Impact of smoking, diabetes and hypertension on survival time in the elderly: the Dubbo Study

Risk factors for serious disease and mortality in elderly people have been documented in observational studies.1-6 The individual contributions of cigarette smoking, diabetes and hypertension have been noted in the clinical sphere;1-3 other factors, such as self-rated health, physical disability, marital status, social support and physical activity, have been noted in the sociodemographic sphere.4-6

The longitudinal Dubbo Study of elderly people commenced in 1988 with the broad aim of studying “healthy ageing” in a group of generally healthy senior citizens residing in the community.7-9 Mortality data for this group are now available for a follow-up period of 15 years. Uniquely, this group was already advanced in age at baseline (mean entry age around 70 years) and represents a substantial proportion of the eligible population in a well defined community over a relatively long period. This report examines the impact of specific risk factors and combinations of risk factors on survival time over the first 15 years of the study.

Over the 15 years, there were 668 (54%) deaths in men and 625 (40%) deaths in women. Mortality rates, by age, sex and major cause, are summarised in Box 1. Total mortality rates and death rates from cancer and respiratory disease were higher in men than women across all age groups. CHD death rates were higher in men than women until the 80-plus age group, while stroke death rates were similar for both sexes.

The initial Cox model included many biological and psychosocial variables that did not approach statistical significance. They included prior stroke history, lipid and lipoprotein levels, urinary incontinence, education level, home ownership, depression score, social support, social activities, physical activity, and pet ownership.

Key baseline variables and the final Cox proportional hazards model for prediction of time to death are presented in Box 2. Variables showing the strongest independent contribution to mortality in both sexes were current cigarette smoking, diabetes, impaired PEF, physical disability, use of BP medication, and having very high blood pressure. Prior CHD was predictive of death in men, as was a poor self-rating of health in women. Several variables independently predicted a lower risk of mortality: alcohol intake (both sexes) and being married at study entry (men only). The interaction of significant predictors with age was significant only for male smokers (HR 2.19 [95% CI, 1.63–2.93] for age 60–74 years versus 1.08 [95% CI, 0.66–1.78] for age 75+ years) (P < 0.002).

The reduction in survival time over 15 years’ follow-up was derived from survival curves in the Cox models and is presented in Box 3. Compared with those who had never smoked, 22 months of life were lost in male smokers and 15 months in female smokers. Men and women with diabetes both lost 18 months of life. There was substantial shortening of survival in both sexes associated with other factors such as hypertension, impaired PEF, physical disability and zero alcohol intake. Men (but not women) with prior CHD or who were not married at study entry also experienced a reduction in survival.

When factors such as smoking and taking any alcohol were combined, the adverse impact on survival was somewhat lessened, in line with the opposite effects of smoking and alcohol on survival. The reduction in survival was exacerbated in people who smoked while having impaired respiratory function, in those with diabetes who were also taking BP medication, and in those having physical disability combined with impaired respiratory function.

The predominant cause of death in elderly Australian men and women is CHD, with lesser but important contributions from cancer, stroke and chronic respiratory disease. Our multivariate model identified key predictors of mortality based on data collected 15 years earlier. The unique feature of our study is its assessment of the impact of specific risk factors and combinations of risk factors on survival time over 15 years of follow-up in a group with a mean entry age of about 70 years. The burden of disease attributable to risk factors has been the subject of other Australian research;13 however, these studies have been based on a cross-section of the population, and, for this and other reasons, direct comparisons with our study are not feasible.

The Dubbo Study has certain limitations. Although it involves a well defined population, to gain entry to the study Dubbo senior citizens needed to be non-institutionalised and able to attend baseline assessment. Most of the 27% of the target population who did not participate stated that they were “too old, too tired or too unwell”.7 In other words, we are studying the ageing process in a group of healthier “survivors” who have been willing and able to participate. On the other hand, we have succeeded in maintaining contact with 97% of the cohort.

Moreover, baseline observations were made at a single visit in 1988–89 and the Cox model assumes that the broad characteristics of the group will remain relatively unchanged over the time interval (eg, that people with diabetes will always behave as diabetics, or that those with an impairment in ADL will continue to be disabled). Although no treatments were offered by the study, interventions may have been offered by local medical officers or others. A variable such as marital status is quite likely to change over 15 years, so we are effectively measuring the effect of marital status before study entry. Furthermore, some of the associations found (or not found) between potential risk factors and mortality may have arisen through confounding by factors measured or unmeasured. Finally, some of the baseline data are based on self-reported behaviour and this may have introduced bias. Many of these variables have well- studied measurement effects.4-6

The demonstrated impact of smoking, diabetes, hypertension, impaired respiratory function and zero alcohol intake on mortality confirms the findings of previous studies.1-3,14-17 But the duration of follow-up is a key factor. In an earlier report on the Dubbo Study, at 10 years’ follow-up we calculated that any alcohol intake improved survival time by 8 months in men and 3 months in women.18 By 15 years, this had extended to 9 and 5 months, respectively. While we have confirmed that this finding is robust, there must be a point at which alcohol or other factors no longer impinge on survival! In our study, alcohol has been shown to have a protective effect on mortality across all apparent levels of consumption, in contrast to the “U”-shaped relationship described by others.19 This contrast can not readily be explained, except to note that a “U”-shaped relationship has not been a general finding in the elderly, possibly because these cohorts contain an excess of “healthy survivors” who do not maintain very high intakes of alcohol.17

The lack of association between some potential risk factors and mortality deserves comment. Lipid and lipoprotein levels at baseline certainly predict heart attack in the Dubbo cohort,20 yet many other factors contribute to CHD in old age and only about a third of deaths relate to CHD. Prior stroke at baseline failed to predict mortality; however, people with prior stroke comprised only 6% of the cohort, and they were clearly mild cases still living in the community. It is intriguing that prior CHD (found in 25% of men and 19% of women) showed significant prediction of mortality only in men. Most likely, this is not a real biological difference, but may be related to the heterogeneous criteria used to define prior CHD.

The Dubbo Study is an observational study. Where a risk factor for mortality has been identified, we can only hypothesise that intervention may be beneficial. Studies in Seventh-Day Adventists have shown that a lifestyle making appropriate choices in diet, exercise and cigarette smoking does indeed add years to life expectancy.21 Other observational studies have confirmed this potential benefit.22-24 Controlled intervention studies in hypertension25 and diabetes26,27 do suggest that a benefit can be realised. In our study, the reduction in survival among elderly Australians in the presence of cigarette smoking, diabetes and hypertension highlights the potential benefit from intervention. This may be capable of improving survival and hopefully improving quality of life, but further work is needed to assess this potential.