What actually happens to our bodies when we get moving?

If he were to list the benefits of exercise, Tom Thomas says there would be 1,000 entries, no sweat.

High on the list would be a decreased risk of heart attack and stroke. And lower blood pressure. And a boost in high-density lipoprotein, the good cholesterol.

And there's no doubt that Thomas, who directs the exercise physiology program at the University of Missouri-Columbia, would mention increased insulin sensitivity, a lack of which often leads to Type 2 diabetes. And a probable decreased incidence of some cancers. And weight control.

He'd point out the benefits to bone density, and to muscle maintenance. And then there's the whole cognitive and emotional piece.

"If exercise could be made into a pill, it would be the most-prescribed pill we have," said Cedric Bryant, an exercise physiologist and a vice president (for educational services) with the American Council on Exercise in San Diego.

"It affects virtually every system in the body," chimed in Catherine Jackson, who chairs the department of kinesiology at California State University in Fresno.

The power of physical activity came home to her in an experiment she conducted a few years ago. She listed the effects of inactivity and those of aging. They were nearly identical.

"Some of the things that occur to you as you get older won't occur as soon if you are active," she said. "A lot of people have said exercise is as close to a fountain of youth as you're going to get."

Indeed, it would be hard to overstate the impact of exercise on human health. But exactly what happens when you get up from your chair and shake it?

Burning the engine

Frank Booth, an exercise physiologist at Missouri, is one of the people trying to get a better handle on that. In the broadest sense, he thinks exercise returns us to a state of nature from which we've strayed as machines have gradually eliminated physical activity from our lives.

He contends that humans evolved in response to a world of cycles. Cycles of hunger and satisfaction, of activity and rest.

"Having those cyclical periods when you turn on your burning process, that seems to be something the body has to do to keep it working," said Booth, who turns his own burners on high by running to and from work every day, a three-mile round trip. He believes human bodies and internal combustion engines have at least one thing in common.

"By not turning on the engines every now and again, things go wrong."

When the human engine is kicked into high gear, everything changes. The blood moves faster, the pulse and blood pressure increase, the chemical factories all over the body switch from producing compounds that can cause disease to compounds that promote health.

Exercise and heart health

You know that exercise improves heart health. But how?

Harold Laughlin, a physiologist at the University of Missouri-Columbia School of Veterinary Medicine, puts pigs on treadmills, then inspects their blood vessels. Pigs that exercise, and presumably people who do the same, produce more of a compound called nitric oxide. It tells the blood vessels to relax and get wider.

Roomier blood vessels can be a lifesaver because they can accommodate more blood and keep the blood flowing even when a vessel is partly obstructed by a big glob of fat.

Heart disease happens when fatty plaques accumulate on the walls of the blood vessels leading to the heart and when clots form in the bloodstream.

Together or separately, these can block blood flow to the heart, causing an attack. But a bountiful supply of nitric oxide seems to prevent the development of fatty plaques and blood clots, Laughlin said.

Fat cells and muscle cells throughout the body also get into the act, Laughlin said. When muscle cells are working and when fat cells are shrinking, they produce hormone-like compounds that keep blood vessels clean of fatty build-up.

On the contrary, when muscle cells are sitting idle and fat cells are swelling, they produce chemicals that cause fats to stick to blood vessel walls, Laughlin said.

People who exercise regularly need more blood to supply their muscles with the oxygen and other nutrients it takes to perform all of that hard work. To that end, a regularly exercised body builds a dense network of the tiny blood vessels known as capillaries to get blood where it's needed.

If a major artery to the heart is blocked with fatty deposits, there's a big payoff to having a healthy network of side roads, said Catherine Jackson, of California State University.

"The amount of (heart) tissue that will die will be greatly diminished because there are other ways to get the blood to the tissues," Jackson said.

Exercise is an important strategy against diabetes, too, and probably stems from the fact that active people have extra mitochondria in their muscle cells, up to 30 percent more than couch potatoes, Booth said.

Mitochondria function like power plants, making food into power packets that fuel muscle activity.

People with more mitochondria burn more fat, Booth said, and that seems to mean less obesity and less diabetes.

Regular exercise also affects the levels of stress hormones, with benefits particularly for the heart and vascular system. Stress hormones were a life-saver back in the days when people faced threats from predators and had to be able to either fight or flee.

Modern-day pressures are more subtle, but they still unleash nasty stress chemicals. The problem is that a sedentary life doesn't do much to dispel those chemicals, so they stay in the bloodstream, pushing the heart rate and blood pressure to troublesome levels.

The body responds to prolonged elevated stress hormone levels by putting cholesterol into the bloodstream, Jackson said.

"It's very common for people to have heart attacks at 2 or 3 in the morning," she said. "That has probably occurred because of stressors earlier in the day, and the cholesterol level got higher and higher until there was enough to plug a coronary artery."

Exercise short-circuits that, she said. Although stress hormone levels increase during exercise, they quickly drop once the exercise is done.

"If you have a stressful day and you plug exercise in, you are using up all those hormones that do very bad things when you allow them to run rampant."

Exercise and fat

There's more. A body accustomed to moving along at six miles an hour or lifting heavy objects is a lot smarter about what it does with excess fat calories.

When sedentary bodies gorge on potato chips and cookies and other high-fat foods, the excess fat tends to get packed away in two bad places: inside the blood vessels and in the gut. Both are major heart disease risk factors.

Exercised bodies, on the other hand, tend to stash excess fat in a place they know it'll come in handy: in the muscles themselves.

Exercise and cancer

Another major health threat, cancer, also seems to be discouraged by exercise. There are indications that exercise may reduce a woman's odds of developing breast cancer, possibly because it tends to tamp down the level of estrogen, which feeds at least some breast cancers.

More solid, however, is the evidence that exercise fends off colon cancer. According to Booth, one compelling study found that formerly active people doubled their risk of colon cancer within 10 years of quitting their exercise habit.

Exercise hastens the movement of food through the gastrointestinal tract, said Bryant, of the American Council on Exercise. That's especially important if you eat foods that contain carcinogens because the less time carcinogens spend in your intestine, the less likely they are to provoke a mutation that can turn on a cancerous growth.

The benefits of exercise go on and on. Activities that put pressure on bones stimulate more bone growth, especially important for girls and women who tend to be susceptible to osteoporosis in their later years.

And of course it maintains and even increases muscle mass, a critical consideration for older people who naturally tend to lose muscle and gain fat tissue.

It pumps out lots of endorphins, those natural feel-good neurochemicals. It's thought to enhance brain function and perhaps to help ward off dementia and Alzheimer's disease.

But the growing body of evidence about the powerful effects of exercise means nothing as long as Americans cling to their sedentary ways. And they do.

According to statistics from the U.S. Centers for Disease Control and Prevention, in the year 2000, 26 percent of Americans reported that they were meeting the exercise recommendations of 30 minutes of moderate exercise five days a week.

The CDC then relaxed its definition of suitable activity to include gardening and house cleaning. Even at that, the percent of people meeting the recommendations was bumped up only to 45 percent.

And the proportion of Americans who reported that, in the preceding 30 days, they'd engaged in no physical activity whatsoever?

Twenty-five percent.

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