Background footage for obesity-related news reports often features shots of bulging bellies surrounded by a predictable assortment of dietary hooligans: French fries, ice cream, burgers and other tasty fare. As these images suggest, health officials, nutrition advocacy organizations and weight-loss "experts" blame these "bad" foods for our excess weight.

But the cause of obesity isn't what you think.

In the largest, most comprehensive study of its kind to date, new research published in the prestigious Journal of the American Medical Association found that fewer than a third of U.S. teens get just the minimum amount of exercise recommended by the government.

It gets worse: On weekends, teens move, on average, merely 35 minutes a day. And even though these figures are only the latest addition to a growing body of evidence -- confirming that physical inactivity is a major factor in the health of our communities -- the study's lead author Dr. Philip Nader explained that "people don't recognize this as the crisis that it is."

And there's a good reason for that. Dietary scaremongers have monopolized the public's attention.

Sure, there have been some changes between modern meals and those of our grandparents. But overall nutrition in the '50s and '60s (when there were half as many restaurants and far fewer convenience foods) was far from superior to our own. Fifty years ago, most people ate a high-fat, high-cholesterol and high-sugar diet. Local grocers sold only full-fat versions of dairy products. Most recipe books listed lard -- a solid block of pig fat -- as a cooking staple. And dinner in many households featured only meat and potatoes.

The dietary difference between generations isn't significant enough to explain the difference in weight. However, the change in activity is.

The most radical transformations in the last half century have more to do with our feet than our mouths. At home, in the office and on the road, technological advancements have undeniably influenced our lifestyles and, consequently, our waistlines too.

According to a study by Dr. James Levine published in Science magazine, the mechanization of society -- replacing manual tasks with machines -- has decreased physical activity. That absence leaves us with a daily surplus of 100-200 calories, unused energy that "potentially could account for the entire obesity epidemic."

Other experts agree. Researchers at the Cooper Institute in Dallas determined that by forgoing automated assistance (like mechanized car washes and riding lawn mowers) in our daily chores, we could increase monthly energy expenditure by almost 9,000 additional calories -- the equivalent of 2.5 pounds of body fat. The study's lead author suggested that "inactivity is the major public-health problem of this century."

But for such a major concern, inactivity barely catches a blip on the public's radar.

For most of us, the term "obesity" doesn't conjure images of drive-through car washes. That makes sense given that most campaigns about weight gain fixate on a different kind of drive-though: fast food. The larger health issue, our increasingly lethargic lifestyles, gets lost amid the diet-focused policies promoted by health officials and nutrition activists. And we're suffering as a result.

Sedentary lifestyle kills someone every 15 minutes.

That's an incredible travesty, especially considering the simplicity of the solution. Move more. Prevention isn't limited to sweating it out at the gym. Walk your dog, wash your car, dance, vacuum, garden or shop: almost anything counts, as long as you move. Though small, these adjustments pack a big health boost.

The same can't be said for many food-focused schemes.

The crazy litany of proposals to tax, legislate and litigate away our extra pounds barely stops short of prying the cheeseburgers straight from our hands. From Los Angeles (where city council members aim to outlaw new "fast food" restaurants) to New York (where the Department of Health is trying to guilt consumers into eating low-calorie lunches), these for-your-own-good policies demand extreme concessions. But there's no evidence they improve the public's health.

We may look back in five years, leaner and healthier, as $200 per barrel oil forces us to abandon our cars, chain saws, escalators and moving sidewalks.

But just as it doesn't take $8 gasoline to teach us energy conservation (take the stairs, walk to school, etc.), we don't need bureaucratic know-it-alls to show us how to eat healthily -- especially when the answer to a slimmer you probably lies in your walking shoes, not in your local restaurant.