How is it possible to eat more cereal than you normally would, by putting less cereal in your bowl? It sounds like a riddle, but researchers at Penn State University have made this a focal point of their nutritional investigations.

National dietary guidelines define recommended amounts of food in volume measures, such as cups. However, these measures do not account for other variations in food characteristics that affect nutritional intake and the amount of food we eat.

“People have a really hard time judging appropriate portions,” says lead researcher Barbara Rolls, professor of nutritional sciences at Penn State.

“On top of that,” she adds, “you have these huge variations in volume that are due to the physical characteristics of foods, such as the size of individual pieces, aeration and how things pile up in a bowl. That adds another dimension to the difficulty of knowing how much to take and eat.”

“This can be a problem,” Prof. Rolls continues, “because, for most foods, the recommended amounts have not been adjusted for variations in physical properties that affect volume, such as aeration, cooking, and the size and shape of individual pieces.

“The food weight and energy required to fill a given volume can vary, and this variation in the energy content of recommended amounts could be a challenge to the maintenance of energy balance.”

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The cereal provided was either standard-sized wheat flakes or the same cereal crushed to 80%, 60% or 40% of its original volume.
Image credit: Penn State University

To test the relationship between food volume and calorie intake, Rolls and her team crushed breakfast cereal into smaller flakes using a rolling pin. The smaller size of the flakes allowed the same weight of cereal to fill a smaller volume.

The team recruited 41 adults to eat cereal for breakfast once a week over 4 weeks. The cereal provided to the participants was either standard-sized wheat flakes or the same cereal variously crushed to 80%, 60% or 40% of its initial volume. The participants were free to pour as much cereal into the bowl as they liked.

The study – which is published in the Journal of the Academy of Nutrition and Dietetics – found that as the size of the flakes got smaller, the participants poured smaller volumes of cereal into the bowl. But despite the smaller volume size, the participants were actually consuming more weight and energy content.

The subjects, however, believed that they consumed about the same number of calories each time.

“When faced with decreasing volumes of cereal, the people took less cereal,” Prof. Rolls says. “Yet, even though they thought they were taking the same number of calories, they ended up significantly overeating.”

The findings corroborate a book Prof. Rolls published in 2012, The Ultimate Volumetric Diet, which provides practical tips for readers on how to manage their calorie intake despite variations in food volume.

Prof. Rolls concludes:

Our research shows clearly that, without us even knowing it, these variations can have a big impact on how much we’re eating. For cereals with small pieces, the recommended serving size should be reduced to account for the uncharacteristically low volume, in the same way that the recommended serving size is increased for voluminous foods, such as puffed cereals and leafy greens.”