An examination of the evidence on the cardioprotective benefits associated with various intensity levels, types, and amounts of physical activity, as well as tips on a beneficial yet realistic exercise program.
A sedentary, overweight, middle-aged woman rolls her eyes when you encourage her to become more physically active. "I don't have time to exercise more than once or twice a week, so what good will it do?" she asks. How would you respond?
Inadequate physical activity is a well-recognized independent risk factor for coronary artery disease (CAD), the leading cause of death in the United States.1,2 More than 60% of American adults are not regularly active, and 25% are not active at all.3 Although it is common knowledge that exercise is healthful, what is less clear is the level of exercise needed to benefit cardiovascular health.
Moderate-intensity activity produces reductions in diastolic blood pressure similar to those achieved with vigorous exercise-and it may produce even greater reductions in systolic blood pressure.4 Moreover, moderate exercise together with modification of diet has been shown to lower the risk of type 2 diabetes.5-7 However, these data still do not establish that moderate physical activity has a cardioprotective effect. Nor do they answer the question: what amount or intensity of exercise is needed to help prevent stroke, myocardial infarction (MI), and cardiovascular mortality?
In this article, we examine the available evidence on the cardioprotective benefits associated with various intensity levels, types, and amounts of physical activity. We also offer tips on how to help your patients initiate-and maintain-a beneficial yet realistic exercise program.
WHAT THE GUIDELINES SAY
Revised guidelines from the American Heart Association (AHA), the CDC, and the American College of Sports Medicine (ACSM) recommend moderate to vigorous aerobic activity (at 50% to 75% of maximum heart rate) for at least 30 minutes on most days of the week, either in a single session or in several 10- to 15-minute sessions.4,8 Examples of moderate- and vigorous-intensity activities are listed in Table 1.
Earlier recommendations were updated to include moderate activities on the assumption that more inclusive recommendations would be easier for the general population to follow. In the past 10 years, however, data have become available that also show potential health benefits of regular moderate-intensity exercise. A few caveats in interpreting the results of studies of exercise are listed in the Box.
BENEFITS OF LOW LEVELS OF EXERCISE IN WOMEN
The Women's Health Initiative (WHI) Observational Study monitored more than 70,000 postmenopausal women from 1994 to 1998 and divided them into 5 groups based on their self-reported exercise level, quantitated by metabolic equivalent (MET)-hours per week.9 (Table 2 lists the METs associated with some common moderate-intensity activities.)
The group with the lowest level of exercise averaged 0 MET-hours per week, and the group with the highest level averaged 32.8 MET-hours per week. The study's primary end point was the incidence of coronary events, which included death from coronary causes, nonfatal MI, coronary revascularization, angina, congestive heart failure, stroke, and carotid revascularization.
There was a stepwise increase in benefit with increasing levels of exercise. In age-adjusted analyses that controlled for other known risk factors, the relative risk of a coronary event declined as the level of exercise increased. From the quintile with the lowest level of exercise to that with the highest, the relative risks (RRs) of a coronary event were 1.00, 0.89, 0.81, 0.78, and 0.72.
Walking was associated with a beneficial age-adjusted reduction in relative risk, regardless of whether vigorous exercise was also engaged in. Women who walked at least 2.5 hours per week had a relative risk reduction (RRR) of approximately 30% compared with those who did not walk. Women who engaged in both walking and vigorous exercise had greater benefit than those who engaged in either alone.
A statistically significant benefit became evident when women walked at a pace of 2 to 3 mph (14% RRR). Easy, casual walking (slower than 2 mph) had no such benefit when compared with rarely or never walking.
The Women's Health Study is another large study that attempted to determine the amount of exercise needed to produce a cardioprotective benefit.10 The study authors monitored the exercise habits of about 40,000 women health professionals older than 45 years from 1992 to 1999. The primary end point was MI, coronary bypass surgery, or coronary angioplasty. A significant benefit was seen with as little as 1 hour of walking per week (RR = 0.49; 95% confidence interval [CI], 0.28 to 0.86), as well as with paces slower than 2 mph (RR = 0.56; 95% CI, 0.32 to 0.97), when compared with not walking regularly. When the data were analyzed for trend, time spent walking (P = .01) predicted degree of risk reduction, but walking pace (P = .55) did not.
OTHER STUDIES OF LOW LEVELS OF EXERCISE
The Harvard Alumni Health Study monitored cardiovascular risk factors, including exercise habits, in 12,000 middle-aged men from 1977 to 1993.11 Study participants were classified on the basis of the amount of energy they expended in a typical week. After the data were adjusted for age and other risk factors, no significant reduction in risk of CAD was associated with increasing levels of energy expended in light- and moderate-intensity activities (defined in METs). A significant risk reduction was associated with walking more than 5 km per week (13% RRR). Nevertheless, increasing amounts of walking-up to 20 km per week-showed no statistically significant additional benefit (P = .08).
LaCroix and colleagues12 monitored 1645 men and women older than 65 years for 4 years. The 2 main outcomes were hospitalization for cardiovascular disease and death. In this study, walking more than 4 hours per week was associated with fewer hospitalizations than was walking less than 1 hour per week (RR = 0.69; 95% CI, 0.52 to 0.90). There was also a lower relative risk of death in the group that walked more than 4 hours per week, although this did not reach statistical significance (RR = 0.73; 95% CI, 0.48 to 1.10).
