrial fibrillation (Ayers et al., 2018). Additionally, CoQ10’s strong antioxidant properties might reduce the effectiveness of pro-oxidant chemotherapies (Yasueda et al., 2016) and could interact with antihypertensive drugs to cause an excessive drop in blood pressure, necessitating careful monitoring and dose adjustments (Zhao et al., 2022).

Spermidine is a naturally occurring polyamine found in all living organisms, playing essential roles in cellular metabolism (Hofer et al., 2021), including cell proliferation and division (Luo et al., 2023). It is critical for maintaining DNA stability and synthesis (Wang et al., 2018) and supports processes such as apoptosis and autophagy, crucial for removing damaged cells and maintaining cellular health (Hofer et al., 2021). Due to its extensive biological roles, spermidine is used to reduce inflammatory responses (Mao et al., 2023), improve cardiovascular health (Wu et al., 2022), and enhance cognitive functions (Pekar et al., 2021).

Spermidine helps regulate the immune system’s balance, crucial in conditions like DED, where inflammation plays a key role. M1/M2 macrophage polarization and the Treg/Th17 cell balance, are processes integral to modulating inflammation (Shapouri-Moghaddam et al., 2018; Zhang et al., 2021). DED is associated with an imbalance in Th17/Treg cells, leading to increased Th17 polarization and secretion of pro-inflammatory cytokines like IL-17, causing epithelial damage (Ratay et al., 2017). As we all know, Macrophage-induced inflammation promotes the differentiation of CD4 T cells into Th17 cells, which inhibits Treg cell function. Spermidine regulates immune cell number and function by inducing macrophage polarization and CD4 T cell differentiation in an autophagy-dependent manner (Liu et al., 2020; Carriche et al., 2021). It also acts as an antioxidant and specific calcium chelator, safeguarding cells from oxidative stress-induced calcium overload and preventing apoptosis when exposed to oxidative or endoplasmic reticulum stress (Kim et al., 2021). Meanwhile, it is primarily found in the brain and retina and is an endogenous scavenger for free radicals. It also exhibits potential inhibitory effects against ROS, as observed in previous studies (Ha et al., 1998).

Spermidine has shown potential in inhibiting the production of pro-inflammatory cytokines in microglia activated by LPS and protecting fibroblasts from oxidative damage induced by hydrogen peroxide (Horrocks and Yeo, 1999; Rider et al., 2007). These properties contribute to enhanced tear film stability and reduced ocular surface inflammation in DED models. In mouse studies, spermidine treatment resulted in decreased IL-17 levels in the corneas and lacrimal glands, stabilizing the tear film and mitigating ocular surface inflammation (Lee et al., 2021).

While spermidine offers therapeutic benefits, its use in treating ocular conditions must be approached with caution due to potential cytotoxic effects. These include the risk of excessive proliferation and migration of retinal pigment epithelium cells, which may lead to complications such as hyperproliferative retinopathy (Han et al., 2022).

Royal Jelly (RJ) is a creamy secretion produced by the glands of worker bees (Wang et al., 2023), known for its antibacterial, antifungal, anti-inflammatory, and antioxidant properties (Ghadimi-Garjan et al., 2023). It supports cell renewal, wound healing, and blood pressure and lipid regulation (Zamami et al., 2008). Given its nutrient richness and biofunctional diversity, RJ is widely used as a dietary supplement and for antibacterial and anti-inflammatory treatments (Baptista et al., 2023).

RJ exhibits natural antioxidative characteristics, significantly reducing ROS and nitric oxide production in macrophages and boosting activities of antioxidant enzymes like SOD and glutathione (Gu et al., 2018). Major Royal Jelly Protein 2, a key RJ component, diminishes oxidative stress and inhibits apoptosis in microbial cells, enhancing cell survival under oxidative conditions (Abu-Serie and Habashy, 2019). Additionally, RJ has shown efficacy in reducing oxidative stress in diabetic animal models by enhancing antioxidant enzyme activities and reducing oxidative damage markers (malondialdehyde levels) (Ghanbari et al., 2016).

RJ’s anti-inflammatory effects are mediated through its fatty acid analogs, including 10-hydroxy-2-decanoic acid, 10-hydroxy decanoic acid, and sebacic acid, which effectively inhibit the release of pro-inflammatory cytokines and other inflammatory mediators (Yang et al., 2018b). These compounds influence critical inflammatory pathways, such as mitogen-activated protein kinases and NF-κB signaling pathways, and promote the production of anti-inflammatory cytokines IL-1ra (You et al., 2020).

RJ has been used in conventional ophthalmic remedies like honey eye drops for corneal wound healing (Azmi et al., 2021) and propolis for protecting retinal neurons (Abd Rashid et al., 2022). Studies have demonstrated that RJ enhances tear secretion by promoting Ca²⁺ mobilization and conserving ATP in lacrimal glands through muscarinic signaling pathways (Inoue et al., 2017). Yamaga et al. (2021) investigated the impact of various bioactive components in RJ on tear production. To determine the critical component responsible for increasing tear production, they utilized a mouse model of stress DED. Their research discovered that a combination of three FAs and acetylcholine played a pivotal role in enhancing tear production (Yamaga et al., 2021). An extensive study comparing various bee products (including raw honey, propolis, RJ, pollen, and bee larvae) found RJ most effective in restoring tear secretion and maintaining mitochondrial health in lacrimal glands (Imada et al., 2014).

A clinical trial also confirmed that oral RJ supplementation significantly improved tear secretion in DED patients without adverse effects (Inoue et al., 2017). Further, systematic reviews have validated RJ’s therapeutic impact on improving DED symptoms (Prinz et al., 2023).

Despite its benefits, RJ may cause allergic reactions, including skin rashes and pruritus, particularly in sensitive individuals (Li et al., 2021). Due to its estrogenic properties, RJ is not recommended for children under 10 years and pregnant women (Ishida et al., 2022). However, no significant adverse reactions have been reported in clinical studies.