That's the conclusion of the American Cancer Society, which estimates that as many as 180,000 Americans each year will die of cancers related to obesity and lack of activity. Overweight people are particularly at risk for colon and breast cancer, researchers say.
Facing an epidemic of obesity, the cancer society is calling on governments, schools and businesses to help promote exercise and healthy living in communities.
"The environment in which we live -- where we work, where we play, where we hang out -- has really become a barrier in terms of making choices," says Colleen Doyle, director of nutrition and physical activity for the cancer society.
Subdivisions without sidewalks. Shopping centers located miles from any homes. Fast-food restaurants on every corner. Cutbacks in school physical education programs. Reduced leisure time in two-income households. These have all contributed to a couch potato society, Doyle says.
"The way we build, it's harder for people to walk places or to find safe places to be active," she says. "The environment really has an impact on the kind of individual choices we make. We need walking paths, bike lanes, buses that have racks for bikes."
The cancer society revises its guidelines on diet and exercise every five years. The latest version, released earlier this spring, calls for society to wage war against obesity.
"Obesity and inactivity are like our version of the infectious diseases of 100 years ago," says Dr. Anne McTiernan, director of the Prevention Studies Clinic at the Fred Hutchinson Cancer Research Center in Seattle and a member of the committee that wrote the cancer society guidelines.
"At least 35 percent of all cancer deaths may be related to overweight and lack of activity," she says. "In order to help people, we are really recommending that communities get involved."
Of course, individuals can still make choices that lead to healthier living. As it always has, the cancer society is urging Americans to eat better and exercise more.
That means eating at least five servings a day of fruit and vegetables and getting moderate exercise for 30 minutes a day, five days a week.
Studies have shown people who maintain a healthy weight are less prone to developing colon and breast cancer, in particular.
All the details aren't known, but McTiernan says exercise is believed to speed the passage of food through the colon, thereby reducing the amount of time that any toxins are in contact with the body.
Overweight people also tend to have more insulin, which promotes the growth of tumors.
For women, exercise reduces the level of estrogen, a hormone linked to breast cancer.
Even after menopause, exercise reduces the estrogen level and the risk of breast cancer, says McTiernan, who is the author of "Breast Fitness: An Optimal Health and Exercise Plan for Reducing Your Risk of Breast Cancer."
What kind of exercise is best?
"Any type of activity is beneficial," says Doyle. "Obviously, if you're a couch potato, you're not going to get up and start running five miles a day."
"But something is better than nothing. Walking, swimming, raking the leaves, throwing the ball with your kid, dancing, gardening -- everything counts," she says.
People over the age of 30 or those with health problems should consult their doctor before beginning an exercise program.
If you're pressed for time, you don't have to do all the exercise at once. Three 10-minute sessions throughout the day provide the same benefit as one 30-minute session.
While moderate exercise is a big step toward overall health, studies have shown the risk of colon and breast cancer is reduced even more among people who exercise longer and more vigorously -- say, for 45 minutes instead of 30.
What constitutes vigorous exercise?
McTiernan says a good guideline for vigorous activity is to exercise until you can feel your heart rate rise and you sweat in normal temperatures.
"For someone who has been sedentary, that may occur in walking a mile in 15 minutes," she says. "For someone who's younger or more fit, they may have to do that through jogging or some form of sport."
"It's difficult to set a hard-and-fast guideline for everyone," McTiernan says. "If a person really has an interest in vigorous activity, they can work with a personal trainer to learn more about maximum heart rates and other aspects of fitness."
What To Do
Read the newly revised guidelines on nutrition and physical activity from the American Cancer Society. Here are some simple steps for starting an exercise program from the American Academy of Orthopaedic Surgeons.