Mental disorders remained among the top ten leading causes of burden worldwide, with depressive disorders being leading contributors to this burden and without evidence of global reduction in the burden since 1990 (1, 2). Approximately 300 million people globally suffer from depression in 2020, and the number of instances of major depressive disorder (MDD) increased by 53.2 million cases globally following the outbreak of the new coronavirus pneumonia pandemic, resulting in 49.4 million disability-adjusted life-years for MDD (2). Moreover, the World Health Organization (WHO) ranked depression as the third leading cause of global burden of disease in 2004, and it is predicted to overtake it and move to first place by 2030. Antidepressants, pharmacotherapy, lifestyle modifications, psychotherapy, electroconvulsive therapy, and vagal nervous stimulation are all options for treating depression (3–6). Despite the fact that various therapeutic options exist to alleviate the symptoms of depression, none of them are sufficiently capable of resolving the unpleasant impact. Conventional formulations for the treatment of depression have several disadvantages, including limited drug penetration, high dose frequency, adverse reaction, and patient compliance concerns (7, 8). Therapies and lifestyle modifications also need a significant amount of effort to demonstrate their feasibility, which may, in turn, result in passive non-compliance. All of these difficulties hinder the adequate application of available conventional treatments for depression (9). Consequently, an objective and reliable approach for evaluating the onset and progression of depressive disorders is highly desirable but still awaits further development.

Multiple risk factors for depressive disorders exist, including stress, behavioral habits, and socio-demographic factors (1). Furthermore, several studies have identified a possible link between depression and dyslipidemia (10, 11). Remnant cholesterol (RC), also known as triglyceride-rich cholesterol, is mostly composed of very-low-density lipoproteins, intermediate-density lipoproteins, and chylomicron remnants in fasting or non-fasting states, which is defined as total cholesterol (TC) minus high-density lipoprotein cholesterol (HDL-C) minus low-density lipoprotein cholesterol (LDL-C) (12, 13). Initially, RC is gaining popularity due to its tight relationship with cardiovascular diseases. Previous studies have shown that RC is also associated with hypertension, diabetes mellitus and its consequences, nonalcoholic fatty liver disease, and chronic kidney disease. Particularly, the main pharmacological treatment for cardiovascular diseases (CVD) prevention is thought to be reducing LDL-C using statins. Though the guidelines now propose a target LDL-C, there are still hazards associated with CVD, which are referred to as “residual risks”. Besides, the potential connection between cardiovascular diseases and depressive disorders has also been well established (14, 15). To explore whether the RC may also shed light on the complex interaction between cardiovascular disease and depression has great importance. Nevertheless, information on the relationship between RC and depressive disorders is currently limited.

Thus, the present study aimed to determine the relationship between RC and the risk of prevalent depressive disorders using data from the National Health and Nutrition Examination Survey (NHANES) and to assess the performance of RC in identifying MDD.

This nationwide population-based study of 9,173 US adults form NHANES 2005-2018 identified the association between RC and the risk of prevalent depression. The findings implied that an elevated RC level were connected with an increased likelihood of prevalent depression and that RC performs well in predicting MDD. Moreover, this link persisted even after taking into consideration all potential confounding variables, with the highest RC quartile in the population associated with a 32% increased risk. Additionally, associations between RC and depression were significantly different across subgroups stratified by diabetes, hypertension, and hyperlipidemia, showing that elevated RC levels may be related to a higher PHQ-9 score and an increased risk of MDD, particularly in patients with diabetes, hypertension, or hyperlipidemia. Our findings revealed that RC may hold promise for clinical utility in assessing MDD risk and evaluating disease severity.

Depression and stress-related disorders are common mental disorders, affecting approximately one-third of individuals during the life course (19). Recently, increasing evidence has supported that metabolic dysregulation may contribute to the development of psychiatric disorders (20–22). A diverse cohort (23) followed for 30 years revealed that a consistent bidirectional relationship between depressive symptoms and metabolic syndrome was observed, with individuals with more depressive symptoms more likely to develop metabolic syndrome, and metabolic syndrome similarly predicting more depressive symptoms. Most importantly, lipid profiles played a potential role in the risk of developing depressive disorders, and dyslipidemia has been proven to be a risk factor for depression (24). Substantial studies have investigated the relationship between lipid profiles and the risk of prevalent depressive disorders. However, studies examining the association between TC, LDL-C, and HDL-C and the risk of developing depression have yielded inconsistent and even contradictory results (25–29). RC is an important component of cholesterol, accounting for one-third of nonfasting plasma cholesterol and the source of approximately half of the cholesterol present in atherosclerotic plaques (30, 31). Previous studies have shown that RC plays a crucial role in the pathogenesis of various diseases including CVD (32), diabetes (33), liver disease (34), CKD (35), peripheral artery disease (36), and cancer (37). But there is still a lack of research on the relationship between RC and psychiatric disorders, and the predictive power of RC on depression remains to be explored.

