Depression and anxiety are prevalent mental disorders affecting a substantial number of individuals. Depression and anxiety have emerged as the foremost mental health challenges globally, impacting an estimated 322 million individuals with depression and 264 million with anxiety (WHO, 2017). Depression is the leading contributor to global disability, accounting for 7.5 % of disability-adjusted life years. In contrast, anxiety disorders occupy the sixth position, contributing to 3.4 % of this measure (WHO, 2017). Moreover, individuals with depression or anxiety disorders face significant challenges throughout their lives because these conditions may cause diseases associated with aging with several potential pathways, leading to substantial personal and medical costs (Chisholm et al., 2016; Révész et al., 2016; Verhoeven et al., 2015). Unfortunately, traditional pharmacological treatments and psychotherapies offer limited effectiveness in addressing these conditions (Sinyor et al., 2010). Consequently, preventive strategies are urgently needed to target modifiable factors, emphasizing the necessity for further exploration and research in this domain.

The dietary inflammatory index (DII) is a metric derived from the existing literature and population-based data. Its purpose is to assess and measure the collective impact of diet on the potential for inducing inflammation. It considers the individual inflammatory effects of up to 45 food parameters (Shivappa et al., 2014a). As an assessment tool for evaluating the inflammatory properties of dietary patterns, the DII has become one of the most widely used instruments. It has been included in several studies over the past decade to investigate the association between anti-inflammatory or pro-inflammatory diets and various health outcomes (Wirth et al., 2014). Past research has indicated a possible association between DII and numerous noncommunicable diseases, including cardiovascular disease and cancer (Ruiz-Canela et al., 2015; Zhu et al., 2020). However, the relationship between DII and depression and anxiety is still unclear, requiring further exploration to validate the findings.

Previous studies have investigated the association between diets characterized by inflammation and the likelihood of experiencing depression (Shivappa et al., 2018). However, these studies have produced ambiguous results due to variations in factors such as patient inclusion criteria, geographic regions, gender distribution, and methods used to calculate inflammatory index. For instance, a prospective study conducted among a French male population revealed a significant association between a pro-inflammatory diet and the risk of depression (Adjibade et al., 2017). Conversely, no significant association was observed in the female population. Similarly, a cross-sectional study of the Irish population reported that higher energy-adjusted DII (E-DII) values were associated with a greater risk of depression in women. In contrast, no similar association was found in men (Phillips et al., 2018). Contrasting outcomes in terms of sex-related differences were also observed in two recent meta-analyses exploring the association between pro-inflammatory diets and depression (Shakya et al., 2021). Moreover, the generalizability of many of these studies was limited, as they primarily focused on specific groups, such as middle-aged women, older adults, female nurses, and university graduates, rather than the general population (Lucas et al., 2014; Sánchez-Villegas et al., 2015; Shivappa et al., 2016; Vermeulen et al., 2018).

Moreover, a cross-sectional investigation conducted within a Korean population showed an association between elevated E-DII and elevated likelihood of experiencing depression and its associated symptoms, shedding light on significant geographical variations (Shin et al., 2020). These regional disparities in the relationship between E-DII and depression may arise from variations in the levels of dietary inflammation prevailing in these regions. Notably, only one study explored the association between the DII and the recurrence of depressive symptoms within the UK population, but the results were inconclusive(Akbaraly et al., 2016). Similarly, the association between the DII and depression within the UK population remains uncertain. Previous research has produced inconsistent findings regarding the association between dietary inflammation and anxiety. Some studies have reported no significant association between DII scores and anxiety (Bergmans and Malecki, 2017; Ghazizadeh et al., 2020), while others have presented contradictory findings (Phillips et al., 2018; Salari-Moghaddam et al., 2019).

At the same time, recent studies have indicated a close association between inflammation and biological aging, simultaneously, biological aging has been demonstrated as a risk factor for the occurrence of depression and anxiety disorders(Baechle et al., 2023; Gao et al., 2023). However, there is currently no relevant research investigating the mediating role of accelerated biological aging in the relationship between E-DII and depression or anxiety disorders. Therefore, we employed biological age as a mediating factor to explore this relationship.

This study utilized data from a substantial cohort, the UK Biobank, to conduct a thorough and objective assessment of the association between a pro-inflammatory diet and depression as well as anxiety. Our primary goal was to investigate the association between the E-DII and various aspects of depression and anxiety, encompassing both fear-based and other forms of anxiety. Previous studies have indicated that the Dietary Inflammatory Index (DII) may have varying effects on different diseases within different populations(Haghighatdoost et al., 2019; Phillips et al., 2019; Phillips et al., 2018). Therefore, subgroup analyses were conducted to ascertain the consistency of the relationship between E-DII and depression and anxiety across diverse demographic characteristics within the population.