Background
The prevalence of obesity in Australian adults continues to increase. However, assessment of obesity commonly relies on body mass index (BMI) without accounting for waist circumference (WC). This study aimed to describe changes in weight and WC, and examine the incidence of obesity as defined by BMI and WC.

Methods
In 1999/2000, 11,247 adults aged 25 years and over were recruited from 42 randomly selected areas across Australia. 44.6% of eligible participants completed follow-up in 2011/12. Height, weight and waist circumference were measured at both surveys.

Results
The age and sex adjusted mean 12-year change in weight and WC was 2.6 kg and 5.3 cm, respectively. When examined according to age, those aged 25-34 years gained an average of 6.6 kg weight and 6.6 cm WC over 12 years. By comparison, those aged 75 and over lost an average of 4.5 kg and gained an average of 0.8 cm. Women had a greater increase in WC than men but did not differ in terms of weight gain. The 12-year incidence of obesity was 14.9% when defined by BMI and 31.6% when defined by WC. Increases in weight, WC and incidence of obesity were generally greater with lower socio-economic position.

Conclusion
Assessment of WC in addition to the traditionally measured BMI should be considered when assessing trends of obesity in the population and the burden of disease associated with excess adiposity.