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The Effect of Increased Portion Size on Energy Intake is Sustained for 11 Days

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Portion Size

When served larger portions for an extended period of time, people consume more food over the entire period, suggesting the body's biological signals for controlling hunger and fullness do not step in to regulate intake.

With rates of overweight and obesity on the rise, Barbara Rolls, Ph.D., set out to determine if larger portions could be a driving force behind the obesity epidemic. Other research has shown that since the 1970's portion sizes of restaurant foods, grocery products and recipes in cookbooks have continued to increase. Dr. Rolls' new study, the longest look at the sustained effect of increased portion size, builds on her earlier work demonstrating the same effect over a short period of time.

"Living in the age of supersize meals and 'huge food,' our study shows that there is a great need for people to be more aware of what and how much food they are served. Consistently being presented with bigger portions can have a sneaky effect on energy intake, even for people who don't regularly clean their plates," noted Dr. Rolls, professor of nutritional sciences at Penn State University in University Park, PA.

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In the study, 23 normal weight and overweight adults were provided all of their meals (food and beverages) for two 11-day periods with a two-week break in between. During one of the 11-day periods, standard portion sizes were served of a menu that changed daily. During the other period, the same meals were served, but all of the portions were increased by 50 percent. The order of the periods was randomized across participants, and all meals were consumed in the laboratory, except on the weekends, when participants took the food home.

At the end of the study, the subjects consumed an average of 16 percent more calories per day - or an extra 4,500 calories over the 11 days - during the large portion period. The effect was sustained over the entire period and did not decline over time. Additionally, the participants experienced a slight decrease in hunger and slight increase in fullness over the 11-day period of large portions, but this did not modify the effect of portion size on intake.

The extra calories consumed in this study translate to a weight gain of approximately 1.25 pounds, assuming no other factors important in weight regulation are changed (e.g., increasing exercise). Participants in this study were not informed about the change in portion size; thus, they were not trying to compensate for eating more food. Interestingly, only about two-thirds of the study participants noticed the change in portion size.

According to Dr. Rolls, the combination of bigger portions and calorie-dense foods is a dangerous one, and people should not feel compelled to clean their plates. Some strategies she suggests to control portion size while eating out are ordering an appetizer rather than an entree, sharing the main course and bringing home a doggie bag.