< Back to Menu

CAN A LITTLE EXERCISE REDUCE THE RISK OF STROKE?

Investigators from the renowned Nurses’ Health Study conclude the answer is yes. They followed 72,000 nurses over an eight-year period, during which time 434 strokes occurred. Of those, 285 were due to blood clots obstructing blood vessels of the brain (ischemic), 109 resulted from bleeding into the brain (hemorrhagic), and, in 40cases, the type of stroke was unclear. The extent of exercise was measured as MET hours a week. A MET (metabolic equivalent task) is the measure of caloric need per hour for any activity. Any exercise raises the body’s metabolic rate, as evidenced by calories used, heat generated, and oxygen consumed, and the MET value increases. Slow walking doubles energy expenditure to about 2METs; very brisk walking increases it to 4METs. Vigorous exercise (tennis, squash, jogging, running, vigorous aerobic exercise) increases energy expenditure to 6MET.

Women were divided into five groups based on calculation of METs per hour multiplied by the hours spent each week in the exercise activities. The five groups ranged from being completely sedentary to a lot of moderate or vigorous exercise each week. When total exercise was examined, those doing the most exercise (the equivalent of 3-1/2 hours a week or more of vigorous exercise or at least five hours of moderate exercise) had about one-half the risk of ischemic stroke. The next highest exercise group (at least two hours of vigorous or three hours of moderate exercise a week) had a 24percent reduction in risk of ischemic stroke. Exercise did not protect against hemorrhagic stroke.

Then the investigators analyzed walking as the exercise of choice. In this analysis, women were divided into four groups: slow pace, leisurely, average speed, and brisk. Those doing only one hour of average pace walking a week showed a 24percent reduction in risk of ischemic stroke. Two hours of average speed walking (2 to 2.9miles per hour) reduced risk by 34percent. Three hours of brisk walking (3 to 3.9miles per hour) or four or five hours a week of average speed walking reduced risk by almost 50percent.

The authors say that the critical variable is total METs per week and that walking is just as good as vigorous exercise so long as the total energy expended per week is equivalent. They concluded "we observed comparable magnitudes of risk reduction with similar energy expenditures from walking and vigorous physical activity".

Commentary: This is one of a series of articles all based on one set of data gathered on these nurses. The investigators use the same basic data to analyze different diseases-coronary heart disease, diabetes, and, in this case, stroke. In each case, the authors want us to believe walking is just as good as vigorous exercise in reducing risk.

A major problem in interpretation is that the groups are very different to start with. The heavy exercisers are much healthier in many ways, so their reduced risk of stroke may be due to factors other than exercise. In this article, the authors gloss over the differences, but in previous reports about these nurses, those who exercise more weighed less; less frequently had a history of high blood pressure, or a high cholesterol level, or diabetes; used post-menopausal estrogens more; and took vitamins more frequently. No matter how hard they try to adjust for these differences, they may not be able to do so and that mandates caution in interpretation. Additionally, they do not have actual values for blood pressure, just a history of high blood pressure. That makes their task of adjusting for the differences in the groups more difficult. It would have been much better if they had information on actual blood pressure values. Furthermore, the only statistically significant reduction in risk was found in those with the greatest amount of total exercise and the only statistically significant reduction in risk related to walking was found in those in the top walking group, a lot of whom must have been walking briskly (at least one mile every twenty minutes) or very briskly (one mile every fifteen minutes).

Although they say average speed walking is as good as vigorous exercise, they also note that regardless of the number of hours spent walking, the walking pace was important; those walking briskly (at least one mile every twenty minutes) had a considerably greater reduction in risk than those doing average pace walking.

The major risk factor for ischemic stroke is high blood pressure, and one of the major factors in elevating blood pressure is being overweight. The beneficial effects found with brisk walking or vigorous exercise could have resulted from some weight loss with resulting modest decline in blood pressure; additionally, exercise by itself is known, independent of weight loss, to have a modest blood pressure lowering effect. Those possibilities were not explored in this study. It is important to emphasize that it is not only an elevated blood pressure that is a risk for stroke; having a blood pressure in the high "normal" range is a risk when compared to having a blood pressure in the lower part of the "normal" range. The higher the blood pressure or the greater the excessive weight, the greater the stroke risk. Conversely, anything that lowers blood pressure or weight a modest amount (as little as five or ten millimeters of mercury for blood pressure or a five or ten pound weight loss) can produce significant benefit in reducing stroke risk.

Ratings:

There are two:

(many encouraging data; but not yet fully proved) for vigorous exercise or brisk walking.

There is a lot of encouraging evidence, but it is not fully documented because there are several negative studies showing no benefit.

for moderate pace walking: multiple studies, but still controversial and unresolved.

We need a lot more convergent evidence.

The Healthful Life recommendation is for vigorous exercise (at least one-half hour three times a week) or brisk walking (at least three-quarter to one hour three times a week), plus being sure your blood pressure is not elevated and, preferably, is in the lower part of the normal range (120/80 or less). If it is above that, exercise and some weight loss may bring it into a desirable range. Salt restriction may help some people. If your blood pressure is in the upper part of the normal range or elevated, you must stop smoking because smoking is a risk factor, though a relatively minor one, for stroke. If the blood pressure is borderline high or high, even after exercise, weight loss, and salt restriction, medication should be considered. An elevated blood pressure should always be under medical supervision.

If your blood pressure is elevated and remains elevated despite exercise, it is not clear whether exercise will do much good in preventing stroke.

Although Healthful Life recommends brisk walking or vigorous exercise, if you cannot manage that degree of exercise, average pace walking (2 to 2.9miles per hour) for an hour three times a week might indeed help in stroke prevention; but, do not count on it. Slow paced walking (less than 2miles per hour) probably will not do any good at all; at the very least, we need a lot more evidence before concluding it will.

In June 2000, the Centers for Disease Control and Prevention (CDC) made the same recommendation in regard to brisk rather than less than brisk walking (Morbidity and Mortality Weekly Report, 49, 560-565).

Hu, F.B. Physical Activity and Risk of Stroke in Women. Journal of the American Medical Association. 283. (June14) Pgs2911-2967. 2000.

Supported by