Health and mortality consequences of abdominal obesity: evidence from the AusDiab study

The methods of the AusDiab study have been described in detail elsewhere.7,8 The AusDiab study was a nationwide, population-based, stratified cluster survey of 11 247 adults (44.9% men) aged 25 years or older conducted between May 1999 and December 2000 (the response rate was 55.3% of those completing a household interview and estimated to be 37% of the eligible population). Between June 2004 and December 2005, 6537 of the 10 788 eligible participants (60.6%) returned for a follow-up physical examination. After excluding 285 participants aged over 75 years (because of the established lack of association between obesity and many health outcomes in older people),9 42 pregnant women, and 138 participants for whom there were no data on waist circumference, 6072 participants (54.7% women) were available for analysis of incident diabetes, the metabolic syndrome and its components.10 Responders to the follow-up physical examination were more likely than non-responders to be non-smokers, tertiary-educated, married and to speak English at home.8

Waist circumference was measured twice, halfway between the lower border of the ribs and the iliac crest on a horizontal plane. If measurements varied by > 2 cm, a third was taken; the mean of the two closest measurements was calculated. A 75 g oral glucose tolerance test was conducted at baseline and follow-up surveys in all non-pregnant participants not using insulin or taking oral hypoglycaemic drugs. Biochemical parameters, height, weight and blood pressure were measured as previously described.8

Diabetes was classified according to WHO criteria,11,12 and the metabolic syndrome was defined according to the International Diabetes Federation (IDF) definition.13 Hypertension and cut-off points for elevated triglyceride levels and low levels of high-density lipoprotein (HDL) cholesterol were as described in the IDF definition of the metabolic syndrome. WHO waist circumference cut-off points representing “increased” and “substantially increased” risk of obesity-associated metabolic complications in Europids were used to represent “overweight” (men, 94 cm; women, 80 cm) and “obesity” (men, 102 cm; women, 88 cm), respectively.14 One-week recall of leisure-time physical activity was assessed with the Active Australia Survey (administered by the interviewer), which has been shown to have good test–retest reliability.15,16 Self-reported television-viewing time over the previous week, smoking status and the level of education achieved were assessed with a questionnaire administered by the interviewer.

Our findings provide further evidence of the serious negative health effects that are a consequence of the high and increasing rates of overweight and obesity in Australia. Previous reports from the AusDiab study showed that in 2000, 60% of adult Australians were overweight or obese,1 with the prevalence of obesity in Australia among the highest of any developed country.25 Follow-up of the AusDiab cohort has now allowed us to report on the impact of what has been described as an obesity epidemic. The results presented here confirm that abdominal overweight and, more particularly, obesity are significant risk factors for multiple negative health outcomes, and demonstrate the serious health consequences of the obesogenic environment in which we live.

We could not fully cover the spectrum of ill health associated with obesity, as several conditions, including osteoarthritis, cancers, chronic obstructive pulmonary disease, gall bladder disease, sleep apnoea and depression, were not assessed in the AusDiab study.6,14 However, we do include four of the top five conditions for which obesity resulted in disability-adjusted life-years lost in the 1996 Australian Burden of Disease and Injury Study.26 Indeed, CVD, hypertension and type 2 diabetes were responsible for 68% of the obesity-related burden of disease. A recent report estimated the annual direct and indirect financial costs of obesity in Australia to be $3.8 billion, with over half of this borne by government and society.27

A WHO report into the consequences of obesity highlighted a lack of relative-risk data as a major evidence gap that prevents more accurate burden-of-disease estimates.14 Obesity-related risk is related to the demographic, behavioural and biomedical risk factor profile of the population, which differs between countries and over time. It is therefore difficult to extrapolate estimates from other populations to the Australian situation. The results presented here help to address this gap in the evidence, and are the first estimates of obesity-related relative risk for these conditions from a national Australian sample. Furthermore, they provide valuable evidence with which to more precisely calculate the total economic and health burden attributable to obesity, and to inform initiatives for addressing the already high levels of obesity present in Australia.

The population attributable fraction estimates presented require careful interpretation. They effectively compare the incidences of outcomes observed in the AusDiab sample with those in a hypothetical population in which obesity is totally absent. Since no intervention currently exists to eliminate obesity, they must be considered purely theoretical.28 Indeed, national obesity rates have not fallen as the result of targeted interventions in any country.29 A more detailed case study of the impact of obesity reduction interventions is obviously required. The risk estimates presented here will help to inform such endeavours in the Australian context.

