Systemic immune-inflammatory index mediates the association between regular walking and depression in the elderly with coronary heart disease

Background Depressive disorders and coronary heart disease (CHD) are significant and pervasive global public health challenges, influenced by various factors such as diet, physical activity, social circumstances, and lifestyle habits. This study aim to investigate the relationship among regular walking, the systemic immune-inflammation index (SII), and depression in CHD patients. Additionally, it seeks to explore the mediating role of SII in the effect of regular walking on depression among these patients. Methods This cross-sectional study involved 464 patients aged ≥ 65 years with CHD, admitted to the cadre ward of the First Hospital of Jilin University from September 2021 to December 2022. A comprehensive geriatric assessment (CGA) inventory was prepared by compiling and screening regular walking, calculated SII, and the Geriatric Depression Scale (GDS). Binary logistic regression was employed to examine the relationship among walking, ln(SII) and depression. After adjusting for confounding factors, linear regression analysis was used to assess the correlation among exercise-related factors, ln(SII) and depression. Sensitivity analyses were performed through stratified analysis, investigating the relationship between walking and depression within the strata of each variable, as well as the P -interaction among stratified variables. Additionally, the Process package in SPSS was used to explore the mediating role of ln(SII) in the association between regular walking and depression. The proportion of the mediating effect was calculated using the formula:(mediated effect / total effect) × 100%. Results Model 2: Elderly individuals who walk regularly are 72.2% less likely to experience depression compared to those who do not engage in regular walking (odds ratio (OR) = 0.278, 95% CI = 0.178–0.433, P  < 0.001). However, one-unit increases in ln(SII) corresponds to an approximately sixfold in the likelihood of depression among elderly individuals (OR = 5.899, 95% CI = 4.183–8.318, P  < 0.001). Model 3: Participants who walk regularly are 69.4% less likely to suffer from depression compared to those who do not walk regularly (OR = 0.306, 95% CI = 0.190–0.491, P  < 0.001). Similarly, a one-unit increase in ln(SII) is associated with an approximately sixfold increase in the probability of depression elderly individuals (OR = 5.976, 95% CI = 4.126–8.656, P  < 0.001). The stratified analysis examining the effect of regular walking on depression in patients with CHD confirms the robustness of these associations. Additionally, SII was found to mediate the relationship between regular walking and depression in CHD patients, accounting for 43.19% of the total effect. Conclusions An interaction exists among SII, regular walking and depression in elderly patients with coronary heart disease. Furthermore, SII partially mediates the relationship between regular walking and depression.