In a cross-sectional study we found evidence for an association between physical activity and depressive symptoms in adolescents; an increase in physical activity of about 1 h a week was associated with an 8% decrease in the odds of depressive symptoms in both boys and girls. There was no statistically significant association between physical activity and depressive symptoms in a longitudinal analysis, although the direction of effect was the same.
This study is novel in that no previous study on adolescents has examined the question of direction of causality. Although it is impossible to prove a causal association with observational data, the Bradford Hill criteria provide a framework for making an overall judgement about whether a causal association is likely [34]. One important criterion is temporal sequence of association. The design of RELACHS provided an ideal opportunity to look at temporal sequence of association and found no evidence for reverse causality. A further criterion for causality outlined by Bradford Hill was evidence of a dose response relationship. The univariable analysis for the full categorical variable suggests that a dose response relationship exists, although some caution must be exercised as the confidence intervals overlap.
The composition of the sample was broadly similar to the school population for the three boroughs in terms of ethnicity and eligibility for free school meals. It is unclear how far the results would be generalisable to adolescents outside East London. A response rate of 84% is reasonable and a number of strategies were used to encourage reliable response for an adolescent sample. The overall follow-up was 75% and there was no difference in follow-up by level of physical activity. However, certain groups were less likely to be followed-up. These were: adolescents who were depressed at baseline, girls, those eligible for free school meals and white pupils. This is most likely to be due to these groups being absent from school on the day that the questionnaire was administered or because they had left the school between baseline and follow-up. Patterns of follow-up may have had an impact on the associations found. Those eligible for free school meals were less likely to undertake regular physical activity. Girls were more likely to have depression and were less likely to undertake regular physical activity; this may have contributed to an underestimation of the strength of association between physical activity and depression at follow-up.
All of the measures, with the exception of height and weight, were self-reported and there are no objective measures with which to compare the pupils’ answers. However, for the main outcome, the SMFQ is a well accepted instrument. It has been found to be a reliable measure of a core depression construct in adolescents to be used in epidemiological studies [35]. Regarding the main exposure variable, it has been questioned by some commentators whether self-report data is an adequate measure of physical activity. It has been suggested that self-reported physical activity is only weakly correlated with actual energy expenditure [36, 37]. Pupils with depressive symptoms may be less likely to recall periods of physical activity than those that are not depressed due to a tendency to underestimate their level of activity/achievement. As students get older, it may be less fashionable to be active; physical activity at follow-up may therefore be underestimated. This may partly account for the failure to find a longitudinal association between physical activity and depressive symptoms. Another important limitation in relation to the physical activity variable is that it measures physical activity outside school; activity taken within the school, during physical education classes and break times is not taken into account. Although physical activity classes taken within the school are probably broadly equal across all participants, intensity may have varied considerably amongst pupils. Some pupils are also likely to be more active than others during break times.
The study may suffer from a lack of power, particularly in the longitudinal analysis. An association between physical activity and depression was found cross-sectionally for boys and girls. A lack of evidence for an association in the longitudinal analysis does not mean that there is not a true association. As discussed above, the self-report measure may not adequately reflect actual energy expenditure. The way in which the variable for change in physical activity was created was approximate and leaves room for error. The combination of these weaknesses may have led to a lack of power to detect the potential longitudinal association between depressive symptoms and physical activity.
A weakness of previous studies has been a failure to control for a number of potentially important confounders including chronic illness and general health measures. This study was able to adjust for both as well as for health behaviours. However, there is still a possibility of residual confounding.
The results of the cross-sectional analysis are in line with previous work in finding an association between depressive symptoms and physical activity [12–20]. Few studies have stratified the analysis by gender. Allison et al. found no evidence for a gender interaction, contrary to their expectations. However, their response rate was low and they do not provide information on the degree of missing data [13]. Brown et al. looked at the odds of suicidal behaviour separately for boys and girls and found a stronger association for boys than for girls [38]. These results closely mirror those reported here.
Only three longitudinal studies have been undertaken to examine the association between physical activity and depression. A Norwegian study found an association between hours of physical activity a week at age 15-16 and depression 3 years later for boys but not for girls [10]. A longitudinal study in Germany found that subjects with regular physical activity at baseline had a lower overall incidence of mental disorder [22]. They found no association between physical activity and major depression. Their findings may be due to a lack of power once the analysis was broken down by specific disorder. Motl et al.’s study in the US found an overall association between a change in the frequency of physical activity and change in depression [21]. No longitudinal association was found in the analyses reported here. However, this may be due to lack of power; the direction of effect is the same as in the cross-sectional analyses.
Further research is needed to establish causality. The most direct test would be an experimental design. However, a problem with these studies is that they use selected samples. Two groups are always underrepresented: those who already exercise heavily and those who are persistently sedentary and do not participate, or drop out. Working within the framework of cohort studies may therefore be the best strategy. A key priority is to be able to look at the relationship between physical activity and depressive symptoms within a cohort with sufficient numbers of participants to detect potential associations. The study reported here was limited by small numbers once the sample was stratified by sex. One way to proceed would be to press for items on physical activity to be included consistently in large longitudinal studies such as the Millennium Cohort Study; the children in this sample are currently 10 years old. Efforts are needed to develop robust items for recording physical activity, with objective as well as self-report measures if possible. Regarding the main outcome, the SMFQ is a well accepted self-report instrument for measuring the presence of depressive symptoms in adolescents [35]. However, future research might consider using a measure involving a structured interview.
Further research is needed into the mechanisms linking physical activity and depressive symptoms. The findings suggest that the association cannot be explained by social class differences within this population; it would therefore be interesting to explore whether any of the major psychosocial hypotheses for the link are supported. The absence of a firm mechanism linking physical activity and depression may be a factor in explaining why physical activity has not been more universally taken up as a means of preventing or treating depression in the UK, although in some countries (for example, Belgium) it has been accepted and established in delivery systems [5]. There is an urgent need to collaborate with medical and neurobiological scientists to explore these mechanisms.
