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Preventing Coronary Heart Disease

The Role of Physical Activity

I-Min Lee, MBBS, ScD; Ralph S. Paffenbarger, Jr, MD, DrPH

Paul D. Thompson, MD
Exercise and Sports Cardiology Series Editor


In Brief: Over the past 50 years or so, many epidemiologic studies have examined the association between physical activity or physical fitness and coronary heart disease (CHD) risk. Their findings have been consistent, showing that physically active or fit men and women experience lower CHD risk than those who are sedentary or unfit. On average, active patients have half the risk of sedentary patients. Data regarding the optimal amount, intensity, and duration of physical activity required to decrease CHD risk have been less clear. It appears that following recent recommendations—at least 30 minutes of moderate-intensity physical activity (such as brisk walking) on most days—is sufficient.

Despite dramatic reductions in coronary heart disease (CHD) mortality rates over the past 25 years, CHD remains the leading cause of death in the United States and most western countries (1). In 1996, CHD was responsible for approximately 476,000 of the more than 2 million deaths in the United States, or about 1 in 5 deaths (2). CHD is the leading cause of death in both men and women. Heart disease rates are much lower in premenopausal women than in age-matched men, but after menopause women's rates increase substantially, so that women account for about half of all CHD deaths in the United States (3).

Many modifiable risk factors for CHD have been identified, including cigarette smoking, high blood pressure, dyslipidemia, obesity, diabetes, and physical inactivity (2). Several mechanisms may be responsible for the protective effects that physical activity confers.

Mechanisms for Decreasing CHD Risk

Physical activity has been postulated to reduce the risk of developing CHD through various mechanisms. Physical activity may have a direct effect on the heart: It increases myocardial oxygen supply, decreases oxygen demand, and improves myocardial contraction and its electrical impulse stability (4). Reduced oxygen demand and myocardial work are reflected in lowered heart rate and blood pressure at rest, and a general reduction in sympathetic tone. Physical activity also increases the diameter and dilatory capacity of coronary arteries, increases collateral artery formation, and reduces rates of progression of coronary artery atherosclerosis (5-8). Additionally, high levels of physical activity are associated with lower systolic and diastolic blood pressures (9), elevated levels of high-density lipoproteins (10-12), perhaps low levels of low-density lipoproteins, and increased insulin sensitivity and glucose tolerance (13-15).

Other likely protective mechanisms include reduced platelet aggregation and increased fibrinolytic activity, possibly resulting from lowered levels of plasminogen activator inhibitor-1 (16,17). Preliminary data (18) suggest that physical activity also may be associated with decreased levels of homocysteine, a risk factor for CHD. Finally, physically active individuals are less likely to be overweight, another CHD risk factor (19),

Physical Activity, Fitness, and CHD Risk Reduction

Reviews. In 120217, Powell et al (20) conducted a qualitative review of all English language articles containing sufficient information to calculate the relative risks of CHD associated with different levels of physical activity. Studies consistently supported an inverse association between physical activity and CHD risk. The inverse association was more pronounced in the better-designed studies. Berlin and Colditz (21) expanded on this work to show that, compared with active individuals, sedentary persons had about double the risk of developing or dying from CHD. Only 12% of the studies, however, included women, but more recent studies confirm that physical activity also decreases women's risk of CHD.

Several studies on physical activity and CHD risk are summarized in table 1 (22-35, 37), While they do not constitute a comprehensive review of the literature, they offer a representative selection.

