It isn't the food we eat that's (mostly) making us fat, says a report by the Center for Consumer Freedom.

We do eat too much of the wrong kinds of food. But more important reasons for the obesity epidemic in America are lifestyle changes that keep us from getting as much exercise as we used to get, the CCF said. The medicines we take and the fact that so many of us have quit smoking also contributes to our national weight gain.

According to a study published in Science magazine in 2003, the typical American aged 20 to 40 has been gaining about 2 pounds a year. That could be prevented if we burned just 100 additional calories per day. We could burn an additional 100 calories each day by walking briskly for 10 to 15 minutes, dancing for 20, or doing housework for 30.

The Center for Consumer Freedom is a nonprofit group financed mostly by restaurants and food manufacturers, who have a vested interest in keeping us (over) eating. But the CCF's report is partially supported by two recent studies published in the Journal of the American Medical Association.

About two-thirds of American adults are overweight, and half of these are obese, according to the National Institutes for Health. Obesity is defined by the NIH as having a body mass index of 30 or more. A person with a BMI of 40 or higher is considered morbidly obese. Body mass index is a ratio of an individual's weight (in kilograms) divided by the square of his or her height (in meters). A BMI of from 18.5 to 25 is considered optimal.

In the first study, published in JAMA Nov. 7, Katherine Flegal and her collaborators found that people who are overweight, but not obese, are at a lower risk for death than are people of normal weight. (Underweight people and the obese are at a significantly greater risk.)

In the second study, published in JAMA Dec. 4, Dr. Xuemei Sui and his collaborators found that senior citizens who keep fit are at a substantially lower risk for death, even if they are overweight.

We're getting fatter because we're more sedentary, says the CCF. Our principal leisure time activities are watching television and playing video games. This is a particular problem with children. Only about 30 percent of children are overweight or obese, half the adult rate, but the proportion of children who are overweight has been growing much faster than for adults.

"Twenty years ago, only one in 20 of the children we saw were clinically obese," said Roger Oxendale, president of Children's Hospital.

Compounding the fitness problem for children is that schools have been cutting back on physical education and recess periods, and discouraging vigorous play during those recess periods that remain, said the Center for Consumer Freedom.

"Between 1984 and 2003, student enrollment in PE classes fell from 65 to 28 percent," the CCF said. "By 2004, almost 40 percent of U.S. public school districts had eliminated recess entirely. And many others had outlawed the most vigorous games: running, tag, kickball and touch football."

Americans work hard, but for most of us, work today consists more of sitting in front of a computer screen than it does of cutting hay on the family farm or moving around on a factory floor putting things together.

"It has been estimated that each two-hour increment in sitting time at work is associated with a five percent and seven percent increase in obesity and Type 2 diabetes, respectively, in a large cohort of U.S. women," said a study published in the International Journal of Behavioral Medicine in 2004.

Labor-saving devices also have reduced the energy we expend at home preparing meals or doing housework.

Some prescription drugs have weight gain as a side effect. A study published in the American Journal of Preventative Medicine in 2004 indicated diabetics treated with multiple injections of insulin experienced a 73 percent increase in the risk of becoming overweight.

The benefits of quitting smoking are indisputable, but studies indicate people who stop smoking are twice as likely to become obese as those who have never smoked.

Dieting also contributes to overweight, the CCF said, by lowering the body's basal metabolic rate, promoting fat storage and energy conservation.

Leslie Bonci, director of sports medicine nutrition for UPMC's Center for Sports Medicine, isn't buying CCF's point that food is blameless. What we eat, how much we eat, and how we eat it is, in her opinion, responsible for half the obesity problem.

"There have been some studies showing that our calorie consumption hasn't changed that much [over the last several decades], but we were consuming too many calories back then, too," Ms. Bonci said.

"People need to eat properly and they need to exercise. You can't separate the two," said Dr. Raymond Vactor, a chiropractor in Wexford who cohosts a weekly health and wellness show on WKHB radio. "I've patients who eat right and are still overweight, because they don't exercise. And I have patients who exercise a lot, but are overweight, because they don't eat right."