1 Department of Nutrition, Harvard School of Public Health
2 Division of Nutrition, St John's Medical College and Hospital
3 Department of Biostatistics, Harvard School of Public Health
4 Department of Epidemiology, Harvard School of Pubilc Health
5 Department of Cardiology, All India Institute of Medical Sciences
Correspondence: Dr Tanuja Rastogi, NEB, Division of Cancer Epidemiology & Genetics, National Cancer Institute, NIH, DHHS, 6120 Executive Blvd, EPS 320, Rockville MD 20852 USA. E-mail: trastogi{at}post.harvard.edu
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Abstract |
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Methods We conducted a hospital-based case-control study and collected data from 350 cases of acute myocardial infarction and 700 controls matched on age, gender, and hospital in New Delhi and Bangalore. We used conditional logistic regression to control for the matching and other risk factors.
Results Of the controls, 48% participated in some form of leisure-time exercise compared with 38% of cases. In age- and sex-adjusted analyses, people in the highest level of leisure-time exercise (>145 metabolic equivalents [MET]-minutes per day, equivalent to 36 minutes of brisk walking per day) had a relative risk of 0.45 (95% CI: 0.31, 0.66) compared with non-exercisers. Multivariate adjustment for other risk factors did not substantially alter the association. We observed a positive association between non-work sedentary activity and CHD risk; people with >3.6 hours per day of sedentary activity (for example, television viewing) had an elevated risk of 1.88 (95% CI: 1.09, 3.20) compared with <70 minutes per day in multivariate analysis.
Conclusion Leisure-time exercise, including as much as 3540 minutes per day of brisk walking, was protective for CHD risk and sedentary lifestyles were positively associated with risk of CHD. Given limited resources for care of CHD in India and the important role of physical exercise in disease risk in urban India, improvements in physical activity should be promoted.
Accepted 7 October 2003
Cardiovascular disease (CVD) is the leading cause of death in India,1 and its contribution to mortality is rising; deaths due to CVD are expected to double between 19852015.24 Regular physical activity reduces the risk of obesity, blood lipid abnormalities, hypertension, and non-insulin dependent diabetes mellitus,5,6 and has been shown to reduce substantially the risk of coronary heart disease (CHD). Conversely, measures of sedentary lifestyles or physical inactivity have been associated with a 1.5- to 2.4-fold elevation in CHD risk.5 It is estimated that US$24 billion or 2.4% of the US health care expenditure is directly related to a lack of physical activity.7
As a result of economic changes and increased mechanization, the prevalence of physical inactivity is increasing in India, particularly in urban areas, to levels comparable with the West.6,8,9 However, the association between leisure-time exercise, sedentary lifestyles, and risk of CHD has not been assessed within India. We conducted a hospital-based case-control study of acute myocardial infarction (AMI) in two major cities in India to address the relation between leisure-time exercise and sedentary activity and risk of CHD. To our knowledge, this is one of the first investigations to date, if not the first, into the relation between physical activity and CHD risk in India.
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Methods |
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Controls
For each case, two controls matched by age (within 5 years), gender, and hospital were obtained from non-cardiac outpatient clinics or inpatient wards. The same exclusion criteria utilized for cases were applied for control selection. We identified approximately 707 eligible control subjects, of whom 7 declined to be interviewed. Controls were relatively healthy individuals with minor ailments or conditions and were obtained from the following wards and clinics (% of total controls): eye (37%), ear, nose, and throat (21%), dermatology (14%), orthopaedic (10%), surgery (7%), general medicine (7%), gynaecology (3%), other (<1%).
Controls were selected using one of two methods depending on the hospital. In the first method, research assistants were assigned to accompany a particular physician during an outpatient clinic, following a weekly schedule of clinics and wards. At the end of each consultation, the physician or the physician's assistant invited the patient to speak with the research assistants about their lifestyle and diet. Patients that were eligible according to study criteria were then informed of the study and asked to participate. In these situations, participation was 100%. In the second method, research assistants independently identified control patients from clinics and wards. Assistants attempted to approach all individuals present during a particular outpatient clinic or in a specified ward. In large clinics, patients were screened for eligibility and invited to participate according to their queue number (highest number first). This method was used to prevent arbitrariness in the selection of controls. Basic demographic information was collected from all those approached. If an individual was eligible, assistants briefly explained the study and asked if they were willing to participate. Among those approached, only seven individuals who were eligible refused to participate.