TABLE 1. Selected Studies Examining the Relationship Between Physical Activity or Physical Fitness and Coronary Heart Disease

Study Findings Comments

British Civil Servants (22-24) Vigorous sports associated with reduced CHD risk First studies to examine association between PA and CHD risk
Harvard Alumni (25-29) Sedentary subjects have higher age-adjusted incidence of first MI (relative risk, 1.64) than more active subjects (25); vigorous activity predicts decreased CHD (27-29) Studies specifically designed to investigate PA and its relationship to chronic diseases, including CHD
Alameda County (30) Lower CHD mortality seen in active subjects; effect is seen in younger and older subjects CHD not specifically studied, but inverse relationship between PA and mortality holds in older subjects
Multiple Risk Factor Intervention Trial (MRFIT) (31) Moderate PA reduces CHD risk Tested multiple-factor interventions for reducing male CHD mortality
British Regional Heart (32) Men with and without CHD have lower rates of heart attacks if they engage in light to moderately vigorous PA Among men without CHD, vigorously active men had higher rates of first heart attacks than moderately active men, but reasons for this are unclear. Among men with CHD, heart attack rates declined steadily with higher PA levels.
Iowa Women's Health (33) The most active women had half the CV mortality of the least active women Demonstrated protective effect of PA for women; inverse PA-CHD risk relationship persisted when diet was considered
Finnish Twin Cohort (34) Significant trend for reduced CVD mortality with increased PA in male and female twin pairs First study to investigate genetic factors in relationship between PA and mortality in same-sex twins
Aerobics Center longitudinal (35) Strong inverse association between fitness level and all-cause mortality in men; trend for CV deaths similar, though few CV deaths in study Largest epidemiologic study of physical fitness; small number of CV deaths precludes a definitive conclusion
Lipid Research Clinics Mortality Follow-up (37) Eightfold difference in CV mortality and sixfold difference in CHD mortality rates between the least- and most-fit men Survival curve divergence makes finding less likely to be due to bias

CHD = coronary heart disease; PA = physical activity; MI = myocardial infarction; CV = cardiovascular; CVD = cardiovascular disease

British civil servants. Some of the earliest demonstrations of an inverse association between physical activity and CHD risk were seen in studies by Morris et al (22-24), In one study (24), they prospectively followed a cohort of 9,376 male British civil servants with sedentary jobs who were between 45 and 64 years of age and free of CHD. These men obtained most of their physical activity during leisure time and were habitual gardeners, do-it-yourselfers, and, less often, participants in recreational sports. Investigators categorized men according to the frequency of their participation in vigorous recreational and nonvigorous sports. Vigorous sports required at least 7.5 kcal per minute or six times the resting metabolic rate (6 METs). Nonvigorous sports were those requiring less energy. About 17.5% of men reported vigorous sports play, while twice that proportion reported nonvigorous sports play. During follow-up, CHD developed in 474 men.

As the frequency of participation in vigorous sports increased, the incidence of CHD declined. Men who did not engage in vigorous sports had a CHD incidence of 5.8 per 1,000 person-years, while those who engaged in more than eight episodes in the previous 4 weeks had an incidence of 2.1 per 1,000 person-years. These differences persisted after accounting for smoking habit, body mass index, and personal and family medical history; data on diet were unavailable. In contrast, no association was apparent with increasing frequency of nonvigorous sports play. Higher energy expenditures on nonvigorous sports play were also unrelated to CHD incidence rates. Physical activity had to be current in order for it to benefit patients. CHD incidence rates were high among men who did not play vigorous sports at study entry, regardless of whether they had played vigorous sports previously.

This study is one of the few specifically designed to investigate the relationship between physical activity and CHD risk. Detailed information collected on physical activity (type, intensity, frequency) allowed investigators to conclude that vigorous physical activity was necessary to reduce CHD risk.

Harvard Alumni Health Study. Paffenbarger and colleagues (25-27) have studied physical activity and risk of developing CHD in a cohort of male Harvard alumni. Physical activity was assessed at multiple time points: during college and at various intervals beginning in 1962, when the men were middle-aged.

In one study (25), investigators followed 16,936 alumni who were between 35 and 74 years old and free of CHD. Participants were studied from 1962 or 1966 (1962/1966) until 1972. The baseline questionnaire asked about the number of flights of stairs climbed each day, the number of city blocks walked each day, the types of sports or recreational activity engaged in, and the time spent on each. To obtain an index of weekly energy expenditure, investigators tallied the energy used. Physical activity was then related to incidence of first heart attack occurring between 1962/1966 and 1972.

