We observed significant positive associations between obesity and a range of mood and anxiety disorders in a nationally representative sample of the US household population. In contrast, substance use disorders were associated with significantly lower risk of obesity. These associations were not explained by confounding due to age, sex, smoking, or comorbid psychiatric disorders. Mood and anxiety disorders each made independent contributions to obesity risk.
The positive associations between obesity and mood or anxiety disorders were generally modest, with odds ratios in the range 1.2 to 1.5. Even these modest associations carry public health significance, though, given the high overall prevalence of obesity (approximately 25%) and mood or anxiety disorders (approximately 25%). The estimated prevalence of lifetime mood disorder in those with BMI below and above 30 translate to a population attributable risk of 24%, that is nearly one quarter of cases of obesity in the general population are attributable to the association with mood disorder. This calculation illustrates the public health importance of the association, but does not indicate a direction for the causal relationship. It is equally correct to state that over one fifth of cases of mood disorder in the general population are attributable to the association with obesity (population attributable risk of 21%). We have no way of distinguishing the direction of the causal relationship between obesity and psychiatric disorders nor the possibility that unmeasured common causes induce an association between them.
The National Comorbidity Survey Replication offers several advantages over previous community surveys used to examine associations between psychiatric disorder and obesity. First, the sample was designed to be an accurate representation of the non-institutionalized population of the 48 contiguous United States. Results can therefore be generalized to this population. Second, the survey assessed a full range of psychiatric and substance use disorders, allowing us to examine associations with anxiety disorders, bipolar disorder, and substance use disorders. Third, mental disorders were assessed using a well-validated structured diagnostic interview, which allowed the association of obesity with these disorders to be assessed with accuracy. Fourth, the assessment considered lifetime diagnoses as well as current state. Assessment of lifetime diagnosis is preferable because an association between weight and psychiatric disorder would be expected to reflect long-term behavioral and/or biological mechanisms.
The associations between obesity and psychiatric disorders in this sample did not vary between men and women. This contrasts with findings in other US national surveys 17-20
that positive associations between obesity and depression exist among women while negative or absent associations exist among men. We are unable to identify any consistent differences in methods between our study and previous studies that would explain why earlier studies found no association between obesity and depression in men while such an association was observed in the NCS-R sample. In one earlier study the magnitude of the odds ratio between obesity and depression in men was similar to that in women, but the lower prevalence of depression in men led to a wider confidence interval and a statistically insignificant result 20
. A similar phenomenon is seen in this sample for both mood and anxiety disorders. Because we observe no statistically significant interaction with sex, we conclude that this discrepancy (significant association in women but not in men) probably reflects differences in statistical power rather than differences in magnitude of the association.
Most previous research has focused on the association between obesity and depression, and various mechanisms have been proposed to explain this relationship 11,12,42
. Some of those mechanisms propose a causal pathway leading from depression to obesity. Increased appetite and weight gain are common symptoms of depression 43,44
, and tendency to gain weight remains stable across depressive episodes 43
. Depression may lead to reduced physical activity 45
, increasing risk for obesity. Depression may increase risk of weight gain through its effect on binge eating 46,47
, especially among women 28,48
. Medications used to manage mood or anxiety disorders may also lead to weight gain 49
. Alternatively, some proposed mechanisms suggest a causal relationship leading from obesity to depression. The stigma attached to obesity (especially for women) may contribute to depression 50,51
. Activity limitations due to obesity or obesity-related chronic illnesses may increase risk of depression by reducing involvement in rewarding or pleasurable activities 30
. Finally, depression and obesity may be linked through some common cause or third factor, either environmental (e.g. childhood abuse 52
) or biological.
We observe interesting variation in the relationship between obesity and mood disorder across sociodemographic groups. The association appeared stronger in younger than older respondents, in non-Hispanic Whites than other racial/ethnic groups, and in respondents with higher educational attainment. Only the interaction with educational attainment, however, was significant at the 5% level. Approximately 40 years ago, The Midtown Manhattan Study in the 1950s 21
also observed that, among women, the association between obesity and depression was confined to those with higher socioeconomic status. In the NCS-R sample, the groups showing the strongest association between obesity and mood disorder were also the groups with the lowest overall rates of obesity. A similar phenomenon has been observed regarding the association between depression and tobacco use; as rates of tobacco use decline, the association between tobacco use and depression grows stronger 53
Our findings are consistent with either direction of causal relationship between obesity and mood or anxiety disorders. If stigmatization of overweight and obesity causes or contributes to mood and anxiety disorders, the effects of stigma might be more powerful in sociodemographic groups with lower obesity rates. Some previous research suggests that self-perception of overweight 54,55
and the perceived stigma associated with obesity 56-59
may both be greater in Caucasians and those with higher income or educational attainment. Alternatively, if mood or anxiety disorders contribute to obesity through an effect on health behaviors (reduced physical activity, increased caloric intake), then this effect could be more easily expressed in sociodemographic groups with lower obesity rates. This cross sectional study does not allow us to distinguish between causal mechanisms or examine how they might differ across sociodemographic groups.
Our findings also do not indicate a specific mechanism or causal direction for the observed negative association between obesity and substance use disorders. While alcohol or other substances may have a direct effect on appetite or caloric intake, a negative association was also observed in those with histories of substance abuse (i.e. lifetime substance use disorder not active in the last 12 months).
We conclude that obesity is meaningfully associated with a range of common mood and anxiety disorders in the general US population. Obesity is associated with a moderately lower risk of substance use disorder. Variation in the obesity-depression relationship by educational level and race/ethnicity suggests an important role for social or cultural factors in mediating or moderating the relationship between obesity and mood disorders. Clarifying the social and cultural influences on the relationship between obesity and mood or anxiety disorders will require additional research in populations with a broader range of race/ethnicity, educational attainment, and income. Clarifying the direction of causal relationships will require alternative research designs, including longitudinal and experimental studies.