UNIVERSITY PARK, PA. — Reducing the size of a cereal flake may lead people to pour less into their bowls, but it also may lead to a greater consumption by weight and calories, according to a study appearing on-line March 21 in the Journal of the Academy of Nutrition and Dietetics. Pennsylvania State University researchers led the study while The National Institutes of Health supported it.

“People have a really hard time judging appropriate portions,” said Barbara Rolls, professor of nutritional sciences and Helen A. Guthrie Chair in Nutrition at the university. “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.”

Ms. Rolls said 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.”

As part of the study, the researchers gave 41 adults (24 women, 17 men) Wheaties cereal for breakfast once a week for four weeks during 2011 and 2012. The cereal was either standard wheat flakes or the same cereal crushed to reduce the volume to 80%, 60% or 40% of the standard. The researchers then 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.

According to the research, as flake size was reduced, subjects poured a smaller volume of cereal, but still took a significantly greater amount by weight and energy content. Despite these differences, subjects estimated that 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 energy intake increased.

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

She added the findings of the study have implications for both portion selection and dietary advice, adding that “there are a lot of variations in food volume that we’re not given much advice about.”

“Our research shows clearly that, without us even knowing it, these variations can have a big impact on how much we’re eating,” she said. “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.”