The age-adjusted incidence rate of first heart attack among alumni who expended less than 2,000 kcal per week was 1.64 times higher than those who expended 2,000 or more kcal per week. This held true for the more sedentary participants regardless of age, whether or not they smoked cigarettes, had lower or higher blood pressures, were lean or not, or had parents who died young. As in the previous study described, only contemporary physical activity was beneficial.

Subsequent studies were done to clarify the role of intensity of physical activity in preventing CHD (28,29), In one study (29), investigators assigned a MET score to every sport and recreational activity. Total energy expenditure per week was the sum of kilocalories expended from blocks walked, flights climbed, and other activities performed. Total energy expenditure was divided into two components: vigorous (>6 METs) and nonvigorous activities (<6 METs).

Researchers found that vigorous, but not nonvigorous, activity significantly predicted decreased CHD risk. Adjusted for several factors (age, cigarette smoking, hypertension, diabetes mellitus, body mass index, and early parental death), the relative risks for CHD mortality declined significantly with increasing levels of vigorous energy expenditure. For nonvigorous activities, no significant trend was seen across categories of nonvigorous energy expenditure. While dietary data were unavailable for all men, subsequent analysis of diet in a subgroup revealed that dietary fat did not influence the findings.

This investigation (27-29) is one of the few specifically designed to investigate the relationship between physical activity and incidence of chronic diseases, including CHD, and examine questions regarding quantity, intensity, and timing of physical activity. The findings corroborate the British finding that reduced CHD risk requires current vigorous physical activity.

The Alameda County Study (30) was done with a random sample of adults from that area in California to determine whether factors associated with increased mortality risk in older men and women were the same as those for younger individuals. While CHD mortality was not specifically examined, a large proportion of deaths in participants would typically be expected to occur from cardiovascular causes.

Subjects were restricted to men and women who were 38 years old or older at baseline in 1965. Those participating completed a questionnaire about demographic, behavioral, social, and psychological characteristics. Physical activity assessment was based on frequency and presumed intensity of leisure-time participation in active sports, swimming, long walks, physical exercise, gardening, hunting, and fishing. Using these data, investigators classified participants as inactive or active.

After investigators adjusted for age, self-reported health status, and factors predictive of mortality in this cohort (cigarette smoking, alcohol intake, weight-for-height, hours of sleep, regular breakfast meals, and snacking), physical inactivity was a significant predictor of mortality among men and women of all ages, even among those 70 or older. When crude survival curves were plotted by age-group over the 17-year follow-up, the difference in mortality between inactive and active subjects was more marked in the oldest age-group.

The Multiple Risk Factor Intervention Trial (MRFIT) was designed to test whether multiple-factor intervention could reduce CHD mortality in 35- to 57-year-old men at high risk because of their cigarette habit, elevated diastolic blood pressure, and increased serum cholesterol levels. All men were free of clinical CHD at baseline (1973 to 1976).

Leon et al (31) followed 12,138 men with acceptable data on physical activity for an average of 7 years. Physical activity at baseline was assessed with the Minnesota Leisure-Time Physical Activity Questionnaire. Men were classified into thirds based on physical activity. Those in the least active third averaged 15 minutes of leisure-time physical activity per day at a cost of 74 kcal per day; those in the middle third, 47 minutes and 224 kcal per day; while those in the most active third averaged 134 minutes of activity and 638 kcal per day. Subjects engaged in mainly light (1.5 to 3 METs) and moderate (3.5 to 4.5 METs) activities and spent, on average, only 19% of total daily energy on heavy activities (>5 METs).

Findings were adjusted for differences in age, treatment assignment, cigarette smoking, diastolic blood pressure, and blood cholesterol. Results showed that as the level of energy expenditure increased, risk of fatal and nonfatal CHD declined. Least active men had an incidence of 71.8 per 1,000, while the most active men had an incidence of 58 per 1,000. Although this study was not specifically designed to address questions on physical activity, it showed that even moderate-intensity physical activity was inversely related to CHD risk.

