Studies on the Role of Exercise Fitness in the Etiology of Coronary Heart Disease

Coronary heart disease (CHD) has a multifactorial etiology, and major 'biological' risk factors include elevated concentrations of blood total and low-density lipoprotein (LDL) cholesterol, reduced concentration of high-density lipoprotein (HDL) cholesterol, high blood pressure, diabetes mellitus, and obesity. In addition, 'behavioral' risk factors for CHD include cigarette smoking, a poor diet, and low levels of physical activity and physical fitness associated with the modern, predominantly sedentary way of living. Among these risk factors, a sedentary lifestyle is by far the most prevalent according to data from both the United States and England (Figure 1).

Scientific verification of a link between an indolent lifestyle and CHD has been forthcoming during the past 40 years, with the publication of more than 100 large-scale epidemiological studies investigating the relationships between physical activity and cardiovascular health. These studies, some of which are summarized in Figure 2, have produced consistently compelling evidence that regular physical activity can protect against CHD.

Pooled data and meta-analyses of the 'better' studies indicate that the risk of death from CHD increases about twofold in individuals who are physically inactive compared with their more active counterparts. Relationships between aerobic fitness and CHD appear to be at least as strong. For example, in a cohort of middle-aged men followed up for an average of 6.2 years, the risk of dying was approximately double in those whose exercise capacity at baseline was <5 METS compared with those whose capacity was >8 METS. For both physical activity and fitness, adjustment for a wide range of other risk factors only slightly weakens these associations, suggesting independent relationships.

Diabetes Hypertension Overweight Smoking Elevated Sedentary serum life style cholesterol

Figure 1 Estimates of the prevalence (%) of the U.S. population with selected risk factors for coronary heart disease and the population from England. In both studies, a sedentary lifestyle was taken as 'no physical activity' or irregular physical activity (i.e., fewer than three times per week and/or less than 20 minutes persession). (From Killoran AJ, Fentem P, and Caspersen C (eds.) (1994) Moving On. International Perspectives on Promoting Physical Activity. London: Health Education Authority, with permission.)

Figure 1 Estimates of the prevalence (%) of the U.S. population with selected risk factors for coronary heart disease and the population from England. In both studies, a sedentary lifestyle was taken as 'no physical activity' or irregular physical activity (i.e., fewer than three times per week and/or less than 20 minutes persession). (From Killoran AJ, Fentem P, and Caspersen C (eds.) (1994) Moving On. International Perspectives on Promoting Physical Activity. London: Health Education Authority, with permission.)

A common weakness of such studies is that they often rely on a single measurement of fitness or activity at baseline, with subsequent follow-up for mortality within the cohort. With such a design, it is difficult to discount the possibility that genetic or other confounding factors are influential in the observed relationship between physical activity/ fitness and mortality. A further weakness in single baseline studies is that subsequent changes in activity/ fitness during the follow-up are not monitored, even though they may affect the observed relationships due to the phenomenon of 'regression to the mean.'

Some prospective studies have overcome these deficiencies by examining the effects of changes in physical activity and fitness on mortality. One study reported on the relationship of changes in physical activity and other lifestyle characteristics to CHD mortality in 10269 alumni of Harvard University. Changes in lifestyle over an 11- to 15-year period were evaluated on the basis of questionnaire

0 L-------------------------------------------------------

Sedentary Low Moderate High

Activity/fitness level

Figure 2 Summary of the results from six studies in which fitness level was determined (three studies) or activity level assessed by questionnaire (three studies) in individual populations. Follow-up was generally between 7 and 9years except in Sandvik's study, which had a 16-year follow-up. The 'low level' group for each study represented in this figure was the activity/fitness level next to the least active/fit group. The 'high level' represents the group that was the most active/fit for the particular study. If the study participants were grouped by quintile, the 'moderate' group is the average of the third and fourth quintiles. (From Killoran AJ, Fentem P, and Caspersen C (eds.) (1994) Moving On. International Perspectives on Promoting Physical Activity. London: Health Education Authority, with permission.)

information, and subsequent mortality was assessed over an 8-year period. In men who were initially sedentary but started participating in moderately vigorous sports (intensity of 4.5 METS or greater), there was a 41% reduced risk of CHD compared to those who remained sedentary. This reduction was comparable to that experienced by men who stopped smoking. The second study examined changes in physical fitness and their effects on mortality. In this study of 9777 men, two clinical examinations (including treadmill tests of aerobic fitness) were administered approximately 5 years apart, with a mean follow-up of 5.1 years after the second examination to assess mortality. Results showed that men who improved their fitness (by moving out of the least fit quintile) reduced their aged-adjusted CHD mortality by 52% compared with their peers who remained unfit. Furthermore, such changes in fitness proved to be the most effective in reducing all-cause mortality when compared with changes in other health risk factors (Figure 3).

