Periodontal disease (PD) is an inflammatory condition closely linked to bacterial infections (1, 2); it develops through complex interactions between harmful bacteria below the gum line and the body's immune response (3). The development and severity of PD are significantly influenced by genetic susceptibility, as well as environmental factors such as smoking and systemic disorders (4). Two predominant forms of PD are gingivitis and periodontitis. The search for the specific bacteria that cause periodontal disease is ongoing; however, recent findings indicate that the body's inflammatory response plays a crucial role in affecting the bacteria in the gums, thereby accelerating the onset of periodontitis (5). Adipose tissue, which was once considered merely a fat storage site, actually secretes various bioactive substances known as adipokines (6), such as interleukin-6 (IL-6), visfatin, and irisin, that can influence inflammatory processes (7). Irisin, a newly identified adipomyokine, mediates the effects of physical exercise, with primary production occurring in adipose tissue and skeletal muscle (8). It regulates metabolic processes and helps maintain the balance between bone growth and resorption (9). Irisin has anti-inflammatory effects and may contribute to metabolic regulation (10). In the context of periodontitis, irisin may help modulate the inflammatory response and potentially protect against periodontal tissue destruction (11). Visfatin, another adipokine primarily derived from adipose tissue (12), but also produced in other tissues including periodontal tissue (13), Visfatin is generally expression elevated in inflammatory conditions (14), This leads to an increase in both pro-inflammatory and anti-inflammatory cytokines, such as interleukin-1 beta (IL-1β) and interleukin-6 (IL-6) (15). IL-6 is produced by various cell types, including adipocytes, macrophages, fibroblasts, endothelial cells, and skeletal muscle, during the early phases of inflammation (16). It serves as a significant mediator in inflammatory processes and regulates acute-phase proteins (17). Moreover, IL-6 plays a vital role in the progression of periodontal disease by promoting the formation of cells responsible for bone breakdown, which increases bone loss while decreasing bone growth (18). Elevated visfatin levels can lead to increased IL-6 release (15), creating a cycle of inflammation that exacerbates periodontal disease. In this context, irisin may disrupt this cycle, potentially lowering IL-6 levels and reducing inflammation (10). To the best of our knowledge, no research has yet investigated the role of irisin in patients with plaque-induced gingivitis. Therefore, this study will be the first to explore the significance of irisin in this context. Additionally, while there are two studies addressing irisin in the case of periodontitis (8, 19), none have examined its relationship with visfatin and interleukin-6 in the etiology of periodontal disease (PD). By understanding these connections, we aim to shed light on the inflammatory mechanisms involved in PD and to identify potential therapeutic targets for intervention.

Inflammatory response to periodontal diseases is a crucial survival mechanism (25). Chronic inflammatory disorders are linked with the activation of detrimental signal transduction pathways and the release of many inflammatory mediators, all of which play their roles in disease development. This study demonstrated significantly higher salivary concentrations of visfatin and interleukin-6 (IL-6) in patients with gingivitis and periodontitis relative to periodontally healthy controls. Irisin concentrations were significantly reduced in the gingivitis and periodontitis groups. Notably, this study is the first to show a link between lower levels of salivary irisin and periodontal inflammation, suggesting that irisin might negatively affect the inflammation processes involved in these periodontal diseases. Irisin, a myokine identified in 2012 and induced by physical exercise, is crucial in regulating glucose and lipid homeostasis (26). On the other hand, pro-inflammatory molecules such as visfatin and interleukin-6 (IL-6) are known to contribute to insulin resistance, unbalanced fat metabolism, oxidative stress, and systemic inflammation (27, 28). Irisin has been shown to provide protective effects by enhancing insulin sensitivity, maintaining metabolic equilibrium, and reducing oxidative stress (29). The findings highlight the inverse relationship observed in our study between irisin levels and those of visfatin and IL-6.

This study employed a case-control design because it offers specific advantages over other study designs. Case-control studies are comparatively dependable and economical. Notably, the salivary gland, serous, and mucous acinar cells are reported to generate irisin (30). Therefore, measuring irisin levels in saliva may provide a more accurate insight into the role of this protein in periodontal diseases. However, to date, no study has evaluated irisin's probable role in the development of plaque-induced gingivitis patients or the pathogenesis of irisin together with visfatin and IL-6 in periodontitis.

