The comparisons between different birth cohorts when they were at the same age show that the birth cohorts born most recently were more likely to be overweight or obese combined, obese alone or severely obese and to have a higher median BMI than those born earlier at most ages. The prevalence of abdominal obesity among the birth cohorts born more recently was significantly higher than those born earlier for only half of the adult age groups.

The comparisons within the same birth cohorts as they aged show that, generally, the prevalence of overweight and obesity combined, obesity alone, severe obesity and abdominal obesity, and median BMI, increased as people aged.

The comparisons of the distribution of BMI values within birth cohorts as they aged show that the BMI distribution for younger cohorts has shifted upwards to a larger extent than for older cohorts. The distribution of BMI values has also widened over time across all birth cohorts.

The largest absolute changes in the prevalence of overweight and obesity combined, and obesity alone with age were among the 1973–1982 birth cohort, while the largest absolute changes in severe obesity and abdominal obesity with age were among the 1953–1962 and 1963–1972 birth cohorts, respectively. This is likely to reflect the ages and life stages of these cohorts at the 3 survey points. For example, the 1973–1982 birth cohort were aged 13–22 at the first survey point and 35–44 at the final survey point—early adulthood is a life stage when the risk of weight gain may increase due to a reduction in physical activity and changes to diet and alcohol consumption (NHMRC 2013).

A limitation of this analysis is that it uses separate cross-sectional birth cohorts measured at different time points, rather than a longitudinal sample of the same people followed over time to see how their BMI changes as they age. There will be some variability in BMI values because each survey samples a different group of people. This means that the evidence regarding changes in overweight and obesity over time is weaker than if a similar analysis was done on a consistent longitudinal cohort.

Possible causes

This web report does not identify the factors contributing to changes in the prevalence of overweight and obesity. Overweight and obesity occur primarily because of an imbalance between energy intake and energy expenditure (which can be influenced by a range of factors, including genetics and environment). An increase in energy intake and/or a decrease in energy expenditure is the most plausible explanation for increases in the prevalence of overweight and obesity combined, and obesity alone among more recently born birth cohorts compared with those born 10 years earlier. Diet and physical activity are, however, difficult to measure, and trend data in Australia are limited.

The authors of a previous birth cohort study proposed that the greater prevalence of obesity among more recently born birth cohorts was likely due to the birth cohorts born more recently having spent greater proportions of their lives in an obesogenic environment (Allman-Farinelli et al. 2008).

An obesogenic environment is one that promotes obesity among individuals or populations, and includes aspects of the:

  • physical environment—such as food availability in supermarkets, workplaces, and schools, and the availability of footpaths and recreational facilities
  • economic environment—such as personal income, and the cost of food
  • political environment—such as policies, regulations, and laws around food labelling and advertising
  • sociocultural environment—such as media and social and cultural norms (Swinburn et al. 1999).

Examples of environmental factors that contribute to overweight and obesity include, among other things:

  • a wide availability of cheap processed foods that provide excess kilojoules
  • larger portion sizes
  • replacement of physically active workplaces with more sedentary occupations
  • longer working hours leaving less time for food preparation and physical activity (NHMRC 2013).

Several policy options included in the WHO’s Global action plan for the prevention and control of noncommunicable diseases 2013–2020 address the obesogenic environment. These include:

  • policies to reduce marketing of less healthy foods to children
  • taxes and subsidies to encourage consumption of healthier foods and discourage consumption of less healthy foods
  • promotion of provision of healthier foods in public institutions, such as schools and workplaces (WHO 2013).

Possible implications

When combined and considered alongside the broader literature, the findings of this web report suggest a range of possible implications, including:

  • a potential earlier onset of obesity-related chronic conditions, with previous research showing a longer duration of overweight and obesity is associated with increased risk of some cancers among women (Arnold et al. 2016)
  • potentially higher health-care costs, with previous Australian research showing:
    • higher hospital admission rates, days spent in hospital, and hospital costs with increasing BMI among adults aged 45–64 and 65–79 (Korda et al. 2015)
    • health-care costs for a severely obese population to be more than double those for the general population, with the difference appearing to be due to greater use of services among the severely obese population (Keating et al. 2012)
  • potential impacts on workforce productivity
  • a potentially greater prevalence of overweight and obesity later in life, as children and adolescents who are overweight or obese are more likely to be overweight or obese as adults than children of a normal weight (Singh et al. 2008)
  • a potential increased risk of mortality and lower life expectancy, with previous research showing:
    • a higher number of years lived with obesity is associated with increased risk of mortality (Abdullah et al. 2011)
    • 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 theirs (Lung et al. 2019).

The analysis showed that at age 5–14, the birth cohort born most recently was only slightly more likely to be overweight or obese, or obese alone, than those born 10 years earlier, and these differences were not statistically significant. This is an encouraging finding and consistent with other findings that suggest that the prevalence of overweight and obesity among children has plateaued in recent years in Australia (Olds et al. 2010).

While it appears to have plateaued, the prevalence of overweight and obesity among children and adolescents remains high by historic standards and remains a major public health issue in Australia. Any stabilisation in the prevalence of childhood overweight and obesity could also be only temporary if efforts are not continued (Olds et al. 2010).


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