Background, data sources and methods
Overweight and obesity is a major public health issue in Australia and globally. Excess body weight among adults is a risk factor for several chronic conditions, such as cardiovascular disease, type 2 diabetes, and some cancers. Children and adolescents who are overweight or obese are more likely to become adults who are obese than children and adolescents of a normal weight (Venn et al. 2007).
What is a birth cohort analysis?
A birth cohort is a group of people born in the same year or years.
Birth cohort analysis can be used to identify birth cohorts that are particularly at risk of a health outcome (Keyes et al. 2010). A birth cohort effect could occur because:
- a population-level environmental cause of the health outcome is unequally distributed across a population and across time (for example, younger people today may have higher access to and consumption of energy-dense foods than younger people of previous generations)
- a population-level environmental cause of the health outcome differentially affects age groups who are in a critical developmental period (for example, exposure to an obesogenic environment may affect younger people more than older people through shaping of lifelong food and physical activity behaviours) (Keyes et al. 2010; Wilson & Abbott 2018).
The prevalence of a health outcome is also affected by age effects (accumulated exposure to a cause and/or physiological changes that occur with aging) and period effects (population-wide exposures that occur at a particular point in time) (Keyes et al. 2010). Age, period and cohort effects are all related—the analysis in this web report does not attempt to separate these effects.
Australian data have shown that, when compared at the same age, more recent birth cohorts were often more likely to be obese than earlier birth cohorts (AIHW: Bennett et al. 2004; AIHW 2017; Allman-Farinelli et al. 2008; Pilkington et al. 2014). For example, the proportion of those born in 1994–1997 who were obese at age 18–21 (15.2%) was almost double the proportion of those born in 1974–1977 who were obese at the same age (8.0%) (AIHW 2017).
The increased obesity prevalence among those born more recently, at younger ages especially, has potentially significant implications—it could result in earlier onset of obesity-related chronic conditions, higher health-care costs, impacts on workforce productivity, greater prevalence of obesity later in life and increased risk of mortality. For example, Australian men who are obese at age 25 are projected to live 8.3 fewer years than their healthy weight peers, while Australian women who are obese at age 25 are projected to live 6.1 fewer years than their healthy weight peers (Lung et al. 2019).
This web report looks at how overweight and obesity prevalence varies between birth cohorts when compared at the same age, and at how overweight and obesity prevalence changes within birth cohorts as they age. The report extends Overweight and obesity in Australia: a birth cohort analysis (AIHW 2017) by including the most recent nationally representative data (2017–18) and analysing new birth cohorts. It focuses on changes over more recent periods, given patterns over the longer term are better established.
The analyses in this report are based on data from 3 surveys conducted by the Australian Bureau of Statistics (ABS):
- 1995 National Nutrition Survey
- 2007–08 National Health Survey
- 2017–18 National Health Survey.
These data sources were chosen because they provided nationally representative measured height, weight, and waist circumference data.
Detailed information about these data sources is provided in Detailed data sources and methods.
This report did not track the same individuals over time—instead, birth cohorts were constructed using cross-sectional survey data representing the Australian population at various time points.
This approach treated, for example, survey participants aged 25–34 in 2007–08, and survey participants aged 35–44 in 2017–18 as representative of the same group of people (those born in 1973–1982) as they aged 10 years between the surveys.
The birth cohorts were constructed in 10-year age groups, from 1923–1932 (aged 75–84 in 2007–08) to 2003–2012 (aged 5–14 in 2017–18).
For each birth cohort, at each survey year, 2 measures of overweight and obesity were calculated: body mass index (BMI) and waist circumference.
- overweight and obesity was classified as a BMI of 25.00 kg/m2 or more
- obesity was classified as a BMI of 30.00 kg/m2 or more
- severe obesity was used to describe the combined prevalence of class II (BMI of 35.00–39.99 kg/m2) and class III (BMI of 40.00 kg/m2 or more) obesity
- abdominal obesity was used to describe a waist circumference of 102 cm or more for men, and of 88 cm or more for women (WHO 2000).
Assessing overweight and obesity among children and adolescents is more complicated due to their growing bodies. For children and adolescents, age- and sex-specific half-year BMI cut-off points were used to classify overweight and obesity (Cole et al. 2000).
Data for age groups that included children and adolescents were not included in analysis of severe or abdominal obesity as there is not yet consensus on the definitions of these among children and adolescents.
Median BMI (or 50th percentile) and other measures of BMI distribution (including 10th and 90th percentiles) were calculated for adults. Data for age groups that included children and adolescents were not included in analysis of BMI distribution, as the age- and sex-specific BMI cut-offs points for overweight and obesity among children and adolescents complicate interpretation of BMI values.
Throughout this report, all reported differences between and within birth cohorts are statistically significant, unless stated otherwise.
Detailed information about the methods is provided in Detailed data sources and methods.
AIHW (Australian Institute of Health and Welfare) 2017. Overweight and obesity in Australia: a birth cohort analysis. Cat. no. PHE 215. Canberra: AIHW.
AIHW: Bennett S, Magnus P & Gibson D 2004. Obesity trends in older Australians. AIHW bulletin no. 12. Cat. no. AUS 42. Canberra: AIHW.
Allman-Farinelli M, Chey T, Bauman A, Gill T & James W 2008. Age, period and birth cohort effects on prevalence of overweight and obesity in Australian adults from 1990 to 2000. European Journal of Clinical Nutrition 62:898–907.
Cole TJ, Bellizzi MC, Flegal KM & Dietz WH 2000. Establishing a standard definition for child overweight and obesity worldwide: international survey. BMJ 320:1240–3.
Keyes KM, Utz RL, Robinson W & Li G 2010. What is a cohort effect? Comparison of three statistical methods for modeling cohort effects in obesity prevalence in the United States, 1971–2006. Social Science & Medicine 70:1100–8.
Lung T, Jan S, Joo Tan E, Killedar A & Hayes A 2019. Impact of overweight, obesity and severe obesity on life expectancy of Australian adults. International Journal of Obesity 43:782–9.
Pilkington R, Taylor A, Hugo G & Wittert G 2014. Are Baby Boomers healthier than Generation X? A profile of Australia’s working generations using National Health Survey Data. PLOS ONE 9:e93087.
Venn A, Thomson R, Schmidt M, Cleland V, Curry B, Gennat H et al. 2007. Overweight and obesity from childhood to adulthood: a follow-up of participants in the 1985 Australian Schools Health and Fitness Survey. Medical Journal of Australia 186:458–60.
WHO (World Health Organization) 2000. Obesity: preventing and managing the global epidemic. Report of a WHO consultation. WHO Technical Report Series 894:i–xii, 1–253.
Wilson R & Abbott JH 2018. Age, period and cohort effects on body mass index in New Zealand, 1997–2038. Australian and New Zealand Journal of Public Health 42(4):396–402.