Bottomless Soup Bowl Study

The Bottomless Soup Bowl Study is an abbreviated title for an academic research study conducted by Brian Wansink, James Painter, andJill North, fully titled “Bottomless Bowls: Why Visual Cues of Portion Size May Influence Intake.” The study was published in January, 2005 in the Obesity Research journal. It is notable for finding that consumers who unknowingly ate from a self-refilling bowl of soup ate 73% more soup than those who were given normal, static amounts of soup. The conclusion suggests that portion size can have a strong influence on food intake.

Background

The size of food packaging has increased over the past 30 years, and research supports that package size can increase consumption. Prior research focuses on the explanation that package size increases food intake, because the consumer feels they should get the full monetary value of the meal or that he/she may run out. However, this rationale does not explain why people tend to eat more from larger portions in situations where the food is unlimited (e.g., buffets and receptions).

The authors of this study offer another possible explanation regarding overconsumption via the idea that people use consumption norms when eating. That is, consumers have estimations of how much food they will consume when the food is presented (e.g., clean plate, 3/4th, ½), letting the amount remaining of the food inform them if they met this decision or not. The authors also suggest that portion size affects consumption monitoring. Portion size can distract consumers from their ability to self-monitor food intake because they rely on personal estimations of how much food has been eaten. When a portion size is larger, the consumer has to rely on visual cues, such as the fill level in the bowl, to determine when they should stop eating. This overreliance may cause the consumer to focus less on physiological aspects (fullness) and to focus more on visual cues (eating half the bowl). Visual cues are not an accurate means to monitor food intake, which creates a discrepancy in amount of food the consumers perceived to be eaten and the actual amount of food.

Hypotheses

Hypothesis 1: Altering visual cues of how much is eaten will influence food intake.

Hypothesis 2: When given accurate or perceptually biased visual cues, a person’s consumption estimation and perception of satiety will be more influenced by the visual cue than by how much they actually consume.

Researchers hypothesized that giving one group automatic refilling soup bowls would result in more ounces and calories consumed, and that they would underestimate how much they consumed.

Methods

Fifty-four participants from the ages of 18-47 (72% male) were recruited from a university in the Midwest to participate. These participants responded to a flyer that informed them of the requirements: eating a soup-only lunch and completing a questionnaire. The study used a between-groups design.

Individuals ate in groups of four to resemble a meal-type setting. The researchers manipulated bowl color (blue or green) and asked distracting questions about color as a means to disguise the true purpose of the study. In each session two participants of the four received a self-refilling bowl and the other two received a normal bowl. Participants were given instructions to not move the bowl from its assigned placement. They were also told that they would be eating a new recipe of tomato soup and to enjoy as much as they would like. After 20 minutes, participants were given a questionnaire asking them to rate the soup and estimate how much they believe they ate in ounces and in calories. Participants were also given another survey about pre-meal hunger and satisfaction from food. To measure soup consumed, the combined weight of the soup remaining in the caldron, the tubing, and the soup bowl with the soup that was originally available was weighed.

The self-refilling bowl was made possible by a constructed self-refilling soup bowl apparatus. It consisted of a heavy 1.9 X 1.2-m restaurant-style dining table (4.6 cm thick) that was designed to accommodate four bowls. Food grade tubing connected the underside of a bowl to its corresponding pot. The pot contained six quarts of soup. To ensure a constant and slow rate, the pot was positioned just above the bowl. Researchers eliminated air pockets prior to the experiment. The refill rate was maintained at rate in which the bowl would completely refill in approximately 20 minutes. Those who were not in the self-refilling bowl were asked if they wanted refills when their bowls dropped to a quarter full; thus, everyone has access to as much soup as they would like.

Results

Analysis was conducted using SPSS. ANOVAs and Pearson correlations were used. Both hypotheses were confirmed. Results of ANOVAs indicated that people who were in the self-refilling condition ate more soup [14.7 vs 8.5 ounces; F(1, 52) = 8.99; P < 0.01] than those eating from normal soup bowls. A difference of 73% of amount of soup and an increase of 113 calories were observed between the self-refilling and normal bowls (267.9 versus 154.9 calories). These findings were consistent across all groups (sex and BMI).

Individuals who rated themselves as hungry before the meal ate more than those who had rated themselves as less hungry, but the visual cue was still significant when pre-meal hunger was used as a covariate. Participants eating from normal bowls believe they had eaten 32.3 calories fewer than they actually ate (122.6 estimated vs 154.9 actual calories). Participants who ate from the self-refilling bowls believed they had eaten 140.5 calories fewer than they actually ate (127.4 estimated vs 267. 9 calories). Across all participants, the amount they consumed was correlated (r = 0.29) with the number of ounces they believed they consumed (r = 0.31), as well as with the number of calories they believed they consumed. Those who were given normal soup bowls were more accurate at estimating calories intake (r = 0.67) than those given self-refilling bowls (r = 0.12). Results of a self-report survey indicated that self-refilling bowl participants’ main strategy for estimating was to guess how much soup the bowl contained and then to use that number for their upper limit. Lastly, the participants in the self-refilling bowl ate 73% more soup, but did not perceive themselves as feeling any more sated than those who had eaten from a normal one.

Discussion

The findings supported that portion size can have a large impact on food consumption. Portion sizes give a visual cue/consumption norm of how much one expects to consume and inevitably does consume. Portion sizes also distracts one’s ability to self-monitor his/her amount already consumed, but does not impact their estimated food intake or satiation. Further, all participants underestimated calories consumed, but those in the self-refilling bowl were even less accurate. These results are consistent across BMI and sex. The implications of this study are that people rely more on visual rules-of-thumb (cleaning the plate) and less on physiological cues (fullness).

This research is valuable because it may encourage individuals to decrease their initial serving sizes in order to avoid this visual bias, and they may become aware of the presence of consumption norms and monitoring in all consuming activities. The researchers suggest using smaller than normal sized plates, bowls, and glasses and to repackage bulk items into smaller servings. Additionally, leaving the remnants of food already consumed (e.g., the empty wine bottle) can remind individuals of the amount already consumed and inhibit seconds.

Awards and Recognitions

The Bottomless Bowl Principle

Brian Wansink is a 2007 recipient of the Ig Nobel Prize in Nutrition for the Bottomless Soup Bowl Study. The Ig Nobel prizes are a parody of the Nobel Prize and are awarded for achievements (or sometimes veiled criticisms thereof) that "first make people laugh, and then make them think." The study has come to be known as the Bottomless Bowl Principle.

Bottomless Soup Bowl in the News

This study has received a significant amount of press and has reached notoriety. It has been referenced by News-Medical, Chicago Tribune, New York Times, International Business Times, and NBC.

Further reading

1. Mindless Eating: Why We Eat More Than We Think by Brian Wansink
2. Slim by Design by Brian Wansink

References

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