Exercise capacity tied closely with life expectancy

One of the best studies to date on why we should exercise was reported in the New England Journal of Medicine on Aug. 4. Yet the terminology and data were so complex that the significance may have gone unnoticed. The study was based on 5,721 heart-healthy Chicago-area women who took a standard stress test in 1992. The speed and incline of the test treadmills were increased every three minutes until the women were fatigued or experienced negative symptoms. Researchers noted the highest treadmill intensity the women were able to sustain during the test and recorded it as their "exercise capacity." The data was then used to create the first formula for predicting fitness norms in women.

The unit used to estimate exercise capacity, the MET, was a source of great confusion to most who read about the study. METs, or metabolic equivalents, are the term exercise physiologists use to describe exercise intensity in the laboratory. A single MET is the amount of energy used sitting quietly for one minute. As activities become more demanding, MET levels go up. Moderate walking uses three to six METs per minute since it requires three to six times more energy than resting. Running a 12-minute mile consumes eight METs, based on the same logic. A maximum effort activity like cross country skiing uphill in hard snow uses an astounding 16.5 METs of energy.

MET values are useful because they correlate roughly with the amount of calories burned per minute during exercise. As exercise intensity rises, so do MET levels and calories burned. At rest, or one MET, a 154-pound person burns about 1.2 calories per minute. When exercising at 5 METs (five times resting rate), he burns about 6 calories per minute (1.2 calories/min x 5).

Researchers used MET scores from the 5,800 healthy study subjects to statistically create an age-based equation for predicting women's exercise capacity. They then used the formula to predict the exercise capacity of 4471 women with cardiac symptoms. After evaluation on a similar stress test, researchers matched survival data for both groups.

In previous studies, exercise capacity had been shown to be a strong predictor of mortality in women. But this study was even more definitive. Women whose exercise capacity was less than 85 percent of the formula-predicted value were twice as likely to die within eight years.

The study offers a powerful reason to pay attention to our own fitness level. A mere 15 percent deficiency can do more than promote weight gain. It can have a profound and serious impact on life expectancy.

How much exercise should a person be able to do for their age? It depends on your gender. This study showed that women have a lower exercise capacity than their male counterparts.

The recommended exercise capacity for women can be calculated through the study formula: Predicted MET = 14.7 n (0.13 x age). Using myself as an example, at 41 years old, I should be able to achieve a MET level of 9.4. This is equivalent to an 11-minute mile run. Men use a different formula to predict exercise capacity, but their levels are roughly 10 percent higher than women's.

The take-home lesson in this complicated study: exercise capacity is directly correlated to life expectancy. Regular activity of sufficient intensity can improve exercise capacity over time. But inactivity and the aging process can erode it. It is nice to know that you determine which side wins. How much exercise is enough to improve capacity? For normal, healthy individuals, accumulating 20 to 30 minutes of exercise at a moderate level of exertion (breathing somewhat harder, but still able to carry on a conversation) at least three days each week may be all you need. And that seems a small investment, considering the payoff.

Lisa Bell is a registered nurse and a personal fitness trainer who lives and works in Newburgh. You can reach her at www.BellBodies.com

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