Changes in Leisure-time Physical Activity and Risk of Death: An Observational Study of 7,000 Men and Women
Peter Schnohr ,
Henrik Scharling and
Jan Skov Jensen
From the Copenhagen City Heart Study, Bispebjerg University Hospital, Copenhagen, Denmark.
Received for publication July 9, 2002; accepted for publication March 19, 2003.
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ABSTRACT
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Associations of regular leisure-time physical activity and changes in leisure-time physical activity with risk of death were studied in 7,023 healthy men and women aged 2079 years in Copenhagen, Denmark. Physical activity was estimated in both 19761978 and 19811983. Men consistently engaging in a moderate or high degree of physical activity, respectively reported at both examinations, had significantly lower risks of death than men reporting low activity at both examinations. Adjusted relative risks were 0.71 (95% confidence interval (CI): 0.57, 0.88; p = 0.002) and 0.61 (95% CI: 0.48, 0.76; p < 0.001), respectively. Similar relative risks were found in women: 0.64 (95% CI: 0.52, 0.79; p < 0.001) and 0.66 (95% CI: 0.51, 0.85; p = 0.001), respectively. Men who increased their leisure-time physical activity from low to moderate or high had a significantly lower risk of death than men reporting low physical activity at both examinations (relative risk = 0.64, 95% CI: 0.50, 0.81; p < 0.001). In this study, maintaining or adopting a moderate or high degree of physical activity was associated with lower risk of death across a wide range of ages in both sexes.
exercise; leisure activities; men; mortality; women
Abbreviations:
Abbreviations: CI, confidence interval; RR, relative risk.
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INTRODUCTION
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Within the past 50 years, several studies have shown an inverse relation between physical activity and mortality, particularly mortality from coronary heart disease. The great majority of these studies were based on a single estimate of physical activity (111); few reported on the effect of changes in physical activity or fitness (1218). Most of the studies focused on middle-aged men, while women were only scarcely investigated. Only two larger studies included women from all age groups (14, 15).
Thus, in an observational population study based on a random sample of 7,000 men and women from Copenhagen, Denmark, we investigated the impact of both regular leisure-time physical activity and altered physical activity on risk of death in healthy younger, middle-aged, and elderly men and women.
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MATERIALS AND METHODS
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Participants
The Copenhagen City Heart Study is a prospective cardiovascular population study comprising a random sample of 19,329 men and women aged 2093 years who were drawn from the Copenhagen Population Register as of January 1, 1976 (19, 20). The first examination was carried out in 19761978 (participation rate = 74 percent) and the second in 19811983 (participation rate = 70 percent). A total of 10,147 persons (4,471 men and 5,676 women) aged 2079 years at the first examination, with no history of coronary heart disease at the second examination and with information on all variables included, attended both examinations. Participants who had been hospitalized (3,124 persons) during the period between the first and second examinations were excluded, which left 7,023 healthy subjects (3,220 men and 3,803 women) eligible for survival analysis.
Survey method
Identical methods were applied in the first and second examinations. Established procedures and examinations for cardiovascular epidemiologic surveys were used (21). A self-administered questionnaire requesting information about physical activity, smoking, alcohol consumption, and socioeconomic status was completed and checked by the staff. Blood pressure, plasma cholesterol and glucose concentrations, and body mass index (weight (kg)/height (m)2) were measured as previously described (19, 20).
Physical activity during leisure time was graded in four levels based on a questionnaire constructed by Saltin and Grimby (22), with minor modifications. Levels of activity were defined as follows: 1) being almost entirely sedentary (e.g., reading, watching television or movies, engaging in light physical activity such as walking or biking for less than 2 hours per week); 2) engaging in light physical activity for 24 hours per week; 3) engaging in light physical activity for more than 4 hours per week or more vigorous activity for 24 hours per week (e.g., brisk walking, fast biking, heavy gardening, sports that cause perspiration or exhaustion); and 4) engaging in highly vigorous physical activity for more than 4 hours per week or regular heavy exercise or competitive sports several times per week. In this analysis, level 1 was considered low physical activity, level 2 was considered moderate physical activity, and levels 3 and 4 together were considered high physical activity. We assumed that low, moderate, and high physical activity corresponded to <4, 46, and >6 metabolic equivalents, respectively. The activity questionnaire has been shown to discriminate between sedentary persons and their more active counterparts with respect to maximal oxygen uptake (23).
