Barbara Rolls, holder of the Guthrie chair of nutritional sciences and director of the Laboratory for the Study of Human Ingestive Behavior, found that when served larger portions for an extended period of time, people consume more food over the entire period.
The research suggests that larger portions could be a driving force behind the obesity epidemic. Other research has shown that since the 1970s portion sizes of restaurant foods and processed products have continued to increase.
"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," she said.
"Consistently being presented with bigger portions can have a sneaky effect on energy intake, even for people who don't regularly clean their plates."
Rolls' research follows calls from the Grocery Manufacturers Association (GMA) to implement uniform portion size standards on all processed food packaging. "Consumers do not always link caloric content to the serving size listed," said GMA senior director of nutrition and regulatory policy Alison Kretser.
"Some simple changes to the Nutrition Facts Panel could help consumers make smarter dietary choices."
Indeed, the 2005 Dietary Guidelines Advisory Committee (DGAC) guidelines, which former USDA secretary Ann Veneman claimed were designed to "address the epidemic of overweight and obesity," specifically targeted portion size as a key determinant in what people eat. The DOAC decided that portion control was of such concern that it devised a new pyramid model using the leanest form of every food product category.
Rolls' study would appear to confirm this contention that portion size plays a significant role in dictating food intake. During the first of two 11-day periods, standard portion sizes were served of a menu that changed daily.
During the second period, the same meals were served, but all of the portions were increased by 50 percent. Participants were not informed about the change in portion size.
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.
Such academic findings are providing a great deal of impetus behind the demand for labeling changes. But any initiative would not come cheap. If the FDA adopts the trade association's suggestions, the cost to food and beverage companies likely would be in the hundreds of millions of dollars.
The FDA estimated that a change to the nutrition label in 2003 cost the food industry $140 million to $250 million.
Rolls presented her work last week at the 2005 Annual Scientific Meeting of NAASO, The Obesity Society.