Previous guidelines from the Center for Disease Control and Prevention (CDC) and other organizations asserted that the majority of health benefits from physical activity are obtained by walking 2 miles briskly most days of the week. This is the energy equivalent of running 8 to 12 kilometers (approximately 5 to 8 miles) per week. These guidelines therefore suggested that the benefits of increasing one's level of activity beyond a modest level is quite small.

More recent (and some older) studies suggest that in fact increasing one's level of exercise beyond the above recommended levels is associated with increased benefit in reducing cardiovascular disease and/or mortality. This has led to updated guidelines which acknowledge that progressive increases in exercise and/or progressive increases in level of fitness gives progressive benefit in reducing cardiovascular disease and/or mortality.

The following are a few studies which support the notion that progressive increases in amount of exercise or level of fitness are associated with progressive reductions in disease.

Reduced Risk of Myocardial Infarction and/or Mortality

Overview
These studies indicate progressive reduction in risk of myocardial infarction or death with increasing amounts of physical activity and/or higher levels of physical fitness.

Lakka TA, Venalainen JH, Rauramaa R, Salonen R, Tuomilehto J, Salonen JT. Relationship of leisure-time physical activity and cardiorespiratory fitness to the risk of acute myocardial infarction in men. New England Journal of Medicine 330: 1549-1554, 1994.

Subjects: 1453 men age 42 - 60 who did not report a history of heart disease or cancer

The Bottom Line: Those whose amount of exercise or degree of fitness were in the top third were approximately one-third as likely as those in the bottom third to have a myocardial infarction:

Amount of Physical Activity	Relative Risk of Myocardial Infarction
Less than 0.7 hours/week	1
0.7 to 2.2 hours/week	1.19
Greater than 2.2 hours/week	0.34

Maximal Oxygen Uptake (higher means more fit)	Relative Risk of Myocardial Infarction
Less than 28 ml/kg/min	1
28 to 33.6 ml/kg/min	0.74
Greater than 33.6 ml/kg/min	0.35

Sandvik L, Erikssen J, Thaulow E, Erikssen G, Mundal R, Rodahl K. Physical fitness as a predictor of mortality amongh healthy, middle-aged Norwegian men. New England Journal of Medicine 328: 533-537, 1993.

Subjects: 1960 healthy men age 40 - 59 yearswho were divided into four groups based on initial level of fitness, and followed for 16 years

The Bottom Line: The risk of death from cardiovascular disease diminished progressively with progressively greater levels of fitness. Death from all causes was also lower in the most fit group.

Fitness Level, by Quartile	Relative Risk of Death from Cardiovascular Causes
1 (lowest)	1
2	0.59
3	0.45
4 (highest)	0.41

Overview
There are a large number or risk factors associated with the development of coronary artery disease (eg. smoking, high blood pressure). Among these risk factors is the amount and types of lipids in the blood: higher levels of HDL (high density lipoprotein) cholesterol are assoicated with reduced risk of coronary artery disease. The following studies indicate increased levels of the "good" HDL cholesterol with increasing amounts of physical activity, which should translate into reduced cardiac disease.

Williams P. High density lipoprotein cholesterol and other risk factors for coronary heart disease in females. New England Journal of Medicine 334: 1298-1303, 1996

Subjects: 1837 women runners recruited at races and through Runners World

The Bottom Line: Increasing levels of the "good" HDL (high density lipoprotein) cholesterol were found with increasing distance run per week:

Distance Run per Week	HDL Cholesterol Level (mg/dl)
0 - 15.9 kilometers	59.2
16 - 31.9 kilometers	62.1
32 - 47.9 kilometers	64.2
48 - 63.9 kilometers	65.9
64 or more kilometers	68.8

Overview
One of the potential concerns about exercise is that myocardial infarctions occur more commonly during or soon after heavy exertion than at rest. This can lead to the argument that heavy exercise should be avoided in order to circumvent this problem. The studies below confirm the increased risk of myocardial infarction during or soon after exercise, but indicate that the risk of this happening decreases substantially the more one exercises on a regular basis.

Mittleman MA, Maclure M, Tofler GH, Sherwood JB, Goldberg RJ, Muller JE. Triggering of acute myocardial infarction by heavy physical exertion. New England Journal of Medicine 329: 1677-1683, 1993.

Subjects: 1228 patients with acute myocardial infarction (both men and women)

The Bottom Line: For the group as a whole, myocardial infarction was 5.9 times more likely to occur within an hour of heavy physical exertion than during periods or mild or no physical exertion. However, the relative risk was progressively reduced with progressively increasing frequency of regular exertion:

Frequency of Regular Exercise	Relative Risk for Myocardial Infarction
Less than 1 time/week	107
1 - 2 times/week	19.4
3 - 4 times/week	8.6
5 or more times/week	2.4

Willich SN, Lewis M, Lowell H, Arntz HR, Schubert F, Schroder R. Physical exertion as a trigger of acute myocardial infarction. New England Journal of Medicine 329: 1684-1690, 1993.

Subjects: 1194 patients with myocardial infarction (men and women)

The Bottom Line: For the group as a whole, myocardial infarction was 2.1 times more likely to occur within an hour of heavy physical exertion than during periods or mild or no physical exertion. However, the relative risk was reduced with increased frequency of regular exertion:

Frequency of Regular Exercise	Relative Risk for Myocardial Infarction
Less than 4 times/week	6.9
4 or more times/week	1.3

Overview
One of the potential concerns about running is that the associated injuries may lead to the earlier development of disability. On the other hand, regular exercise may improve cardiovasculr fitness, strength and bone density, and hence delay the development of disability. The following study suggests that regular running delays the development of disability.

Fries JF, Singh G, Morfeld D, Hubert HB, Lane NE, Brown BW. Running and the development of disability with age. Annals of Internal Medicine 121: 502-509, 1994.

Subjects: 451 members of a running club were compared to 330 community controls; comparisons were also made between 534 "ever runners" and 247 "never runners". The subjects were age 50 - 72 years old at the onset of the study.

The Bottom Line: At the start of the study, the running club members had lower disability scores than the community controls, and the "ever runners" had lower disability scores than the "never runners". Over an eight year follow-up, the running club members had a lower rate of development of further disability than the community controls, and the "ever runners" had a lower rate of development of further disability than the "never runners". (In the table below, a low disability score indicates less disability.)

	Running Club Members	Community Controls	"Ever- Runners"	"Never- Runners"
Initial Disability Score	0.026	0.079	0.028	0.094
Score 8 Years Later	0.071	0.242	0.077	0.286

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