Endpoints
Participants were followed from the second examination in 19811983 to August 31, 2000, or death, using the unique personal identification number in the national Central Person Register. The completion rate of follow-up for death was almost 100 percent.
Statistical methods
To assess the independent contribution of physical activity to death, we used Coxs proportional hazards regression analysis with age as the underlying timescale and delayed entry accordingly. Using age as the underlying timescale ensured optimal adjustment for age (24). Relative risks were calculated as the proportional hazards ratios. Additional covariates measured at the second examination were included as categorical dummy variables: smoking status (none, former, or current), plasma total cholesterol level (<5, 56.9, or
7 mmol/liter), systolic blood pressure (<140, 140159, or
160 mmHg, or use of antihypertensive medication), diabetes mellitus (none, non-insulin-dependent diabetes mellitus/nonfasting plasma glucose level
11.1 mmol/liter, or insulin-dependent diabetes mellitus), alcohol consumption (<1, 12, 34, or
5 alcoholic drinks per day), body mass index (<20, 2024.9, 2529.9, or
30), years of education (<8, 810, or
11 years), and income (low, middle, or high (approximately equivalent to a pretax income of $<1,000, $1,000$2,000, and $>2,000 per month)). Proportional hazards were checked by reference to log-log survival plots for each variable. In two-sided tests, p values less than 0.05 were considered statistically significant. All analyses were performed using SAS (25) and Stata (26) software.
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RESULTS
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Baseline characteristics of the 3,220 men and 3,803 women seen at the second examination in 19811983 (i.e., at the start of follow-up) are presented in table 1. The data show a tendency toward a higher risk factor burden for men and women in the lower physical activity groups. The total numbers of deaths during 17 years of follow-up were 1,424 in men and 1,301 in women.
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TABLE 1. Baseline characteristics of men and women aged 2079 years (n = 7,023) according to leisure-time physical activity level* at two examinations (19761978 and 19811983), Copenhagen City Heart Study
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Table 2 presents the numbers of participants, the numbers of deaths, and adjusted relative risks of death according to leisure-time physical activity at the two examinations, by sex. In general, men had a higher level of physical activity than women. A low activity level at both examinations served as the reference category for relative risk estimation. Among the men, consistently engaging in moderate or high physical activity was associated with a lower risk of death (relative risk (RR) = 0.71 (95 percent confidence interval (CI): 0.57, 0.88; p = 0.002) for consistently moderate activity and RR = 0.61 (95 percent CI: 0.48, 0.76; p < 0001) for consistently high activity). Women exhibited similar risk reductions (RR = 0.64 (95 percent CI: 0.52, 0.79; p < 0.001) and RR = 0.66 (95 percent CI: 0.51, 0.85; p = 0.001), respectively). Men who changed from a low level of activity to a moderate or high level of activity had similar risk reductions (RR = 0.64 (95 percent CI: 0.49, 0.83; p = 0.001) and RR = 0.64 (95 percent CI: 0.47, 0.87; p = 0.004), respectively). Similar results were observed among women (RR = 0.75 (95 percent CI: 0.57, 0.97; p = 0.029) and RR = 0.72 (95 percent CI: 0.50, 1.05; p = 0.091), respectively). If men decreased their physical activity from high to low, the risk of death was increased (RR = 1.82, 95 percent CI: 1.27, 2.61; p = 0.001) (this relative risk, 1.11 divided by 0.61 with consistently high activity as the reference group, is not shown in the table). Conversely, this was not the case among women. No interaction between sex and the nine different physical activity groups was found.
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TABLE 2. Adjusted relative risk of death in relation to leisure-time physical activity level* at two examinations (19761978 and 19811983) in men and women aged 2079 years, Copenhagen City Heart Study, 19762000
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In table 3, the study population is stratified by age at the first examination. Since there was no significant difference in the relative risk of death when participants changed from low to moderate physical activity or from low to high physical activity, the moderate and high activity groups were pooled. In all age and sex strata, increasing ones physical activity from low to moderate/high was associated with a tendency toward a lower risk of death. This was particularly pronounced among elderly men (RR = 0.46, 95 percent CI: 0.27, 0.78; p = 0.004). When middle-aged men decreased their physical activity from moderate/high to low, the risk of death was 1.57 (95 percent CI: 1.15 2.14; p = 0.005) (this relative risk, 1.27 divided by 0.81 with consistently moderate/high physical activity as the reference group, is not shown in the table). No interaction was found between the four different groups of physical activity and age in men or women. There was no interaction between the four different groups of physical activity and other risk factors under study.
