Inactivity - page 5

That means that strength training preserves bone better than walking or running.

"Load is more important than repetitions, so a small number of repetitions with a heavy load stimulates bone formation more than walking or jogging, where you can do thousands of repetitions but the load isn't very much," explains Hurley.

How much is enough? "A good rule of thumb is that you shouldn't be able to do more than 15 repetitions," says Hurley. "On the other hand, if you can't do 8 reps, it's probably too much of a load."

8 Mental Health

"People who are active are less likely to develop depression," says researcher Steven Blair.

In a study of nearly 2,000 residents of Alameda County, California, people who were more active were nearly 20 percent less likely to be diagnosed with depression over the next five years than less active residents. (13)

What's more, says Blair, "physical activity helps if you're already depressed."

To see if it was exercise, not socializing, that lifted spirits, the Cooper Institute's Andrea Dunn and colleagues "had young adults with mild to moderate major depressive disorder do aerobic exercise by themselves in a room," explains Blair. "As a placebo, they had similar people just do light stretches."

Sure enough, exercisers reported greater reductions in the symptoms of depression, and those who cycled or ran on a treadmill for 180 minutes a week--about 30 minutes a day--felt better than those who cycled or ran only 80 minutes a week. (14)

At least in that small study, says Blair, "the drop in depressive symptoms in people who exercised 180 minutes a week was as good as what you'd get with antidepressant drugs or cognitive behavioral therapy."

9 Weight

It's no surprise that couch potatoes start looking like potatoes.

For example, the risk of obesity is 68 percent higher in women who watch TV for 20 to 40 hours a week and twice as high in women who watch more than 40 hours a week than in those who watch no TV. (1) Likewise, each hour a day of brisk walking cuts the risk of obesity by 24 percent.

"If you don't exercise, you're going to go to pot," says Blair. "You're going to gain weight and lose fitness."

It's especially easy to gain visceral fat--the inner layer of abdominal fat that's linked to a higher risk of heart disease and diabetes.

Researchers at Duke University Medical Center in Durham, North Carolina, and elsewhere assigned 175 middle-aged overweight men and women to walk or jog 11 miles a week, jog 17 miles a week, or continue their usual lifestyle. (15)

After about six months, the 17-miles-a-week joggers lost almost 11 pounds of fat. Those who either walked or jogged for 11 miles a week lost about 4 1/2 pounds of fat (the walkers) or 6 pounds of fat (the joggers). Most telling: the non-exercisers gained roughly 2 1/2 pounds of fat.

"In most people who are overweight, it's a slow but steady gain of one to five pounds over the course of a year," says Duke University exercise physiologist Cris Slentz, who coauthored the study.