The Honolulu Heart Program classified 2678 men older than 70 years into 3 groups based on the distance they walked each day (less than 0.25 mile, 0.25 to 1.5 miles, or more than 1.5 miles).13 The men were monitored for up to 4 years. After adjustment for age and other risk factors, the risk of CAD was 2.3 times greater in the men who walked less than 0.25 mile per day than it was in the men who walked more than 1.5 miles per day (P < .001). The risk of CAD in the men who walked 0.25 to 1.5 miles per day was double the risk in the men who walked farther (RR = 2.1; 95% CI, 1.2 to 3.6). There was also a significant trend of increasing reduction in risk of CAD with increasing distance walked (P = .002).
Finally, the Railroad Study monitored more than 3000 railroad workers for over 20 years and correlated their exercise habits with incidence of CAD.14 This study found that light to moderate activities had a protective benefit. There was a 30% to 40% greater risk of dying of CAD or of any cause in participants who were sedentary than in those who expended more than 1000 kcal per week in leisure-time physical activity. The latter level of energy expenditure would be equivalent to a 175-lb man swimming a total of 90 minutes in a week, running 6 miles in a week at a 9-minute mile pace, or walking 2 miles in 30 minutes every day of the week.
The investigators found that an increase of 73 kcal per day beyond the amount expended by study participants in the least active group was associated with a 16% reduction in CAD mortality and a 14% reduction in all-cause mortality. In participants who expended 150 kcal more per day (in light to moderate activity) than did participants in the least active group, mortality from CAD was 27% lower and all-cause mortality was 19% lower. Moreover, the effects of light to moderate activity on CAD death remained significant after adjustment for potential confounders. The results of this study are especially significant, since 62% of participants reported no vigorous activity.
WHICH SPORTS PROVIDE THE GREATEST BENEFIT?
The Health Professionals' Follow-up Study provides the best information on the health benefits of various sports played by middle-aged men.15 This study monitored more than 44,000 men between the ages of 40 and 75 years, analyzing their exercise habits over a 2-year period. A significant reduction in the relative risk of cardiovascular disease was seen with running for at least 1 hour per week, lifting weights for at least 30 minutes per week, rowing for 1 hour per week, and walking at a brisk pace for at least 30 minutes per day. Surprisingly, those men who swam, cycled, or played racquet sports for at least 1 hour per week showed no benefit compared with those who did not.
Participants in the Health Professionals' Follow-up Study were also classified on the basis of the number of MET-hours they clocked walking each week. A significant preventive benefit (reduction in relative risk of CAD) was seen only for the men in the highest quintile, who clocked at least 14.75 MET-hours per week walking (the equivalent of walking 5 hours per week at an average pace of 2 to 2.5 mph). In contrast to the results of the Women's Health Study, walking pace in this study was strongly associated with reduction in cardiovascular risk, suggesting that the intensity of a walk is more important than how long it lasts.
10,000-STEP PROGRAMS: THE EVIDENCE SO FAR
Studies such as the WHI Study and the Honolulu Heart Program that have shown the health benefits of walking have contributed to the promotion of organized walking programs. The largest organization to encourage walking is Shape Up America!, a nonprofit supported by ex-surgeons general C. Everett Koop and David Satcher.
Shape Up America! promotes walking to maintain physical fitness by encouraging Americans to take at least 10,000 steps per day (about 5 miles) in the course of normal daily activities. Studies have shown that the average American now takes fewer than 6000 steps per day.16 The 10,000 Steps Program encourages participants to purchase a pedometer and to slowly increase their daily step total until they reach the goal of 10,000 per day.
Step-counting programs such as that used by Shape Up America! are more convenient than other kinds of exercise programs in that they enable participants to incorporate exercise into their everyday lives. In a small study by Hultquist and colleagues,17 women instructed specifically to walk 10,000 steps per day clocked more steps than those instructed to walk at a brisk pace for 30 minutes each day.
To date, however, the evidence of the health benefits of such programs is limited. Hatano18 suggests that the accumulation of 10,000 steps per day is comparable to 30 minutes of moderate physical activity per day. In his study, he found that the energy required to take 10,000 steps in a day was about 300 to 400 kcal-which is double the amount that the US Surgeon General indicates is associated with health benefits.3 Also, the AHA/CDC/ACSM recommendations appear to correlate well with 10,000-step programs. In the Hultquist study, those participants who walked at a brisk pace for 30 minutes a day took approximately 9500 steps per day.17 Another Japanese study showed that hypertensive patients who increased the number of steps they took each day to more than 10,000 lowered their blood pressure and reduced daily fluctuations in blood pressure.19 However, no large observational study of a 10,000 steps-per-day program has yet been published.
WHAT TO TELL YOUR PATIENTS
Based on the proven cardioprotective benefit of moderate-intensity exercise, we recommend encouraging patients to participate in such activity. Benefits may be seen with as little as 2.5 hours of walking per week, and at paces slower than 2 mph. For a more robust benefit, recommend an average daily expenditure of 150 kcal (a weekly total of more than 1000 kcal), which can be achieved by walking briskly for 30 minutes per day or by running for 15 minutes per day at a speed of 10 minutes per mile.
Although some data suggest that increasing the number of steps taken each day has health benefits, particularly lowered blood pressure, such a regimen has not yet been shown to reduce mortality. Larger studies of step-counting programs need to be completed before definitive recommendations can be made.
The area in which primary care practitioners may have the greatest impact on the burden of cardiovascular disease may be in helping to change the daily habits of the 25% of American adults who are not active at all. Many studies suggest that the greatest difference in risk of CAD is between persons who are sedentary and those who exercise regularly.
The US Preventive Services Task Force recommends a variety of strategies for motivating patients to improve their exercise habits,20 including:
Whichever strategy or strategies you adopt, communicating to patients that even a modest exercise program can provide real cardioprotective benefits should help provide some of the needed motivation.
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