Excess body weight in childhood can lead to negative health conditions such as impaired glucose tolerance, increased CVD risk factor prevalence, asthma, orthopaedic complications and hepatic stenosis (excessively fatty liver), as well as psychosocial problems( Reference Reilly, Methven and McDowell 1 ). Furthermore, an overweight child is more likely to become an overweight adult and thus at increased risk of encountering associated health issues in adulthood( Reference Reilly, Methven and McDowell 1 ). The systematic monitoring of overweight and obesity prevalence in population-based studies of children and adolescents is important, firstly to assess the effectiveness of current public health interventions in addressing this issue and secondly to forecast the future potential burden on health-care systems. Internationally, recent research has indicated some levelling off in the overweight prevalence in girls( Reference Martínez-Vizcaíno, Martínez and Pacheco 2 ) and boys( Reference Sjöberg, Lissner and Albertsson-Wikland 3 ). However, a reduction in overweight prevalence has been noted in girls( Reference Sjöberg, Lissner and Albertsson-Wikland 3 ). A recent review identified that the prevalence of overweight and obesity in Australian children (aged 2–18 years) increased rapidly between 1985 and 1996; however, the rate of increase was not maintained between 1996 and 2007( Reference Olds, Tomkinson and Ferrar 4 ). Rates of change in overweight and obesity prevalence during this time were not significantly different between boys and girls. This levelling off has likewise been noted in Europe, Japan and the USA( Reference Rokholm, Baker and Sørensen 5 ). The aim of the present study was to examine changes in underweight, overweight and obesity prevalence in Western Australian school-aged boys and girls between 2003 and 2008.
Study sample and recruitment
The Child and Adolescent Physical Activity and Nutrition Survey (CAPANS) was undertaken on two occasions (2003 and 2008) and included a cross-section of Western Australian school-aged children and adolescents. The protocol and measurement procedures for the 2008 and 2003 surveys were comparable and full details can be found elsewhere( Reference Martin, Rosenberg and Miller 6 , Reference Hands, Parker and Glasson 7 ). The sample was structured to obtain proportional representation according to the State's general population figures. The target CAPANS study sample size (2880 students) was calculated based on: (i) the ability to detect a change of 5 % in physical activity level in follow-up surveys, (ii) to accommodate intra-cluster correlations and (iii) consideration of class sizes. The study was conducted according to the guidelines laid down in the Declaration of Helsinki and all procedures involving human subjects were approved by the applicable university ethics committees and the Western Australian Department of Education and Training. Written informed consent was obtained from all subjects.
Stratified random sampling was used to select schools. Western Australian schools were divided into the following categories: primary/secondary, metropolitan/non-metropolitan and government/non-government. Schools were proportionally randomly selected from each stratum. In 2008, thirty-four from seventy-four (45·9 %) schools agreed to participate, while in 2003, thirty-six from sixty schools (60·0 %) agreed to participate.
Thirty schools in total participated in the project in 2008 and thirty-six in 2003. In primary schools class groups were selected at random from grades 3, 5 and 7. In secondary schools random class recruitment was not possible, thus the principal or school liaison chose classes to be involved (based on class timetabling) from grades 8, 10 and 11. All students in selected classes were invited to participate in the study with the aim of recruiting approximately twenty-five students from each year group at each school.
Written parental consent was gained for each student to participate, and students were free to withdraw at any time. In 2008, 79·5 % of primary (n 961) and 46·2 % (n 747) of secondary students consented to participate in the study and have their anthropometric measures taken; whereas in 2003 consent rates of 58·8 % for primary (n 876) and 52·1 % for secondary (n 817) students were achieved.
Trained researchers collected data during school visits in school terms 3 and 4 (July to December) in both survey years. Demographic data (sex, birth date, home postcode) and anthropometric measurements were collected from each student. Physical activity and dietary behaviour were also assessed, results are reported elsewhere( Reference Martin, Rosenberg and Miller 6 ).