The British Regional Heart Study (32) assessed cardiovascular disease among 7,735 men between the ages of 40 and 59 who were randomly chosen from general medical practices in socioeconomically representative communities in the United Kingdom. Subjects with and without preexisting CHD (n=7,630) provided information about leisure-time physical activities (regular walking or cycling, recreational activity, and sporting activity) and other health habits. Based on the frequency and intensity of their physical activities, men were classified into six ordinal groups: 9% were considered inactive, 31% occasionally active, 23% lightly active, 16% moderately active, 15% moderately vigorous, and 7% vigorously active. During 8 years of follow-up, 480 men had at least one major heart attack: 242 occurred in men without CHD, and 238 occurred in men with preexisting CHD.

Among men without preexisting CHD, rates of first heart attack declined with increasing physical activity until the moderately vigorous category. The rates for moderate or moderately vigorous men were less than half the rates of those for inactive men. Vigorously active men actually experienced higher rates—about equal to rates for men who were occasionally or lightly active—though this finding was based on a small group of men. These analyses were adjusted for additional social and health variables, but not for dietary differences.

Among men with preexisting CHD, rates of heart attack generally declined with increasing physical activity levels, with no increased rates among men in the most vigorous category. The findings among men without preexisting CHD are in contrast to most other studies that have observed CHD risk to decline progressively with higher levels of physical activity. The reasons for this observation remain unclear.

The Iowa Women's Health Study was conducted with postmenopausal women recruited from a random sample of 55- to 69-year-old Iowa women drivers. Kushi et al (33) analyzed 40,417 women for whom physical activity and cigarette smoking status was available at study entry. In 120216, women completed a baseline questionnaire on physical activity and other health habits. Physical activity was assessed using two questions that asked about the frequency of moderate activity (eg, bowling, golfing, light sports or physical exercise, gardening, or long walks) and vigorous activity (eg, jogging, racket sports, swimming, doing aerobics, or strenuous sports). In analyses, women were categorized according to a three-level physical activity index. Women in the high-activity category participated in vigorous activities at least twice a week, or moderate activities at least four times a week. Women in the medium-activity category engaged in vigorous activities once a week or moderate activities one to four times a week. The low activity category included all other women. In separate analyses, researchers also examined cardiovascular mortality rates according to frequency of participation in moderate and vigorous activities.

Among all women, all-cause mortality rates declined as the physical activity index level increased. To minimize bias in assessment of cardiovascular mortality, investigators conducted separate analyses among women who were free of CHD and cancer at baseline and who had survived at least 3 years into the study (about 81% of the participants). The most active group had about half the cardiovascular mortality as the least active group. Women who engaged in moderate activities four or more times a week had a 47% lower risk than those who did so rarely or not at all. But those participating in vigorous activities four or more times a week had an 80% lower risk than those who did so rarely or never, though the trend was only marginally significant since few women participated regularly in vigorous activities. The inverse association persisted even after dietary differences were taken into account.

Finnish Twin Cohort Study. The study cohort included all same-sex twins born before 1958 and with both members alive in 1967. This analysis (34) examined physical activity and mortality in 7,925 men and 7,977 women who were between 25 and 64 years old in 1976 and free of CHD, cancer, and chronic obstructive pulmonary disease. Additionally, subjects completed questionnaires about their leisure-time physical activities (including frequency, duration, and intensity) and other health habits. Investigators classified subjects into three categories: sedentary, occasional exercisers, or conditioning exercisers. Sedentary subjects reported no leisure physical activity. Conditioning exercisers exercised at least 6 times a month, for an average of at least 30 minutes each session at a vigorous walking or jogging intensity. All others (69% of men and 78% of women) were deemed occasional exercisers. During follow-up from 1977 to 1994, 1,253 subjects died. Researchers did not specifically analyze mortality from CHD, but CHD was the most common cause of death (319 of 1,253, or 25%).

