UNIVERSITY PARK, Pa.—In an effort to combat obesity, manufacturers of ready-to-eat cereals have made positive strides to reduce sugar, sodium and calories without compromising taste. Now, new research published in the Journal of the Academy of Nutrition and Dietetics suggests product developers may want to focus attention on flake size to keep calories in check.

Researchers at Penn State found that when flakes are reduced by crushing, people pour a smaller volume of cereal into their bowls, but still take a greater amount by weight and calories even though they think they were taking the same number of calories.

“People have a really hard time judging appropriate portions," said Barbara Rolls, professor of nutritional sciences and Helen A. Guthrie Chair in Nutrition. “On top of that 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."

National dietary guidelines define recommended amounts of most food groups in terms of measures of volume such as cups. “This can be a problem 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," she said. “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."

For the study, the researchers tested the influence of food volume on calorie intake by systematically reducing the flake size of a breakfast cereal with a rolling pin so that the cereal was more compact and the same weight filled a smaller volume. In a crossover design, the team recruited 41 adults to eat cereal for breakfast once a week for four weeks. The cereal was either standard wheat flakes or the same cereal crushed to reduce the volume to 80%t, 60% or 40% of the standard. The researchers provided a constant weight of cereal in an opaque container and participants poured the amount they wanted into a bowl, added fat-free milk and non-calorie sweetener as desired and consumed as much as they wanted.

They found that as flake size was reduced, participants poured a smaller volume of cereal, but still took a significantly greater amount by weight and calorie content. Despite the differences, participants estimated they had taken a similar number of calories of all versions of the cereal. They ate most of the cereal they took, so as flake size was reduced, breakfast calorie intake increased.

Research has shown that lowering the calorie density—or calories per bite—of food can help people feel full while eating fewer calories.

“There are a lot of variations in food volume that we're not given much advice about," Rolls said. “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."

For more information on how food manufacturers are formulating better-for-you breakfasts, check out First Things First: Building A 21st-Century Breakfast on Food Product Design.