This investigation of obesity-mental disorder relationships among thirteen countries found statistically significant relationships, adjusted for age, sex and education, between obesity and depressive disorder, and between obesity and anxiety disorder, in pooled analyses across countries. These relationships are concentrated among those with more severe obesity (BMI 35+), though they are significant for the total obesity group (BMI 30+). Subgroup analysis confirmed suggestions from earlier research that sociodemographic variables are important moderators of obesity-mental disorder relationships, with associations between total obesity and both depressive and anxiety disorders occurring in females but not in males. Associations between obesity and anxiety disorders were stronger among younger and older persons. Education had variable effects across depressive and anxiety disorders. No relationship was observed between obesity and alcohol use disorders.
A salient feature of the associations observed in this study between obesity and both depressive and anxiety disorders is that although they are statistically significant, they are modest. Three observations need to be made about this. First, as Simon et al (9
) point out, the small size of the odds ratio may belie relationships of considerable public health significance in countries where the prevalence of obesity and mental disorders is high and therefore where their overlap, even though small, amounts to substantial numbers of the population with obesity attributable to mental disorder, or mental disorder attributable to obesity. Second, given the inconsistency with which prior observations of population level relationships between obesity and mental disorder have been observed, it is all the more remarkable that the current associations should occur in data pooled across this range of countries, diverse in level of development, obesity prevalence, mental disorder prevalence, and size of survey sample. Third, it is clear that the relationship between obesity and mental disorder is stronger among those with more severe obesity. Small sample sizes for a number of the countries precluded our investigating the relationship between severe obesity and mental disorders in demographic subgroups, so the odds ratios reported here for those subgroups are underestimates of the strength of the relationship with mental disorder for those with BMI 35+.
A significant limitation of this study is the fact that height and weight were self-reported which has been found to result in underestimates of the prevalence of obesity (22
), though as noted above, this did not appear to occur in the New Zealand survey. Nonetheless, the degree of underestimation of obesity in the other surveys is unknown, and the effect of depressed mood on estimates on self-reported weight is also unclear. To the extent that weight underestimation is motivated by distress about weight, the associations between obesity and mental disorder reported here could be attenuated relative to their true magnitude in the population. On the other hand, if depressed mood leads to overestimated weight among those with weight concerns, this would have the effect of elevating the true magnitude of the association.
Onyike and colleagues (5
) measured height and weight and observed an odds of past year depression amongst obese persons of 1.4, and an odds of past month depression of 1.9. This latter finding suggests that stronger associations between depression and obese persons may have been obtained in the current study had we used a measure of past-month depression rather than past year, but again the small sample sizes in some of the surveys included precluded this. A further limitation imposed by the small sample size of some of the surveys is that it prevented testing for interactions between population subgroups in the relationship between obesity and mental disorder (e., between age and sex).
A strength of this study is that the estimates are pooled across a large number of consistently conducted surveys. The individual surveys might appear to yield disparate results if examined individually, yet whether or not the country-specific odds ratios are statistically significant is greatly influenced by sample or cell size. More important is the fact that the country-specific odds ratios do not typically differ significantly from each other, allowing confidence in the pooled estimates.
There are two components of these findings that are of particular note. The first is that while depressive disorder has been the focus of prior research on this topic, these results indicate that anxiety disorders, too, are associated with obesity at greater than chance levels. Alcohol use disorders are not related to obesity. It is emotional disorders then, rather than depressive disorders specifically, or mental disorders generally, that appear to have a connection with obesity.
The second finding of note is that this relationship between obesity and emotional disorder is confined to women. This supports the hypothesis of Friedman and Brownell (4
) and the findings of other investigators (5
). It is a contrast, however, to the recent finding from the NCS-R (9
) where no sex difference was found in the associations between obesity and either mood or anxiety disorders. Simon and colleagues suggest that the sex difference sometimes observed in prior research may be a function of differences in statistical power (because fewer males have emotional disorders). The current results do not support that explanation, given that the pooled odds of association between obesity and either depressive or anxiety disorders for males did not exceed 1.0. There are several analytical differences between the Simon et al. study and the current study (use of lifetime versus 12 month disorders, the number and type of disorders included in mental disorder groups, the inclusion or exclusion of those with BMI less than 18.5); at this point it is not possible to be conclusive about the exact source of the discrepancy between results. It is noteworthy though, that of the five studies (including the current study) that have investigated the relationship between diagnosed mental disorders and obesity in general population samples (5
), four have observed a sex difference in the relationship between obesity and mental disorder.
There are a number of possible mechanisms that may explain the relationship between obesity and emotional disorder for women. Women appear to be more troubled by obesity than men, for although the prevalence of obesity is fairly similar across men and women, women are much more likely to present for treatment for obesity (4
). They also experience more stigma in association with obesity (28
). Women are under more pressure to be thin, and experience greater body dissatisfaction (14
); these factors may trigger or maintain obesity through mechanisms such as the paradoxically disinhibiting effects of dietary restraint (33
), or emotional eating (34
). Women are known to be more likely to engage in binge eating unaccompanied by compensatory behaviour (31
). The relationship between obesity and emotional disorders may represent a particularly uncomfortable juncture for some women between the pressures of the “toxic environment” (36
) that fuel the global rise in obesity on the one hand, and the sociocultural pressures that encourage body dissatisfaction and a drive for thinness among women, on the other.
This does not presuppose any particular direction in the relationship between obesity and mental disorder. This study cannot inform on that issue, and the mechanisms cited here can be viewed as pathways from both obesity to emotional disorder (eg, through the effects of stigma (37
), or obesity-related disability (38
)) and from emotional disorder to obesity (eg, through psychologically-mediated disordered eating (27
), the effects of psychotropic medication (41
) or reduced physical activity (6
)). It seems plausible that given the heterogeneous population of the obese, both pathways occur. Moreover, for some individuals the association of obesity and emotional disorder may be a function of other factors altogether, either biological (42
), genetic (7
) or environmental (6
The effects of age were less consistent in this study than the effect of sex. The only other population study investigating age in the association between obesity and DSM mental disorders (9
) found no significant interaction between age and obesity in the odds of either mood or anxiety disorders. However, that study did find that the one age group to show a significantly higher odds of anxiety disorder among the obese was the 60 years and over group with an odds ratio of 1.64 (1.02, 2.64). This is similar to the pooled odds observed in the current study of anxiety disorder among the obese aged 65 years and over of 1.7 (1.2, 2.3). This finding may warrant further research, but it needs to be interpreted in the context of insignificant overall effects of age in the relationship between obesity and anxiety disorders in both the NCS-R study, and for individual countries in the current study where the odds of anxiety disorder among the oldest obese were relatively high (New Zealand, France, United States: data not shown but available on request).
This first cross-national study of the relationship between obesity and mental disorders is suggestive of a modest relationship between obesity (particularly severe obesity) and emotional disorders for women, in the general population, in diverse nations. The study cannot clarify the direction or nature of that relationship, but it may indicate a need for a research and clinical focus on the psychological heterogeneity of the obese population (43