Mechanisms of Effect

Exercise appears to reduce the risk of CHD through both direct and indirect mechanisms. Regularly performed physical activity may reduce the vulnerability of the myocardium to fatal ventricular arrhythmia and reduce myocardial oxygen requirements. Aerobic training also increases coronary vascular transport capacity via structural adaptations and altered control of vascular resistance. Risk of thrombus formation may also be reduced with regular exercise through its effects on blood clotting and fibrinolytic mechanisms. Regular endurance exercise may also improve the serum lipid profile (particularly in favor of an enhanced HDL: total cholesterol ratio) and have beneficial effects on adipose tissue lipolysis and distribution. Regular exercise may also reduce postprandial lipemia, increase glucose transport into muscle cells, and improve the elasticity of arteries.

Exercise Prescription

For protection against CHD and other diseases associated with inactivty, exercise needs to be habitual, predominantly aerobic in nature, and current. Evidence from work carried out on British civil servants suggests that to be cardioprotective, exercise should be moderately vigorous (>7.5kcalmin-1 (>31.4kJmin-1) or 6 METS, equivalent to walking at approximately 3 miles per hour up a gradient of 1 in 20) and performed at least twice weekly. However, other studies have indicated that lower intensity activity is also effective as long as the total accumulated exercise energy expenditure is greater than approximately 2000 kcal week-1 (>8368 kJ week-1).

Thus, recommendations from the U.S. Surgeon General suggest that everyone older than the age of 2 years should accumulate 30 minutes or more of at least moderate-intensity physical activity on most— preferably all—days of the week. Such activity may embrace everyday tasks such as stair climbing and walking, recreational physical activities, and more

BMI Systolic BP Cholesterol Smoking Fitness

(27.0 kg m-2) (140 mm Hg) (6.2 mmol l-1) (any amount) (least fit

Figure 3 Relative risks (adjusted for age, family history of coronary heart disease, health status, baseline values, and changes for all variables in the figure, and interval in years between examinations) of all-cause mortality by favorable changes in risk factors between first and subsequent examinations. The analyses were for men at risk on each particular variable at the first examination. Cutoff points designating high risk are given parenthetically at the bottom of the figure. The number of men at high risk (and the number of deaths) for each characteristic were as follows: body mass index (BMI), 2691 (66); systolic blood pressure (BP), 1013 (55); cholesterol, 2212 (79); cigarette smoking, 1609 (45); and physical fitness, 1015 (56). (From Blair SN, Kohl HW, Barlow CE, Paffenbarger RS, Gibbons LW, and Macera CA, (1995) Changes in physical fitness and all-cause mortality. A prospective study of healthy and unhealthy men JAMA, 273: 1093-1098, with permission.)

Metabolic equivalents 21 24.5 28 31.5 35 38.5 42 ml per kg per min

Figure 4 Age-adjusted, all-cause mortality rates per 10000 person-years of follow-up by physical fitness categories in 3120 women and 10224 men. Physical fitness categories are expressed as maximal metabolic equivalents (work metabolic rate/resting metabolic rate) achieved during the maximal treadmill exercise test. One metabolic equivalent equals 3.5 ml kg-1 min-1. The estimated maximal oxygen uptake for each category is shown also. (From Blair SN et al. (1989) Physical fitness and all-cause mortality. A prospective study of healthy men and women. Journal of the American Medical Association 262: 2395-2401, with permission.)

formal aerobic exercise programs and sports. Intermittent or shorter bouts of activity (of at least 10 minutes duration) may be accumulated throughout the day to confer similar benefits to single, continuous 30-minute bouts of exercise. A consistent finding is that previous exercise that has been abandoned confers no benefit.

Desirable aerobic fitness levels have also been described for women (maximal aerobic power of approximately 9 METs [32.5 ml kg-1 min-1]) and men (10 METs [35 ml kg-1 min-1]) (Figure 4).

The Donts of Treadmill Buying

The Donts of Treadmill Buying

Though competitive runners are advised to run on the road, there are several reasons why you should buy treadmills anyway. You might have a family which means that your schedule does not have the flexibility it once had.

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