While you wait: the cost of inactivity.

Planning to exercise more? Been

It's easy to put off, at least today, at least this week, at least until you (choose one) get new sneakers, get past the holidays, get some free time, get a new job, get the kids off to college, whatever.

But as the years slip by, your body isn't just frozen in time, waiting to get moving. It's fading.

"If muscle isn't stimulated, your body senses that you don't need it," explains Miriam Nelson of Tufts University in Boston. "Metabolically, it's expensive to keep up so you start to lose it."

Muscle is just the beginning, she adds. "Inactivity affects the brain, heart, blood vessels, bones, liver, gut, sleep, anxiety, depression, self-esteem, your ability to use glucose, and more."

"From the top of your head to the bottom of your toes, being physically active is the stimulus that gets most organs in the body to work at their best," says Tufts University exercise expert Miriam Nelson.

"If you're not active, it affects all body systems, literally down to the cellular level, where your ability to transfer oxygen from the bloodstream to cells is diminished and the number of power-producing mitochondria in your cells is less."

"If you can't get as much oxygen out of your blood, you can't walk up a flight of stairs as easily as you get older." And that's just one system that suffers if you're sedentary.

The good news: it's never too late to start moving. "Well into your 90s, all of these systems can be stimulated," notes Nelson. "It's quite remarkable."

Here are 10 ways inactivity can take a toll on your body.

1 Diabetes

"The one thing that seems to deteriorate quickest with inactivity is insulin sensitivity," says Ben Hurley, a professor of kinesiology at the University of Maryland at College Park (and husband of Nutrition Action's Jayne Hurley). Fortunately, "it also responds most consistently when you train."

Type 2 diabetes--by far the most common kind--occurs when the body be comes insensitive, or resistant, to insulin in the blood. When insulin stops working, blood sugar levels rise and diabetes sets in.

Regular exercise reverses the damage.

"It increases insulin sensitivity and makes the cells better at taking in glucose and processing it," explains I-Min Lee, an associate professor of epidemiology at the Harvard School of Public Health in Boston.

In a study of more than 50,000 nurses, every two hours a day of TV-watching was linked to a 14 percent increase in the risk of diabetes. (1) Every two hours of sitting at work was linked to a 7 percent increase. In contrast, every hour of brisk walking per day was linked to a 34 percent lower risk.

What's more, when researchers told people with high-but-not-yet-diabetic blood sugar levels to do aerobic exercise for at least 2 1/2 hours a week and lose at least 7 percent of their body weight, their risk of diabetes was 58 percent lower than similar people who didn't exercise or lose weight. (2)

"The data are striking," says Hurley. And it's not just an issue for adults. "Type 2 diabetes used to be a disease of middle age," he adds. "But now we're seeing it in young people. It's a sedentary disease."

Hurley sounds like researcher Steven Blair talking about the metabolic syndrome, which raises the risk of both diabetes and heart disease. Doctors diagnose the syndrome when people have a large waist, low HDL ("good") cholesterol, and elevated (though not necessarily high) blood pressure, blood sugar, and triglycerides.

"The metabolic syndrome is misnamed," says Blair, who is president of the Cooper Institute in Dallas, Texas. "It ought to be called the inactivity syndrome."

2 Cancer

"The evidence is fairly clear now that men and women who are physically active have a 30 to 40 percent lower risk of colon cancer compared to individuals who are not active," says Harvard's I-Min Lee, who examined dozens of studies. (3)

Experts have several theories that might explain how physical activity protects the colon. "It increases transit in the intestine, which makes food flow through fast," says Lee. "So any carcinogens in the intestine have less contact with the cells that line the intestine."

Another possibility is that regular exercise shores up the immune system. "That would protect the body from any cancer, including colon," she adds,

Hormone-like substances called prosta glandins might also play a role. "Activity can decrease prostaglandin E2 levels in the intestinal cells," says Lee. "Prostaglandin E2 is associated with a higher risk of colon cancer," she notes, probably because it makes colon cells proliferate faster and slows intestinal motility.

Then there's the obvious: "Physical activity prevents weight gain, and the overweight have a higher risk of colon cancer," says Lee.

How much movement is enough? "We don't have precise data, but it looks like you need 30 to 60 minutes a day of moderate-intensity physical activity."

Regular exercise also appears to lower the risk of breast cancer by about 20 percent. (4) "But the research has been somewhat inconsistent," says Walter Willett, chair of the nutrition department at the Harvard School of Public Health.

