Until recently, smoking was estimated as the leading independent risk factor contributing to the burden of disease in Australia.1,2,3 However, the prevalence of overweight and obesity is increasing in Australia,4 which now has one of the highest rates of overweight and obesity in the world.5 Concurrently, the prevalence of tobacco smoking has significantly decreased as a result of public health efforts.
This letter presents results of a recent investigation into the contribution of selected risk factors to the burden of disease in Western Australia (WA) in 2006. The study was based on the comparative risk assessment (CRA) approach developed by the Global Burden of Disease study.6 This approach represents a systematic evaluation of the changes in population health, which would result from modifying the distribution of exposure to a risk factor or group of risk factors in the population. Central to CRA is the calculation of the Population Attributable Fraction (PAF), reflecting the proportion of current disease burden attributable to current and past exposure to a risk factor. The PAF is determined from the relative risks (RR) of disease in those exposed (compared to those not exposed) and the prevalence of the risk factor in the population. The RRs used in the calculation of PAFs in the WA study were consistent with those used in the Australian 2003 study,1 while estimates of the prevalence of high body mass and tobacco use were obtained from the WA Health and Wellbeing Surveillance System.7
The population burden was estimated in Disability Adjusted Life Years (DALYs). DALYs provide comparable information on the years of life lost to premature mortality and the equivalent ‘healthy’ years lost due to disability, thus reflecting both fatal and non-fatal burden. Mortality data for WA in 2006 were used to determine mortality burden. Disability burden was derived by applying the Australian 2003 baseline disease models, derived for the Australian 2003 study1 to more recent WA data derived from death and hospitalisation trends
In 2006, high BMI was the leading independent risk factor contributing to the burden of disease in WA (8.7% of the total burden), contributing 2.2% more than the next leading risk factor, tobacco (6.5%). The BMI burden impacted at a significantly younger age than the tobacco burden. High BMI and tobacco were followed by physical inactivity (6.1%), high blood pressure (6.0%), high blood cholesterol (5.0%), alcohol harm (3.8%), inadequate fruit and vegetables (2.3%), illicit drugs (1.6%) and unsafe sex (0.53%). Full results will be published in a forthcoming report.
The changing impact of high BMI and tobacco on the WA population is documented in Table 1. Methodological differences do not allow a direct comparison of the proportion of burden attributed to risk factors between studies. However, a decreasing trend in the burden contributed by tobacco is consistent with the decreasing smoking prevalence.
|Ranka||% total DALY||Ranka||% total DALY|
The increase in the proportion of total burden attributed to high BMI in the Australian 2003 and WA 2006 studies was partly due to the use of advanced CRA methods, which included examining BMI as a continuous risk with a hypothetical minimum mean of 21 kg/m2.
Some assumptions were made in the estimation of the burden due to high BMI. Type II diabetes estimates rely on projections of incidence and assumptions of case fatality, mostly based on the somewhat dated AusDiab Study.8 Uncertainty in the calculated type II diabetes burden could result in an imprecise estimation of the burden attributed to high BMI because type II diabetes accounted for almost half of the BMI burden in WA. These factors will impact on the proportion of total disease burden attributed to high BMI.
The increase in the proportion of disease burden attributed to high BMI is most likely a combination of methodological improvements in burden estimation and the increased prevalence of high body mass in WA, resulting in high BMI overtaking tobacco as the leading single contributing risk factor to disease burden. While updated Australian diabetes prevalence estimates could reduce uncertainty of the results, the findings highlight the urgent need to reverse the increasing prevalence of high BMI to improve the health of the WA population. The extent that these findings are also true throughout Australia will be evident as this type of study is replicated for other states.