Data source and study population
We used baseline questionnaire data from the 45 and Up Study, a large-scale collaborative cohort study of people aged 45 years and over in NSW, Australia’s largest state. The study includes 266,848 people aged 45 years and over, who were selected at random from the Medicare Australia (Australia’s national health insurance system) database. Its methods are described in detail elsewhere . In brief, participants were randomly sampled from the database of Australia’s universal health insurance provider, Medicare Australia, which provides virtually complete coverage of the general population, including some temporary residents and refugees. The study over-sampled, by a factor of two, individuals aged 80 years and over and people resident in rural areas; all residents of remote areas were sampled . Around 11% of the entire NSW population aged 45 years and over (approximately 18% of those approached) were included in the sample. Participants joined the Study by completing a baseline questionnaire between Jan 2006 and April 2009 and giving signed consent for follow-up and linkage of their information to a range of health databases. The Study’s baseline questionnaire captured information on a wide range of factors relating to healthy ageing, including disease risks, quality of life and other health outcomes, exposures such as life style factors, environmental determinants, socioeconomic status and demographic factors including country of birth (for more information see: http://www.45andup.org.au).
The 45 and Up Study received ethical approval from the University of NSW Human Research Ethics Committee for its collection of baseline data.
Definitions, classification and exclusions
This analysis used self-reported data from the 45 and Up Study baseline questionnaire (available at https://www.saxinstitute.org.au/our-work/45-up-study/). Type 2 diabetes was defined on the basis of answers to the question ‘Has a doctor EVER told you that you have Diabetes?’ If YES “please cross the box and give your age when the condition was first found.” Participants were classified as having type 2 diabetes if they responded ‘yes’ and reported an age at diagnosis of ≥25 years.
Age was grouped into four categories: 45–55, 55–65, 65–75, and ≥75 years. Country of birth was classified according to the Australian Bureau of Statistics (ABS) Standard Australian Classification of Countries (SACC) , as follows: Australia, New Zealand, Rest of Oceania and Antarctica, United Kingdom, Germany, Netherlands, Italy, Greece, Rest of Europe, Egypt, Lebanon, Rest of the Middle East and North Africa, Vietnam, Philippines, China, India, Sri Lanka, Rest of Asia, Americas, and South Africa. Each of these groupings included 100 or more people with diabetes. People born in countries of Sub-Saharan Africa were excluded from the analysis because these comprised less than 100 people with diabetes.
Variables related to socio-economic status included educational qualification, work status, income and private health insurance. Educational qualification was classified as no formal qualification, school intermediate education, trade, apprenticeship, and university and higher education. Work status was categorized as in paid work, retired and other. Annual household income was categorized as more than $AU50,000, $AU20,000 to < $AU50,000, <$AU20,000 and not disclosed. Body Mass Index (BMI) was classified into four categories: underweight (BMI < 20 kg/m2), healthy weight (25 kg/m2 > BMI ≥ 20 kg/m2), overweight (30 kg/m2 > BMI ≥ 25 kg/m2) and obese (BMI ≥ 30 kg/m2). BMI values of less than nine and more than 50 were considered as invalid and these participants were excluded from analysis. Physical activity was defined as sessions per week including the sum of the total number of sessions spent walking and doing moderate activity and double the number of sessions of vigorous activity. This was based on the questions: “In the last week how many times have you walked continuously for at least 10 minutes for recreation or exercise or to get to or from places?”; “how many times did you do any vigorous physical activity that made you breathe harder or puff and pant?”; and “In the last week how many times did you do any other more moderate physical activity that you have not already mentioned?” Sessions of physical activity were grouped into four categories: None, ≤5, >5- ≤ 20, and more than 20 sessions per week. Where more than 200 sessions per week were reported, this was considered as invalid and the record was excluded from analysis.
Smoking status was classified into two categories: never being a regular smoker or ever being a regular smoker.
Analysis was performed using the STATA statistical package Version 11. Univariate analyses were used to describe the general characteristics of the study population. Multivariable logistic regression models with self-reported diabetes as the dependent variable were used to estimate odds ratios (ORs) with 95% confidence intervals (CI). Three models were built: adjusted for age and sex only; adjusted, for age, sex, country of birth and measures of socioeconomic status (household income, educational qualification, work status); and adjusted for age, sex, country of birth, measures of socioeconomic status and lifestyle-related factors (BMI, physical activity, and smoking). Significance was considered at the 5% level.