Although BMI is the most frequently reported index of obesity, and a measure routinely used in WHO obesity surveillance initiatives, the recently announced Australian national obesity campaign (“Measure Up”) is based on the promotion of waist circumference measurement to identify obesity.30 Waist circumference is easily measured and has been shown to be both a better indicator of abdominal adiposity and a stronger predictor of many health outcomes than is BMI.5,14,31 Evidence-based cut-off points for waist circumference in different ethnic groups are lacking, and therefore those used in this report are appropriate only for Europid populations. For comparative purposes, analyses using BMI to categorise obesity are presented in the Appendix. We included waist-to-hip ratio in the analysis of mortality because it has been shown in two other population-based Australian cohorts to be more closely associated with mortality than was waist circumference.23,24 This trend was also present in the AusDiab cohort, even though the increased HRs did not reach statistical significance, most likely because the follow-up period was only 8 years.

It is important to interpret this report in the context of the inherent limitations of the survey. Firstly, the risks associated with lesser degrees of overweight and obesity, particularly for myocardial infarction and mortality, may not become apparent without considerably longer follow-up than the 5 years (and 8 years for mortality) used here. Other appropriately conducted and analysed studies with longer follow-up and more deaths have shown a strong and independent relationship between abdominal obesity and mortality.24,32,33 Secondly, rates of non-response to the surveys at baseline and at follow-up mean that the results are from a population-based, but not necessarily representative, sample of Australians. Finally, small numbers of Indigenous Australians in the sample mean that obesity-related risks for this population cannot be estimated.

Previous reports from the AusDiab study have shown strong associations between abdominal obesity and time spent in both physical activity and television viewing,1 and have also shown the effects of these behaviours and time spent being sedentary on markers of cardiometabolic risk.34-36 Tackling the obesity epidemic will require environmental and policy initiatives that provide realistic and achievable opportunities for Australians to be more active, to avoid too much time spent sitting and to avoid obesogenic food environments.37 A recent report from the Obesity Working Group of the national Preventative Health Taskforce has highlighted the multisectoral approach required to achieve the goal of preventing unhealthy weight gain in Australia.29

2 Sex-specific adjusted* odds or hazard ratios† (and 95% confidence intervals) among Australian adults aged 25–75 years for the development of various clinical outcomes over 5 years in those classified as obese or overweight on the basis of waist circumference at baseline, compared with those with a normal waist circumference at baseline

HDL = high-density lipoprotein. * Analysis adjusted for age and smoking status (current or ex-smoker/never smoked), and, in the case of myocardial infarction, for self-reported history of cardiovascular disease. † Hazard ratios for myocardial infarction only. ‡ Cut-off point, ≥ 1.7mmol/L. § Cut-off points, < 1.0 mmol/L for men and < 1.3 mmol/L for women. ¶ Defined as blood pressure ≥ 130/85 mmHg or reported use of antihypertensive medication. ** Defined as two or more of the non-obesity components of the International Diabetes Federation definition.13

3 Sex-specific adjusted* odds ratios and 95% confidence intervals for the development of various clinical outcomes and biomedical markers of cardiometabolic risk over 5 years by quintiles of waist circumference at baseline among Australian adults aged 25–75 years

HDL = high-density lipoprotein. * Adjusted for age and smoking status (current or ex-smoker/never smoked). † Cut-off points, < 1.0 mmol/L for men and < 1.3 mmol/L for women. ‡ Defined as blood pressure ≥ 130/85 mmHg or reported use of antihypertensive medication. § Defined as two or more of the non-obesity components of the International Diabetes Federation definition.13 ¶ Cut-off point, ≥ 1.7mmol/L.

4 Adjusted* hazard ratios and 95% confidence intervals for all-cause mortality over 8 years by quintiles of baseline waist-to-hip ratio and waist circumference in Australian adults aged 25–75 years

* Adjusted for age, self-reported history of cardiovascular disease (angina, myocardial infarction or stroke), self-reported cancer (excluding skin cancer) and smoking status (current or ex-smoker/never smoked). Those who died within 2 years of attending the baseline survey were excluded from the analysis.