Trends in overweight and obesity by socioeconomic status in Year 6 school children, Australian Capital Territory, 2006–2018

Overweight and obesity in children continues to be a major public health concern worldwide [1, 2]. Overweight children and adolescents are likely to become overweight adults [3, 4]. Excess weight gain during childhood and adolescence is associated with increased risk of numerous noncommunicable diseases (such as cardiovascular diseases, type 2 diabetes and some cancers) throughout the life-course, resulting in a significant economic burden on healthcare systems [2, 5‐8]. In addition to the immediate and long-term adverse health consequences, childhood overweight and obesity also has negative impacts on children’s self-esteem, confidence and academic performance [9, 10].

The prevalence of childhood overweight and obesity has increased substantially since the 1970s and has reached an alarming level in many countries [1, 11]. In 2017–18, almost one in four (25%) Australian children and adolescents aged 5–17 were overweight or obese [12]. Consumption of sugar-sweetened beverages and fast food, inadequate physical activity, and excessive screen time have been identified as risk factors for excess weight gain in children [13]. In developed countries, socioeconomic disadvantage in childhood has been shown to be associated with increased risk of overweight and obesity [14‐18]. The mechanisms through which socioeconomic background influences weight status in children are still unclear [15].

Given the significant public health implications, it is essential to monitor the weight status of children and associated risk factors. Although some recent reports have shown that, in Australia and other developed countries, the weight status of some age groups of children and adolescents are stabilising [1, 2, 19‐22], other studies suggest that the trend may vary by socioeconomic status (SES) [13, 23, 24]. Public health policies and intervention programs designed to change the obesogenic environment and promote lifestyle change in children may have different outcomes across socioeconomic subgroups [25].

Using data from a triennial survey of Year 6 students, the aim of the current study was to examine the trends in prevalence of overweight and obesity among children in the Australian Capital Territory (ACT) between 2006 and 2018. Differences in prevalence and trends across sex and socioeconomic groups, as well as behavioural factors associated with excess weight gain in children (such as physical activity, sedentary behaviour, and consumption of SSD and fast food), are investigated. Few studies of recent trends in weight status of children by socioeconomic status have been reported in Australian populations [23, 26]. A better understanding of the socioeconomic and behavioural factors associated with excess weight gain in children will enable more effective prevention strategies to be developed.

A total of 6729 Year 6 school children were surveyed between 2006 and 2018. Complete data on height and weight measurements were available for 6384 (94.9%) participants; of these, 1139 (17.8%) were classified as overweight and 303 (4.8%) as obese. Table 1 summarises the characteristics of the study population.

Over the 12-year period, the estimated prevalence of overweight and obesity remained stable for girls, from 22.5% (95% CI: 18.6–27.0%) in 2006 to 21.6% (CI: 17.5–26.3%) in 2018 (Fig. 1a). For boys, however, prevalence of overweight and obesity significantly declined from 27.8% (CI: 23.1–33.0%) in 2006 to 17.9% (CI: 15.2–21.0%) in 2018 (Fig. 1a).

Trend analyses by ICSEA showed that the gap between the highest and the lowest socio-economic groups has widened in recent years (Fig. 1b). From 2009 onwards, prevalence of overweight and obesity remained highest in the lowest ICSEA quintile, whereas a significant downward trend was observed in the highest ICSEA quintile (Fig. 1b). For the moderate ICSEA groups (quintiles 2–4), prevalence of overweight and obesity was stable between 2009 and 2018.

There was a slight non-significant increase in the proportion of children reporting doing at least 60 min per day of physical activity for 5–7 days per week (Fig. 2a). The proportion of children spending less than 1 h per day watching TV and using a computer on weekdays also increased significantly, from 16.0% (95% CI: 14.0–18.2%) in 2006 to 38.5% (CI: 35.1–42.1%) in 2018 (Fig. 2b). There were significant increases, too, in the proportions of children reporting consuming less than one sugar-sweetened soft drink (SSD) per week (from 54.3% (CI: 49.8–58.8%) in 2006 to 75.0% (CI: 71.0–78.6%) in 2018, Fig. 2c) and never eating from a fast food outlet (from 11.7% (CI: 9.5–14.4%) in 2006 to 19.5% (CI: 15.3–24.6%) in 2018, Fig. 2d).

Levels of physical activity differed by sex, with 62.0% (95% CI: 59.2–64.8%) of boys reporting doing physical activity for 5–7 days per week, compared to 49.0% (95% CI: 46.3–51.6%) of girls. Boys (30.0%; CI: 28.0–32.1%) were more likely than girls (18.8%; CI: 17.3–20.4%) to meet the physical activity guideline of at least 60 min every day.

Figure 3 shows the prevalence of overweight and obesity by selected demographic and behavioural factors. Prevalence was significantly higher among Indigenous children, those in the lower ICSEA quintiles, and those reporting lower levels of physical activity, higher amounts of screen time and greater fast food and SSD consumption (Fig. 3). The unadjusted and adjusted relative risks for being overweight or obese associated with selected demographic and behavioural risk factors are presented in Table 2. Fast food and SSD consumption, physical activity, and sedentary behaviours were independently associated with overweight and obesity (Table 2). In the model adjusting for sex, Indigenous status, ICSEA quintiles and selected behavioural factors, higher screen time and lower physical activity were associated with a significantly increased risk of being overweight or obese (Table 2). However, after adjusting for other factors, the associations with SSD and fast food consumption were no longer significant, so they were not included in the final multiple regression model (Table 2). This change in statistical significance is likely due to the co-occurrence (correlation) of health-related risk factors in children.

