Relationship Between Serum Apelin-13 and Apelin Receptor Levels and the Severity of Disease in Patients Diagnosed with Obstructive Sleep Apnea Syndrome

<b>Background and Objectives:</b> Apelin-13 and its receptor (APJ) are increasingly recognized as key regulators of metabolic pathways that may contribute to the pathophysiology of obstructive sleep apnea (OSA) syndrome. This study aimed to investigate the relationship between circulating apelin-13 and APJ levels with disease severity in patients diagnosed with OSA, considering the impact of obesity. <b>Materials and Methods:</b> A total of 105 subjects were enrolled: 35 obese patients with OSA, 35 non-obese patients with OSA, and 35 healthy controls. Demographic data, polysomnographic parameters, metabolic markers, Apelin-13, and APJ levels were compared across groups. Patients were further classified as mild-moderate, or severe OSA for subgroup analysis. Correlations between Apelin-13, APJ, BMI, minimum oxygen saturation (Min SaO₂), and apnea-hypopnea index (AHI) were assessed. ROC analysis was used to examine the potential of Apelin-13 and APJ to predict severe OSA. <b>Results:</b> Apelin-13 levels were significantly higher in obese patients with OSA compared to non-obese OSA and controls (<i>p</i> < 0.001), whereas APJ levels were lowest in obese OSA subjects. Apelin-13 showed significant positive correlations with BMI (r = 0.63, <i>p</i> < 0.001) and AHI (r = 0.33, <i>p</i> = 0.005), and a negative correlation with Min SaO₂ (r = -0.35, <i>p</i> = 0.004). Conversely, APJ levels were negatively correlated with BMI (r = -0.60, <i>p</i> < 0.001) and AHI (r = -0.40, <i>p</i> = 0.002) and positively correlated with minimum SaO₂ (r = 0.40, <i>p</i> = 0.002). In severe OSA, insulin and HOMA-IR levels were significantly higher than in mild-moderate OSA (<i>p</i> = 0.02 and <i>p</i> = 0.003, respectively). However, there was no significant difference in Apelin-13 and APJ levels by OSA severity category. ROC analysis revealed that neither Apelin-13 nor APJ demonstrated sufficient diagnostic performance to predict severe OSA (AUC = 0.50 and 0.63, respectively). <b>Conclusions:</b> Apelin and APJ levels are correlated with key metabolic and hypoxic parameters in OSA, indicating that the apelin/APJ system may play a compensatory role in mitigating hypoxia-induced and metabolic complications. However, neither marker alone provides sufficient predictive value for disease severity, emphasizing the need for further studies to clarify the mechanisms and potential clinical applications of this system in OSA management.