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TABLE 3. Adjusted relative risk of death in relation to leisure-time physical activity level* at two examinations (19761978 and 19811983) in men and women, by age group, Copenhagen City Heart Study, 19762000
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DISCUSSION
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This observational study of 7,023 healthy persons has shown that both men and women who regularly engage in low levels of physical activity during leisure time have a significantly higher risk of death than persons engaging in moderate or high levels of physical activity. Furthermore, it has shown that change in physical activity is associated with risk of death. The major observation is that both men and women aged 2079 years who increased their leisure-time physical activity from a low level to a moderate level had a significantly reduced risk of death. No further risk reduction occurred when physical activity was increased to a high level. The benefit of increasing physical activity was particularly pronounced among men aged 6579 years.
In men, risk of death was highly increased when the level of physical activity was reduced from high to lowfor instance, because of disabling disease that occurred between the two examinations. However, through exclusion of all participants who had been hospitalized during the period between the two examinations, we partly ruled out this possibility. Furthermore, we repeated the analyses after including a lag period of 2 years at the beginning of follow-up and found similar results.
Assessment of leisure-time physical activity is difficult. A major strength of our study is the application of the same self-reported questionnaire at both examinations 5 years apart. The questionnaire discriminates sedentary persons from their more active counterparts with respect to maximal oxygen uptake (23). Furthermore, the questionnaire has revealed significant associations between physical activity level and mortality in other population studies (10, 14). The questionnaire divided the present study population into three physical activity groups: low, moderate, and high. The former two groups were defined according to the amount of time per week used for physical activity, either less than 2 hours or 24 hours. The latter group was additionally defined according to intensity. In general, our results indicated a benefit of increasing ones physical activity level from low to moderatefor example, average walking for 24 hours per weekwith no further longevity obtained by increasing to a high level of physical activity.
No interaction was found between leisure-time physical activity and body mass index, smoking, plasma cholesterol level, blood pressure, alcohol consumption, education, income, or diabetes. Therefore, the effect of changing ones physical activity level can be considered independent of these factors.
Most studies describing the association between physical activity and all-cause mortality or coronary heart disease have focused on middle-aged men. The present study involved a large number of both women and men and confirmed the findings of a previous study by Kaplan et al. (15), which had a similar design and magnitude. In contrast to our findings, another Danish study of elderly men showed that increased physical activity was associated with increased risk of coronary heart disease (nonfatal and fatal) (27). However, other studies have found reduced risk of coronary heart disease with increased physical activity, also in elderly persons (1218). Walking has been associated with a lower risk of death (28), while other studies have demonstrated benefit from vigorous exercise only (111). This is probably a consequence of physical activity per se, but it could also be due to other associated lifestyle factors.
There are plausible mechanisms for a link between increased physical activity and decreased risk of death. Physical activity increases insulin sensitivity, improves lipid profiles, lowers blood pressure, reduces platelet aggregation, increases fibrinolytic activity, improves cardiac function, increases maximum oxygen uptake, and lowers resting heart rate. Other mechanisms have also been described. The beneficial effects of physical activity do not appear to be mediated by genetic factors (29).
Because our study was observational and not a randomized trial, we can demonstrate associations but not causalities. However, we stress that our cohort was a large random sample that included both men and women from the general population of Copenhagen, and the attendance rate of those invited to participate was high. Furthermore, the risk estimates were adjusted for diabetes, alcohol consumption, body mass index, education, and income.
In conclusion, these prospective data from a large observational population study of men and women indicate that risk of death is significantly higher in sedentary men and women than in their more active counterparts and that sedentary persons who increase their physical activity to a moderate level (e.g., average walking for 24 hours per week) increase their survival rate. This applies to all age strata and both sexes. Thus, it is never too late to take up physical exercise, and from a public health policy viewpoint, all men and women should be encouraged to engage in at least a moderate level of physical activity.
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ACKNOWLEDGMENTS
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This study was supported by grants from the Danish Heart Foundation and the Danish Medical Research Council.
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NOTES
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Reprint requests to Dr. Peter Schnohr, Copenhagen City Heart Study, Bispebjerg University Hospital, Entrance 33, DK-2400 Copenhagen NV, Denmark (e-mail: peter{at}schnohr.dk). 
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