During physical activity, peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) in skeletal muscle releases fibronectin type III domain-containing protein 5 (FNDC5). Once FNDC5 is released, it undergoes cleavage by an unknown protease, creating irisin (31). Since its discovery, irisin has received much interest due to its proposed critical role in several biological processes, including inflammation. Reports suggest that irisin may have differential effects on NF-κB, depending on the biochemical pathways involved (10). In context of inflammation, irisin might reduce the activity of NF-κB via a reduction in toll-like receptor 4/nuclear factor erythroid 2–related factor (TLR4/NRF2) and AMPK signaling pathways. This could lead to lower release of inflammatory markers such as COX-2, IL-1β, iNOS, TNF-α, and IL-6 (32). It is plausible that decreased irisin levels in individuals with inflammatory conditions could contribute to heightened inflammatory responses associated with impaired NF-κB regulation. This may help to explain our observation of a negative correlation between irisin and IL-6. In contrast, visfatin, produced in human gingival fibroblasts (hGFs) and human periodontal ligament cells (hPDLCs), is thought to regulate pro-inflammatory and pro-degradative factors partially through the NF-κB signaling pathway, thus promoting an increased expression of IL-6 and increased inflammation (15). This is consistent with our finding of a positive correlation between visfatin and IL-6. Notably, this activation of NF-κB may suppress irisin production, while irisin could counteract their effect by inhibiting NF-κB activity. We propose that irisin may play a pivotal role in regulating the balance between adipokines such as visfatin and IL-6 within the context of periodontal disease. It is hypothesized that an increase in irisin levels could potentially mitigate the inflammatory effects of visfatin and IL-6, suggesting a possible protective mechanism against the systemic inflammation often associated with periodontal disease. Supporting this notion, a study by Yuksel Ozgor et al. demonstrated that the mice injected with irisin showed decreased pro-inflammatory adipokines, weight gain, and lower visfatin levels (33). This aligns with our findings of a negative correlation between irisin and visfatin. Moreover, Khajebishak et al. found that irisin levels in obese people with type 2 diabetes were considerably lower than in controls, and a negative correlation between irisin and IL-6 was observed (34), reinforcing our observation of a negative correlation between irisin and IL-6. Furthermore, research on metabolic disorders, including Gestational Diabetes Mellitus (35, 36), has suggested that reduced irisin levels may be linked to obesity and diabetes, which are implicated in the etiology of periodontitis (37). While several studies have shown reduced irisin levels, such as myocardial damage associated with severe hypothyroidism (38), cardiovascular diseases (39), and rheumatoid arthritis (40), which align with our findings, other research has indicated elevated irisin levels (8) as a comenstary mechanisim to inflammation however despite variations in study outcomes, a prevailing hypothesis reduced irisin levels in persons with inflammatory metabolic disorders.

For the first time, the study identifies a negative correlation between irisin and all periodontal clinical parameters, alongside positive correlations between clinical periodontal parameters and saliva visfatin and IL-6 levels, which agrees with prior research (41). Irisin levels are known to rise with increased visceral adiposity (42); however, our subjects exhibited a waist-to-height ratio (WHtR) below 0.5, a metric regarded as a more precise predictor of visceral fat distribution than body mass index (BMI) (43). Our findings support the use of WHtR as a more effective anthropometric measure than BMI, which, despite its widespread use, is often critiqued for inadequately representing adiposity distribution (44). Interestingly, our study found no statistically significant correlation between the examined biomarkers and BMI or WHtR. These findings align with specific prior publications (2, 45), while they diverge from others (8). Therefore, more research with bigger and more varied groups is warranted to elucidate the potential link between these biomarkers and anthropometric measures such as BMI and WHtR. One limitation of this study is the lack of quantitative data on participants' physical activity. Our results may be confounded by irisin, a myokine whose circulating concentration is affected by acute and chronic physical activity. Although all participants were instructed to remain seated and relaxed for one hour prior to saliva sampling to minimize acute exercise-related fluctuations in irisin, we did not assess habitual levels of physical activity. Prior evidence shows that exercise can significantly alter baseline irisin concentrations: Boström et al. observed a 2–3 fold transient increase following acute exercise (46), while subsequent meta-analyses reported that chronic exercise interventions elevate circulating irisin by ∼20%–100% depending on exercise modality and intensity (47, 48). Thus, unmeasured variability in habitual physical activity may have biased our results, and future studies should objectively quantify physical activity to better disentangle its effects from the associations observed. Another limitation of this study is its small sample size. It is essential to acknowledge that, despite the irisin concentrations in this study being measured with a commercially available ELISA kit validated by the manufacturer for high specificity and absence of cross-reactivity with FNDC5 or other irisin analogues, the dependability of irisin quantification through ELISA continues to be a subject of ongoing discourse in the literature. The lack of a standardized assay and the variability among various commercial kits may affect the comparability of results across studies. This must be taken into account when interpreting the current results. An important consideration in interpreting our findings is the potential influence of confounding factors on circulating irisin levels. In the present study, we were able to adjust for age, BMI, and WHtR, and our results remained consistent after controlling for these variables, which strengthens the validity of our conclusions. However, other relevant factors such as detailed exercise habits, dietary intake, body fat percentage, and muscle mass were not available in our dataset. These variables may also contribute to variation in irisin secretion, and their absence represents a limitation of the current work. Therefore, our results should be interpreted with this limitation in mind, and we recommend that future research incorporate these additional measures to provide a more comprehensive understanding of the independent relationship between periodontal diseases and irisin levels.

With more precise testing methods in the future, the identification and detection of salivary irisin, visfatin, and IL-6 may serve as potential biomarkers to predict susceptibility to periodontal diseases.