The portion-size effect (PSE) refers to the notion that food intake is influenced by the amount of food served. In the most general sense, larger portions lead to greater intake, and it has been estimated that a doubling of portion size results in a 35% increase in intake. Moreover, the idea that “mindfulness” can aid in overcoming the PSE has been shown to be false. Although there is substantial evidence that the PSE exists, little has been discussed regarding why it occurs. As such, Peter Herman and colleagues from the University of Toronto, Canada set out to tackle the “mechanism” issue head-on.
In a previous review on the PSE, two prominent mechanisms underlying its effects were proposed: value for money (larger portions represent a better economic value) and portion distortion (larger portions have become normative). However, as Herman points out, in most laboratory studies on the PSE, there is no economic benefit to eating more when presented with larger portions. This makes sense considering that if there was an economic benefit then participants would eat as much as possible in all the conditions; but research clearly shows that smaller portions lead to less consumption.
Herman favors the second argument more and states that it relies on the presupposition that hunger and satiety take a backseat to people’s reliance on portion size as a guide for food intake. It appears that people assume whoever determined the portion size selected that particular size after some deliberation rather than randomly. There are limits of course, as a family size bag of Doritos would obviously be seen as too much, but in general the appropriateness of serving sizes extends across a broad range. For instance, it has been shown that smaller and larger (150% of smaller) portions are perceived as equally appropriate and lead participants to believe that the same amount of food was consumed, supporting the notion that people are not monitoring their intake in any absolute way, although perhaps they are monitoring their intake in relation to the size of the portion.
On this note, Herman points out that people often do not eat everything they are served. Perhaps people restrict themselves to a specific fraction of what they are served, which raises interesting (and unanswered) questions about what fraction of the portion is considered appropriate, if different people use different fractions, and how conscious people are of these judgements. Nonetheless, it continues to support the PSE in that larger servings would still lead to greater intake because one half of two is still greater than one half of one.
So when does the appropriateness norm form? Early work showed the PSE to be present in 5-year-olds but not 3-year-olds, and others have noted that despite increasing portion sizes over the past 20 years, 2-year-old’s intake has remained relatively stable. However, others have found that self-monitored food intake is only good until 12 months of age, after which it starts to decline. Indeed, the PSE has been noted in 2-year-olds. But as Herman asks, are we prepared to accept the proposal that 2-year-olds behave in a norm-governed fashion? If not, then perhaps the PSE reflects some other mechanism outside of environmental and social norms. It is difficult, after all, to specify how aware children are of portion size and their response to it.
Two other possible mediators of the PSE are visual cues and bite size. It is reasonable to assume that people use visual cues as an element in the calculation of how much they should eat. It has been shown that people eating in the dark do consume more food than those eating in the light, which personally raises questions about late-night snacking. The biting mechanism, on the other hand, is more sensory than normative. We know that large portion sizes increase bite size, and that this is influenced by both gender and weight status. Is it possible that the PSE arises because larger portions induce larger bites which in turn lead to greater intake? This may help explain why people who eat larger portions tend not to report greater satiety afterwards.
Bottom line
The concern about large portions as a major contributor to the obesity epidemic, with resultant attempts to reduce portion size, appears to have distracted much of the research community from the basic question of why larger portions induce greater food intake in the first place. The PSE has become somewhat accepted as fact – of course larger portions make you eat more – but no one yet knows why. More work will definitely be needed and I for one look forward to what is discovered.