Overweight and obesity from childhood to adulthood: a follow-up of participants in the 1985 Australian Schools Health and Fitness Survey

Childhood obesity has become more common in Australia in recent decades. Key evidence for this comes from two national surveys: the 1985 Australian Schools Health and Fitness Survey (ASHFS), which measured 8498 children aged 7–15 years;1,2 and the 1995 National Nutrition Survey, which measured 2962 children aged 2–18 years.3 The prevalence of overweight and obesity, defined on the basis of standard international age-specific and sex-specific body mass index (BMI) cut-points,4 increased substantially in 7–15-year-olds in the 10 years between surveys.5 For boys, overweight increased from 9.3% to 15.3% and obesity increased from 1.4% to 4.7%, and, for girls, overweight increased from 10.6% to 16.0%, and obesity increased from 1.2% to 5.5%.

Evidence that childhood obesity persists into adulthood comes from large cohort studies conducted in the United Kingdom6-8 and the United States9,10 and from smaller studies in Australia and New Zealand.11-13 Most observed moderate to strong correlations between childhood and adult BMI, but only a minority of obese adults had been obese or overweight in childhood. When population-attributable fractions were calculated,11 they indicated that between 8% and 22% of adult obesity could be accounted for by obesity in childhood.

In this study of a large national sample of young adults, we present up-to-date Australian estimates of the risk of adult obesity associated with overweight and obesity in childhood.

The 1985 ASHFS collected data from a nationally representative sample of Australian children aged 7–15 years. Details of the sampling strategies have been described elsewhere.1,2 Electoral rolls, telephone listings, the National Death Index, and contact with classmates were used to trace participants in 2001 and 2002 for the Childhood Determinants of Adult Health (CDAH) study. Traced individuals were contacted in 2001–2005 and invited to participate in the follow-up study. Participants gave their informed consent and the study was approved by the Southern Tasmania Health and Medical Human Research Ethics Committee.

In 1985, participants’ heights were measured with a KaWe height tape (KaWe Kirchner & Wilhelm, Asperg, Germany) or rigid measuring tape while they were bare-foot. Weight was measured with beam or medical spring scales with participants dressed in light clothing. Residential postcode was used to derive a measure of socioeconomic status based on the Australian Bureau of Statistics Socio-Economic Indexes for Areas Index of Relative Socio-Economic Disadvantage and the 1986 census data.14

At the time of consenting to participate in the follow-up study (2001–2005), participants completed a brief enrolment postal questionnaire or a telephone interview seeking selected demographic and health information, including height and weight. However, questions about height and weight were not included in the early months of data collection. Participants were informed that they would have an opportunity to attend a study clinic; these were held in 34 sites around Australia in 2004–2006; for those who attended, measurements of height and weight were made by trained staff using a Leicester height measure (Invicta, Leicester, UK) and Heine scales (Heine, Dover, NH, USA). An assessment of the accuracy of self-reported height and weight was made by asking a subsample of participants to report their height and weight before measurements were taken by clinic staff.

BMI (kg/m2) was calculated from height and weight. For children, overweight and obese categories were defined according to international standard age-specific and sex-specific BMI cut-points.4 These cut-points do not permit classification of very obese children. Children who were not overweight or obese were defined as being healthy weight. For adults, overweight was defined as a BMI in the range 25.0–29.9 kg/m2, obese as a BMI of 30 kg/m2 or more, and very obese (a subset of obese) as a BMI of 35 kg/m2 or more. Healthy weight in adults was defined as having a BMI of less than 25.0 kg/m2. Underweight was not defined as a separate category for the purposes of this study.

Of the original ASHFS participants, 6840 (80.5%) were traced and contacted, and 5170 (75.6%) agreed to participate in the follow-up study. Of these, 2208 men and 2363 women self-reported their heights and weights. The mean age of participants was 29.3 years for men and 29.2 years for women. Mean heights for men and women were 179.6 cm and 165.6 cm, and mean weights were 83.4 kg and 66.0 kg, respectively. Twenty-nine per cent of men and 23% of women reported current regular smoking.

A comparison of participants who did and did not enrol into the CDAH follow-up study, based on their characteristics at baseline in 1985, showed that men who enrolled had a significantly lower mean BMI at baseline than those who did not enrol (18.1 kg/m2 v 18.2 kg/m2), but were not significantly different in age or socioeconomic status. Women enrolled in the follow-up were significantly older at baseline (mean age, 11.4 years v 11.3 years), of lower mean BMI (18.2 kg/m2 v 18.4 kg/m2), and higher socioeconomic status (26% v 21% in the highest quarter by postcode of residence) than those who did not enrol. While these differences were statistically significant (P < 0.01), they were small in absolute terms.

Our follow-up of children who participated in the 1985 ASHFS has confirmed that overweight and obesity in Australian children are strong predictors of obesity in early adulthood. However, most of those who were obese and overweight at follow-up were a healthy weight in childhood.

This study has important strengths in its size, national sampling and the quality of height and weight measurements at baseline. It also has some limitations in that not all children were followed up and we were unable to measure height and weight objectively in all adult participants. Compared with the general population of Australian adults aged 25–34 years,17 participants in this study had a higher socioeconomic status and lower mean BMI. However, these differences were modest and are unlikely to have substantially influenced estimates of the association between childhood and adult obesity. The prevalence of current smoking in our sample (29% in men and 23% in women) was similar to that in the general population (31% in men and 24% in women).

The use of self-reported height and weight at follow-up may have resulted in some misclassification of overweight and obesity in adulthood. We, like others,18 found that participants tended to overestimate their height and underestimate their weight. However, because we had objectively measured height and weight in a large sub-sample of participants at follow-up, we were able to account for these errors and estimate true BMI values, thus improving the accuracy of our findings. RR estimates derived from clinic-measured heights and weights were not significantly different to those derived from adjusted self-report measures taken at enrolment, providing further reassurance that bias was minimal.

While the prevention of overweight and obesity in childhood is an important public health priority, our findings serve as an important reminder that most obese young adults in our community were not obese as children. The period from adolescence to early adulthood is associated with major life-stage transitions. These are likely to be important considerations in the search for effective strategies to engage young people in the healthy diet and physical activity behaviours that are the key to preventing obesity.