Adolescence is a critical period, whether for weight problems or for depression. Obesity and depressive disorders in adolescence in particular are risk factors for chronic pathologies in adulthood, the medical and economic consequences of which are central public health issues 1 2 3 4 . Numerous covariates of the association between variation of weight and depression have been described, especially in adolescence, such as age, gender, socioeconomic status of the parents, family structure, level of education, race/ethnicity, body image, 5 6 7 8 9 . A multifaceted approach to explaining child and adolescent variations in weight has suggested that the home environment and socioeconomic status are important considerations 10 6 .

While only longitudinal studies are likely to demonstrate the complex underlying mechanisms linking variation in weight and depression, such as a bidirectional association, with potential moderators such as physical activity, eating patterns, or depression severity 11 12 13 , cross-sectional studies in a large-scale community sample do contribute to describing the characteristics of the association between weight and depression 14 . To date, all kinds of associations have been described at different ages: a positive association between depression and obesity (more severe depression is associated with greater obesity) 15 16 17 18 , a negative association (more severe depression is associated with a lesser degree of obesity) 19 , no association 20 21 . We found few studies in which the existence of a U-curve was tested 22 23 24 . According to Wardle, the case for a relationship between adolescent obesity and depression is not yet proven. The heterogeneity of results in this area could be related to different factors. Firstly adolescence is a period of bodily change, with a particular impact of weight change, probably more complex than in adults. Secondly, covariables envisaged differ from one study to another. And finally heterogeneity in results may also arise from the different methods used to measure depression 4 19 .

Indeed, the variability of criteria to define categories of weight and depression from one study to another render interpretation of results rather difficult. There is also a problem with the use of depression measures that contain an item or items concerning appetite or weight, which could bias results.

To date, there has been no study on this question in any European country on a large sample of adolescents aged 17 and representative of that population. The aim of the present study, in the largest-ever representative sample of French adolescents aged 17 years-old, was i) to determine the prevalence of overweight and obesity, ii) to determine the prevalence of depression, using a validated tool, the Adolescent Depression Rating Scale (ADRS) 25 , which contains no item on weight, appetite, or body image, and iii) to investigate the association between BMI and depression controlling for working and marital status of the parents.

Of the total sample (n = 39542), 49.7% were girls and 50.3% were boys. 92.8% of the parents had a professional activity and 71.2% were living together.

Our results show that the distribution of the sample across BMI categories differed significantly between boys and girls (p < 0.001). The prevalence of « thin » subjects was higher among girls than boys (4.0% versus 2.2%), the prevalence of overweight was higher among boys than girls (12.9% versus 8.7%) while the difference in the prevalence of obesity was not statistically significant (2.1% versus 1.8%) (Table 1). Prevalence of overweight and obesity was higher when the parents were not working (p < 0.001) for both genders. We found a significant difference according to parental matrimonial status for the girls (p = 0.044) but not for the boys (Table 2).

Concerning depression, the mean score on the ADRS was significantly higher for girls compared to boys (p < 0.001). The proportion of depressed girls (ADRS score ≥ 6) (10.4%) was greater than that for boys (4.5%) (p < 0.001, Table 1). The prevalence of depression was significantly higher when parents were not working (p < 0.001), and when parents were separated (p < 0.001) for the two depression groups, sub-threshold and depressed for boys and for girls (Table 3).

Depression and BMI were significantly associated in both the total sample, and in boys and girls separately (P < 0.0001 in all three samples). In addition, the association was not linear and the curve presented a different shape for boys and girls. (Figure 1A and 1B). Among boys, the U-shaped curve demonstrated that both obesity and underweight were associated with an increased level of depression (Figure 1A). Among girls, the curve showed a first part of the curve describing a U shape and then inverted convexity for higher BMI levels (Figure 1B) indicating that overweight adolescent girls were more likely to be depressed than obese adolescent girls.

In order to test whether this inversion of convexity observed in girls with BMI between 25 and 35 was statistically significant, we used a linear regression model with orthogonal polynomials. More precisely, for girls with BMI between 25 and 35, the ADRS score was regressed against BMI expressed with a linear quadratic and cubic component. The inversion of convexity corresponds, from a statistical point of view, to the cubic component, which was statistically significant here with p = 0.003 (the linear and quadratic components are not significant at the 5% level). The association did not alter when working and marital status were controlled for.

Obesity and depressive disorders are central public health issues and the relationship between depression and weight remains unclear 1 2 .

Our study shows that irrespective of gender, the prevalence of overweight and obesity at 17 years of age is respectively 10.8% and 1.9%. Figures in France at age 14 are 7% for overweight and 4% for obesity 30 31 . According to gender, the present results show a prevalence of overweight (12.9% for boys and 8.7% for girls) that is comparable to that found in the international literature, ranging from 5.2% to 28.9% for boys and from 8.1% to 31.0% for girls 19 . As in the literature, we found differences in prevalence according to parental professional status and parental marital status 6 30 31 .

