The researchers served a free soup lunch to 54 adults, half of whom ate from normal 18-ounce soup bowls, while the other half ate from identical bowls that, unbeknownst to the participants, were slowly refilled through tubing connected to out-of-sight soup cauldrons.
Those who ate out of the refilling bowls consumed 73 percent more soup than did participants who ate from the normal soup bowl during the 20-minute lunch.
Although they averaged 113 more calories than those eating from normal bowls, those eating from the bottomless bowls believed they consumed the same number of calories as the other participants and rated themselves as being no more full.
"People use their eyes to count calories and not their stomachs," lead researcher Brian Wansink, professor of marketing and of nutritional science at Illinois, said. "This can be dangerous to our diets."
Because we appear to judge our food intake by visual cues, such as an empty bowl, Wansink said that people worried about overeating should carefully consider the size of portion servings in restaurants and in their kitchens.
He suggested, for example, repackaging snacks and other bulk foods into small plastic bags. The visual cues from the filled bags can lead families, especially children, to think that a smaller-than-normal serving was a satisfying full serving.
The paper, titled "Bottomless Bowls: Why Visual Cues of Portion Size May Influence Intake," was co-written by James E. Painter, a professor of family and consumer science at Eastern Illinois University, and Jill North, a graduate student in food science and human nutrition at Illinois.
Journal
Obesity Research