Dr. Barbara Rolls, who holds the Guthrie Chair of Nutrition in Penn State's College of Health and Human Development and who led the studies, says, "You can't rely on somebody else to put the right amount of food on your plate, in your sandwich or in your snack pack. And you can't depend on your appetite to tell you when to stop eating, either, since our experiments show that the study participants often didn't notice changes in portion size even when they were given 50 percent or 100 percent more food.
"Those who want to reduce their risk of gaining weight should realize that when they are served big portions of high-calorie foods, they're likely to end up overeating," she adds.
Rolls' group is the first to publish a study of the effects of portion size on the patrons of a public restaurant. The study, "Increased Portion Size Leads to Increased Energy Intake in a Restaurant Meal," is detailed in the current issue of the journal, Obesity Research. The authors are Nicole Diliberti, former Penn State graduate student, Dr. Peter Bordi and Dr. Martha T. Conklin, faculty in Penn State's School of Hotel, Restaurant and Recreation Management; Liane Roe, research nutritionist; and Rolls.
In this experiment, on different days, the size of a baked ziti portion served in a public restaurant was varied between a standard portion and a larger serving containing 50 percent more. The price for the meal, which included a pesto-stuffed tomato and a roll and butter, remained the same. Customers who ordered the meal were also asked to rate their satisfaction and the appropriateness of the portion size. In addition, their food intake was measured by weighing each entree in the kitchen before and after the meal.