Exercise & Sport Sciences Reviews:
Physical Activity and Cardiovascular Disease Prevention in Women: How Much Is Good Enough?
Bassuk, Shari S.1; Manson, JoAnn E.1 2
1Division of Preventive Medicine, Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, and 2Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts
Accepted for publication: May 5, 2003.
Address for correspondence: Shari S. Bassuk, ScD, Division of Preventive Medicine, Brigham and Women’s Hospital, 900 Commonwealth Avenue East, Boston, MA 02215 (E-mail: firstname.lastname@example.org).
BASSUK, S. S., and J. E. MANSON. Physical activity and cardiovascular disease prevention in women: How much is good enough? Exerc. Sport Sci. Rev., Vol. 31, No. 4, pp. 176–181, 2003. Epidemiologic data suggest that 30 min ·d−1of brisk walking can reduce cardiovascular disease incidence in women and men. In a sedentary society, public health initiatives that promote moderate increases in physical activity may represent the optimal balance between efficacy, feasibility, and safety to achieve the desired cardioprotective effect.
Until recently, cardiovascular disease (CVD) was widely perceived to be less of a public health problem for women than for men. Although coronary heart disease (CHD) incidence in women trails that of men by 10 yrs for total CHD and by 20 yrs for more serious clinical events such as myocardial infarction, CHD nevertheless emerges in the United States as the leading killer of men by 45 yrs of age and of women by 65 yrs of age. In fact, as soon as overt CHD develops in a woman, she has a markedly worse prognosis than a man; case-fatality rates are higher for women after both myocardial infarction and coronary revascularization, with 38% of heart attacks in women but only 25% of heart attacks in men followed by death in 1 yr. Moreover, in 63% of women but in only 50% of men who died suddenly of CHD, there were no previous symptoms of the disease. Secular declines in cardiovascular mortality also have been less marked among women than among men (Fig. 1). Boosting prevention and treatment efforts among women is necessary to forestall potentially widening gender disparities in cardiovascular outcomes. One potent cardioprotective factor that should be targeted for promotion is physical activity.
There is general consensus among researchers that physical activity provides cardiovascular benefits. A 1990 metaanalysis of 27 cohort studies in mostly male populations concluded that physically active individuals have approximately half the CHD risk of those who are sedentary, and studies conducted within the last 15 yrs also suggest that physically active women experience lower CHD rates than their inactive counterparts. There is still debate on the amount or “dose” (a function of intensity, frequency, and duration) of physical activity required for optimal health. Such a debate serves little purpose, however, if the general public cannot be persuaded to adopt a physically active lifestyle. A recent national survey conducted by the U.S. Department of Health and Human Services found that only 27% of American women and 34% of men exercise regularly (Fig. 2), and 41% of women and 35% of men engage in no leisure-time physical activity at all.
PHYSICAL ACTIVITY GUIDELINES
The long-held belief that physical activity must be vigorous to be salutary has been overturned in the last decade by epidemiologic studies showing otherwise. Indeed, earlier guidelines advocating vigorous exercise for at least 20 min three times weekly have been supplemented by a widely publicized 1995 recommendation by the Centers for Disease Control (CDC) and the American College of Sports Medicine (ACSM) that adults engage in 30 min of moderate-intensity physical activity on most, preferably all, days of the week. This has also been the standard endorsed by the U.S. Surgeon General since 1996. In 2002, the influential Institute of Medicine (IOM) concurred that moderately intense activity is beneficial, but it doubled the daily 30-min goal, concluding that 1 half-hour is not sufficient to maintain a healthy weight (i.e., a body mass index [BMI] of 18.5 to 25.0 kg·m−2) nor to achieve maximal health benefits. To prevent weight gain as well as to accrue additional, weight-independent health benefits of physical activity, the IOM recommends 60 min of daily moderate-intensity physical activity. These 60 min are in addition to the activity involved in carrying out normal routines of everyday life. The guideline was issued as part of a detailed report focused primarily on setting forth diet and nutrition goals for the American public; the full text of the report is available online at http://www.nap.edu/books/0309085373/html./
Although the IOM is to be commended for highlighting the importance of physical activity as part of a healthy lifestyle, its recommendation fails to balance the issue of efficacy with feasibility and safety considerations, all of which are crucial in achieving a public health goal. By allowing “the perfect to be the enemy of the good,” this well-intentioned but problematic guideline will likely undercut most of the motivation the sedentary public—already largely unable to adhere to the 30-min goal—might muster to adopt a more active lifestyle.
