The Geelong Osteoporosis Study (GOS) is a population-based cohort study of adults aged 20 years and over, randomly-selected from the Commonwealth electoral rolls for the Barwon Statistical Division in south-eastern Australia . In Australia, the electoral roll is a register of all eligible voters and provides the most comprehensive available list of adult Australian citizens. At baseline, an age-stratified sample was recruited so that there were approximately 100 individuals for each 5-year age-group 20–24, 25–29, 30–34, 35–39, 40–44, 45–49, 50–54, 55–59, 60–64 and 65–69 years, and approximately 200 individuals for each of the age groups 70–79 years and 80 years and older. Baseline assessments for women enrolled in the GOS occurred during the period 1993–7 (1,494 recruited, 77% response) and again approximately a decade later, 2004–8 (882 of the eligible women were assessed at the 10-year follow-up with 82% response). A further sample of 194 women aged 20–29 years was also randomly generated using the same methods employed for the baseline recruitment (2005–8, 82% response) and included in this study. Thus, data from 1,076 women were included in analyses for the 2004–8 time period. Most of the cohort (99%) was Caucasian; details of the study have been provided elsewhere . All participants gave written, informed consent. The Barwon Health Human Research Ethics Committee approved the study.
Weight was measured to the nearest 0.1 kg using electronic scales, standing height was measured to the nearest 0.001 m using a wall-mounted stadiometer and BMI expressed as weight/height2 (kg/m2). An adult with a BMI of 40.0 kg/m2 or over is described as being morbidly obese (class III obesity) but the term can also refer to those who have a BMI over 35 kg/m2 with obesity-related co-morbidities . We identified morbid obesity using BMI criteria alone. Class II obesity corresponded to BMI 35.0-39.9 kg/m2, class I obesity to BMI 30.0-34.9 kg/m2, overweight to BMI 25.0-29.9 kg/m2, normal weight to BMI 18.5-24.9 kg/m2 and underweight to BMI <18.5 kg/m2 . Area-based socioeconomic status (SES) was determined using the Socio-Economic Index for Areas (SEIFA) values based on census data (for 1996 for 1993–7; and for 2006 for 2004–8) from the Australian Bureau of Statistics (ABS). SEIFA values were used to assign an Index of Relative Socioeconomic Disadvantage, which was categorised into quintiles according to cut-off points for the study region.
In order to compare the prevalence of each BMI category in 1993–7 and 2004–8, both time period datasets were stratified into age-groups and age-standardised to the 2006 census population figures for the Barwon Statistical Division (ABS Catalogue No. 2001.0). The standardized morbidity ratio (SMR) for morbid obesity was calculated using the 2004–8 dataset as the standard.
The distribution of BMI was skewed, but was normalised by natural log-transformation. Differences in mean BMI between the two time periods were determined using linear regression models. The variable of interest was the period of assessment (1993–7 or 2004–8) and the models were adjusted for age and SES tested as a potential confounder. Differences in the likelihood of morbid obesity during the two time periods were determined using logistic regression models. The outcome was morbid obesity and the exposure variable of interest was the period of assessment; models were adjusted for age and SES tested as a confounder. This method enables a comparison of the likelihood of morbid obesity between women of the same age and SES in the different time periods. In the statistical models, interaction terms were tested as effect modifiers and retained in the model if p < 0.05. Statistical analyses were performed using Minitab (version 15; Minitab, State College, PA).