Height and weight measurement adhered to the protocol of the International Society for the Advancement of Kinanthropometry. Students removed their shoes, socks and any other heavy clothing (e.g. jumpers/sweaters/coats). Every measurement was repeated; if the two measures differed by >0·2 kg for weight or >5 mm for height, additional measurements taken until these criteria were met. Weight scales and stadiometers were checked between school visits to ensure that all scales were measuring within ± 1·0 % of the calibrated weight and ± 0·2 % of the calibrated height.
BMI (weight in kilograms divided by the square of height in metres), a valid estimate of body fatness in groups of children and adolescents( Reference Pietrobelli, Faith and Allison 8 ), was calculated for each student. International BMI thresholds specific for sex and age (to 6 months) were used to categorise children as underweight( Reference Cole, Flegal and Nicholls 9 ), healthy weight, overweight or obese( Reference Cole, Bellizzi and Flegal 10 ). Each student's home neighbourhood postcode was used to assign a home neighbourhood socio-economic index (Socio-economic Index for Areas – derived from the 2006 Australian Census data) as an estimate of level of social and economic well-being.
Descriptive statistics for 2008 and 2003 were weighted to the Western Australian population standardised age and generalised estimating equation models were created for each sex for both primary and secondary school samples. Each model (adjusted for school clustering as well as age and home neighbourhood socio-economic status) compared 2008 and 2003 proportions of students within BMI categories (overweight/obese v. not overweight/obese; underweight v. not underweight). Analyses were conducted using the statistical software package SPSS Statistics version 17·0.
Participation rate in the anthropometric component of the study was higher among primary-school students in 2008 than in 2003; whereas these rates for secondary-school students were similar.
The prevalence of overweight and obesity in primary students in Western Australia in 2008 was similar to that in 2003, whereas a smaller proportion of secondary students were overweight or obese in 2008 than in 2003.
In both 2008 (mean age 10·1 (sd 1·6) years) and 2003 (mean age 10·0 (sd 1·6) years), the majority of primary students surveyed were within the healthy weight category (Table 1). Similar proportions of primary boys and girls in 2003 and in 2008 were classified as overweight or obese (Table 1). A higher proportion of primary girls were underweight in 2008 than in 2003 (P < 0·001); primary girls had a 2·20 (95 % CI 1·43, 3·37) increased odds of being underweight in 2008 compared with 2003 (Table 1). Home neighbourhood socio-economic status was not associated with being overweight or obese or with being underweight for primary-school boys or girls.
Descriptive statistics for 2008 and 2003 weighted to the Western Australian population standardised age and home neighbourhood socio-economic index (Socio-economic Index for Areas; derived from the 2006 Australian Census data). Generalised estimating equation models were created for each sex for both primary- and secondary-school samples and each model was adjusted for school clustering as well as age and home neighbourhood socio-economic status.
Significant difference: **P < 0·005, ***P < 0·001.
†Analysis within school type (primary or secondary) between 2003 and 2008 in the proportion underweight compared with all other weight categories combined in the same gender, after adjustment for school clustering as well as age and home neighbourhood socio-economic status.
‡Analysis within school type (primary or secondary) between 2003 and 2008 in the proportion overweight or obese compared with all other weight categories combined in same gender, after adjustment for school clustering as well as age and home neighbourhood socio-economic status.
In 2008 (mean age 14·1 (sd 1·3) years) and 2003 (mean age 14·2 (sd 1·3) years), the majority of secondary-school students were classified as healthy weight (Table 1). The proportion of secondary-school girls classified as overweight or obese was 7 % lower in 2008 than in 2003 (P = 0·002; Table 1); secondary girls had a 0·54 odds (95 % CI 0·36, 0·80) of being overweight or obese in 2008 compared with 2003. Secondary girls had a 0·57 odds (95 % CI 0·37, 0·89) of being overweight or obese if they resided in a higher socio-economic neighbourhood compared with a middle or low socio-economic neighbourhood. The odds of being overweight or obese for secondary boys in 2008 compared with 2003 was 0·72 (95 % CI 0·48, 1·01). Secondary boys had a 0·50 odds (95 % CI 0·27, 0·92) of being overweight or obese if they resided in a middle socio-economic neighbourhood compared with a high or low socio-economic neighbourhood.
