This study has shown the emergence of an association between obesity, and emotional and behavioural problems in very young children. At age 3, obesity was associated with conduct problems in boys, while obese girls displayed more prosocial behaviours. However, obesity was associated with a greater range of problems at age 5, suggesting a strengthening of the relationship. At this age, obese boys had more conduct problems, hyperactivity and inattention problems, peer relationship problems, and total difficulties. Increased risk of abnormal scores at this age also captured the clinical, not simply statistical, meaningfulness of these associations. Contrary to our hypothesis, obese girls displayed less psychological impairment as this weight status was only associated with peer relationship problems. Finally, by using the longitudinal nature of the data, we have shown that obesity at age 3 was associated with risk of peer relationship problems in boys at age 5, even after controlling for their early emergence and current weight.
Most previous research in this area has used cross-sectional study designs. In general, while older obese children (from approximately 8 years of age) and adolescents display more impaired well-being compared with normal weight counterparts (11–15), the results of studies investigating younger children are more varied (11, 12, 16, 17). For example, Lawlor et al. (11
) found that overweight was not associated with behavioural problems at age 5, in boys or girls. However, their study combined overweight and obese groups and so did not focus on obese children. While Drukker et al. (12
) also failed to report significant associations between obesity and SDQ scores, analyses were carried out on a combined sample of both boys and girls. Given that we found very few problems in obese girls, aggregating data may have led to reductions in effect sizes. In contrast, other cross-sectional studies have reported evidence of psychological problems (16
). Indeed, our findings most closely resemble those of Sawyer et al. (17
) who used the SDQ in a nationally representative sample of Australian children. Obese 4 to 5-year-old boys had more conduct problems and total difficulties (teacher-reported, but not parent-reported) than normal weight boys. Obesity was also only associated with peer relationship problems in girls, similar to our cross-sectional findings at age 5.
Our observation of a relationship between early obesity and later peer relationship problems in boys contrasts with the null findings of two other longitudinal datasets (11
). These differences in outcome may be related to the instruments used to measure total behavioural problems. By using the SDQ and examining behavioural problems independently (i.e., the subscales of total difficulties) we have explored which of these were (peer problems) and were not (e.g., conduct problems) longitudinally associated with obesity. Additionally, the 9-year gap in one of the studies (11
) may have been too long to establish a prospective association.
Our study has a number of strengths, in addition to reporting on cross-sectional and longitudinal associations. The sample was large and drawn from a contemporary, nationally representative, cohort of young children. Weight and height was measured using trained interviewers and standardised protocols. The analysis adjusted for a wide range of factors, including an indicator of the mental and physical health status of the cohort child's mother. Furthermore, the parent-proxy report version of the SDQ is a valid and reliable instrument for assessing a range of mental health problems in children (30
), and the clinical relevance of our findings is shown through the ability of the SDQ to assess abnormal scores. Nevertheless, it is important to acknowledge that parents completed the SDQ, and have been observed to overestimate other psychological problems in obese children compared with children's self-assessments (7
). Parents of obese children are also more likely to be overweight or obese themselves than parents of normal weight children (31
). Consequently, their own weight history and recollection of childhood may impact on how they parent their own children and how sensitive they are to their children's experiences. As a consequence, triangulation of views from parents, teachers and, at an older age, from children may provide a more complete picture of the psychological morbidities associated with child obesity.
The scale items themselves give clues about the behaviours that mothers are reporting on behalf of their obese sons. Hyperactivity scale items ask about restlessness and distractibility. Conduct scale items include temper tantrums, dishonesty, and bullying other children. The peer problems scale asks about solitary play and being bullied. The overall description is of boys more likely to be seen as unsettled, erratic, physical in some peer relationships, and excluded from others. Some of these qualities may be exaggerations of gendered stereotypes of boy's behaviour and reflect different parenting styles used by their mothers. Others may be behavioural consequences following stigmatisation by peers, parents or even educators. Aggressive coping strategies have been observed in slightly older obese children (33
). Impaired peer relationships in both boys and girls are consistent with observations on victimisation. For example, we have previously shown that obesity is predictive of victimisation in preadolescent children, using data from a different UK longitudinal cohort (32
). These findings therefore emphasize the necessity for carers of young obese children, particularly boys, who are experiencing emotional or behavioural problems, to be extra vigilant of difficult social experiences that may be contributing to these problems.
Further research is needed to examine effect modifiers of the associations found in our study. For example, initiatives within schools to reduce stigmatisation towards obese youth (34
) may help to protect obese boys from friendship problems. Additional attention should be paid to potential mediators, such as low self-confidence, which can result from obesity and can also be a barrier to developing new peer relationships (33
). Relationships in the opposite direction also need examination, to evaluate whether emotional and behavioural problems are risk factors for obesity. Such associations have been reported in adolescents and the idea of a bi-directional relationship between obesity and affective disorder is gaining momentum (35
). Similar complex conceptualisations should not be restricted to older age groups.
Overall, this research is a valuable reminder of how early relationships between obesity and well-being can emerge. If psychological problems are a barrier to treatment uptake and adherence (36
), then action should follow. Accepting this justifies strategies that recognise and reduce psychological distress in obese youth, within both community and clinical settings.