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By Dónal P. O’Mathúna, PhD
Everyone seems to know that some exercise is good for you. Yet the evidence also shows that only about 20% of U.S. adults engage in the recommended amount and type of exercise, and this has not increased since the mid-1980s.1 For those who accept the recommendation, many questions remain about how much, how often, how intense, and in what ways exercise helps. Evidence has been accumulating that exercise helps reduce the risk of Type 2 diabetes, osteoporosis, obesity, depression, and breast and colon cancer.2 Cardiovascular disease (CVD) in particular is influenced by exercise. Since 1992, physical inactivity has been accepted as an independent risk factor for CVD.1 Yet, CVD remains a leading killer in the United States and other developed countries. Much remains to be done to reduce CVD risk using exercise.
How Does Exercise Bring Cardiac Benefit?
The precise mechanisms by which exercise impacts CVD are not fully understood. The physiological effects of exercise are many, complex, and interrelated (see Table).3 Physical activity has been associated with reduction in obesity, improved distribution of body fat, and lower incidence of Type 2 diabetes. Blood pressure is lower for 8-12 hours immediately after physical activity, and regular exercise results in sustained reductions in blood pressure and heart rate. Studies examining the impact of exercise on lipid profiles have had variable results, but generally show improvements. These lipid changes occur at lower exercise intensities than those needed to improve fitness. Numerous studies also show that exercise improves blood-clotting mechanisms, although much remains to be elucidated here.4 Physical activity helps improve muscle strength, which can improve cardiac performance. Muscle blood flow is occluded during exercise, which limits cardiac ejection fraction and stroke volume, ultimately leading to improved peak cardiac performance.3 Additionally, physical activity often leads to other beneficial lifestyle changes regarding CVD, such as smoking cessation and weight loss.
How Much of What Sort of Exercise?
When exercise became connected with health promotion, as opposed to simply an enjoyable activity, the recommendation was to get 20 or more minutes per day of vigorous aerobic exercise such as jogging, swimming, cycling, or aerobics.5 Since the mid-1990s, however, research has reassessed the amount and intensity of activity necessary for cardiovascular benefit. For these purposes, physical activity is defined as any bodily movement produced by skeletal muscles that results in energy expenditure.3 Exercise is defined as those types of physical activities that are planned, structured, repetitive, and designed to improve or maintain physical fitness. Physical activity of lesser intensity is now recommended by the Centers for Disease Control and Prevention and the American College of Sports Medicine.6 In 2003, this same recommendation was reaffirmed by the American Heart Association.2
This recommendation is that people engage in 30 minutes of moderate-intensity physical activity such as brisk walking on most, and preferably all, days of the week.6 The particular type of activity does not appear to be crucial. For example, one study examined the different outcomes among 40 obese women randomized to two interventions.7 Both groups adhered to the same weight-loss diet and had similar support groups. One group included three classes per week of vigorous step aerobics, gradually increasing from 15 to 45 minutes per class. The second group added 30 minutes per day of moderate-intensity physical activities such as walking instead of driving, or taking stairs instead of elevators. After 16 weeks and 1 year, both groups had significantly reduced body weight and CVD risk factors, such as total cholesterol, triglycerides, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and resting systolic blood pressure. Maximum oxygen uptake improved significantly in both groups. The two groups did not differ significantly in any of these measurements.
Another similar randomized controlled trial examined two exercise interventions in 235 sedentary men and women.1 The structured exercise intervention was supervised and individually designed at a fitness center, with attendance increasing from three to five days per week. The lifestyle intervention instructed subjects on how to gradually add 30 minutes of moderately intense physical activity to most, or preferably all, days. The interventions were monitored actively for six months, with follow-up for an additional 18 months. Both groups had significantly reduced systolic and diastolic blood pressure and percent body fat, but not body weight. Cardiovascular fitness also increased. Again, the outcomes did not differ with the type of activity. Total cholesterol, LDL-C, HDL-C, and triglycerides did not change in either group, probably reflecting the lack of dietary changes. The lifestyle intervention had a mean monthly cost one-third that of the structured intervention due to fitness center costs.8
Exercise for People with CVD
A number of studies have shown that patients with stable chronic CVD can benefit from exercise programs.9 Hence, over the last several years, exercise programs have become standard in cardiac rehabilitation centers. The most recent meta-analysis reported more than 50 studies in support of this trend.10 Most of these programs were supervised for two to four months, and follow-up averaged 2.4 years. Cardiac rehabilitation programs involving only exercise reduced cardiac mortality by 31%, while more comprehensive programs involving exercise along with psychosocial and educational interventions reduced cardiac mortality by 26% (both statistically significant). However, an important limitation with this meta-analysis is that it included studies only up to 1999. Thus, many were conducted prior to widespread use of several newer pharmaceuticals for CVD, questioning whether the same impact would be seen in patients treated with these drugs today.
