Main findings, interpretations and comparisons with previous studies
In this study, data from a large national survey among adults were used to examine the association between MDE and AUD and the effects of socio-economic factors on this association. In agreement with several studies [5, 19, 20, 4, 21–23] our study found a significant association between AUD and depressive disorder. Such significant and positive relationship could be explained in terms of shared common genetic and environmental factors in the comorbidity of alcohol-use disorder and depressive disorder, as investigated in other studies [24–26]. It could also be explained in terms of causality where depressed persons may turn to alcohol as a self-medication for their symptoms and develop AUD afterwards [27]. However, confirming this explanation is beyond the scope of this study.
Also in agreement with other studies [4], the association between MDE and AUD differed across different levels of socio-economic status. Our results show that the association of MDE with harmful-dependent drinking was smallest among people in the middle socio-economic tercile but similar between the lowest and highest terciles. Inequalities in the prevalence of AUD and MDE between levels of wealth index, education, employment and urban vs. rural populations were also found, which supports studies investigating social inequalities in mental health conditions within a country [28, 7, 15, 29].
The reason that the strength of association between MDE and AUD among those in the lowest and highest socio-economic classes were higher than that in the middle class is unclear. On the one hand, it could be explained in terms of social difficulties where people of the lower class often face the most difficulties, which enhance susceptibility to negative health outcomes when exposed to risk factors [30, 8, 31]. The higher prevalence of both MDE and AUD among those in the lowest SES class could lend support to this finding. On the other hand, people in the highest SES class may have more means to reach out to alcohol or other drugs for self-medication when they get depressed. This thus explains the strong association between the two conditions. However, chance cannot be ruled out as an explanation for the observed findings due to the low statistical power as both AUD and MDE are rare in this general population study. Furthermore, the classification of wealth index derived from the PCA might not differentiate people well, thus spurious differences could also be the case [7].
For both groups of AUD, the significant and strong associations with MDE were found only among those completing secondary school education or higher but not among those with primary school education. The reason for this may be similar to the above explanation for the high socio-economic class and the finding further supports the impact of SES on the relationship between the two disorders. Education is known to causally influence health through mechanisms such as creating greater sense of control, better working conditions, increased social capital and improved health behaviors [32]. If it is true that highly educated people turn to alcohol when they are depressed to alleviate their dysphoria, it is possible that a depressive condition deforms their sense of control, leading to such poor health behaviour and education does not protect against this outcome. This finding highlights the importance of comprehensive assessment of the co-occurrence of depressive disorder and alcohol-use disorder and providing appropriate treatment and care in all individuals regardless of their education level.
No modifying effect of employment status was found on the association between AUD and MDE. The inconsistency of the findings by three SES indicators warrants further study to explicate mechanisms underlying the socio-economic inequality in the relationship between AUD and MDE. As mentioned in other studies, socio-economic inequalities in health may vary depending on the indicators used to measure SES and no single indicator can provide a full picture of SES of the population [4, 33].
Living in a rural area was found to be protective against harmful-dependent drinking in a univariate analysis and a stronger association was found between harmful-dependent drinking and MDE for people living in urban areas compared to those living in rural areas. Evidence on the relationship between urbanicity with depression and alcohol use varies in the literature [34, 30, 35]. In Thailand, people living in rural areas often have close ties and shared social activities. The poor in these rural areas also live better lives than do those in the urban areas as they have more ready access to food and green space in their surroundings. People living in urban areas can face a lot of hardships relative to urban living, for example, faster pace, higher crime rates, more crowded environment, limited green space and higher levels of pollution [36]. Such urban environments may enhance susceptibility to negative health outcomes when exposed to risk factors and increase the risk of poor mental health [37].
Depressive disorders and alcohol use disorders are common public health problems in Thailand [38] and priority conditions identified in the WHO Mental Health Gap Action Programme (mhGAP) [39]. The present findings should be considered in terms of the social context of Thailand where income inequality is in the middle range (GINI coefficient ranges between 37.50 and 39.40 since 2010) [40]. Our findings among the general Thai population suggest that mental health care and promotion would not help to improve the mental health of Thai people to its highest limit should concurrent efforts to reduce social inequalities not be implemented.
Study strengths and limitations
The main strength of this study is the large sample size and probability sampling method making it representative of the general population of the whole country. Furthermore, the possible role of confounding factors such as socio-demographic factors and the presence of chronic medical diseases was taken into account in the analyses. However, there are some limitations which deserve mention. A cross-sectional study is a useful design for obtaining the prevalence of AUD and MDE among the general population, but it cannot establish a causal relationship. The direction of causality is indeterminate; AUD and depression could be reciprocally related to each other by a feedback loop in which drinking increases the risk of depression and the depression leads to an increased consumption of alcohol and related problems [41]. In our study, AUD and MDE were self-reported and the questionnaire used for measuring both conditions in our study was a screening instrument (not a diagnostic instrument), assessing symptoms that occurred in the past 12 months, which may not fit the full criteria of major depressive or alcohol-use disorder. Therefore, our results have limited comparability with studies that used diagnostic measures. Finally, due to unavailability of data, our study did not take into account other potential confounding factors, e.g. stressful life events and personality profile, which may possibly be associated with both AUD and MDE.