Data collection
The study was approved by the relevant institutional review boards. Interviews were conducted in hospital wards or clinics by one of four research assistants and lasted approximately 25 minutes. Research assistants collected data on socioeconomic status, smoking history, history of hypertension, diabetes, hypercholesterolaemia, family history of CVD (including CHD, angina, myocardial infarction, hypertension, diabetes, stroke, sudden death, bypass surgery), dietary intake, types of fat or oils used in cooking, nutritional supplement use, and physical activity. Next, anthropometric measures (height, weight, hip and waist circumferences) were obtained and body mass index (BMI) (weight (kg)/height (m2)) and waist to hip ratio (WHR) were calculated. Waist and hip measures were assessed using a standardized tape measure with waist measures taken at the midpoint between the costal margin and iliac crest and hip measures taken at the widest circumference. Study questionnaires were coded and double entered.
Physical activity questionnaire
Physical activity levels were assessed using a validated physical activity questionnaire specific for the Indian population that focused on occupational and other non-leisure time activities, in addition to leisure-time exercise.11 It was validated by comparing energy expenditure (determined by the questionnaire) with energy intake as measured by 24-hour dietary recalls. A significant positive correlation was reported (r = 0.33, P = 0.02) which was comparable with other validation studies where energy expenditure was assessed using a physical activity questionnaire.1113 Subjects were asked to report the average time spent at work and average frequency of activities related to leisure or recreation, household chores, as well as sedentary and daily activities over the last month. The intensity or metabolic equivalents (MET) of the reported activities were obtained from the Compendium of Physical Activities.14 For those activities not listed in the Compendium, the MET of a similar activity was assigned. The questionnaire provided an estimate of overall energy expenditure per day and the energy expenditure of specific categories of activities (work, leisure-time exercise, hobbies, chores, and sedentary activities). Metabolic equivalent-minutes (MET-min), a measure of both intensity and duration of specific activities, were also derived to assess levels of leisure-time exercise.
Statistical analysis
To assess the potential for confounding, mean values of CHD risk factors were examined across levels of leisure-time exercise (assessed in met-minutes), sedentary activity (minutes), and work-related activities (minutes) among controls. Continuous covariates were categorized to avoid assumptions of linear associations with the outcome and to minimize the effect of outlying values. Participants were grouped into quartiles (sedentary activity), or into tertiles (leisure time exercise), or two categories depending on the distribution of each variable and the number of subjects within each category. To evaluate the relation between leisure-time exercise and risk of CHD, we used conditional logistic regression, first controlling only for the matching factors (age, sex, hospital), and then, in addition, other potential risk factors. Analysis of leisure-time exercise compared risk associated with different levels of exercise to non-exercisers. Similar analyses were performed for sedentary (non-work) and work-related activities. For work-related activities, total time spent at work, and average time spent sitting, standing, walking, and in strenuous activities at work were assessed. We also examined whether the associations observed with leisure-time exercise and sedentary activity and CHD risk were modified by: age, gender, cigarette and bidi smoking, BMI, WHR, alcohol intake, education, or income. All analyses were conducted in Statistical Analysis Software (version 8).
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Results |
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The mean 24-hour energy expenditure among control subjects was 10 536 kJ (standard deviation 2939). Men had higher energy expenditure than women (10 844 compared with 8356 kJ/day). On average, the relative contributions of major type activities to total energy expenditure among controls were 32.2% for work-related activity, 7.7% for sedentary activity, and 4.3% for leisure-time exercise. Approximately, 48% of all controls were participating in some form of leisure-time exercise. This included walking, jogging, yoga, gardening, as well as sports and games such as badminton, swimming or those recreational activities that had MET ranging from 3 to 8. On the other hand, cases spent more time in sedentary activities or behaviours such as watching television, listening to music, reading and writing, or other activities with MET of 2. The majority of sedentary time was spent watching television (47%).
The mean values of CHD risk factors across levels of leisure-time physical activity, sedentary, and work-related activity are presented in Table 1. People in the highest level of leisure-time exercise were older, more educated, consumed fewer cigarettes, and had less family history of CHD, and lower BMI and WHR than non-exercisers. The higher prevalence of history of hypertension and diabetes among exercisers may reflect doctors' recommendations to hypertensives and diabetics to improve physical activity. People with the most sedentary lifestyles were older, had higher socioeconomic status and increased BMI, and consumed more cigarettes yet less bidis (small unfiltered cigarettes) and were less likely to be involved in manual labour than the least sedentary. They also spent less time in work-related activity, and had higher prevalences of hypertension, diabetes, and family history of CHD than the least sedentary. People in the highest level of work activity were usually younger, more educated men who smoked more and were involved in less sedentary activity, but also in less leisure-time exercise than those who had no work-related activity.