CARDIOVASCULAR BENEFITS OF WALKING 30 MINUTES PER DAY
The public may not only be dispirited by the new recommendation but also perplexed by it, given that well-publicized epidemiologic studies in the past several years have lent strong empirical support to the 30 min·d−1 prescription. Most recently, among 73,743 postmenopausal women aged 50 to 79 yrs participating in the Women’s Health Initiative, walking briskly for at least 2.5 h·wk−1 (i.e., 1 half-hour five times per week) was associated with a 30% reduction in cardiovascular events over 3.2 yrs of follow-up in analyses adjusted for multiple potential confounders, and the results did not vary substantially according to race, age, or BMI (Fig. 3) (10). Brisk walking and more vigorous exercise were associated with similar risk reductions in cardiovascular events after adjustment for total exercise energy expenditure.
The cardiovascular benefits of modest amounts of walking—the most common leisure activity among U.S. adults—also have been demonstrated in other studies of middle-aged and older women. In the Nurses’ Health Study, which followed 72,488 healthy middle-aged female nurses for 8 yrs, women who walked briskly for 3 h·wk−1 or, alternatively, exercised more vigorously for 1.5 h·wk−1, had a 30% to 40% lower rate of myocardial infarction than did sedentary women (11). In the Women’s Health Study, a 7-yr follow-up of 39,372 healthy middle-aged female health professionals, walking at least 1 h·wk−1 was associated with a 50% reduction in CHD risk in women reporting no vigorous physical activity (8). Among 1564 middle-aged University of Pennsylvania alumnae followed for 30 yrs, walking 10 or more blocks daily as compared with walking fewer than four blocks daily was associated with a 33% reduction in CVD incidence (13). Among community-dwelling women and men aged 65 yrs and older, walking more than 4 h·wk−1, as compared with walking less than 1 h·wk−1, significantly reduced the risk of hospitalization as a result of CVD (7). Benefits of walking have been observed in male populations as well, albeit somewhat less strongly, perhaps because of generally higher physical activity levels for men as compared with women. Nevertheless, in the Health Professionals’ Follow-up Study, a 12-yr follow-up of 44,452 male health professionals aged 40 to 75 yrs, a half-hour per day or more of brisk walking was associated with an 18% reduction in CHD incidence (15). In the Honolulu Heart Program, men aged 71 to 93 yrs who walked 1.5 miles·d−1 experienced half the risk of CHD of those who walked less than 0.25 mile·d−1 (2).
HOW MUCH EXERCISE IS GOOD ENOUGH?
How do we reconcile the apparent contradiction between the CDC–ACSM and the Surgeon General’s 30-min physical activity guideline and the IOM’s 1-h recommendation? The answer is by recognizing that there is in fact no actual contradiction, only an apparent one resulting from differences in which aspect of empirical findings are emphasized and the way the public health message has been framed by the media. Moderately intense exercise for 1 half-hour daily confers significant and measurable cardiovascular health benefits, as the data on walking demonstrate. It is also true, however, that a dose-response relationship between physical activity and cardiovascular outcomes exists such that another half-hour of exercise would, on average, be expected to confer additional protection against the development of CVD. This additional benefit was recognized explicitly by the CDC and the ACSM and by the Surgeon General, even as they issued their 30 min·d−1 guideline: “People who already meet the recommendation are also likely to derive some additional health and fitness benefits from becoming more physically active.” By contrast, the IOM, in advocating a higher dose of exercise for the optimization of health, does not sufficiently acknowledge the importance of more modest physical activity achievements with respect to disease risk reduction.
The primary basis for the IOM’s recommendation appears to be the observation that 1 half-hour of exercise daily has not been shown consistently to ensure weight maintenance within the healthy BMI range of 18.5 to 25.0 kg·m−2 or to promote weight loss in the absence of curtailing caloric intake. Some evidence does suggest that 1 h of activity daily may be necessary to control weight when dietary restraint is not exercised concurrently. In an IOM-compiled database of some 400 stable-weight adults whose energy expenditures had been estimated with the doubly labeled water method, considered the gold standard of energy expenditure measurement, individuals with a BMI between 18.5 and 25.0 kg·m−2 expended a daily energy equivalent of at least 1 h of moderate physical activity—or, as the IOM put it, walking at least 4.4 miles per day at the rate of 2 to 4 mph. (This energy expenditure also could result from lighter activity over a longer duration or more vigorous activity over a shorter duration.) Moreover, descriptive studies of formerly obese individuals suggest that 80 min·d−1 of moderately intense activity or 35 min·d−1 of vigorous activity is required for long-term maintenance of weight loss. In the National Weight Control Registry, a sample of 629 women and 155 men who lost an average of 30 kg and maintained a minimum weight loss of 13.6 kg for 5 yrs, the self-reported median weekly energy expenditure was 11,830 kJ·wk−1 (404 kcal·d−1), a level that corresponds to 1.5 h·d−1 of brisk walking for a 65-kg woman (http://www.nwcr.ws/).
