In recent decades, the association between obesity and depression has been examined in a considerable number of studies 12. Both conditions are associated with increased risk of disability, reduced quality of life, increased mortality and co morbid conditions such as cancer, diabetes and coronary heart disease. The prevalence of both obesity and depression is very high, and both are associated with an enormous individual burden and huge economic costs 3.

Weight gain is for the most part influenced by decreased physical activity and increased intake of calorie-dense food. The development of obesity depends on genetic, metabolic and environmental factors 14. Depression is caused by a combination of biological, psychological and social factors, and most researchers support vulnerability-stress models. In these models the development of a depressive disorder is triggered when a vulnerable (psychological and/or biological) individual experiences a life event or severe stress 56789.

The exact underlying mechanism for the relationship between depression and obesity is not clear. Depression may cause obesity, for example through changing eating patterns or reduced physical activity 101112. But it is also possible that obesity may cause depression, for example through the negative body image which is the result of obesity 13. Depression and obesity may also be caused by a third underlying factor. Socio demographic factors may moderate the association between depression and Body Mass Index (BMI, weight in kilograms divided by height in meters squared) 14.

Before causal pathways can be explored further, it is necessary to establish the exact pattern in which depression and obesity are associated with each other. Until now, three hypotheses have been suggested: a positive association between depression and obesity (higher depression is associated with more obesity) 151617, a negative association (higher depression is associated with lower obesity), and no association 181920.

From a psychiatric point of view, however, it could be expected that both overweight and underweight are associated with depression. According to the DSM-IV 21, in which the diagnoses of mental disorders are described, eating problems (eating too much or eating too little) and changed physical activity (increased or decreased) both constitute core symptoms of a major depressive disorder. Even though an increase in food intake might be more likely than a decrease in food intake, there is not much evidence to support this assumption. Furthermore, even though a decrease in food intake might occur less frequently, it might still affect large numbers of people since depression is such a prevalent disorder. Based on the assumption that food intake might decrease as well as increase, one would not expect a linear association between depression and obesity, but a U-curved association in which people with underweight and overweight report more depressive symptoms, compared to people with a normal weight. Although this seems an obvious association, we could find only few studies in which the existence of such a U-curve was tested 2223. The first study 22 only found a U-curve for the unadjusted data. The second study 23 found a U-curve when comparing normal weight, overweight and obese category, but they did not include an underweight category. Therefore, we decided to examine in a large population based sample whether we could find evidence for the existence of such a U-curve.

Of the total sample (44,374), 4,574 (10.3%) persons were considered obese, 15,230 (34.3%) were overweight, 22,660 (51.1%) had a normal weight and a total of 800 (1.8%) had underweight, while 1,100 (2.5%) had missing values. The mean MHI score of the total sample was 21.2 (SD: 15.4; Range: 100), 10,349 (23.3%) persons had missing values.

Table 1 shows the distribution of the socio demographic variables (gender, age, ethnicity, level of education, year of the study) and their association with BMI. As expected, the χ² analysis for depression and the socio demographic variables showed all significant (p < 0.05) associations.

The goal of this study was to explore if there is a U-shaped trend in the association between BMI and depression. In this community based sample of 43,534 participants we indeed found evidence of such a positive U-curve. Both BMI categories and BMI continuous BMI are nonlinear (U-shaped) associated with depression. We demonstrated that both obesity and underweight are associated with an increased level of depression, even after controlling for various socio demographic variables. Furthermore we found a difference in the association between men and women.

The results of this study emphasize the importance of distinguishing between (the four) different BMI categories when we investigate the nature of the association with depression. The underweight population should be examined as a distinct category because there could be a higher level of depression.

For example, in a previous study that focused on the association between obesity and depression, comparisons of depression levels were made between a sample of obese and a sample of non-obese subjects which included the underweight sample 32. Our findings lead us to conclude that if one compares the levels of depression between the obese and non obese groups in this way, the results might be less significant because of the risks of high levels of depression in the underweight group. According to the DSM-IV, depression is associated with both increased and decreased food intake, and increased or decreased physical activity 21. Therefore it seems logical that increased levels of depression are associated with obesity and also with underweight. It is notable that underweight was much less prevalent then overweight and obesity. A possible explanation might be that that decreased food intake is less prevalent then increased food intake as a symptom of depression. However these prevalences are not related to the existence of a U-curve and therefore do not affect our results.

Most studies focus on linear- (positive, negative) or no trends in the association between obesity and depression. Those studies investigate whether depression increases or decreases at higher levels of BMI 2.

There are studies that put forward the fact that a subset of the depressed is actually losing weight as a possible reason why the association between obesity and depression is not found in many studies 13. Our study shows that this is indeed a plausible consideration.

The first strength of our study is that it contains a large sample of participants (44,374) and a broad variety of socio demographic variables, which we could use as covariates. The second strength is that the sample is random conducted in the Netherlands, which makes the results generalizable for the general population.

There were also several limitations. For the assessment of depression symptoms we used, the MHI. It is a valid instrument to measure depressive symptoms 2728, but can not be used as a diagnostic instrument for depression. Furthermore, there were a considerable number of missing values on the MHI (23.3%) and BMI (2.5%) score which might have influenced the results. Especially if respondents suffering from depressive symptoms did not report the MHI score and people with underweight or obesity did not report their BMI score could that have caused bias in the results of this study. For the assessment of BMI we used self reported data. People tend to underestimate their weight and overestimate their height, this could jeopardize the validity of the results 3334. Furthermore the sample had a considerable amount of non-response (35–40% each year). This could possibly cause bias in the generalizability of the results when for instance the non response is larger among respondents with obesity, underweight or depression. Like many other studies concerning the association between BMI and depression, this study is cross-sectional. Therefore we can't explore the onset and causality and the reciprocal effects in the association between depression and BMI. Longitudinal studies are needed to examine the course of the depression and possible effect on BMI. Depression is known to appear in different episodes during a life span, so we need data on life time prevalence to study the association.

We conclude that our findings clarify the nature of the association between BMI and depression. We found a U-shaped trend in the association. Longitudinal and experimental studies are needed to explore possible explanations of the relationship and the direction of causal relationships between BMI and depression.

The authors declare that they have no competing interests.

LdW wrote the first draft and the revisions of the manuscript. LdW and MvH conducted the analyses. AvS, PC and BP critically read each draft and contributed to the further drafting and revisions of the manuscript.