SES gradient, as measured by school-level ICSEA, was highly associated with risk of being overweight or obese (Table 2, Fig. 3). Children in the lowest ICSEA quintile (most deprived) were more likely to be overweight and obese compared to those in the moderate and highest ICSEA quintiles (Table 2). Children in the lower ICSEA quintiles also reported lower physical activity, higher screen time and greater fast food and SSD consumption compared to those in higher ICSEA quintiles (Fig. 4).

There was large variance in the prevalence of overweight and obesity among schools. The multilevel regression analysis showed that the selected demographic and behavioural factors explained the majority (71.2%) of variance across schools.

Based on ACTPANS data for ACT Year 6 children, aged mainly 11–12 years, from 2006 to 2018, prevalence of overweight and obesity remained stable in girls but declined in boys. The reasons for the differences by sex in trends of overweight and obesity in this study are unclear. Boys and girls differ in body composition, patterns of growth, hormone biology, as well as susceptibility to certain social, cultural, and environmental factors [31, 32]. However, studies of gender differences in weight status and lifestyle behaviours at various childhood developmental stages are limited [31, 33, 34]. In the current study, levels of physical activity differed by gender, with a higher proportion of boys (30.0%) meeting the physical activity guidelines compared to girls (18.8%). Differences between boys and girls in levels of physical activity and other lifestyle choices have also been reported previously [33‐35]. It has been suggested that gender differences should be considered in intervention programs designed to promote behavioural change in children [33, 36].

There is also evidence from other recent studies conducted in Australia and other developed countries of a plateau or, even, a decline in prevalence of overweight and obesity in children and adolescents [1, 19‐23, 37]. It has been postulated that these trends reflect a positive effect of public health campaigns designed to prevent excess weight gain in children at the local, state and national level [38, 39]. Since 2012, a range of health promotion programs have been developed and implemented in the ACT to promote physical activity and healthy eating in school children [40]. For example, the Ride or Walk to School program was launched in 2012 to build capacity of schools to actively support and encourage students to ride or walk to school. Additional funding was committed to expand the program and, as of May 2019, 65% (84 of the 128) of primary and high schools in the ACT are participating. Further, the ACT Public School Food and Drink Policy 2015 was implemented to prevent the sale of sugary drinks in ACT government schools [40]. Some non-government schools have also adopted this policy, although it is not mandated.

Analysis of the ACTPANS data indicated a reduction in unhealthy behaviours that contribute to overweight and obesity between 2006 and 2018. For example, concurrent with a slight increase in physical activity, screen time and frequency of fast food and SSD consumption decreased over the same period. Consistent with other studies [13, 41], our findings confirm that physical activity, screen time, and consumption of SSD and fast food are important predictors of childhood weight status. However, because health-related risk behaviours tend to co-occur in children and adolescents [13, 41], the relative importance of individual factors in the development of overweight and obesity is difficult to determine.

In the current study, socioeconomic status, based on school ICSEA, was significantly associated with excess weight in ACT Year 6 school children. The socioeconomic gradient in childhood overweight and obesity has been reported previously [14‐18]. Of greater concern is the widening socioeconomic inequalities in recent years. Since 2006, the estimated prevalence of overweight and obesity has remained high in the lowest SES groups, but a concurrent downward trend was observed in the highest SES group, leading to increasing disparity between SES groups.

The effect of SES is generally understood to be mediated through differences in weight-related behaviours [17, 37, 42, 43]. Compared to those from high SES background, ACT Year 6 children of low SES background reported lower levels of physical activity, higher levels of screen time and greater fast food and SSD consumption. These differences may reflect difficulties experienced by lower socio-economic groups in receiving or actively responding to health promotion messages [23, 25, 37]. This lack of effectiveness in low SES groups, which has the unintended effect of widening inequality [23, 44], suggests that more targeted intervention strategies or regulations are needed. The context in which a child lives involves complex interactions among individual, family, school and community factors. Intervention studies suggest that effective healthy lifestyle promotion in children requires multiple setting approaches, combining education, environmental change and family involvement [15, 45‐47]. Several studies highlight the importance of family influence in promoting healthy life behaviours in children and support the development of health promotion strategies and interventions involving families [43, 47‐50].

In the current study, diet, physical activity and sedentary behaviours were assessed using self-reported questionnaires, which are prone to recall bias. School-level ICSEA was used to approximate student’s SES background because family SES measures, such as parents’ education, occupation and income, at the individual level were not collected in the ACTPANS data. ACT schools have higher ICSEA scores relative to the national average. The ICSEA values for sampled schools ranged from 954 to 1184 with an average of 1095, whereas the national average is 1000; thus, the socioeconomic effect may not be accurately represented in these results. Over time, there was a slight increase in the proportion of children from non-government schools in the sample; however, this has been adjusted for using post-stratification sampling weights. Due to the correlation among weight-related risk factors, the relative importance of each factor is also not well captured. Strengths of our study include the large representative sample, high response rate and measured, rather than self-reported, height and weight. The multilevel modelling method used in the study allows for the adjustment of factors at different levels that may confound the association between weight status and the selected exposures.

Recent trends in childhood overweight and obesity in the ACT are promising and may reflect the positive effect of public health efforts to halt the epidemic rise in overweight and obesity in children. However, prevalence of overweight and obesity remains unacceptably high, especially in those from low socioeconomic backgrounds. Additional prevention efforts are required to achieve substantial improvement and address the socioeconomic disparity.