That may be because most studies aren't long or large enough to see the link, he adds. "It's a modest effect, but it does look like it's there, mostly for postmenopausal women."

And it looks like women can protect their breasts at the same time they're maintaining other body parts.

"We're not clear how much physical activity you need to reduce the risk of breast cancer," says Lee. "But it's in the same ballpark as for colon cancer--between 30 and 60 minutes of moderate-intensity physical activity a day."

As for some other cancers, she adds, "there's some suggestive evidence, but it's not as conclusive."

3 The Brain

It's bad enough that inactivity can turn your muscles to Jell-O. Can it do the same to your brain?

"The evidence is fairly solid that people who are more physically active are at lower risk for cognitive decline and dementia," says Constantine Lyketsos, director of the division of geriatric psychiatry and neuropsychiatry at Johns Hopkins University School of Medicine in Baltimore.

For example, among more than 3,000 older men and women in the Cardiovascular Health Cognition Study, those who reported engaging in at least four activities (like walking, household chores, gardening, and jogging) during the previous two weeks were half as likely to be diagnosed with dementia five years later than those who took part in no more than one activity. (5)

However, exercise had no link with either Alzheimer's or other dementias in people in the study who had the [epsilon]4 version of the apolipoprotein (ApoE) gene. In contrast, a Finnish study found that exercise protected ApoE [epsilon]4 carriers more than people without the gene. (6) (About 20 percent of people have at least one copy of the [epsilon]4 version of the gene.)

"People with the gene seem to get Alzheimer's about 8 to 10 years earlier than non-carriers," explains Lyketsos, who coauthored the Cardiovascular Health Cognition Study. That usually means it starts in their 70s rather than their 80s.

With or without ApoE [epsilon]4, he acknowledges, the evidence on exercise isn't conclusive. "It's always possible that people are reducing their physical activity because they're in the early stages of dementia."

But clues from animal studies are compelling.

"There's a fairly strong neurobiological basis to suggest that the more physically active you are, the less likely you are to develop dementia," says Lyketsos.

For example, the brains of physically active mice have more nerves, more connections between nerves, fewer clogged arteries, more oxygen flow, and better ability to utilize glucose, he explains.

"All are probably factors in helping prevent cognitive decline and dementia."

Recent studies used mice that are prone to acquire the amyloid plaques that are found in the brains of Alzheimer's patients.

"At least one study suggests that if you take these mice out of their traditional cages, where there's little to do, and put them into stimulating cages with more colors, objects, brighter areas, and little mouse treadmills, you find fewer amyloid deposits in the brain," says Lyketsos.

Why? In stimulated mice, an enzyme that degrades amyloid deposits is more active. (7) "So there may be a specific anti-Alzheimer's effect of physical activity and social stimulation," he explains.

4 The Heart

Every year, 1.2 million Americans have a heart attack. It's no surprise that couch potatoes have a higher risk.

"Exercise affects the function of the heart muscle, but it also affects the blood vessels, from the large aortic artery to the veins and the small capillaries," says Tufts University's Miriam Nelson.

Researchers have long known that regular exercise can boost HDL ("good") cholesterol, which is the cholesterol that's on its way out of the arteries. But in recent years, they've learned that physical activity also makes the lining of blood vessels--the endothelium--more flexible.

"If partially blocked arteries are more elastic, they can relax better and send more blood to the heart muscles," explains Harvard's I-Min Lee. "It's like pumping blood through a rubber hose instead of a concrete pipe."

And you don't have to be an athlete to protect your heart.

In a study that tracked nearly 40,000 women for five years, those who walked briskly for at least an hour a week were half as likely to be diagnosed with heart disease as those who did no regular walking. (8) The risk was even lower for women who jogged or did other vigorous activity.

Scientists have good reason to believe that regular exercise protects the heart. "We know that physical activity has beneficial effects on risk factors for heart disease like lipids, blood pressure, and insulin sensitivity," explains Lee.

What's more, researchers have tested the impact of exercise training on people who already have heart disease.

"If they are assigned to an exercise program, they have a lower risk of dying and of dying from heart disease," says Lee.

5 Stroke

People who are active are 25 percent less likely to have a stroke than their sedentary counterparts. (9) How can exercise keep strokes at bay?

"By lowering blood pressure, raising HDL cholesterol, and reducing the risk of blood clots," says Lee.

