To the Editor: The recent article by Venn et al reported that childhood overweight carries through into adult overweight and obesity, but that most obese young adults in their study were “healthy” weight as children in 1985.1 As demonstrated by National Health Surveys, age is one of the strongest predictors of overweight,2 with body mass index (BMI) increasing as we grow older. However, there are two additional time-related components influencing obesity.

Since 1985 (when Venn et al reported the prevalence of overweight and obesity in children was less than 10%), the environment appears to have become more obesogenic — a 2004 survey in New South Wales showed that 26% of children were overweight or obese.3 It is not only children who are vulnerable — the percentage of overweight adult Australians increased for almost all age groups from 1990 through 2001, and the mean BMI at which Australians enter adulthood has increased with each subsequent survey. For example, for women aged 20–24 years, mean BMI increased from 22.1 kg/m2 (1990) to 22.5 kg/m2 (1995) to 23.2 kg/m2 (2001) to 23.3 kg/m2 (2004). As the heights and weights were self-reported in these surveys, true BMI values may be even higher.

We recently reported that year of birth (birth cohort) also predicts prevalence of overweight and obesity, independent of age and survey period; the prevalence of overweight and obesity in adults increased progressively with birth cohorts born since 1960.4 This birth span includes the cohort in the study by Venn et al.1 While obesity begins in childhood for only a small proportion of adults, the so-called healthy weight children now have a higher mean BMI, giving little margin for the seemingly inevitable increases in weight with ageing, before the population mean BMI reaches the cutpoint for overweight and later obesity. The 2004–2005 National Health Survey showed that men reached the overweight cutpoint at 25–29 years and women reached it at 30–34 years.5

Given increasing child and adult obesity, the need for allocation of public health resources to improve dietary and physical activity habits is undisputed. However, these data1,4 indicate that efforts should be directed to the hard-to-reach group, young adults, to prevent weight gain at this point. This will pose considerable challenges, because this group has minimal contact with health services, and perceives the threat of chronic illness as irrelevant. However, swift intervention is required, not only for their own health and that of their children as they become parents, but also because they will become overconsumers of health care for chronic diseases within a generation.

In reply: Allman-Farinelli et al make an important point about the influence of age, survey period and cohort effects on the prevalence of overweight and obesity. While age and cohort effects could not be clearly separated in the 1985 Australian Schools Health and Fitness Survey, the prevalence of overweight and obesity increased with age in 7–15-year-olds.1 Our data collected from 4571 of the individuals in that survey at follow-up about 20 years later also showed an increase in the prevalence of overweight and obesity with age, although these findings were not presented in our report.2

In the Box, we show the distribution of body mass index (BMI) values for men and women in three age groups (24–27 years, 28–30 years and 31–34 years). Mean BMI values across the age groups were 25.2 kg/m2, 25.6 kg/m2, and 26.5 kg/m2 in men and 23.5 kg/m2, 24.2 kg/m2, and 24.6 kg/m2 in women. The prevalence of obesity (BMI ≥ 30 kg/m2) increased with increasing age as follows:

• age 24–27 years — men 12.2%, women 9.9%;

• age 28–30 years — men 12.3%, women 12.0%; and

• age 31–34 years — men 15.6%, women 14.6%.