Overweight and obesity

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 groups 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
• differentially affects age groups that 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 and Abbott 2018).

Health outcomes are also affected by age effects (either accumulated exposure to a cause or physiological changes that occur with ageing or both) and period effects (population-wide exposures that occur at a particular point in time) (Keyes et al. 2010).

Australian data have shown that, when compared at the same age, more recent birth cohorts were often more likely to be living with obesity than earlier birth cohorts (AIHW 2017; Allman-Farinelli et al. 2008; Bennett et al. 2004; Pilkington et al. 2014)

The increase in obesity prevalence among those born more recently, especially at younger ages, has potentially significant implications and impacts on both individuals and the population. This could result in earlier onsets of obesity-related chronic conditions, with more people living with these conditions and more years lived in ill-health. This would increase healthcare costs, have impacts on workforce productivity, as well as increased mortality. For example, Australian men and women who are living with obesity at age 25 are projected to live 8.3 and 6.1 fewer years, respectively, than their healthy weight peers (Lung et al. 2019). 

Differences between birth cohorts

This section compares the estimated prevalence of obesity, severe obesity and abdominal obesity between different birth cohorts.

For each age group, prevalence estimates in the birth cohort born most recently (measured in 2022–24) is compared with the birth cohort born 10 years earlier (measured in 2011–12), and where possible, 27 years earlier (measured in 1995), at the same age.

Children, adolescents and young adults

Obesity (measured by BMI)

This section considers obesity in the 5–14 and 15–24 age groups.

For the 5–14 age group, when comparing those born most recently with those born in earlier birth cohorts, data showed that for those born most recently (in 2008–2017) (Figure 17):

• an additional 3 in every 100 children were living with obesity (8.4%) compared with those born in 1981–1990 (5.1%) at that same age
• the proportion living with obesity did not differ significantly when compared with those born in 1998–2007 (7.2%) at that same age.

For those in the 15–24 age group, when comparing those born most recently with those born in earlier birth cohorts, data showed that for those born most recently (in 1998–2007) (Figure 17):

• about 5 additional adolescents and young adults in every 100 were living with obesity (13%) compared with those born in 1971–1980 (8.4%) at that same age
• the proportion living with obesity was similar compared with those born in 1988–1997 (both 13%) at that same age.

For females aged 15–24, the proportion of adolescents and young adults living with obesity in those born most recently in 1998–2007 (12%) was smaller when compared with those born 10 years earlier in 1988–1997 (15%). However, for males aged 15–24, the proportion living with obesity was higher in those born more recently – 15% of those born in 1998–2007 were living with obesity, compared with those 11% of those born in the 1988–1997 cohort.

Adults

Obesity (measured by BMI)

Among adults, those born most recently were more likely to be living with obesity than those born 10 years earlier. The largest absolute differences were seen in the (Figure 18):

• age group 75–84, where an additional 10 in every 100 adults were living with obesity at age 75–84 in 2022–24 (37%) compared with those at the same age in 2011–12 (27%)
• age group 25–34, where almost additional 9 in every 100 adults were living with obesity at age 25–34 in 2022–24 (29%) compared with those at the same age in 2011–12 (21%).

By sex

For almost all age groups, when comparing those born most recently to those born 10 years earlier, the increase in the proportion of adults living with obesity was much greater in males than females.

In male adults, across all age groups, except those aged 55–64, there was a greater proportion living with obesity when comparing those born most recently to those born 10 years earlier. The largest absolute differences were seen in the (Figure 18):

• age group 75–84, where an additional 14 in every 100 males were living with obesity at age 75–84 in 2022–24 (38%) compared with those at the same age in 2011–12 (24%)
• age group 25–34, where an additional 9 in every 100 males were living with obesity at age 25–34 in 2022–24 (30%) compared with those at the same age in 2011–12 (21%).

In contrast, in female adults, the absolute differences in the proportion living with obesity between birth cohorts were smaller. The largest absolute differences were seen in those (Figure 18):

• aged 25–34, where an additional 8 in every 100 female adults were living with obesity at age 25–34 in 2022–24 (28%) compared with those at the same age in 2011–12 (20%)
• aged 75–84, where an additional 7 in every 100 female adults were living with obesity at age 75–84 in 2022–24 (37%) compared with those at the same age in 2011–12 (30%).

Severe obesity (measured by BMI)

In this section, severe obesity is defined as having a BMI of 35 kg/m² or more. The analysis of severe obesity is limited to adults due to a lack of consensus on the definition for children and adolescents.