In the total cohort, physical activity was inversely related to all-cause mortality. The relative risks for dying during follow-up, adjusted for age, occupation, cigarette smoking, and alcohol consumption, were highest for sedentary twins (1.00); lower for occasional exercisers (0.80), and lowest for conditioning exercisers (0.76). The trend across categories was statistically significant. When analysis was restricted only to the 434 same-sex twin pairs discordant for death, a similar trend was observed in both sexes. Analyses did not consider monozygotic (identical) and dizygotic (fraternal) twins separately because there were too few monozygotic twins for meaningful analyses.

This study is the first one that considered genetic factors when examining the relationship between physical activity and mortality. These data provide some evidence for a causal relationship between higher levels of physical activity and greater longevity rather than for a selective advantage that confers both the capability for high levels of physical activity and low CHD risk.

The Aerobics Center Longitudinal Study. Unlike in the previous studies, Blair et al (35) investigated physical fitness rather than physical activity. Physical activity and physical fitness are interrelated (36), Regular physical activity can improve physiologic fitness over time, while physiologic fitness limits the amount of physical activity that may be performed. Thus, physical activity and physical fitness each may act independently to influence the risk of CHD.

This study (35) is the largest epidemiologic investigation of physical fitness to date. Investigators enrolled 10,244 men and 3,210 women, ages 20 to older than 60, who had received a preventive medical examination between 1970 and 120211 at the Cooper Institute for Aerobics Research in Dallas. Investigators used total treadmill test time, specific for each sex and age-group, to classify subjects into fifths of physical fitness. Researchers then followed subjects for an average of more than 8 years, during which time 240 men and 43 women died. Of these deaths, 66 in men and 7 in women were due to cardiovascular causes.

There was a strong inverse association between fitness level and age-adjusted all-cause mortality rates in men (figure 1). Although the number of deaths in women was small, a similar pattern emerged. After adjustment for cigarette smoking status, systolic blood pressure, serum cholesterol levels, serum glucose levels, body mass index, and parental history of CHD, the inverse associations persisted in both sexes. Further, investigators noted that the inverse association between physical fitness and all-cause mortality held within categories of each of these risk factors. While the small number of cardiovascular deaths in either sex precluded a definitive conclusion about the links, there also appeared to be an inverse association between physical fitness and cardiovascular mortality rates. The difference in cardiovascular mortality rates between subjects in the extremes of physical fitness appeared more pronounced for women than men.

Lipid Research Clinics Mortality Follow-up Study. The purpose of the original study (37), conducted between 1972 and 1976, was to describe the lipid profiles of men and women in North America. Participants were recruited at 10 different centers and examined up to two times. The follow-up study was initiated to examine the relationship between factors ascertained at the second visit and mortality among participants 30 or older.

Ekelund et al (37) enrolled 3,106 healthy white men, 30 to 69 years old at baseline, who had valid exercise test data. Women were excluded because the small number of deaths among them precluded meaningful analyses. Physical fitness was assessed using a submaximal treadmill exercise test, according to a modified Bruce protocol (37), and men were then classified into fourths according to physical fitness (the heart rate at stage 2 of the exercise test). During follow-up (average, 8.5 years), 45 men died from cardiovascular disease.

Among participants with higher levels of physical fitness, crude death rates from cardiovascular disease were lower. There was a more than eightfold difference in cardiovascular mortality rates between the least and most fit men. A similar pattern was observed for CHD mortality rates: Men in the extreme categories of physical fitness had a more than sixfold difference in CHD mortality rates. Findings were similar when analyses factored in age, cigarette smoking, systolic blood pressure, and high-density and low-density lipoprotein cholesterol levels.

Plots of cumulative cardiovascular disease mortality, adjusted for study variables, were examined to confirm the validity of the lower cardiovascular mortality rates among men who were more fit and to look for potential bias from inclusion of men with subclinical illness. In 8.5 years of follow-up, the curves for the least and most fit men diverged (figure 2), suggesting that the benefit of physical fitness was unlikely to be an artifact.

