Waist circumference and risk of cardiovascular disease, type 2 diabetes and mortality among Aboriginal adults in an Australian community

Abstract

Introduction Chronic diseases such as cardiovascular disease (CVD) and diabetes are the greatest contributors to the rapidly increasing public health burden worldwide. About two thirds of mortality reported globally was attributed to chronic diseases, to which these two conditions contribute significantly, and the disadvantaged are mostly affected, dying prematurely. Accelerating rates of deaths from chronic diseases have been reported in developing countries and poorer population groups in developed countries. This situation relates to the Aboriginal people of Australia, who suffer about two and half times greater burden of disease than the general Australian population with chronic diseases accounting for 70% of the health gap, to which CVD and diabetes are chief contributors. It is widely documented and accepted that elevated waist circumference (WC), causes, or aggravates a number of medical conditions, and is also associated with reduced life-expectancy. Aboriginal people have experienced higher and rising rates of diabetes and related outcomes such as CVD and kidney disease. And while WC has been linked to increased risk of CVD and Type 2 diabetes in the Aboriginal population, cut-off points which are often used for defining central obesity for population screening and health promotion are not available for Australian Aboriginal people. This thesis has four objectives: 1. Assess the relationship WC has with morbidity outcomes (CVD and Type 2 diabetes) and all-cause mortality in a remote Aboriginal Australian community 2. Determine which of WC, body mass index (BMI) and waist-to-hip ratio (WHR) has the strongest association with CVD and Type 2 diabetes; and which of WC and BMI had stronger association with all-cause mortality 3. Provide estimates of gender-specific absolute risks of CVD and Type 2 diabetes for specific WC and age values and 4. Generate gender-specific WC values derived from equivalent BMI points with same absolute risks of CVD and Type 2 diabetes. Methods Three datasets collected from Aboriginal people in the Tiwi Islands in the Northern Territory of Australia were utilized: 1. Baseline dataset: collected at a community screening program from 1992 to 1998, including 976 adult participants. 2. Hospitalization dataset: consisting of Northern territory hospital records of participants from 1992 to 2012. 3. Mortality dataset: collected from 1992 to 2010 from death registry records. Eligible participants for the studies in this thesis were adults (18 years and over), who had WC measures and were free of the outcome of interest at baseline screening. Survival analysis was conducted in all the studies, which included eligible participants being followed up prospectively for up to 20 years from the baseline screening measurement until the occurrence of the end point of interest for each participant as recorded on the hospitalization and/or mortality datasets. Statistical methods used include: Cox proportional hazards regression model to test the associations of WC with the risk of CVD, Type 2 diabetes and all-cause mortality; and Weibull accelerated failure-time model to provide absolute risk estimates of CVD and Type 2 diabetes using WC values. Results Objective 1: WC was statistically significantly associated with CVD and Type 2 diabetes in crude and multivariable analyses. Association of WC with all-cause mortality was statistically significant in the crude analysis and in multivariable analysis after adjusting for BMI and other covariates. There was no statistically significant difference between the genders in the association between WC and any study outcome. Objective 2: Waist circumference compared to BMI or WHR had the strongest association with CVD in females; and with Type 2 diabetes in both males and females. WC had stronger associations than BMI with all-cause mortality. Objective 3: Absolute risk of CVD and Type 2 diabetes increased as WC and age increased for males and females. Objective 4: For CVD, coronary artery disease (CAD) and heart failure (HF), WC equivalent to overweight BMI (>=25 kg/m2) ranged from 91 to 93 cm; and obesity BMI (>=30 kg/m2) ranged from 99 to 103 cm for males and females. The derived WC for equivalent overweight (BMI of 25 kg/m2) were 91.5 for males and 90.9 cm for females; and for obesity (BMI of 30 kg/m2), equivalent WC were 105.7 cm and 102.3 cm for males and females respectively for Type 2 diabetes. Conclusions The findings of this thesis show that WC was associated with CVD, Type 2 diabetes and all-cause mortality. The absolute risks of CVD and Type 2 diabetes increased as WC and age increased. Despite their high WC averages, females were not different from males in their risk of disease or death in relation to WC. The absolute risk findings are useful in creating awareness and educating the Aboriginal people in the study community on the risk associated with elevated WC and CVD and Type 2 diabetes. Future studies should examine the risks in other Aboriginal populations to further contribute to guidelines required for the recommendation of WC cut-off points for Aboriginal people in Australia.