Exercise also has been shown to reduce the risk of Type 2 diabetes. People with diabetes have a very high incidence of CVD. One study examined survey data on the impact of walking on mortality among diabetic patients.11 Compared to sedentary people with diabetes, those who walked at least two hours per week had 39% lower mortality from all causes, and 34% lower CVD mortality. Lower mortality (54%) was found for those who walked three to four hours per week, but longer duration afforded no additional benefits. Walking that led to moderate increases in heart and breathing rates gave lower mortality than walking that greatly increased these rates. The researchers estimated that one death per year could be prevented for every 61 diabetic patients who were persuaded to start walking two hours per week.
Exercise is not without its risks, although several practical steps can be taken to reduce the risk of injury. The most common type of risk in adults is musculoskeletal injury, with between one quarter and one half of adults reporting some sort of musculoskeletal injury within a year of commencing exercise.2 The risk of injury increased with obesity, amount of exercise, and exercise intensity, especially if involving competitive sports. The risk was reduced with higher baseline fitness, supervision, stretching exercises, and wearing protective equipment such as bike helmets. Walking is a low-risk exercise where increased duration did not lead to more injuries.12 Increasing the exercise intensity gradually resulted in fewer injuries.
Exercise also can increase risk of myocardial infarction and sudden cardiac death. A number of highly publicized deaths of high school, college, and professional athletes have drawn attention to this area. The incidence of these sudden deaths is not well documented, but is greatest in those who are least physically active and who were performing unaccustomed vigorous exercise.2
A significant body of evidence supports the cardiovascular health benefits of physical activity and exercise. Exercise has an important role in both preventing CVD and, when medically supervised, assisting in recovery from CVD. Thirty minutes of physical activity per day are recommended, although research is showing that some benefits are obtained with less activity. Therefore, people should be encouraged to gradually increase their activity by manageable amounts. This should help prevent discouragement through failing to attain overly ambitious goals or through injury. At the same time, people can be encouraged by the variety of ways they can include physical activity in their lifestyles. In the words of one researcher, "There is no single best exercise. Depending on initial fitness, health status, personal preferences, and lifestyle, any of several physical activity choices may be the right one for a given individual."13 The important point is that people be encouraged to get active—for their heart’s sake.
Dr. Mathuna, Visiting Professor of Bioethics, University of Ulster, Coleraine, Northern Ireland, is on the Editorial Advisory Board for Alternative Therapies in Women’s Health.
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7. Andersen RE, et al. Effects of lifestyle activity vs. structured aerobic exercise in obese women: A randomized trial. JAMA 1999;281:335-340.
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9. Hambrecht R, et al. Effects of exercise on left ventricular function and peripheral resistance in patients with chronic heart failure: A randomized trial. JAMA 2000; 283:3095-3101.
10. Jolliffe JA, et al. Exercise-based rehabilitation for coronary heart disease (Cochrane Review). In: The Cochrane Library, Issue 4, 2003. Chichester, UK: John Wiley & Sons, Ltd.
11. Gregg EW, et al. Relationship of walking to mortality among US adults with diabetes. Arch Intern Med 2003;163:1440-1447.
12. Hootman JM, et al. Epidemiology of musculoskeletal injuries among sedentary and physically active adults. Med Sci Sports Exerc 2002;34:838-844.
13. Pratt M. Lifestyle and structured interventions to increase physical activity Letter. JAMA 1999; 282:1517.