Inactivity - page 5

Both the men and the women laid down belly fat. "We studied postmenopausal women, so they were gaining visceral fat aggressively," he notes. Before you know it, your slacks are snug.

"People don't realize that inactivity is not a steady state," says Slentz. "They think it would be nice to start exercising, but if they think about it for six months, they've gained another pound. or two and an 8 percent increase in visceral fat."

How much activity is enough to stop gaining? It depends on who you are.

"If you're trying not to become overweight or obese, it looks like 45 to 60 minutes a day," says Harvard's I-Min Lee. But the story is different for people who used to be chubby.

"Individuals who are overweight and have lost weight need more physical activity to maintain the weight loss than people who don't want to gain weight, because if you've already become overweight, you're in an at-risk group," she notes. "Those people probably need 60 to 90 minutes a day to avoid regaining weight."

If that sounds about as realistic as going to medical school in your spare time, don't despair. "That's based on formerly obese individuals who lost 30 to 50 pounds and kept it off," says Blair. "You don't need 90 minutes a day to keep five pounds off."

What's more, it's not as though half an hour a day doesn't count. "Just 30 minutes a day is sufficient to reduce the risk of many chronic diseases," says Lee.

10 Immune System

Regular exercise may boost the immune system ... unless you overdo it. One indicator: how often people get colds.

"In moderate amounts, physical activity reduces the risk of upper respiratory tract infection," says Lee.

In studies that use older or middle-aged individuals, 30 to 45 minutes of walking about five days a week is moderate, she adds.

But the tables are turned if you go to extremes.

"Marathon runners have a higher risk of colds after a race, because immune function goes down after a prolonged intense bout of exercise," says Lee.

The picture is similar for inflammation, which can be a sign of weakened immune function.

"It's very complicated, but it looks like regular exercise can decrease inflammation," says the University of Maryland's Ben Hurley.

On the other hand, playing a hard game of basketball or soccer once a month can boost inflammation if that's all you do.

"The weekend athlete who does something strenuous once in a while can do more harm than good to a lot of systems," says Hurley. "You have to perform vigorous activity at least two or three times a week to avoid making things worse."

The Bottom Line

* To lower your risk of heart disease, diabetes, and colon cancer, shoot for at least 30 minutes a day of moderate-intensity aerobic exercise like brisk walking, cycling, jogging, or swimming.

* To keep off substantial amounts of lost weight, shoot for 60 to 90 minutes a day of moderate-intensity aerobic exercise.

* To build muscle and prevent bone loss, do strength training exercises at least twice a week using weights you can lift no less than 8--but no more than 15--times.