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The First 20 Minutes

Do you have many aches and pains? Is your mood generally good? How much time do you spend jogging? Do you ever visit the gym? Periodically, the good folks at the Division of Adult and Community Health, at the Centers for Disease Control and Prevention, in Atlanta, check in with Americans to see how they're feeling about their health, asking them such questions as part of the ambitious and sweeping telephone poll known as the Behavioral Risk Factor Surveillance System or, more familiarly, the BRFSS (gesundheit!). This survey asks Americans about their physical activities, including whether they engage in any exercise, and about how they subjectively feel about their "health-related quality of life."

After the raw data from one of the recent BRFSSs became available, CDC researchers decided for the first time to cross-correlate the information about, on the one hand, people's activity levels and, on the other, their health-related quality of life, on a monthly basis. The researchers had anticipated, as they wrote in their published report, that "physical activity" would be "associated with increasing benefits to health," both physiological and emotional, although as they also recognized, the "dose-response relationships between physical activity and many health benefits remains unclear." In other words, the researchers felt confident that exercise was good for you, but they weren't quite sure how much was necessary to receive benefits.

Their report, published in Medicine & Science in Sports & Exercise, the official journal of the American College of Sports Medicine, somehow managed to muddy the issue further. They found that of the 175,850 adults whose health information was parsed, 18 percent engaged in effectively zero planned physical activity (i.e. exercise), while 66 percent completed at least 30 minutes a day of moderate physical activity (such as walking or easy bicycling) and 42 percent said they exercised vigorously (jogging, for instance) at least once or twice a week for 20 minutes or more. (Many in this group were also moderate exercisers on other days.) These are more impressive numbers, in terms of activity, than in many recent studies of Americans. In those, particularly when the studies relied on hard measurements, such as pedometers, to gauge activity, the percentage of Americans who were even moderately active on most days of the week barely reached 50 percent.

The more eye-opening BRFSS data, though, came from people's estimates of their health-related quality of life in the month preceding the survey. People who exercised moderately reported fewer "unhealthy" days, during which they felt fatigued, unhappy, ill, anxious, achy, or otherwise "off," than people who didn't exercise. Almost 30 percent of the sedentary respondents, in fact, said they'd felt puny on at least 14 days in the prior month. Far more surprising, though, was that more than 20 percent of the people who said they worked out vigorously multiple times during the week also reported 14 or more "unhealthy days" in the month. Specifically, since this was a study overseen by health statisticians, the scientists wrote that "a poor HRQOL [health-related quality of life] was always more likely among those with no physical activity, usually more likely among those who had daily (7 days a week) activity, almost always more likely for those with activity of short duration (less than 20 minutes a day) and more likely more than half the time for those with very long duration (more than 90 minutes a day)."

Or, to be blunt, the issue of just how much exercise people need and how much may be either too little or too much is, from a scientific standpoint, a big fat mess.

Not Stepping Up

There was a time when the question of how much exercise a person required was moot. The cows needed seeing to; the corn needed tending. As we all know, prior to World War II, most Americans lived outside cities and were active almost all the time, whether they wished to be or not. A recent study of activity levels among a group of modern Old Order Amish families, whose lifestyles are considered representative of a past America (apart from the boomers), found that Amish men spent more than 10 hours a week in vigorous activity, on top of almost 43 hours a week of moderate activity and 12 hours a week of walking. They averaged almost 18,500 steps per day, or about nine miles of walking every day of the week except Sunday. The Amish women were relatively slothful, covering only about 7.5 miles per day, on average.

By comparison, according to 2010 statistics, most American adults take about 5,000 steps a day, which pales in comparison not only with the Amish but also with activity levels in other countries. The happy-go-lucky Australians average about 9,700 steps a day, the highest total in the Western world. The Swiss, number two, yodel through 9,650 steps a day and, despite the ready availability of Lindt chocolate, have a national obesity rate of barely 8 percent. In America, that rate is 34 percent and rising.

But while those figures make it clear that most Americans don't move enough, they don't tell us how much each of us should be moving, because, frankly, no one really knows. "Science and common sense tell us that, without a doubt, it's unhealthy to sit and be sedentary all day," says William Haskell, Ph.D., an emeritus professor of exercise physiology at Stanford University and one of the country's experts in exercise dosing and longevity. "But precisely how much exercise is required for health, fitness, or athletic performance is difficult to determine."

Health, fitness, and athletic performance are, after all, distinct aims with distinct demands, and each of us must resolve, for ourselves, which we're trying to achieve. We also must decide how much we're willing to do, realistically, to reach those standards. Health may seem the most achievable goal, but in reality health is a slippery term, defined often by its absence. Having high blood pressure, rotten cholesterol numbers, too much blood sugar, a wide waist, or actual illnesses, from colds to cancer, is un-healthy. Not experiencing those same conditions is good health. Activity can, if chosen wisely, improve health.

Fitness is something else, although health and fitness are often automatically joined together. If you ask an exercise physiologist, fitness refers to cardiovascular or cardiorespiratory fitness (the two terms are almost but not quite synonymous-cardiorespiratory includes measures of lung function-but close enough). Physical fitness in this sense is a measure of how efficiently you transport oxygen to laboring muscles and maintain movement. A physically fit person has strong lungs, a robust heart, and sturdy muscles. She may or may not be clinically healthy. Some people blessed with high marks on fitness can have miserable cholesterol profiles or rotund waistlines. A surprisingly large portion of any given person's biological fitness is, in fact, innate. According to several large recent studies, 30 percent or more of a person's cardiovascular fitness may be genetic. You are born either more or less physically fit than the next person. But how you augment or diminish that inheritance is up to you.

Finally, there's athletic performance, an ambition unto itself, capable, in some instances, of mitigating the other two. Walking three miles a day on a regular basis will almost certainly improve most people's health and fitness. Running four marathons in a year might not. Unless it does. "There is considerable variability in people's responses to exercise, at any dose," Dr. Haskell says.

Which raises the most central and pressing question in this entire book: Yes, fine, all those studies are very interesting; but what about me?

How Low Can You Go?

Recently researchers in Scotland trawled through a vast database of survey data about the health and habits of men and women in that fair land, similar to the BRFSS survey. In this case, the scientists were looking to see how much exercise was needed to keep the average Scotsman or -woman from feeling dour (or in technical terms, experiencing "psychological distress"). Scots are not famed for being blithe-hearted, and many of us might have expected that firm measures and lots of sweat would be required. But as it turned out, researchers found that a mere 20 minutes a week-a week!-of any physical activity, whether vigorous or easy, improved the respondents' dispositions. The activities in question ranged from organized sports to walking, gardening, and even housecleaning, the last not usually associated with bliss. The researchers found that, in general, more activity did confer more mental-health benefits and that "participation in vigorous sports activities" tended to be the "most beneficial for mental health." But overall their conclusion was that being active for as little as 20 minutes a week was sufficient, if your specific goal happened to be a sanguine temper.

The question of just how little activity people can get away with has preoccupied exercise scientists in recent years, in part because so many of us have proven so resistant to any exercise. There was a time, in the 1970s and 1980s, when most exercise guidelines, including those from the American College of Sports Medicine (ACSM) and other groups, aimed at athleticism; they recommended that people engage in prolonged, uninterrupted, vigorous activity for an hour or more, multiple times a week. Basically, people should...