Article: Chiadi U. Onyike, Rosa M. Crum, Hochang B. Lee, Constantine G. Lyketsos, and William W. Eaton. (2003). Is Obesity Associated with Major Depression? Results from the Third National Health and Nutrition Examination Survey. American Journal of Epidemiology, 158, 1139-1147.
I will start looking at obesity and depression in children by looking at the topic in adults. The figure reports rates of depression in the past month for men and women, as a function of BMI. The data come from 9,997 respondents to the National Health and Nutrition Examination Survey (NHANES), an interview survey of the US population. Becoming heavy and becoming depressed. Depression was assessed using the Diagnostic Interview Schedule, based on the criteria for major depression outlined in the DSM-III. In this figure I plot depression rates people who were “underweight” (BMI < 18.5), “normal weight” (BMI 18.5–24.9), “overweight” (BMI 25.0–29.9), “obesity class 1” (BMI 30.0–34.9), “obesity class 2” (BMI 35.0–39.9), and “obesity class 3” (BMI ≥ 40). The latter three classes were created by the authors.
The key finding is that obesity is associated with increased rates of depression. However, being overweight or even mildly obese is not associated with substantially increased rate of depression, it is the severely obese (BMI ≥ 40) who suffer markedly higher rates. As one always finds, women are more likely to be depressed at all levels of BMI. There is also a suggestion that the obesity effect on depression on depression starts for women at a milder rate of obesity, but the authors did not find this difference to be statistically significant.
This study provides no evidence about what causes what. However, it seems very likely that causation runs in both directions here. On the one hand, depression can cause obesity, if only because depressed people may be less able to engage in weight control programs. On the other hand, severe obesity can cause depression, because severe obesity is physically and socially stressful.
This is the NHANES survey from 1988-1994, so the data are now at least 15 years old. The severely obese were a relatively small group in that era (2.7% of the NHANES respondents). If the same relationship holds, and then the the distribution of the population would have shifted and a greater proportion of the population will have shifted to the right hand side of the graph, then there will be more adults in the population with obesity-caused depression, and the public health importance of obesity as a factor in depression will have increased. One might also argue that the data suggest that there is little need to worry about mild obesity and depression. I think this misses the dynamics of obesity. No one is born at BMI ≥ 40. You get there through a developmental process that includes a point when you have BMI = 35. There is nothing to be said in favor of mild obesity, and preventing that is a wholly effective way of preventing severe obesity.