To the Editor: While the rates of overweight and obesity among Australian adults, as determined by the Australian Diabetes, Obesity and Lifestyle Study (AusDiab),1 may be alarming to some, they may in fact be underestimates of the true prevalence of overweight and obesity.

The AusDiab study design2 and its low response rates indicate that the results will need to be interpreted with caution.

Firstly, the AusDiab study design excluded rural and predominantly Indigenous census collection districts (CDs). In Queensland, all CDs selected were capital city or other major urban centres (Rural and Remote Areas Classification, categories 1 and 2);3 thus, people living in major rural centres (such as Rockhampton or Bundaberg) or major remote centres (such as Mt Isa) were excluded (ie, in Queensland, about 20% of the population were excluded).

Secondly, another potential bias may have been introduced by the Socio-Economic Indexes for Areas (SEIFA) scores of the CDs sampled in the AusDiab study. For example, the overall SEIFA score for the CDs included in Queensland was 1035 (73rd percentile), well above the state average.

Finally, the response rates in the AusDiab study were low: only 29% of those estimated to be eligible, and only 52% of those invited, actually completed the study.

Our analysis of risk factors of Queensland-AusDiab participants suggests that these participants may have been more health conscious than the general Queensland population. Rates of smoking reported for men and women were considerably lower in the Qld-AusDiab cohort compared with those in the Queensland phase of the 2001 National Health Survey4 (17.3% and 14.5% v 28.4% and 19.8%, respectively).

Compared with results of a Queensland Omnibus telephone survey5 conducted at about the same time, higher proportions of Qld-AusDiab participants reported greater intakes of vegetables (≥ 4 serves/day: 27.4% Qld-AusDiab v 16.4% Omnibus) and fruit (≥ 2 serves/day: 28.9% v 24.3%), and less frequent consumption of fast foods (> 1 day/week: 37.3% v 49.5%).

Given the low response rate and possible selection bias in the AusDiab study, we suggest that the overweight and obesity data should be interpreted with caution. Several indicators suggest that these data could be underestimates of the true prevalence of overweight and obesity, and that the AusDiab population may have been of higher socioeconomic status, more health conscious (lower rates of smoking, better dietary intake), and more willing to participate in a lengthy examination than the general Australian population. These factors may all be associated with lower rates of overweight and obesity, and therefore future national surveys will need to take these factors into consideration to obtain more accurate estimates of important determinants of health.

In reply: Coyne suggests that, based on comparisons within Queensland, the national prevalence of obesity in our article1 is an underestimate. It should be noted that the Australian Diabetes, Obesity and Lifestyle Study (AusDiab) was designed primarily to produce national, not state-specific, data. Forty-two census collection districts (CDs) were selected Australia-wide, with only six CDs selected within each state. The primary objective of this sample selection was to obtain a nationally representative population, not necessarily one representative of each state.

Coyne states that none of the Queensland CDs were in major provincial centres. Of the six Queensland CDs, four were outside Brisbane. From the national sample, 17 of 42 CDs (40.5%) were outside capital cities. As a comparison, 36% of the Australian population lives outside capital cities.2

Regarding selection of CDs, we excluded only those in Statistical Local Areas defined as 100% rural, and those where the Indigenous population made up 10% or more of the overall population.3 This excluded only 5.8% of the total eligible population. If the prevalence of obesity among this group was double the overall prevalence, this would not significantly alter the national rate.

While the smoking rates in AusDiab were lower than reported elsewhere, the prevalence of obesity, hypercholesterolaemia and hypertension were in line with trends in a series of surveys over the past 20 years.4 In an extensive analysis of food consumption between AusDiab and the 1995 National Nutrition Survey, the rates of fruit and vegetable consumption were within 4% between the surveys for those most commonly eaten.

Since our conclusion was that obesity has increased, the possibility of an underestimate only reinforces our message.