However, a recent randomized trial that assigned 173 sedentary, overweight, postmenopausal women to a year-long program of 45 min of moderate-intensity exercise 5 d·wk−1 or to a stretching control group found significant reductions in adiposity even among women whose exercise amounts fell short of the prescribed intervention (5). All participants were asked to maintain their usual diet. Women in the intervention group, who exercised a mean of 3.5 d for 176 min·wk−1, experienced a mean BMI reduction of 0.3 kg·m−2, whereas BMI of the controls increased by 0.3 kg·m−2. Moreover, women who were highly active (exercised more than 195 min·wk−1) or moderately active (135 to 195 min·wk−1) each lost significantly more total body fat and intraabdominal fat than did women in the control group. Indeed, another recent study using nationally representative data has estimated that a mere 100 kcal·d−1 change in energy balance could prevent weight gain in most U.S. adults aged 20 to 40 yrs, and that modest increases in physical activity, such as 15 min·d−1 of walking, or reductions in caloric intake, such as eating fewer bites at each meal, would be sufficient to achieve the desired change (3). Thus, these new findings suggest that 1 h of exercise daily may not be necessary to reap measurable benefits with respect to weight control.
CARDIOPROTECTIVE MECHANISMS OF PHYSICAL ACTIVITY
Physical activity reduces cardiovascular risk not only by promoting weight maintenance or loss but also by mechanisms independent of weight regulation. Although these pathways are not clearly understood, regular exercise has been shown to lower blood pressure, raise glucose tolerance and insulin sensitivity, and improve lipid levels (particularly plasma levels of high-density lipoprotein cholesterol). Observational and clinical trial data suggest that 30 min·d−1 of moderate-intensity activity may be sufficient to produce favorable changes in at least some of these physiologic parameters (for a detailed review, see Bassuk and Manson (1)). In the Nurses’ Health Study, for example, walking briskly for 3 h·wk−1 was associated with a 20% reduction in the incidence of type 2 diabetes, after adjustment for BMI and other potential confounders.
Interestingly, findings from randomized trials indicate that moderate-intensity exercise lowers systolic blood pressure as or more effectively than does high-intensity exercise; such effects occur in both normotensive and hypertensive individuals and largely are independent of weight change. Moderate- and high-intensity exercise also may be equally efficacious in improving insulin sensitivity, glycemic control, and the metabolic profile among both diabetic and nondiabetic populations. In the Nurses’ Health Study, brisk walking and more vigorous activity resulted in comparable reductions in diabetes incidence, after adjustment for total exercise energy expenditure (4). In contrast, stronger dose-response relationships between exercise intensity and blood lipids—specifically, high-density lipoprotein cholesterol and triglycerides—have been reported in observational studies. Nevertheless, a recent 8-month trial that assigned overweight, dyslipidemic women and men to various exercise regimens found that improvements in lipoprotein profile were related more strongly to the amount, rather than the intensity, of exer-cise (6).
The cardioprotective effect of physical activity also may be a result of its influence on inflammation and hemostasis. Exercise has been linked to reduced levels of the inflammatory marker C-reactive protein in both women and men (1). Moderate-intensity exercise also has been associated with reductions in plasma fibrinogen level and platelet aggregation and elevations in plasma tissue plasminogen activity in men; such relationships have not yet been studied in women (1).
These observations suggest that cardiovascular benefits derived from physical activity are not solely a function of weight control. Indeed, in the Nurses’ Health Study, walking was associated with a reduced incidence of CVD even after adjusting for changes in BMI over time. Moreover, clinical trials suggest that in dieters who exercise, a more favorable lipid profile develops than in dieters who do not exercise, even when weight loss is equivalent between the two groups (14). Thus, focusing on excess weight—as powerful a CVD risk factor as it is—as the primary guidepost yields an incomplete picture when assessing exercise’s impact on cardiovascular health.
