In the present study we found no evidence of marked differences in either habitual physical activity or sedentary behaviour between SES groups. Thus, our observations do not support the hypotheses that more deprived children are less physically active or more sedentary than more affluent children, despite marked differences in obesity risk associated with SES.2,4,5
Our failure to observe differences in physical activity and sedentary behaviour in the predicted direction could relate to lack of power, but with the paired design, study 2 was powered to detect quite small differences in engagement in total physical activity (100 cpm between groups)—differences that may be too small to be biologically meaningful.12,14,15,17
Study 1 was one of the largest studies to use objective methods to measure physical activity in preschool children to date.
The present study suggests that social patterning of childhood obesity in the UK may be due to socioeconomic differences in dietary intake rather than differences in habitual physical activity or sedentary behaviour. This perspective on the aetiology of obesity is reductionist as it considers the origin of obesity purely from the point of view of energy balance, the difference between energy input and output. We accept that the aetiology of obesity is more complex than this in reality, and that research which considers the many possible behavioural or biological determinants of socioeconomic differences in obesity will be helpful in providing an improved understanding of its social patterning in future.
At present very few studies have reported on objectively
measured physical activity and sedentary behaviour in young children. The determinants of physical activity in childhood remain unclear,18,19
and SES has not emerged strongly as a predictor of physical activity in the literature.19
Predictors of objectively measured sedentary behaviour in young children are even less well understood since this topic has been little researched, only emerging in the literature as an important variable relatively recently.8,15
The cut‐offs we applied to accelerometry output to determine time spent in different intensities of activity have been validated against direct observation of behaviour and energy expended on activity in children in independent studies,15,16
and so should have provided accurate measurement of time spent sedentary and in MVPA. However, it is possible that subtle differences in the types
of activity undertaken by children in different socioeconomic groups, not measured by accelerometry, might be important. It is also possible that habits associated with sedentary behaviour (such as eating/snacking while watching TV)20
might have differed between the socioeconomic groups, but these would not have been identifiable using accelerometry. Short sleep duration has recently been suggested as a possible risk factor for childhood obesity,21
and it is conceivable that this might differ between socioeconomic groups, but we did not set out to measure sleep duration precisely in the present study, or to power our study to detect differences in sleep duration between groups.
In the present study we considered habitual physical activity and sedentary behaviour as separate constructs, as is now the norm.8,15
However, we accept that in circumstances where one form of behaviour (in this case sedentary behaviour) predominates, this may displace opportunities for other forms of behaviour such as moderate–vigorous physical activity.14,17,22
It is possible that there are differences in habitual physical activity between ethnic groups in the UK, or that relations between physical activity and adiposity might differ between ethnic groups, or between boys and girls. Children who participated in the present studies were almost entirely from the majority ethnic group in Scotland, and so we could not address the issue of ethnicity. We found evidence that social patterning of sedentary behaviour might differ between boys and girls in study 2, but this needs to be confirmed in future studies in other samples and settings. One recent study found a possible gender difference in the relation between physical activity and adiposity in 7 year old British children.22
In the present studies, we used a uniaxial accelerometer (designed to measure activity predominantly in the vertical plane). While in theory measurement in two or three planes of movement might provide greater accuracy, empirical studies comparing uniaxial versus biaxial or triaxial accelerometry do not support this.23,24
We summarised accelerometry output in one minute measurement intervals (epochs). In theory, shorter epochs might provide more accurate quantification of more vigorous activities, but again empirical tests do not support this hypothesis, and use of shorter measurement intervals does not provide an advantage in practice.25
The main practical impact of this approach to the interpretation of accelerometry output appears to be a small systematic misclassification of some vigorous activity as moderate intensity activity;25
this is one reason why we summarised both categories in combination.
The results of the present studies are not consistent with the hypothesis that differences in physical activity or sedentary behaviour underlie socioeconomic variations in obesity risk in early childhood. The present study leads to the prediction that socioeconomic differences in dietary intake underlie the social patterning of obesity in young British children.
What is already known on this topic
- Children from more socioeconomically deprived families are at much greater risk of obesity than those from wealthier families
- The reasons for this social patterning in childhood obesity are unclear, but must lie in socioeconomic differences in physical activity, dietary intake, or both
What this study adds
- Habitual physical activity is not influenced by socioeconomic status
- Socioeconomic differences in obesity risk are likely to be due to social patterning of dietary intake