Metabolic syndrome (MS) is a complex metabolic disorder characterized by a cluster of interrelated risk factors, including abdominal obesity, hypertension, hyperglycemia, and dyslipidemia. Driven largely by adverse lifestyle factors, the global prevalence of MS continues to rise, positioning it as a significant risk factor for the development of cardiovascular diseases and diabetes (1). Depressive states are primarily characterized by persistent low mood, sadness, or distress. When these symptoms evolve into a chronic mood disorder that substantially interferes with daily functioning, they are clinically recognized as depressive symptoms. The World Health Organization predicts that by 2030, depressive symptoms will be the leading cause of disease burden in high-income countries (2). Both MS and depressive symptoms present substantial public health challenges. Emerging evidence increasingly suggests a significant association between MS and depressive symptoms. A long-term study spanning 21 years has shown that the incidence of hyperglycemia and dyslipidemia is closely linked to the development of depressive and anxiety disorders (3). Koponen et al. (4). observed that the incidence of developing depressive symptoms after a diagnosis of MS is twice as high as in individuals without MS. Research has shown that insulin resistance, elevated cortisol levels, and immune-inflammatory activation associated with MS are closely linked to the onset of depressive symptoms. Moreover, people with comorbid MS and depressive symptoms tend to experience more complex symptomatology, lower recovery rates, and frequent recurrence of episodes (5).

Compared to other indices reflecting dietary quality, such as the Healthy Eating Index-2015 (6), the Dietary Inflammatory Index (DII) is specifically designed to objectively integrate evidence from diverse research methods and nutritional assessments across various populations. The DII score provides a measure of the inflammatory potential of an individual’s diet, with higher scores indicating a greater pro-inflammatory effect (7). The DII pattern can promote systemic inflammation, which is closely associated with abnormal levels of inflammatory markers in the bloodstream, including C-reactive protein, tumor necrosis factor-α, and interleukin (IL)-1β. This relationship underscores the critical role of diet in modulating inflammation and its potential impact on various health outcomes (8).

The pathogenesis of MS primarily involves insulin resistance, impaired lipolysis, and excessive accumulation of fatty acids (1). Dietary factors play a crucial role in these processes, with diets low in saturated fats and rich in fiber, monounsaturated oils, vitamins, and minerals exhibiting protective effects against the development of MS (9). Conversely, diets high in meat consumption and fried foods, characteristic of Western dietary patterns, have been linked to the onset of MS (10). Unhealthy dietary habits can induce adverse biological effects that contribute to the initiation and progression of MS. For instance, high-fat diets disrupt energy metabolism by promoting lipid accumulation, which adversely affects adipose tissue metabolism and contributes to obesity (11). A systematic review has shown that higher DII scores are associated with key components of MS, including hypertension, hyperglycemia, abdominal obesity, and dyslipidemia (12).

Despite the incomplete understanding of the etiology of depressive symptoms, current research has shown a significant correlation between depressive symptoms and the expression of inflammatory molecules (13). Studies indicate that an imbalance between pro-oxidants and antioxidants increases the production of reactive oxygen species, leading to oxidative stress, which may be linked to the pathogenesis of depressive symptoms (14). Foods rich in various vitamins exhibit anti-inflammatory and antioxidant properties, and they can also modify gut bacteria strains, thereby regulating gut inflammation associated with depressive symptoms (13). Cross-sectional studies have shown that breastfeeding women can reduce the incidence of postpartum depressive symptoms by adjusting their dietary patterns to lower the inflammatory score of their diet (15). Thus, it is evident that an inflammatory dietary pattern can influence MS and depressive symptoms. However, it remains unclear whether such a dietary pattern may exacerbate depressive symptoms in people with MS. To address this gap, we utilized data from the National Health and Nutrition Examination Survey (NHANES) from 2007 to 2018 to investigate the relationship between the DII and the likelihood of depressive symptoms in U.S. adults with MS.

In this cross-sectional study, we utilized a representative national sample from the United States. Our findings indicate a positive correlation between the DII and depressive symptoms in participants with MS, which persists even after adjusting for covariates. This association remains consistent across various subgroups, including age, smoking status, race, PIR, diabetes, hypertension, abdominal obesity, educational level, and marital status. RCS analysis further illustrates the nonlinear relationship between the DII and depressive symptoms in participants with MS. This correlation remained significant both before and after adjustment for covariates. The RCS curve suggests that higher DII levels are associated with an increased likelihood of depressive symptoms, implying that dietary-induced inflammation may serve as a potential predictor for the onset of depressive symptoms in participants with MS.