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Leisure-time exercise and sedentary activity variables were not highly correlated in our study population (r = 0.09 among controls and r = 0.16 among cases), we therefore were able to examine their joint effect. We conducted this analysis using a multivariate model that included indicator variables for each combination of leisure-time exercise and sedentary activity. Figure 2 illustrates the relation of leisure-time exercise to CHD risk across levels of sedentary activity: people in our study population with the highest amount of leisure-time exercise and the least amount of sedentary activity had over a 70% reduction in risk in multivariate analysis compared with people who were the most sedentary with the least amount of leisure-time exercise. This finding was not altered after further control for work-related activity.
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Discussion |
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Potential sources of bias in our investigation include the selection of controls and a differential recall among cases compared with control subjects. While the use of population-based controls would be ideal, we addressed selection bias that may arise with use of hospital-based controls by obtaining control subjects from seven different outpatient clinics and inpatient wards in hospitals. Therefore, if an association exists between the exposure of interest and the disease status of one control group, the bias that may result would be diluted.16 Additionally, we used a systematic method of control selection that would avoid arbitrary selection of controls within wards and clinics. To the extent that physical activity is also protective for these conditions, such bias would attenuate the association observed between physical activity and CHD risk in our study. Selection bias among case subjects, where only cases who survived long enough to be interviewed were included, would also be minimized as only 25 cases did not survive to be included in the study. Although health conscious individuals may have been more likely to give consent for the study, overall participation was high, and bias from this source is likely to be modest. Controls in our population were slightly more educated and had lower incomes than cases, but controlling for socioeconomic factors did not alter our findings. Differential recall of physical activity is also a potential concern; however, research assistants asked case subjects to specifically report on their level of activity and exercise prior to their myocardial infarction. While the potential for recall bias may exist, awareness of heart disease prevention and health consciousness may not be as high among our study population in India as its is in Western countries; hence the likelihood of recall bias may be lower. We also excluded all those with any prior heart disease from the study. As well, while we cannot exclude the possibility that not all activities were reported, interviewers were trained to conduct thorough interviews on all activities done during the day including leisure-time and activities of daily living. While future prospective research within India will adequately address these biases, the case-control design had the advantage of being cost, resource, and time efficient.
The apparent protective effect we observe with moderate-intensity exercises such as brisk walking at a frequency of 3540 minutes per day is consistent with US recommendations stating that individuals accumulate at least 30 minutes or more of moderate-intensity physical activity on most, or preferably all, days of the week.5,6,9 As well, findings from a large prospective study of US women indicate that approximately 3 hours per week of brisk walking was associated with a 3040% reduction in CHD risk,17 and in a cohort of US men, individuals expending the equivalent of 30 minutes in moderate-intensity activities had a 20% lower risk for CHD.18 Moreover, recent findings from a cohort of post-menopausal US women indicate that the reduction in CVD risk that is associated with vigorous exercise is similar for walking.19 The elevation in risk observed with increased sedentary activity independent of leisure-time exercise is consistent with research that has described physical activity and physical inactivity (such as television viewing) as different risks.20,21 While reduced levels of physical activity lead to lower energy expenditure and lower lean body mass, physical inactivity or sedentary activities may be related to obesity and risk for chronic disease through increase in energy intake and not just lower energy expenditure. Research on work-related activity has generally not shown an association with risk. Data from the Israeli Ischemic Heart Disease Study indicated that among middle-age men leisure-time exercise, but not work-related activity, was associated with a significant reduction in risk for CHD and all-cause mortality.22 Work-related activity was also associated with a slightly higher all-cause mortality risk among a cohort of Swedish men, but the association did not remain after further control for smoking, occupational class, and alcohol abuse.23
Several biological mechanisms could explain the beneficial effects of physical activity on CVD risk, including lowering of blood pressure, elevation in high density lipoprotein levels, increased insulin sensitivity, improved endothelial function, and reduced atherogenic cytokine production.5,6,2434
Our findings are the first to highlight the adverse health consequences of physical inactivity and the importance of leisure-time exercise in the prevention of CHD risk among Indians. Given limited resources for the care of CHD and the potential benefit of exercise in urban India where physical inactivity levels are now comparable with the West, population-based health strategies should focus on promoting regular physical exercise.
KEY MESSAGES
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Acknowledgments |
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References |
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