Setting and participants
Data were derived from the Men, Women and Ageing project, which incorporates data from the 1921–1926 birth cohort of the Australian Longitudinal Study on Women’s Health (ALSWH) and from the Health in Men Study (HIMS). Detailed methods for both studies have been described elsewhere [
16,
17].
Briefly, the ALSWH is a repeated survey of the health and well-being of three cohorts of women who were born in 1973–1978, 1946–1951 and 1921–1926 when recruited in 1996. The women were randomly selected from the Australian national health insurance database (Medicare), which includes all citizens and permanent residents, with intentional over-sampling of women from rural and remote areas16. The project uses mailed questionnaires to collect self-reported data on health and related variables every three years. In the 1921–26 birth cohort, 39,000 women were initially invited to participate; of these 1,100 were not contactable and 2,366 were ineligible. Of the 35,534 remaining women, 12,432 responded to the first survey in 1996. Because the physical activity questions in the first survey differed from those in the HIMS survey, data used in the current analyses were drawn from the second survey conducted in 1999, to which 10,430 women responded. The ethics committees of the University of Newcastle and the University of Queensland approved the research protocol.
The HIMS cohort was formed from men screened for abdominal aortic aneurysm in a randomised controlled trial conducted in Perth, Western Australia in 1996. In this trial, eligible men were aged 65–83 years, resident in Perth (the capital of Western Australia), and not in long stay institutional accommodation. A list of eligible men was drawn from an electronic copy of the electoral roll in 1996 (enrolment to vote is compulsory for adult Australians) and, after excluding 8,801 who were no longer resident in Perth and 2,296 who had died before the study began, the remaining men were randomised into a screening group (n = 19,352) or a control group (n = 19,352). Of those invited to be screened, 1,836 were ineligible, 5,303 did not respond or refused, and 12,203 were screened between 1996 and 199917. These 12,203 screened men formed the HIMS cohort and have been followed since their recruitment. The HIMS research protocol was approved by the ethics committee of the University of Western Australia.
Measurements
A postal questionnaire was used to collect data from the women, and all variables, including height and weight and smoking status, were self-reported. Data from the men were obtained using a postal questionnaire that was reviewed during a face-to-face interview by a research nurse at a clinic visit. During this visit, physical measures including height and weight were taken, and participants were asked about their current smoking status.
Age was calculated from the date at which the survey forms were returned. Because the age range was much greater for the men (65 to 83 years) than for the women (71 to 79 years), those men outside the age range of the women were omitted from the analysis (N = 6068 omitted).
Smoking status was categorized as ‘never smoked’, ‘ex-smoker’ or ‘current smoker’. BMI was calculated from height and weight (kg/m
2) and categorized according to WHO recommendations as ‘underweight’ (<18.5), ‘normal’ (18.5 to <25), ‘overweight’ (25 to <30) and ‘obese’ (> = 30) [
18]. Due to small numbers of participants in the underweight group and the possibility that these people were already suffering from significant disease, they were excluded from the analysis (N = 87 men and N = 319 women).
Participants in both ALSWH and HIMS were asked about their usual frequency and quantity of alcohol consumed and very few of these older people reported drinking large quantities of alcohol. Comparable categories of alcohol consumption were derived for women and men [
19]. These were: drinking alcohol less than weekly (including those who never or rarely drink), or drinking weekly or more often.
In both cohorts, participants were asked to report the duration of time spent in
vigorous leisure activity/exercise (that makes you breathe harder or puff and pant) in the last week (women) or a usual week (men)
. A MET (metabolic equivalent) value of 6 was applied to responses to these questions, in line with estimates for 'hard' physical activity in this age group [
20]. In addition, the women were asked to report time spent
walking briskly (for recreation or exercise, or to get from place to place), and in
moderate leisure activity (like golf, bowls, social tennis, moderate exercise classes), while the men were asked to report on
non-vigorous exercise for recreation or health and fitness (e.g. slow walking, slow cycling, Tai Chi, yoga, etc.) Responses to the women’s walking and moderate activity questions were combined to calculate time in non-vigorous exercise and a MET value of 3 was applied to the responses for non-vigorous activity from both men and women. A physical activity score was calculated from total minutes per week in each of the two categories of physical activity and the MET values: (non-vigorous minutes per week x 3.0 MET + vigorous minutes per week x 6.0 MET). Scores were categorized as ‘inactive’ (<600) or ‘active’ (≥600). The 600 MET.minutes/week threshold equates to 200 minutes of moderate activity/week, which is commensurate with current physical activity guidelines [
21].
Due to the differences in data collection methods missing data were more common from the women than the men. All participants with missing data were excluded from the analyses. The missing data were as follows: for smoking, women n = 781, men n = 0; for BMI, women n = 1389, men n = 9; for alcohol, women n = 1294, men n = 482; and for physical activity, women n = 1045, men n = 25; with some people having missing data for more than one of these variables. The resulting analysis data set was for N = 7438 women and N = 6053 men.
Outcome variable
The outcome variable was death, from any cause, within 10 years of collection of the risk factor data. For ALSWH participants, deaths were identified by matching identification information to the Australian National Death Index [
22]. For HIMS participants, all of whom lived in Western Australia at recruitment, death information was obtained through the Western Australian Data Linkage System [
23], which provides electronic linkage to the state’s population health data collections and includes records from the death register.
Statistical methods
Logistic regression models were fitted for each sex separately. Each of the explanatory variables was treated as categorical and age (three categories), smoking status (three categories), BMI (three categories) and physical activity (two categories) were modelled simultaneously. The models were assessed using various measures of goodness of fit and were then used to estimate the predicted proportion of deaths within 10 years for each combination of explanatory variables. Absolute risk estimates were calculated by inverting logits of predicted proportions of deaths. Sensitivity analyses were conducted for the physical activity variable using different multipliers for the MET values for women and men separately and together. All analyses were performed using Stata/SE 11.0.