Study Population
The sample was drawn from the 2006-2016 waves of the New Zealand HWR study, a prospective cohort study of community-dwelling older adults. The HWR commenced in 2006 as a biennial postal survey of a sample aged 55-70, randomly selected from the New Zealand electoral roll. An over-sample of adults of Māori decent was undertaken to ensure adequate representation of this section of the older population. The core questionnaire assesses domains of health and wellbeing; family and social support; work and retirement; financial wellbeing; and cultural identity. Cases included in the current analyses participated in the baseline survey and at least one subsequent follow up period and provided information on health-related quality of life at two or more time points. The original survey had a response rate of 53% (n = 6662), and of these, 2632 consented to participate in subsequent follow up. Compared to those who dropped out, those who completed at least one wave of follow up were older (60.8±4.5 vs. 61.1±4.6, p = 0.033), non-Māori (50.6% vs. 58.2%, p < 0.001), and more male (45.1% vs. 46.6%, p = 0.211). Of those willing to participate in longtidunal follow up, 1609 (41%) were lost to follow up over the five biennial follow up waves (212 to death, and remaining unknown). For the current analysis, participants with a diagnosis of diabetes at baseline who responded to at least one subsequent survey were selected, A comparison group of those without diabetes at the baseline who remained free from DM in all follow- up surveys (2008-2016) were selected. We excluded incident cases of diabetes (n=192). Figure one presents a flow chart illustrating inclusion criteria and attrition 2006-2016.
Measures
Sociodemographic variables
Demographic variables were as follows: age as two categories of 55-64 years and 65 and over and as a continuous variable, marital status in three groups of married/living with a partner, divorced/ separated/single, and widowed, ethnicity as Māori (indigenous New Zealanders) and non-Māori (Europeans, Asians, Pacific people and others) according to the priority ethnic groups in NZ. Education was categorised as no secondary, secondary, post-secondary and tertiary. Annual personal income was categorised as 0-25000, 25001-50000, 50001-70000 and >70000 NZ$.
Clinical variables
A list of self-reported doctor-diagnosed physical and mental conditions for the current analysis includes cardiovascular diseases (Heart conditions, stroke), neurologic diseases (epilepsy, Alzheimer/dementia, Parkinson, migraine headache), musculoskeletal (arthritis, osteoporosis, hip/knee replacement), asthma and Chronic Obstructive Pulmonary Diseases (COPD)), chronic liver diseases (cirrhosis), depression and other mental illness, and cancer. The sum of chronic conditions was calculated and categorised as none, one, two and more conditions. Hypertension, hearing and eye problems or sleep disorder, were considered separately as dichotomous variables. Height and weight were measured in the 2008 survey wave and was used to calculated BMI as healthy weight (<25), overweight (25-29.9), and obese (≥30).
Health behaviours
Current smokers were those who identified themselves as a regular smoker. Alcohol consumption was classified into two categories: regular alcohol consumption (2 or more drinks per week), and non-regular alcohol drinkers (≤1 drink per week). Physical activity (moderate/brisk walking or vigorous activity) over the last seven days were categorised into two levels: two or more times per week (sufficient), once per week/none (insufficient).
Health Related Quality of Life
Physical HRQOL was assessed using the SF12v2 (22). Ten items of the SF12v2 are rated and a scale of 1-5 and two items on a scale of 1-3. Standardised norm based orthogonal factor weights are used to calculate a PCS [positive weights for physical functioning (2 items), role physical (2 items), pain and general health] and a MCS [positive weights for vitality, social functioning, relationships (2 items) and mental health (2 items)] with reference a New Zealand population mean of 50 and standard deviaiton of 10 (23).
Statistical analysis
Data were extensively screened. Missing data for chronic conditions such as hypertension, stroke, and so on in some waves were replaced with available data from the preceding or the subsequent waves over the study period. All possible comparisons of the age/year at diagnosis in each wave were checked to ensure that the most reliable list was utilised.
Descriptive analyses were usd to describe the characteristics of the sample. The absolute changes in PCS and MCS were calculated by subtracting the follow-up score from the baseline score, which was the first SF12 completed for each person. For those with multiple scores, the average over time was considered.
A Generalised Estimating Equation (GEE) model was performed to examine the association between demographic, clinical and health behaviours variables with and HRQOL over time to account for longitudinal within-subject correlations. Variables were introduced as fixed (sex, ethnicity, education, personal income, BMI) or time-varying (other variables) into the models based on availability and completeness of the data in each wave. Two models were fitted separately for the physical and mental dimensions of the SF12; the comparison group for each model was people without diabetes. As a sensitivity analysis, the difference between SF12 scores for DM and non-DM participants was also estimated for its baseline values in both crude and adjusted models. Data were analysed using the STATA statistical package version 14; all estimates were reported with 95% confidence interval and a significance level of 0.05.