Across all age groups, except those aged 35–44, those born most recently were significantly more likely to be living with severe obesity than those born 10 years earlier. The largest absolute differences were seen in those (Figure 18): 

• aged 45–54, where an additional 6 in every 100 adults were living with severe obesity at age 45–54 in 2022–24 (16%) compared with those at the same age in 2011–12 (10%)
• aged 25–34, where an additional 5 in every 100 adults were living with severe obesity at age 25–34 in 2022–24 (12%) compared with those at the same age in 2011–12 (7.1%).

By sex

For almost all age groups, when comparing those born most recently to those born 10 years earlier, the increase in the proportion of adults living with severe obesity was greater in males than females (Figure 18).

Abdominal obesity (measured by waist circumference)

Those born most recently were significantly more likely to be living with abdominal obesity than those born 10 years earlier. This was seen across all adult age groups. 

The largest absolute differences were seen in those (Figure 18):

• aged 75–84, where an additional 13 in every 100 adults were living with abdominal obesity at age 75–84 in 2022–24 (66%) compared with those at the same age in 2011–12 (54%)
• aged 45–54, where an additional 9 in every 100 adults were living with abdominal obesity at age 45–54 in 2022–24 (55%) compared with those at the same age in 2011–12 (46%)
• aged 25–34, where an additional 9 in every 100 adults were living with abdominal obesity at age 25–34 in 2022–24 (36%) compared with those at the same age in 2011–12 (27%).

By sex

For almost all age groups, when comparing those born most recently with those born 10 years earlier, the increase in the proportion of adults living with abdominal obesity was much greater in females than males.

In male adults, the largest absolute differences was seen in those (Figure 18):

• aged 75–84, where an additional 12 in every 100 males were living with abdominal obesity at age 75–84 in 2022–24 (58%) compared with those at the same age in 2011–12 (46%)
• aged 25–34 and 45–54, where almost 7 additional males in every 100 adults were living with abdominal obesity at age 25–34 (30%) and at age 45–54 (48%) compared with those at the same age in 2011–12 (24% and 41%, respectively)

For female adults, the largest absolute differences were seen in those (Figure 18):

• aged 75–84, where an additional 13 in every 100 females were living with abdominal obesity at age 75–84 (74%) in 2022–24 compared with those at the same age in 2011–12 (61%)
• aged 25–34 and 45–54, where an additional 12 in every 100 females were living with abdominal obesity at age 25–34 (42%) and at age 45–54 (62%) in 2022–24 compared with those at the same age in 2011–12 (30% and 50%, respectively).

Different female life stages could explain some of the greater increase in abdominal obesity in women, such as pregnancy and menopause. Evidence suggests that changes in female hormone profiles during and after menopause are associated with increased adiposity and shifts in body fat distribution (Hirschberg 2012). 

Cohort changes over time

This section compares estimates of the prevalence of obesity, severe obesity and abdominal obesity within the same birth cohort as they aged over time.

When following the same 10-year birth cohort over time, between 1995 and 2022–24, the prevalence of obesity, severe obesity and abdominal obesity increased significantly for almost all birth cohorts as they aged over this period (Figure 19). The prevalence of abdominal obesity (measured by waist circumference) had greater increases as each birth cohort aged over time, compared with the increase in the prevalence of obesity (measured by BMI) over time.

When looking at change over the last 5 years, the prevalence of abdominal obesity increased significantly for almost all 5-year birth cohorts as they aged between 2017–18 and 2022–24 (Figure 19). In contrast, the prevalence of obesity and severe obesity showed stabilising trends for birth cohorts born before the late 1960s to mid-1970s. For birth cohorts born after this period (that is, the late 1960s to mid-1970s), the prevalence continued to increase between 2017–18 and 2022–24.

By age group

When compared at the same age, younger birth cohorts (those born more recently) generally had a higher prevalence of obesity, severe obesity and abdominal obesity than those born in earlier birth cohorts (Figures 18). For detailed information, see Differences between birth cohorts.

By sex

Between 1995 and 2022–24, when looking at 10-year birth cohorts, the absolute increase in the prevalence of abdominal obesity was much greater for all female birth cohorts as they aged across time, compared with males. In contrast, male birth cohorts had larger increases in the prevalence of obesity as they aged across time, compared with females (Figure 19).

The birth cohorts with the largest increase in prevalence of abdominal obesity by sex were: 

• in the female 1968–1977 birth cohort, where the prevalence of abdominal obesity increased from 11% in 1995 when they were aged 18–27, to 62% in 2022–24 when they were aged 45–54.
• in the male 1968–1977 birth cohort, where the prevalence of abdominal obesity increased from 8.0% in 1995 when they were aged 18–27, to 48% in 2022–24 when they were aged 45–54.