The study, the first to focus on the combined effects of both portion size and calorie density or the calories per ounce, showed that calorie density and portion size add together to affect caloric intake. Tanja Kral, doctoral candidate in nutritional sciences who conducted the study as part of her dissertation, says, "Even though the study participants consumed 221 fewer calories when offered a smaller meal of lower calorie density, they felt just as full and satisfied as when they had consumed a larger meal of higher calorie density."
Kral's dissertation adviser is Dr. Barbara Rolls, who holds the Guthrie Chair of Nutrition in Penn State's College of Health and Human Development. "The fact that the participants in this study didn't notice when they were given lower calorie density food offers evidence that the food industry could change their products to make them healthier without causing customer dissatisfaction," Rolls says. "Small reductions in the calorie density of foods will allow people to eat satisfying portions without consuming too many calories which, in turn, may help them with weight management."
Kral presented her results in a paper, "The Combined Effects of Energy Density and Portion Size on Food and Energy Intake in Women," today (Oct. 13) at the annual meeting of the North American Association for the Study of Obesity in Fort Lauderdale, Fla. Her co-authors are Liane Roe, research nutritionist, and Rolls. Kral is one of five finalists selected for the Ethan Sims Young Investigator Award to be announced at the meeting.
In the study, 39 normal weight and overweight women ate breakfast, lunch and dinner once a week for six weeks in Penn State's Laboratory for the Study of Human Ingestive Behavior. The breakfasts and dinners were standardized but the main entree at lunch was formulated to vary in calorie density as well as portion size.
The lunch entree was a pasta bake made from medium shells, zucchini, broccoli, carrots, onions, tomato sauce and parmesan, mozzarella and ricotta cheese. The calorie density was changed by varying the proportions of ingredients. The amount served was also varied from two cups to two and three-quarters cups to three and a half cups.
"Portion size alone increased calorie intake by 20 percent. Calorie density alone increased intake by 26 percent," Kral says. "Together, portion size and calorie density increased calorie intake by 56 percent." Rolls notes, " In practical terms, the study shows that big portions of high calorie foods put people at greater risk of overeating than big portions alone. If you like big portions, stick to water-rich foods that don't have too much fat. Other research in our laboratory has shown that big portions of a low-calorie salad as a first course can even help lower the total amount of calories you consume."
The study was supported by a grant from the National Institute of Diabetes, Digestive and Kidney Diseases.