OTHER EFFICACY AND FEASIBILITY CONSIDERATIONS
Indeed, if obesity per se rather than CVD is the outcome of interest, then it should be kept in mind that energy output from physical activity is just one half of the energy balance equation. Energy input is the other half, and, indeed, this topic, rather than physical activity, is the focus of the aforementioned IOM report. The most effective weight-loss programs combine controlled caloric intake with physical activity. Given the trend of ever-increasing portion sizes, reinforcing the importance of dietary moderation is crucial to halt the rising prevalence of obesity. Otherwise, recommended exercise doses will creep ever upward in an attempt to keep waistlines in check. It is plausible that 1 half-hour of exercise daily, in combination with modest-to-moderate dietary restraint, will confer cardiovascular benefits of equal magnitude to those conferred by 1 h of exercise daily in the absence of caloric restriction. Such a prescription may be more feasible to achieve in our time-starved but not food-starved society.
Because exercise must be current and continued to confer cardiovascular protection, being able to fit it into one’s daily schedule is essential. Especially welcome news to time-pressured individuals should be findings from studies that show that short durations (as short as 10 min) of exercise favorably affect cardiovascular risk factors. A recent prospective study has examined the relationship between short bouts of exercise and risk of CVD. In a 5-yr follow-up of 7337 middle-aged and elderly male Harvard alumni, exercising in 15-min bouts, 30-min bouts, or 45-min bouts all offered equal protection against CVD after controlling for total energy expenditure (9). This knowledge may help motivate busy people to view exercise as a manageable part of their daily routine rather than as a time-consuming activity reserved only for rare occasions.
Another recent finding is that resistance exercise, also known as strength training, reduces CHD incidence. In the Health Professionals’ Follow-up Study, men who trained with weights for at least 30 min·wk−1 were 23% less likely to experience CHD over an 8-yr follow-up period (15). Resistance exercise is known to improve glycemic control and also may improve lipoprotein profile and reduce the risk of hypertension. Resistance exercise also has been shown to confer noncardiovascular benefits, such as improved musculoskeletal function, which may be particularly important for women, who are more susceptible to osteoporotic fractures than are men. It is therefore of great concern that only 16% of American women aged 45 to 64 yrs and less than 10% of women aged 65 yrs and older report ever engaging in strengthening activities (Fig. 4).
Because of a dearth of methodologically rigorous research, the U.S. Preventive Services Task Force to date has been unable to evaluate the effectiveness of behavioral counseling by primary care clinicians in leading to sustained increases in physical activity. However, data from randomized trials suggests that so-called “lifestyle interventions” can be more effective than structured exercise programs in increasing activity levels and improving cardiorespiratory fitness and cardiovascular risk profiles (12). Additional studies are needed to further our understanding of which specific characteristics of individual- and community-level interventions are required to foster lasting changes in exercise behaviors.
In addition to issues of efficacy and feasibility, initiatives to boost physical activity must address safety concerns adequately. Exercise is not an unalloyed boon with respect to either cardiovascular or musculoskeletal health, especially among individuals with preexisting heart disease or diabetes or who are not accustomed to exercising. Risks of overexercise, especially vigorous exercise, include acute cardiovascular events such as sudden cardiac death, fall-related injuries, other musculoskeletal injuries, and, for women, menstrual and reproductive dysfunction. Although physical inactivity and overeating clearly exact a far greater toll on the health of U.S. adults than do excessive exercise and overzealous dieting, the latter risks must not be neglected by clinicians and public health professionals in the quest to get America into shape.
To bring about tangible improvements in health, the results of scientific research require skillful translation into clinical practice, public health policy, and public health recommendations, and the balancing of efficacy, feasibility, and safety concerns. The challenge to clinicians and policy makers is determining how best to promote appropriate levels of regular physical activity to the general public. Given the high prevalence of sedentary lifestyle in America, the identification of strategies for facilitating sustained exercise at a level sufficient to result in measurable improvements to public health should be a top priority. We believe that the message should be “30 min per day is good, and more is better, to a reasonable extent.” As the eminent British epidemiologist Geoffrey Rose noted in his classic text The Strategy of Preventive Medicine, when a large segment of the population adopts modest improvements in health behaviors, the overall disease burden in the population is likely to be reduced more dramatically than if a modest segment of the population adopts large improvements.
This article, with a more complete list of references than that provided here, can be obtained from Dr. Shari S. Bassuk, Division of Preventive Medicine, Brigham and Women’s Hospital, 900 Commonwealth Avenue East, Boston, MA 02215 (E-mail: email@example.com).
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