This Christmas, requests for second helpings may be drowned out by a new set of demands: No sugar, no meat, liquid-only, or fat-free. “What’s your diet?” has replaced “what’s your sign?” in small talk today.

Americans are placing increasingly more importance on what to eat and what to avoid. But our waistlines refuse to budge.

As many as 97 percent of dieters eventually regain their lost weight and then some, according to a recent government review. Based on those odds, most adults have a better chance of losing their minds before they lose those last 5 pounds. Obviously, overweight people don’t continue to enter this battle with food because of its losing track record. People choose to diet because they rarely hear about any other options.

Nutrition zealots dominate the obesity debate. They have singled out “junk food” as the culprit behind our burgeoning behinds. Endlessly revising the guilt-food du jour, their campaigns target cupcakes one day and chow mein the next. This food-focused hype keeps dietary issues in the limelight.

But these dinner-plate debates have distracted us from the couch-potato culture that has become the norm.

A quick inventory of our homes and offices provides more insight into our weight woes than any nutrition survey. Elevators in almost every building enable tenants to avoid climbing any stairs. Moving sidewalks in airports help travelers make their connections without breaking a sweat. And TV sets in 99 percent of U.S. households ensure that time between work and sleep can be spent comfortably on the sofa.

The evidence linking these creature comforts to the bathroom scale is growing.

Walking, the most basic physical activity, has been largely engineered out of daily life. Whether to work, to school, or to the house down the street, car trips have replaced most travel by foot. Though this change may seem small, it makes a big difference.

One 2007 study calculated that by walking just one hour each week — the activity difference between average drivers and pedestrians — a person could lose 28 pounds over a decade. (That’s more than double the average American weight gain.)

Inactivity doesn’t end with our daily commutes. The human body – programmed to survive incredibly laborious work -faces few challenges in the modern office environment.

Hunting woolly mammoths and foraging for wild fruits tested the physical limits of our ancestors. Even as recently as 30 years ago, security staff patrolled on foot, retailers checked inventory by hand and many jobs in the labor force involved actual labor.

The electronic age changed all that. Video monitors have replaced walking patrols, spreadsheets have simplified inventories, and other manual tasks can now be done with the click of a mouse. The only physical limits we test today are those of our belts.

Machines are picking up the slack in the home, too, thanks to food processors, dishwashers and other appliances. Studies suggest that the “electrification” of household chores, such as riding instead of pushing a lawnmower, may explain some of our overall weight gain. How much? Researchers at the Mayo Clinic estimate that the energy-burning difference between active tasks and automated ones can run up to 8, 800 calories per month.

Viewed separately, changes in transportation, occupations and leisure time almost seem irrelevant to the weight debate. But, collectively, over generations, these small lifestyle changes have re-engineered us as weight-gaining machines. That’s bad news.

The good news is that we’re free to choose.

Obesity is a trade-off between conveniences of modern life and metabolisms inherited from our great-grandparents. It’s not as black and-white as salad versus steak. Our days are speckled with countless decisions that give us plenty of wiggle room in our diets, and even some in our belts.

Ultimately, it boils down to this: Would we rather minimize our body weight or to maximize our comfort and convenience?

Deciding whether to walk or drive is just as important as the decision to go back for second helpings. Do we e-mail a colleague or walk to her office? Should we grab a pail and hose or drive through a carwash? Choosing wisely, we can have our cake and eat it, too.