Portion Size Matters: Given Too Much, We Eat It

In the first systematic, controlled study of the response to portion size in adults, the researchers found that the bigger the portion, the more the participants ate. On average, they ate 30 percent more from a five-cup portion of macaroni and cheese than from one half its size – without reporting feeling any fuller after eating. Dr. Barbara Rolls, who holds the Guthrie Chair of Nutrition in Penn State's College of Health and Human Development, led the study. She says, "Men and women, normal-weight and overweight individuals, restrained and unrestrained eaters, all responded to larger portion size by eating more."

The response to larger portions was not influenced by who determined the amount of food on the plate, the study participant or the researchers.

The study is detailed in a paper, "Portion Size of Food Affects Energy Intake in Normal-weight and Overweight Men and Women," in the current (December) issue of the American Journal of Clinical Nutrition. Rolls's co-authors are Erin L. Morris, Penn State master's degree recipient in nutrition, and Liane S. Roe, research nutritionist.

In the study, 51 normal-weight and overweight men and women, 21 to 30 years of age, had lunch one day a week for four weeks in Penn State's Laboratory for the Study of Human Ingestive Behavior. The lunch included water, carrot sticks and a snack-size chocolate bar as well as macaroni and cheese made in the "light" version from a well-known commercial mix.

Each week, the participants were served the macaroni and cheese in one of four portions ranging from two and a half to five cups. The participants were required to eat all of the carrots and chocolate bar but could eat as little or as much of the macaroni and cheese as they wanted.

One group received the different amounts of macaroni and cheese pre-portioned on a dinner plate from which they ate. Another group received the different portions in a serving dish and could scoop as much of the entrée as they liked onto their plates. In both cases, the participants ate more when more food was available but didn't report feeling any fuller after eating.

Rolls says, "Our research shows that pretty much everyone is susceptible to the influence of portion size. However, it's not increased portion size alone that is contributing to the American obesity epidemic but rather eating large portions of high-calorie, high-fat foods.

"Large portions of foods low in calories and fat such as vegetables, fruits and broth-based soups can aid weight management by providing satisfying portions with few calories," she adds.

Her strategy is described in detail in her best-selling book, "The Volumetrics Weight-Control Plan," to be published soon in a new low-cost paperback edition by Avon.