Recently, not only has cardiovascular conditions received increased attention, but mental health has also been given more emphasis. Findings of a new study (13) indicated that even when LDL-C was lowered to recommended levels, there was still a substantial residual risk of ischemic heart disease, which may be explained by the association between elevated levels of RC and low-grade inflammation. Furthermore, the relationship of CVD and psychiatric disorders has also been confirmed, with recent data from 21,942 individuals showing that an ideal level of cardiovascular health contributes to the risk reduction of depression (38). Therefore, further investigation of the connection between RC and the risk of prevalent MDD to determine a dependable predictor and to assess the occurrence and development of depressive disorders is of crucial importance. Notably, our study preliminarily offered the population-based evidence that an elevated RC was positively connected with a greater prevalence of MDD and each 1 mmol/L increase in RC corresponded to a 32%. In our subgroup analyses, we found that the relationship between RC and MDD was broadly consistent across a range of demographic and clinical subpopulations, including age, gender, BMI, smoking, CVD, and CKD, suggesting a strong connection that is not significantly affected by these factors. Conversely, individuals with diabetes, hypertension or hyperlipidemia have a higher likelihood of developing depression. However, current studies on the effect of diabetes on the risk of developing depression have produced conflicting results. A nationwide population-based cohort study (39) of about 2 million adults in Korea demonstrated that diabetes severity was positively connected with an elevated risk of developing MDD, and it is feasible to consider screening for depression in those with higher diabetes severity scores. Further studies confirmed that variants of the melatonin receptor 1B gene (MTNR1B) and the prolactin receptor gene (PRLR) were identified as the genetic risk factors for the comorbidity of diabetes and depression (40, 41). Inconsistent with the results, findings from a bidirectional Mendelian randomization study (42) showed a negative correlation between diabetes and the risk of depression onset, indicating that diabetes might reduce the risk of depression. Moreover, hypertension is well known as one of the key risk factors for depression, and the bidirectional and causal relationship between hypertension and depression has also been confirmed in recent years (43). Comorbid hypertension and depressive symptoms were considered to be associated with a higher risk of elevated C-reactive protein levels (44). Experimental study (45) showed that Mfn2-mediated mitophagy activation played a significant role in mitigating depression-like behaviors in hypertension rats. Besides, substantial studies have demonstrated that depression was associated with deteriorating lipid control and higher risk of dyslipidemia (46, 47). Study of 72,235 participants (48) further revealed that the risk of CVD was 1.24-fold higher in dyslipidemia patients with depression than in those without depression.

Several previous studies have suggested the potential mechanism for the close link between elevated RC and the increased risk of depression. RC-induced low-grade inflammation and endothelial dysfunction may be considered as triggers for arterial stiffness (13, 49), and a cross-sectional epidemiological study (50) of 1,510 participants indicated that arterial stiffness of the carotid arteries was found higher in the presence of depression. In addition, due to the lesser size along with high cholesterol content, and increased residence period in blood the remnant lipoproteins, RC is more readily captured and absorbed by macrophages than LDL-C, resulting in faster foam cell formation and inflammatory cytokines secretion (51). Further research found that inflammation may increase the risk of depression through the hypothalamic-pituitary-adrenal axis, and lower levels of inflammatory cytokines may contribute to alleviating depressive symptoms (52, 53). Moreover, the microbiome-lipid metabolic axis has also been regarded as a potential approach to ameliorating depression (54).

There are various strengths to this study. Firstly, NHANES, a population-based sample database that was conducted nationally and adhered to a set process, provided the data utilized in this investigation. Proper NHANES sampling weights were taken into account for all analyses in order to increase the representativeness of the study samples. Furthermore, the represent study computed the cut-off point and identified segmented effects between variables to provide more solid evidence, while concurrently evaluating nonlinear correlations between RC and depressive disorders after controlling for confounding variables. Nevertheless, it is important to note several limitations of the current study. Firstly, we are unable to establish causality between RC and the prevalence of depression due to the cross-sectional study methodology. There is a potential for circular causality and confounding variables in interpreting the link between remnant cholesterol and depressive symptoms. Factors like inflammation, lifestyle, and metabolic conditions might both influence and be influenced by the variables measured, creating complex interdependencies that limit straightforward causal interpretations. Further large-scale prospective studies are still required to confirm our findings. Secondly, information bias can persist even if precautions were made to prevent systematic errors during data collection. Thirdly, although this study provided a timely analysis of RC measurements, the results may not reflect the long-term reality of the included patients. While depression is a more perennial data, data over time in relation to RC could be more useful for this topic.

The present study demonstrated that elevated RC levels were associated with higher PHQ-9 scores and a greater prevalence of prevalent MDD. Our findings highlight the importance of the management of RC even in the presence of normal lipid testing in identifying patients at risk of MDD. Furthermore, managing comorbidities such as diabetes, hypertension, and hyperlipidemia may assist to minimize the risk of developing MDD. Nevertheless, further large-scale prospective studies are still required to provide more robust evidence to validate our findings.