Weighing the Evidence

All of the epidemiologic studies examining the relationship between CHD and physical activity or physical fitness have been observational in their research design. Most studies have observed lower CHD risk among persons with higher levels of physical activity or physical fitness; on average, about a twofold difference in risk exists (21), Studies of physical fitness indicate more pronounced differences (35,37),

Observational vs experimental studies. Observational epidemiologic studies cannot presume cause and effect (38), Evidence from observational epidemiologic studies is weaker than that from randomized clinical trials, but randomized clinical trials with the end point of CHD are unlikely to be conducted because of cost constraints and difficulty of maintaining high compliance with physical activity during the study. However, randomized clinical trials have instead examined the association between physical activity and risk factors for CHD such as blood pressure (9,39,40) and lipid profile (10,11,39,40), Such trials have shown that physical activity is capable of improving risk factors for CHD. The implicit assumption is that improvements in risk factors will translate to a lower CHD risk. But this assumption may not always hold true (41); therefore, evaluation of observational studies directly assessing the association between physical activity or physical fitness and CHD risk is important.

Bias and confounders. From the observational studies, can we conclude that higher levels of physical activity or fitness lower CHD risk? One must consider potential biases. Bias resulting from incomplete follow-up is unlikely, since all the studies reviewed had good follow-up. Another concern is a selective process (such as a genetic predisposition) that might render an individual capable of high levels of physical activity or fitness and bestow a low CHD risk. Data from the Finnish Twin Cohort Study (34) make this less likely. If an inverse association had been observed among monozygotic twins, the argument against selection bias would have been even stronger; however, there were insufficient numbers of such twins for separate analyses. A third source of bias is subject selection. Study subjects might place themselves in the lower spectrum of physical activity because of occult CHD and produce the (artifactual) observation of higher CHD rates among the inactive. Longer follow-up, however, dilutes such bias, because those with occult disease would be diagnosed relatively early in the study. Moreover, the findings of Ekelund et al (37), which show divergence in cardiovascular mortality rates over time between the least and most fit subjects, make this bias unlikely to account for results observed (see figure 2).

Study validity can also be compromised by confounding variables. Individuals who exercise or are physically fit likely differ in other health habits. Perhaps these other health habits, and not physical activity, are responsible for lower CHD rates. Investigators, however, controlled for the various other predictors of CHD risk in their analyses (2,42), and the inverse association between physical activity or physical fitness and CHD risk persisted. Thus, neither bias nor confounding are likely to explain the observational study findings.

Requirements for Preventing CHD

How much activity is enough? The recent physical activity recommendation, which calls for accumulation of at least 30 minutes of moderate-intensity physical activity on most days of the week, has fueled debate about the optimal length, intensity, and duration of physical activity to prevent CHD (43), For most healthy adults, moderate-intensity activities represent the equivalent of brisk walking at 3 to 4 mph (3 to 6 METs). In contrast, previous recommendations typically have prescribed vigorous exercise, for at least 20 minutes continuously, at least three times a week (44), Vigorous exercise increases the heart rate and causes sweating; such activities generally require 6 or more METs and include jogging and running. The recent recommendation differs from previous recommendations in two main ways: a concession to moderate-intensity activity and an allowance for accumulated short bouts.

Very few studies have attempted to quantify physical activity required to decrease CHD risk. Paffenbarger et al (25) found that heart attack rates declined progressively, beginning with physical activity at the 500 to 999 kcal per week level. Once activity reached 2,000 kcal per week, the benefit began to plateau. Thus, the new physical activity recommendation, which would require about 1,000 kcal per week, would probably suffice to decrease CHD risk, at least in terms of energy expenditure.

