Nutritional Pearls: Portion Size Perception

Larry is a 45-year-old overweight man struggling to lose weight. He recently brought up his difficulty determining appropriate serving sizes for both snack foods and meals.

How would you advise your patient?
(Answer and discussion on next page)

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Answer: If you are working on your weight, become familiar with standard portion sizes.

A study looked at people's perception of their usual portion size and whether that amount might be larger for those who are considered clinically overweight or obese.¹ Not only did the authors find that those who were overweight tended to have larger personal usual portion sizes than those who were of clinically normal weight, but all of the participants, regardless of their weight status, considered their usual portion size to be larger than the standard portion sizes.

The implication of this and other studies of portion size has been that because people have been exposed to oversized portions so frequently, they are no longer able to correctly assess portion sizes by sight. In short, they're used to larger portion sizes, so they look normal.

The Research

Authors in the UK devised a series of studies to see if that might be the case.² In the first study, carried out online, the researchers used 3 different pictures of 3 different serving sizes of spaghetti bolognese, all depicted on white plates that were identical in size and shape. A third of the participants were first shown a picture of the smallest serving size of spaghetti, a third were first shown a picture of the largest serving size, and the final third were shown a picture of a common household object. All 3 groups were then shown a picture of an intermediate serving size (what would be considered a standard serving size) of either spaghetti bolognese or another familiar food (chicken curry), and asked whether their "ideal" serving size would be smaller, larger, or the same as that intermediate serving size shown.

In a second study, the authors eliminated the household object arm of the study in order to focus solely on the food comparisons. This study used only 55 men and women from the university as well as the surrounding community.

In the final study, the authors showed 68 adult female participants photographs of either larger or smaller portion sizes of different brands of potato chips, then a photograph of a normal size, and were asked to evaluate whether a normal serving would be larger, smaller, or the same as the photograph.

The Results

After rating the images, the participants were asked to serve themselves a portion of the same food as a snack. How much they chose to serve themselves and how much they actually ate was weighed and compared to a "normal" serving size.

Somewhat unsurprisingly, in the first 2 studies the participants who were shown smaller servings before evaluating the intermediate serving tended to think that a normal serving was smaller than the intermediate-sized serving. On the other hand, those who were shown the largest serving size tended to think that a normal serving was larger than the intermediate serving size.

In the case of the chips, however, how people evaluated the intermediate serving size had nothing to do with how much of the chips they served themselves or how much of them they ate.

What’s The “Take Home”?

The problem I see with the third study is that the participants were evaluating a snack food and not a main course dish. It's quite possible that people's perceptions of what might constitute an appropriate serving size might have little to do with how much they personally would eat. Evaluating spaghetti bolognese would have been a better choice.

The first 2 studies are much more interesting, suggesting that exposure to various serving sizes might set up a mental norm—and that might affect consumption. If you are working on your weight, you would do well to accustom yourself to standard portion sizes.

References:

1. Lewis HB, Forwood SE, Ahern AL, et al. Personal and social norms for food portion sizes in lean and obese adults. International Journal of Obesity. 2015 April 14 [epub ahead of print]. doi:10.1038/ijo.2015.47.
2. Robinson E, Oldham M, Cuckson I, et al. Visual exposure to large and small portion sizes and perceptions of portion size normality: Three experimental studies. Appetite 2016;98:28-34.