Food Portion Sizes Have Grown -- A Lot

— -- More and more Americans are losing their battle against obesity, and a study out this week in the Journal of the American Medical Association suggests a major reason why: Their plates are stacked against them.

Analyzing data from three national surveys involving more than 60,000 Americans, researchers at the University of North Carolina at Chapel Hill found that serving sizes have grown over the past 20 years, not only at fast-food places, but at other restaurants and even in homes.

"Between 1977 and 1996, food portion sizes increased both inside and outside the home for all categories except pizza," wrote the study's authors, Samara Joy Nielsen and Barry M. Popkin. "The sizes of the increase are substantial."

The data revealed that over the past 20 years:

Hamburgers have expanded by 23 percent; A plate of Mexican food is 27 percent bigger; Soft drinks have increased in size by 52 percent; Snacks, whether they be potato chips, pretzels or crackers, are 60 percent larger.

Not surprising, the prevalence of adult obesity in the United States has increased from 14.5 in 1971 to 30.9 percent in 1999.

What Consumers Want

Other researchers say food portions have been gradually getting larger because that's what many consumers want. It's called "value sizing" — getting more food for the dollar.

The problem is, whether you want so much food or not, the more you're served, the more you eat.

At Penn State University's College of Health and Human Development, that theory was put to the test. Volunteers were given a different amount of macaroni and cheese each day for lunch. Researchers then watched to see if larger portions resulted in greater consumption.

The study, published last month in the American Journal of Clinical Nutrition, was led by Barbara Rolls. "It didn't matter if it was men or women, dieters or non-dieters, people who were overweight or not, people who habitually clean their plates or not," she said. "Everyone responded to the increased portion size by eating more."

On average, the volunteers ate 30 percent more from a five-cup portion of macaroni and cheese than from a serving one-half its size, without reporting feeling fuller after eating.

Perhaps even more troubling, most of the volunteers never even noticed when the portions were getting larger.

"I think it's quite astounding," said Rolls, "because we were serving them alone in a little booth in a lab where they had nothing to do but pay attention to the food. Think what would go on in a restaurant when you're distracted by your friends and all the other things going on. You're even less likely in that situation to notice portion sizes."

Less likely to notice, perhaps, until larger portion sizes become larger body sizes.