The proportion of underweight secondary-school girls was significantly higher in 2008 than in 2003 (P = < 0·001; Table 1), with a 2·24 (95 % CI 1·16, 4·33) increased odds of being underweight in 2008 compared with 2003. Higher odds of being underweight was evident for secondary girls in a high (1·82 odds; 95 % CI 1·03, 2·86) and middle socio-economic neighbourhood (1·94 odds; 95 % CI 1·31, 2·89) compared with a low socio-economic neighbourhood.
Our study results indicate that overweight/obesity prevalence over time has plateaued in Western Australian primary children and reduced in secondary children. Internationally, overweight/obesity prevalence over time has been indicated as stabilising in youth in France( Reference Péneau, Salanave and Maillard-Teyssier 11 ) and reducing in girls in Sweden( Reference Sjöberg, Lissner and Albertsson-Wikland 3 ).
While a natural saturation of the proportion of children reaching a high BMI may account for the plateau in the proportion of overweight/obese in Western Australian primary children, the observed reduction in the proportion of overweight/obese secondary students suggests that recent intensive Western Australian public health interventions promoting increased activity and healthy eating could have impacted adolescent behaviour. Despite these encouraging results, the continuing high proportion of overweight/obese youth indicates the need for continued public health initiatives targeting children and adolescents.
Survey results indicate an increase in the proportion of primary- and secondary-school girls who were underweight. This is cause for concern as youth who are underweight are more likely to encounter adverse health conditions including delayed puberty, negative body image and fatigue, as well as an increased risk of osteoporosis and reduced fertility as an adult( Reference Luder and Alton 12 ). Furthermore, although there is no current evidence that obesity prevention programmes impact on eating disorder rates( Reference Schwartz and Henderson 13 ), body image has been identified as the top concern of Australian youth( 14 ), indicating that an integrated approach to prevention and treatment of weight-related disorders is appropriate( Reference Schwartz and Henderson 13 ).
There are several limitations to be considered when interpreting these study results. Students volunteered to participate in the study and secondary-school classes were not randomly selected. Students or their parents may have self-selected participation based on their weight status. A higher proportion of primary students consented in 2008 compared with 2003. While data analysis controlled for socio-economic status, response rates within the two ‘school types’ (i.e. government v. non-government funding) may have differed between 2003 and 2008 (this could not be determined as these data were not available for 2003). A major strength of our study is that it captured objectively measured data from a representative sample of school-aged students living in metropolitan and non-metropolitan locations at two time points using a consistent methodology.
The current study provides evidence that overweight and obesity prevalence may have plateaued in primary and declined in secondary students in Western Australia; however, rates remain high. The study findings also suggest that the proportion of underweight girls in Western Australia increased significantly between 2003 and 2008. Integrated efforts to address the ongoing environmental conditions and behavioural practices leading to weight-related disorders are needed to optimise the physical and social health of youth and decrease future disease burden.
Sources of funding: The Child and Adolescent Physical Activity Survey 2008 (the research underpinning this report) was commissioned by the Physical Activity Taskforce (PATF) and was funded through Healthway and the Western Australian Departments of Education, Health, Planning and Infrastructure, and Sport and Recreation, The Government of Western Australia. The data were collected by research teams at Edith Cowan University (2008) and Notre Dame University (2003). The funders had had no role in the design, analysis or writing of this article. Conflicts of interest: No potential conflicts of interest identified. Authors’ contributions: main researcher and guarantor: M.R., M.M. and K.M. Study methods and instrumentation: all authors. Study protocol: K.M., M.R. and M.M. Data collection coordination: K.M., M.R. and M.M. Statistical analysis: K.M. Drafted manuscript: M.R., I.S.P. and K.M. Critical review for scientific content and approval of final version: all authors. Acknowledgements: The authors thank all students who participated in the study and the school staff who assisted with facilitating this. They would also like to acknowledge the assistance of the field team who participated in recruitment and data collection, Professor Max Bulsara and Dr Gavin Pereira for their statistical advice, and Ms Estee Lambin and Catherine Coletsis for manuscript formatting.