How Food Portion Size Affects How We Eat

Portion size may be determined by several factors unrelated to hunger

/ Author:  / Reviewed by: Chris Galloway, M.D. Beth Bolt, RPh

People who are dieting are commonly told to reduce portion size. However, a new review shows that changing portion sizes to affect calorie intake is a more complicated issue.

The review looked at several previous studies on how portion sizes have changed, how they affect food intake and how people decide on their portion size.

The researcher examined evidence showing that the portion sizes of many common, high-calorie foods have increased. This researcher found that packaging, labeling and size of the food affected how much people chose to eat.

The author of this review wrote that a person's hunger level is only one of the many factors that determine what a person eats and how much.

"Talk to a nutritionist about appropriate serving sizes."

David Benton, BSc, PhD, of the Department of Psychology in Swansea University, Wales, wrote the review in order to point out what we know and what we need to know about portion sizes.

A portion size is how much food is put on a plate or eaten at one time. Dr. Benton referenced evidence that shows that portion and packaging sizes have increased over the past 30 years, which has been suggested to be a factor in increasing rates of obesity.

He suggested that discussing portion size is important, because it may play a crucial role in how much people eat.

First, Dr. Benton presented evidence showing that portion sizes have indeed grown and are often more than what is recommended for a single serving.

One study in 2003 showed that, between 1977 and 1994, the average portion size of some of the most calorie-dense foods like french fries, hamburgers, soft drinks and desserts, increased. Additionally, those foods were 18 percent of the calories consumed in the US toward the beginning of the study, but were 27.7 percent of the calories by the end.

Dr. Benton noted that consuming larger portion sizes also has been tied to dining in restaurants, especially fast food restaurants.

The review then looked at how larger portion sizes affected the way people ate. Previous studies have looked at how many calories people eat when they have differing portion sizes.

One of those trials found that people with the largest portions consumed 30 percent more calories than people with the smallest portions. However, hunger levels were similar for all participants regardless of how much they ate.

The review also looked at the unit size of food consumed. Two studies examined food items of different sizes placed in a public area. When larger items, like whole pretzels, were provided, people ate more by weight than when they were given smaller pretzels. In other words, people ate the same number of food items, whether the foods were big or small.

Dr. Benton also looked at evidence that the "expected satiation" of a food, or how full a person expects to be after eating, matters for portion size.

In one study, participants expected 200 calories worth of pasta and 894 calories of cashews to result in the same level of fullness. According to this review, participants' expectations affected how much food they decided to put on their plates.

The review found that several factors influenced how people decided on the size of their portions.

Among those factors were visual cues, or the volume of food or drink that people see. In one study, adolescents pouring themselves drinks poured 88 percent more into a short and wide glass than into a tall and narrow glass of the same volume. 

Labeling also affected portion sizes, according to the review. Dr. Benton referenced studies that compared portion sizes of foods with labels that read "low-fat," low-choelsterol" or "organic." In one of those studies, participants ate an average 28.4 percent more candy if they believed it was a low-fat version.

The size of the package also affected how much people ate. A study comparing small and large containers of spaghetti found that participants used 29 percent more pasta when it came from a big container. However, Dr. Benton noted that other studies have contradicted that finding, and other factors like the perceived value of the food may have had an influence.

Similarly, the National Institutes of Health recommend that people eat on smaller plates because they may eat less. But this idea has not held up in some trials comparing calorie intake to plate size.

Dr. Benton concluded that several factors besides hunger levels may determine how much food people put on their plate, and in turn, how much they consume. He called for more research to be conducted on the importance of portion size and factors that affect eating decisions.

This review was published in Critical Reviews in Food Science and Nutrition on September 20.

The research was funded by Sugar Nutrition UK. The author disclosed no conflicts of interest.

Review Date: 
October 11, 2013
Last Updated:
October 14, 2013