In the U.S., most strokes occur when a blood clot gets stuck in a partially clogged artery that feeds the brain. "Physical activity has the same effect on arteries to the brain as on arteries to the heart," she adds.

The evidence is less solid that regular exercise wards off less-common hemorrhagic strokes, which occur when a blood vessel in the brain bursts. However, if exercise prevents hemorrhagic stroke, says Lee, "it's probably by lowering blood pressure."

High blood pressure, or hypertension, is the biggest risk factor for any stroke. And researchers are now finding that a regular spin on the bike or walk on the treadmill doesn't lower blood pressure in everyone.

"It's a mixed bag," says the University of Maryland's Ben Hurley. "Regular aerobic exercise reduces blood pressure in aboperformers ut 75 percent of people."

In others, blood pressure doesn't budge. (10) "Our group found that blood pressure improved in people with one gene profile, but not in others."

You can't easily get tested to see if you have the gene variations that make blood pressure respond to regular exercise. But that shouldn't keep you off your feet, says Hurley.

"You may not respond with lower blood pressure, but you may improve more than others in other areas such as insulin sensitivity," he explains. "We've never seen a case where someone doesn't have the right genotype for any benefit."

6 Muscles

It happens earlier in women than in men, but in both genders muscle starts to wither away if it's not used.

"Muscle atrophy for people with average activity levels starts at age 40 for women and in the late 50s for men," says Hurley. The muscle wasting probably starts even earlier, he adds, but so slowly that it isn't easy to detect.

"For every decade after about age 50, you lose some 6 percent of your muscle mass, which comes with a 10 to 15 percent loss of your strength," says Hurley.

But anyone can build muscle back up with strength training exercises. (11) "After two months of training, we see a 40 percent increase in strength," says Hurley.

"The earlier you start, the better," he adds. "But even people over 100 years old can partially reverse some of the loss that occurs with aging."

The trick is to not just use the muscle, but to overload it. "You have to make it work harder than it's accustomed to," explains Hurley.

"If you overload it in a gradual, progressive way, you can make the muscles bigger and stronger by making each muscle fiber thicker."

If you think of strong muscles as a luxury, think again. They can ward off the frailty that makes older people lose their independence, either because they can't take care of themselves or because they fall and fracture a hip.

"Regular exercise is the best way to stay out of a nursing home," says the Cooper Institute's Steven Blair.

7 Bones

Fragile bones--osteoporosis--cause more than 1.5 million fractures each year in the U.S. But bone starts to disintegrate decades before it cracks.

"Bone is like any other part of your body," says Harvard's I-Min Lee. "If you stress it, it responds."

If you don't, the ongoing balance between bone buildup and breakdown shifts towards a net loss. But strength training can make a difference.

"The research doesn't consistently show that you can increase bone, but you can prevent loss," says Hurley.

The best studies randomly assign people to strength training or a non-training "control" group. (12) "In postmenopausal women, the control group loses almost 1 percent of their bone mineral density in a year," notes Hurley. "But the group that trains either stays the same or has a slight increase."

Whether you actually gain bone or hold on to it may depend on how much you stress, or overload, the bone.

"We reported a 3 percent increase in density in the femoral neck bone at the top of the thigh, we think because the leg press--an exercise that strengthens the major leg muscles--has the greatest load," says Hurley. "We didn't see an increase where the load wasn't very big."

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.

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.

* For tips on strength training, a useful guide (for women and men) is Strong Women Stay Young, by Miriam E. Nelson (Bantam, 2000).

(1) Journal of the American Medical Association 289: 1785, 2003.

(2) New England Journal of Medicine 346: 393, 2002.

(3) Medicine & Science in Sports & Exercise 35: 1823, 2003.

(4) Archives of Internal Medicine 159: 2290, 1999.

(5) American Journal of Epidemiology 161: 639, 2005.

(6) Lancet Neurology 4: 705, 2005.

(7) Cell 120: 701, 2005.

(8) Journal of the American Medical Association 285: 1447, 2001

(9) Stroke 34: 2475, 2003.

(10) Journal of the American Geriatrics Society 53: 204, 2005.

(11) Journal of the American Geriatrics Society 49: 1428, 2001.

(12) Journal of the American Medical Association 272: 1909, 1994.

(13) American Journal of Epidemiology 156: 328, 2002.

(14) American Journal of Preventive Medicine 28: 1, 2005.

(15) Journal of Applied Physiology 99:1613, 2005.