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Depression and anxiety, defined as psychological distress when manifesting as a comorbidity, are common mental health conditions (Arvidsdotter et al., 2016; Choi et al., 2020). Psychological distress can burden the affected individual, in addition to their family and wider society (Othman et al., 2019). This is demonstrated by the inverse relationship which exists between psychological distress and educational attainment, full-time employment, and quality of life (Brenes, 2007). The global increase in psychological distress prevalence disproportionately affects low- and middle-income countries (LMICs; Javed et al., 2021). This burden is largely attributed to the existing treatment gap (of up to 93%) in LMICs, which is caused by inadequate treatment availability and access (Mascayano et al., 2015). For example, Sri Lanka is a LMIC that has experienced natural disasters, civil conflict and political unrest in recent decades (Kathriarachchi et al., 2019). In the context of these, a high proportion (almost 20%) of children aged 13 to 18 years in Sri Lanka are reported to have experienced a psychological disorder (Perera, 2009). Similarly, adults who were exposed to the civil war reported increasing levels of psychological distress for up to 5 years post-conflict (Jayasuriya et al., 2016). Despite the introduction of a decentralised public health system, a significant mental health treatment gap persists (van den Broek et al., 2021). Unique barriers to accessing mental healthcare in Sri Lanka include: a failure to recognise symptoms or available services, social stigma and/or a negative perception to treatment efficacy (Fernando et al., 2017). The existing mental health treatment gap necessitates a review of alternative risk and protective factors for psychological distress that can be targeted for preventive intervention. Socioeconomic status (SES) is strongly associated with psychological distress (Maselko, 2017). Two underlying mechanisms facilitate this relationship. Social causation suggests that mental disorders arise due to the adverse social and financial stress of lower SES (Lund et al., 2010; Wickham et al., 2017). Conversely, social selection finds individuals with mental disorders to have a greater likelihood of drifting into a low SES (Chiu et al., 2017; Saraceno et al., 2005). A recent longitudinal study of SES and depression in China (a LMIC) found these mechanisms to work concurrently (Jin et al., 2020). Modifiable protective factors that mediate the link between low SES and psychological distress can be targeted for preventive interventions to help break the debilitating cycle of low SES and psychological distress (Jin et al., 2020). One possible proactive intervention is dietary diversity. A growing body of research in LMICs links dietary diversity with SES and mental health outcomes, whereby low SES is associated with reduced dietary diversity (Mozaffari et al., 2021; Onyango, 2003) while transitioning from a homogeneous diet to one with a variety of food groups is protective against psychological distress (Poorrezaeian et al., 2015, 2017; Yin et al., 2019). Thus, low dietary diversity may mediate the association between low SES and psychological distress, however, this has not been previously investigated. Although we suggest dietary diversity as a mediator of the relationship between low SES and psychological distress, we recognise the possibility of alternative relationships. For example, psychological distress may be associated with compensatory hyperphagia (carbohydrate craving which is associated with decreased fruit and vegetable consumption; Razolli et al., 2017; Matsungo & Chopera, 2020), and thus mediate the association between low SES and reduced dietary diversity. An alternative mechanism is that SES, low dietary diversity and psychological distress are jointly influenced by a common factor. For example, the civil conflict and consequent economic hardship in Sri Lanka may increase poverty, serve as a source of stress with consequent psychological distress, disrupt food supply which would make a varied diet expensive and inaccessible (Conklin et al., 2016; Siche, 2020). The evidence base for these possibilities is relatively slim, hence the decision to investigate dietary diversity as a mediator of low SES and psychological distress. However, the direction of causal effects may be better tested in future studies. Furthermore, considering that genetic and environmental factors influence individual differences in depressive and anxiety symptoms (Nas et al., 2021), and SES (e.g., through active gene-environment correlations, Scarr & McCartney, 1983; Trzaskowski et al., 2014); it is possible that their covariance and the proposed mediation by dietary diversity are confounded by correlated genetic and environmental influences. In which case, interventions targeting dietary diversity to reduce the risk for psychological distress among those from low socioeconomic backgrounds will not be sufficient. In contrast, the absence of confounding by correlated aetiological (genetic and environmental) influences will further reinforce the utility of targeting dietary diversity for mental health preventive efforts. The present research, thus, aims to deepen the understanding of the interplay between SES, dietary diversity and psychological distress and disentangle overlapping genetic and environmental influences on these relationships. Considering that most mental health studies using genetically sensitive designs are carried out in higher-income countries which may not generalise to LMICs, our findings can serve as a first step in identifying targets for preventive mental health interventions in LMICs to reduce the existing treatment gap. Furthermore, the recent history of civil conflict in Sri Lanka may make our findings generalisable to LMICs with high levels of environmental stress. The present study will test 1) whether SES and dietary diversity are associated with psychological distress, 2) whether the association of SES with psychological distress symptoms is mediated by dietary diversity, 3) whether the direct phenotypic mediation effects are a better explanation for the observed covariances of the latent factors.

This study investigated the genetic and environmental contributions to emotional overeating (EOE) and depressive symptoms, and their covariation, in a Sri-Lankan population, using genetic model-fitting analysis. In total, 3957 twins and singletons in the Colombo Twin and Singleton Study-Phase 2 rated their EOE behaviour and depressive symptoms, which were significantly associated (men: r = 0.11, 95% confidence interval (CI) 0.06–0.16, women: r = 0.12, 95% CI 0.07–0.16). Non-shared environmental factors explained the majority of variance in men (EOE e2 = 87%, 95% CI 78–95%; depressive symptoms e2 = 72%, 95% CI 61–83%) and women (EOE e2 = 76%, 95% CI 68–83%; depressive symptoms e2 = 64%, 95% CI 55–74%). Genetic factors were more important for EOE in women (h2 = 21%, 95% CI 4–32%) than men (h2 = 9%, 95% CI 0–20%). Shared-environmental factors were more important for depressive symptoms in men (c2 = 25%, 95% CI 10–36%) than women (c2 = 9%, 95% CI 0–35%). Non-shared environmental factors explained the overlap between depressive symptoms and EOE in women but not in men. Results differed from high-income populations, highlighting the need for behavioural genetic research in global populations.

Description of measures included in the questionnaire of CoTaSS 2. Table S2. Number of participants for each component and respective response rates. (DOCX 43Â kb)