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Are Large Portions to Blame for Obesity?

It turns out there is very little evidence linking bigger portion sizes to increasing waistlines.
(Photo: George Redgrave/Flickr)

(Photo: George Redgrave/Flickr)

Have you ever mindlessly eaten an entire family-size bag of chips, or a jumbo bucket of popcorn at the movies, just because, you know, it was there? Well, science has a name for that. It's called the portion size effect. It's a simple concept: The more food people are served, the more they tend to eat. It is perhaps unsurprising that the massive portion sizes characteristic of modern society have been blamed for the obesity epidemic. To be fair, it's a cognitively satisfying theory; larger portions mean we eat more calories, an excess of which leads to weight gain. But according to the authors of a new study published in Physiology & Behavior, there is actually little evidence larger portions are responsible for our larger waistlines.

Here's the researchers' reasoning: In order to even be considered as a possible cause of the obesity epidemic, the factor in question—portion size—has to meet two requirements:

1. The timelines for increasing portion sizes and increasing human sizes must overlap.

2. There must be empirical evidence that larger portions increase weight.

As it turns out, there's plenty of evidence for the former—the 1950's "king-sized" 12-ounce Coke, for example, is today just a kid-sized drink, and even portions of homemade foods have been climbing as obesity rates began to soar in the late 20th century. But there is surprisingly little evidence for the latter.

Most of the research that's been carried out on the portion size effect only follows its effect on intake over the course of a single meal. The question remains whether or not people will continually eat more when presented with larger portions, or if they will compensate for past excesses by eating less in the future. Another pitfall: Many studies on the effect are conducted in environments where it's difficult to set food aside for later. "It is at least conceivable that larger portions at home could simply mean more leftovers," the authors write.

Future studies that examine the effect of portion size on intake and weight over longer time periods might yet find stronger evidence for a causative link, the authors concede. But they present at least one other reason to be skeptical such a link exists: The obesity epidemic has not struck the population evenly. Mean weight has increased faster than median weight, which means the heavier end of the spectrum has become much heavier, while the lighter end has barely budged. What data we have on the portion size effect so far indicates that it does not discriminate; people of all shapes and sizes fall victim to the psychological trap, so larger portion sizes alone can't explain the pattern of obesity we see today.

The focus on portion size, the authors argue, blinds us from targeting other potential culprits of obesity, such as the increase in meal frequency—another well-documented trend.


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