One “trick” dieters often use is to put their food on a smaller plate. The idea is to fool yourself into thinking you’re eating more food than you really are. But doesn’t our stomach tell us how full we are?
Actually, it doesn’t. Brian Wansink has devoted his career to studying how perception of food intake relates to actual eating behavior. Together with James Painter and Jill North, he’s come up with a dramatic demonstration of how wrong our stomachs can be.
Volunteers were recruited to participate in a soup-only lunch in a room adjoining the school cafeteria. They filled out a form asking about color preferences, then were seated a table with four different-colored bowls. The colors were just a distraction: the real purpose of the study was to see how much people would eat when their soup bowls refilled automatically.
Two of the participants ate from self-refilling bowls; the other two had their bowls refilled by a server. Everyone was encouraged to eat as much as they wanted. The self-refilling bowls involved a fair bit of cooking technology — plastic tubes connected a soup pot next to the table to the underside of each bowl. The refill rate of the bowls was adjusted so that the bowls could be filled completely in 20 minutes — the duration of the study. Technically the bowls could be nearly empty by the end of the session, but each bowl held 18 ounces of soup, so this would have required consuming over a quart of soup!
Despite the fact that everyone’s bowls were refilled, the people eating from self-refilling bowls ate 73 percent more soup. Even more surprising is that they didn’t feel any different from people who ate from manually-refilled bowls:
None of these other measures were significantly different — even though the people eating self-refilled soup indicated that it seemed they couldn’t possibly eat all their soup, they didn’t estimate they’d eaten significantly more than those who had the visual cue of a server refilling their bowl every time it was less than 25 percent full.
When asked to rate hunger on a 1-9 scale, again, there was no significant difference between the two groups. In all, a dozen ratings were collected, asking questions about whether they monitored their food intake during the study, whether they generally try to clean their plate, and how the presence of others affects their eating. In every case, there was no difference between the two groups — the only difference was how much they ate.
The team also controlled for gender, body mass index (BMI), and other factors, and still found the same results (though since they didn’t study an extremely wide range of BMIs, the results might be different for dramatically over/underweight individuals).
Wansink et al. argue that this demonstrates that the primary way people decide how much to eat is visual: when there is a visual indicator of how much food is consumed, then people are accurate at determining how much to eat. The problem comes when social norms of “reasonable” portions change: as portion sizes in restaurants and stores increase, people expect to eat more at each meal–leading to unhealthy eating. The team argues that restaurants and retailers should present food in smaller portions to reinforce the idea of eating less. Parents could repackage snacks for their kids in individual bags to reinforce the idea that just a small portion is reasonable.
And in no case should you install a self-refilling soup bowl in your kitchen!
[For more on the idea that we rely on our perception and memory of what we eat to decide when we're full, check out How do we know when we're hungry?]
Wansink, B., Painter, J.E., & North, J. (2005). Bottomless bowls: Why visual cues of portion size may influence intake. Obesity Research 13(1), 93-100.