The prevalence of depression in the present study on the basis of the ADRS measure specific to adolescents was 4.5% among boys and 10.4% among girls. In the literature, the data on prevalence of depression varies, in particular according to the type of measure used (measures of intensity, or DSM-IV-based) as well as according to whether or not gender differences are sought. In France, using criteria for depression according to a standardized assessment (CIDI short form), the prevalence of depression for the age of 15 to 24 years old as been reported to range from 6.3% for boys, to 11.2% for girls 32 . A review of recent epidemiological studies showed prevalence rates ranging from 1.8% to 5.9% in the United States 33 34 35 . For the years 2005-2006, the Center for Disease Control found a prevalence of depression between 4% and 6.4% for ages 12 to 17, using the PHQ-9, but without testing for gender differences 36 . Many epidemiological and clinical studies have shown that girls have typically been found to display higher levels of depressive symptoms than boys 37 38 . This has been attributed to genetics, increased prevalence of anxiety disorders in females, biological changes associated with puberty, cognitive predisposition and socio-cultural factors 39 .

The most important result of our study concerns the non-linear and significant association between depression and BMI, with no effect of parental working and marital status. In addition, the shape of the association is different for boys and girls. We have a U- shaped association for boys, which shows that boys that are too thin or too fat have higher levels of depression. For girls, we have a beginning of a U shape, and then an inversely convex curve, which means, first, that depression levels in underweight girls are higher than in normal weight girls, and second, that depression levels are higher in overweight adolescent girls than in obese girls. This can be related to the "fat and jolly" hypothesis, which has been discussed for older women 40 41 42 . Our results recall the study by Wardle et al 19 , who found, in a small sample of adolescents, that regardless of moderators reports of depressive symptoms were more frequent in overweight than in obese adolescent girls of 14, and who found no difference for depressive symptoms between normal weight and obese girls. Using splines, Cortese 5 in a small sample found curves of different shape for boys and a U-shaped curve for girls aged 11 to 14, showing that underweight girls had an equivalent level of depression to that of obese girls, this not being true for the boys. While some previous studies have explored the association between obesity and depression without questioning the gender difference 23 43 44 , according to the recent meta-analysis of community-based studies by de Wit et al 14 there is some support for the hypothesis that there is a difference between males and females in the association.

The "fat and jolly" hypothesis, which we suggest for the adolescent girls of 17 years old, is generally thought to be related to hormone and neuro-endocrine issues, but it could be envisaged as a possible dynamic in adolescent girls. It can indeed be thought that the difficult struggle against excess weight could let up when the subject reaches the obesity stage, as if, once they give up the struggle, they may feel less depressed. In all events, obesity, despite creating negative social experiences in adolescence, does not in itself cause depression. The effect of moderators needs to be explored in the association between weight and depression, and in particular the complex association between weight and the perception of weight, as a risk factor of depression 45 46 .

Several limitations of the study must be considered. BMI was calculated from self-report, with the attendant risk of under-estimation of weight. The degree of divergence between subjective and objective BMI has been described for different ages, different countries and according to gender 47 . However, most studies use self-report data. The underestimation tends to be greater among obese adolescents, so that the prevalence in our study may well be underestimated. Secondly, as our study is cross-sectional, we cannot demonstrate the mechanisms of the association found. Only longitudinal studies are able to explore the different effects of the potential moderators of the association, and investigate the possible predictive relationship between overweight and depression. Indeed, longitudinal studies can provide evidence for depression predicting future obesity or weight gain or the reverse. Published research to date has shown a mixed picture 11 48 49 . Thirdly, we have no data available about race or ethnicity because no studies in France are allowed to ask questions about theses major factors in public health. Moreover, the present data only concerns French nationals from metropolitan France (i.e. excluding overseas territories). However the sample nevertheless represents 5.2% of the French metropolitan population aged 17, and is representative of it.

Overall, since the results are derived in part from the visual examination of regression splines, they should be considered basically as exploratory, and therefore as requiring replication. Despite these limitations, this study provides valuable information on account of the exceptionally large size of the sample. To our knowledge, this is the very first survey worldwide on such a large representative sample of a total population of adolescents. And another important point is that while most studies use depression measures comprising items concerning the body, food, or weight, which are confounders in the exploration of the links between depression and weight, in the present study we used the ADRS measure of depression, which is the only measure that does not contain these confounders.

These results point to the importance of distinguishing between girls and boys when we investigate the nature of the association between depression and the four different BMI categories. It also shows that overweight adolescent girls are at higher risk of depression than the obese adolescent girls.

The authors declare that they have no competing interests.

ARL wrote the first draft and the revisions of the manuscript. MS, CH and CB, BF conducted the statistical analyses. MS, IG, BF, MRM critically read each draft and contributed to the further drafting and revisions of the manuscript. All authors read and approved the final manuscript.