Multiple studies have highlighted a significant association between MS and depressive symptoms, contributing to a substantial economic burden (19). Existing research suggests that inflammation serves as a primary mechanism for the comorbidity of MS and depressive symptoms, particularly in relation to the activation of inflammatory factors such as IL-6 (20). In the context of obesity, the increase in white adipose tissue leads to a state of low-grade inflammation. This inflammatory state is a consequence of the accumulation of various inflammatory cells, including macrophages, CD8+ T cells, IL-17 producing CD4+ T cells, and natural killer cells (21). Furthermore, levels of adiponectin (an anti-inflammatory mediator) are decreased in obesity and increased with caloric restriction (22).

In research on MS, various dietary patterns have been investigated, including the Mediterranean diet, plant-based diets, and anti-inflammatory diets. Key components of these dietary patterns significantly influence processes such as glucose metabolism and insulin sensitivity, which are closely linked to the development and progression of MS. Furthermore, many neurotransmitters that regulate mood and behavior are synthesized from dietary precursors, making adequate intake of proteins, vitamins, and minerals essential for the production and function of these neurotransmitters (23). Higher inflammatory dietary patterns have been associated with an increased risk of type 2 diabetes, obesity, MS, depressive symptoms, and certain cancers, with this phenomenon being more pronounced in older adults (24). Cross-sectional studies have shown that a legume-nut dietary pattern is linked to an increased risk of MS, particularly among individuals with irregular physical activity (25). In contrast, a “vegetarian” dietary pattern has been shown to improve depressive symptom scores, psychosocial functioning, quality of life, body mass index, cholesterol levels, low-density lipoprotein, and diastolic blood pressure in participants with depressive disorders related to MS (26).

As far as we know, no previous studies have explored the relationship between the DII and depressive symptoms in participants with MS. To address this research gap, our study aimed to investigate the association between depressive symptoms in participants with MS and DII. The results suggest that higher DII scores are positively correlated with an increased likelihood of depressive symptoms in these participants. Subgroup analysis revealed no significant differences among Hispanic groups, which may be related to variations in dietary patterns. Previous studies have indicated that traditional Hispanics consume more fruits than non-Hispanic Blacks and non-Hispanic Whites (27). Since fruits are rich in anti-inflammatory compounds, this dietary habit may help mitigate the risk of depressive symptoms in participants with MS. Interestingly, the impact of the DII on depressive symptoms was more pronounced in men, which contrasts with earlier studies suggesting that women are more likely to experience depressive symptoms associated with MS (28). While our study also found a higher odds of depressive symptoms in females, this may be due to the predominance of men in the study population used for DII assessment. Research has shown that women often cope more effectively with lifestyle disruptions and tend to make more restrictive food choices, whereas men are more likely to follow dietary patterns associated with inflammation (29). Furthermore, studies have suggested that emotional disorders such as depressive symptoms and anxiety are significant risk factors for the development of MS in men, while in women, age and marital status are often more prominent risk factors (30). The primary focus of this study is MS, and given that men tend to exhibit more emotional disorders, gender differences in the prevalence of depressive symptoms are noteworthy. Several studies have shown that male gender is more strongly associated with depressive symptoms in individuals with MS, reinforcing the findings from our study (31, 32).

However, further experimental research is necessary to validate this hypothesis. Our study has several limitations. First, due to the inherent constraints of cross-sectional study design, we are unable to establish a causal relationship between the DII and depressive symptoms in individuals with MS. Therefore, large-scale prospective studies are needed to better understand the temporal relationship between these variables. Second, dietary data were based on self-reported recall from questionnaires, which may introduce potential biases. Although we averaged data from two separate sessions to minimize error, reliance on self-reported dietary intake inevitably introduces recall bias. Additionally, missing dietary data may lead to an underestimation of the DII score. Finally, since our study sample was limited to U.S. adults, the generalizability of our findings to other populations may be limited.

This study identified a positive correlation between higher consumption of pro-inflammatory diets and an increased likelihood of depressive symptoms in individuals with MS. These findings suggest that dietary interventions designed to reduce the DII may help mitigate inflammation and lower the odds of depressive symptoms. However, MS with comorbid depressive symptoms is a multifaceted condition influenced by numerous factors. Therefore, a comprehensive and personalized approach to treatment is necessary to address the underlying complexities of this comorbidity.