The main finding of our study is that NLTPA is independently associated with lower mortality risk among elderly Taiwanese, even after adjusting for age, gender, educational level, smoking and drinking habits, living status, medical history, BMI and LTPA. Subjects who engaged in more NLTPA had lower mortality risk especially in the less LTPA group. In our study, NLTPA can predict the survival of older people, and its predictive power was no less than that of LTPA.
Our results are consistent with three cohort studies which suggest that NLTPA is independently associated with lower all-cause mortality among the elderly[12, 19, 20]. In the United States, Glass et al. conducted a 13-year study on elderly subjects in New Heaven which showed that engagement in productive activities, such as gardening, preparing meals, and shopping, could reduce the mortality risk as much as fitness activities did with adjustment of sociodemographic variables, functional disability, and medical history. Morgan et al. also demonstrated a positive influence of customary activities, predominantly NLTPA (including indoor and outdoor housework, shopping, purposeful walking and so on) on 10-year survival among British subjects aged 65 years or over. Arrieta and Russell estimated that mortality risk reductions were 34% and 38% for moderate and high NLTPA respectively, compared with low NLTPA in person aged 60-74 over a 20-year period. Arrieta and Russell's study was derived from the National Health and Nutrition Examination Survey (NHANES I, 1971-1975) and its Epidemiologic Followup Study NHEFS, a large, nationally representative sample of non-institutionalized U.S. adults aged 35-74 at baseline.
Although the exact mechanisms relating NLTPA to longevity are not well known, several hypotheses have been suggested, and psychosocial pathways are thought to be in operation in addition to direct physical functioning. The psychological effects of NLTPA, such as the maintenance of normal mental states, distraction from negative emotions, and a sense of self-mastery, are proposed to have a positive health impact that goes beyond the benefit of fitness[19, 26]. Engagement in social and productive activities may enhance the sense of having a meaningful social role, which then promotes a feeling of self-efficacy and social support. In addition, purposeful social roles have been found to be associated with decreased mortality in later life[27, 28]. Secondly, increasing evidence suggests that total energy expenditure through any activity, rather than a specific level of intensity, may confer a survival advantage in older adults[1, 17, 29]. For example, Manni et al. showed that community-dwelling older adults experienced a 32% lower risk of mortality for every 287 kcal/day in free-living activity energy expenditure. Short bouts of activities, like NLTPA, can be achieved easily by most elderly people. Finally, the beneficial effects of doing housework may be similar to occupational therapy, which can prevent disability and promote quality-of-life in older adults.
Because the content of NLTPA is culturally sensitive, research on different ethnic groups is necessary. For example, washing clothes by hand is a common activity for the elderly in our study, but is less common in the West. Previous studies provide tentative evidence for a link between NLTPA and elderly longevity among British and American subjects[12, 19]. This study has examined the effects of productive and transportation activities on the mortality risk among Taiwanese elderly, and the results corroborate the earlier findings that increased physical activity, including NLTPA, can decrease the mortality rate in the elderly.
There are several limitations associated with this observational prospective study that warrant consideration. First, there is a potential non-response bias, since the non-responders were older and had a higher prevalence of widow/widower status, and also might be expected to have a higher mortality rate and lower participation in NLTPA and LTPA. Therefore, our results might under-estimate the negative association between NLTPA and mortality in this relatively younger elderly population. Second, some information bias might be inevitable when communication problems occurred in the interviews, perhaps due to hearing or cognitive impairment, which might lead to a non-differentiated misclassification of our data collection. Such problems may have underestimated the real excess risk. Third, we only semi-quantify the amount of NLTPA by frequency of activities, and thus results from this might not be very precise. If additional components of NLTPA, such as duration or amount, are available, the associations between activity and mortality might be more apparent. Further, the information about physical activities was collected for only the two weeks before the interview, and therefore misclassification might occur if the subjects had recently changed their lifestyle or health status. However, such misclassifications may weaken the apparent survival advantage of physical activities, so the real benefits will be more significant than seen in the present analysis. Since the residual confounders cannot be completely eliminated by multiple risk factors adjustments, the results of this study are not sufficient to infer causal relation between survival and NLTPA/LTPA. Finally, three Cox models failed to provide a good discriminatory power with a low C statistic-index. This may be due to the relatively small sample size and other unmeasured mortality risk factors.
The strengths of our study include that we provided a simple and useful culture-sensitive questionnaire to predict eight-year mortality of elderly Taiwanese subjects by level of NLTPA. The six-item questionnaire can be applied in community-based screening programs and identify the highest risk group among sedentary elderly who are engaged in neither NLTPA nor LTPA. Furthermore, all our data were collected by a qualified interviewer, and this minimized the measurement bias.
Our findings have several important implications. In research, the unique relationships between different types of physical activity and mortality underscore the necessity of including multiple domains of activity in epidemiologic studies. In health policy, we can identify the people in the greatest need of intervention among the sedentary elderly. According to the Taiwan National Health Interview Survey, up to 51.4% of elderly men and 42.2% of elderly women never exercise regularly. As a public health priority, we should categorize those individuals who do not engage in NLTPA among the irregular exercise group as a high-risk group of interest for intervention. In practice, healthcare professionals might also recommend a broader range of activities for older people, and encourage them to keep engaging in housework if they already do so. However, in Taiwanese social convention, it is not considered filial if children allow their elderly parents to do housework. We thus propose that the public health messages related to physical activity in this context must emphasize a broader range of activities for older people.