More contentious is the intensity and duration of bouts. Two randomized trials (39,40) have compared an intervention of lifestyle physical activity with an intervention of structured exercise. The lifestyle intervention promoted the new physical activity recommendation, while the structured exercise intervention promoted previous exercise recommendations. Both lifestyle physical activity and structured exercise interventions improved physical fitness, blood pressure, lipid profile, and body weight. However, observational epidemiologic studies assessing whether moderate or vigorous physical activity is required to decrease CHD risk have been less clear. Although many studies have examined the association between physical activity and CHD risk (20,21), relatively few have investigated the activity types and intensities associated with lowering risk. Studies are about evenly divided between those that find vigorous-intensity physical activity required (23,24,29,45,46) and studies that find moderate-intensity activity sufficient (31-33,47-49),

Study variability. The observed inconsistencies can have several possible explanations. First, studies enrolled different populations. Some studies were conducted in men, others in women. Subjects also belonged to different age-groups and likely had different basal levels of physical fitness. Perhaps populations with lower levels of fitness might benefit more with moderate-intensity activity, while populations with higher levels of fitness might need more vigorous activity. Hence, different findings might reflect the populations' heterogeneity. Second, in all the observational epidemiologic studies, subjects were asked to recall physical activities. Vigorous activities, such as running, are probably more accurately remembered than moderate activities such as gardening or dancing. Perhaps the studies that found no association with moderate activities merely reflect the imprecision of subject activity recall. In addition, no uniform scheme exists for classifying the intensity of activities so that a particular activity may have been classified as moderate in one study, vigorous in another. Finally, all the studies assessed intensity on an absolute level. For example, brisk walking usually is assigned a value of 4 METs, or moderate intensity, regardless of subject fitness. This might actually represent a vigorous activity for an older, unfit person, but only a light effort for a young, fit marathoner.

With regard to the duration of exercise needed to decrease CHD risk, several clinical trials have investigated exercise of different durations in relation to risk factors for CHD. The two trials (39,40) that tested lifestyle physical activity against structured exercise did not provide data about actual duration per episode of physical activity. Presumably, subjects assigned to the lifestyle group accumulated short bouts of activity, while those assigned to the structured exercise group engaged in continuous, longer sessions.

In another study, Ebisu (50) reported that young men running 6 miles a day, whether in one, two, or three bouts, experienced similar increases in physical fitness after 10 weeks, while high-density lipoprotein-cholesterol levels increased most in the three-bout group. DeBusk et al (51) observed that physical fitness improved after 8 weeks among middle-aged men who jogged 30 minutes a day, whether they jogged in one or two bouts. Those who jogged in one session improved more. Even with these results, it should be noted that no study has directly examined exercise episode duration and risk of developing CHD.

Parting Advice

Americans are extremely sedentary as a group. Data from the Behavioral Risk Factor Surveillance Survey indicate that about 30% of men and women do not engage in any physical activity during their leisure time (19), a proportion that has not changed very much with time. If adults who engage in irregular physical activity are included, the proportion of sedentary adults jumps to an alarming 60%! (52) Therefore, clinicians and other health professionals should aggressively promote physical activity.

Moderate-intensity activity is likely to be more attainable and sustainable than vigorous exercise for sedentary individuals (53), Therefore, we believe that moderate-intensity activity of at least 10 to 15 minutes duration should be promoted to those reluctant to exercise because it represents a more realistic goal. Moderate physical activity is likely to improve CHD risk factors and also reduce CHD risk. However, vigorous activity of longer duration should receive no less emphasis for health promotion if it is not medically contraindicated. In today's fast-paced world where time is a precious commodity, a half hour of vigorous activity expends as much energy as moderate activity carried out for twice or three times as long—and it can provide greater CHD benefits.


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Dr Lee is an assistant professor of medicine at Harvard Medical School and an assistant professor of epidemiology at the Harvard School of Public Health in Boston. Dr Paffenbarger is a professor emeritus (active) of epidemiology at Stanford University School of Medicine in Palo Alto, California, and an adjunct professor of epidemiology at the Harvard School of Public Health. Address correspondence to I-Min Lee, MBBS, ScD, Brigham and Women's Hospital, 900 Commonwealth Ave E, Boston, MA 02215; e-mail to [email protected].

This article was adapted from the recently published book: Thompson PD (ed): Exercise and Sports Cardiology, New York City, McGraw-Hill Medical Publishing, 2021 (to order: 1-800-262-4729 [ISBN:0-07-134773-9]).