While poor diets and lack of exercise are typically blamed for the expansion of the average American's waistline, new research published this week in the journal PLoS One has found another culprit: The workplace. Nearly two-thirds of Americans are overweight or obese, and that's due, in part, to our sedentary jobs. Here, a brief guide:

What did the study find?
Fifty years ago, workers burned 120 to 140 more calories each day. That's because Americans used to be more active — say, plowing fields instead of sitting at desks. At the same time, the amount of leisure-time physical activity hasn't changed much in recent decades. According to the study, expending 100 fewer calories per day would be in line with the weight gain seen in Americans since 1960.

How did they come up with these calorie counts?
Researchers looked at U.S. Bureau of Labors Statistics figures on the prevalence of different occupations over time, and cross-referenced those numbers with a broad national database on body weight.

Were most jobs really so active 50 years ago?
Yes. Five decades ago, nearly half of private-industry jobs were in physically demanding fields like construction, farming, mining, and manufacturing. Now, with the rise of jobs in business, education, and retail, less than 20 percent require physical activity.

So we should blame our jobs?
Not entirely. But the decrease in work-related physical labor is a component of the obesity epidemic. "There are a lot of people who say it's all about food," says Dr. Timothy S. Church, the study's lead author. "But the work environment has changed so much we have to rethink how we're going to attack this problem."

Haven't we heard this before?
Sort of. It's long been known that Americans get less exercise at work than they used to. But this study is thought to be the first to quantify the change in daily caloric expenditure.

What should I do?
Doctors say we need to be more conscious of the physical activity, we are, or in most cases aren't, getting at work. "We need to think about physical activity as a more robust concept than just recreational physical activity," says Dr. Ross Brownson, whose 2005 research was included in the new report. "In many ways we've engineered physical activity out of our lives, so we've got to find ways to put it back into our lives, like taking walks during breaks or having opportunities for activity that are more routine to our daily lives, not just going to the health club."

Sources: U.S. News & World Report, New York Times