It's time again for the A Moment of Science mailbag. A listener writes:

Dear A Moment of Science, I keep reading about how the obesity epidemic is happening because people eat way more than they used to. That seems to make sense, but is it right?

Restaurant Portions

Well, it's certainly true that food is more available than it used to be, at least in the United States. It's also the case that Americans eat out more than they used to, and that restaurant portion sizes have gotten bigger.

But it's complicated. There's certainly evidence that people eat more today than they did several decades ago. One study claims that we eat nearly 600 more calories per day than people did in the mid 1970s.

Exercising Less

At least one other study, though, claims that Americans actually don't eat substantially more now than we used to. Even though restaurant portion sizes are bigger, it's possible that people are compensating by eating less in

other ways.

Which seems totally counter intuitive, given the fact of people consuming more calories than ever at restaurants.

The bottom line, it seems likely, is that obesity has more than one cause. For example, forty years ago, many more Americans did work that required physical labor, like in agriculture and factory jobs. Today, the bulk of those jobs has gone overseas and given way to more sedentary office jobs that mostly involve sitting at a computer.

Office Workers

According to some studies, the average American office workers burn 140 fewer calories per day than they used to. Over months and years, those extra calories can add up. And if we're also eating more these days, that seems like a good recipe for serious weight gain.

Read More:

• The Art of Work: Avoiding the Pitfalls of a Sedentary Job (Mark's Daily Apple)