Energy density, portion size and eating occasions contribute to obesity epidemic in the U.S.

Over the past 30 years U.S. adults have been eating larger portions and eating more often, according to a new study by University of North Carolina at Chapel Hill researchers.

The findings help illustrate that how Americans are eating contributes to the country's obesity epidemic.

"First, the food industry started 'super sizing' our portions, then snacking occasions increased and we were convinced we needed to drink constantly to be hydrated," said Barry Popkin, Ph.D., the study's senior author and W.R. Kenan Jr. Distinguished Professor of Nutrition at the UNC Gillings School of Global Public Health.

"This study shows how this epidemic has crept up on us. The negative changes in diet, activity and obesity continue and are leading to explosions in health-care costs and are leading us to become a less healthy society," said Popkin, a member of the Carolina Population Center.

The study is believed to be the first to examine the combined contribution of changes in three key factors (portion sizes, food energy density and eating frequency) on people's total calorie consumption. The findings appear in the June 2011 issue of the journal PLoS Medicine.

In the study, Popkin and Kiyah Duffey, Ph.D., postdoctoral fellow at the UNC Interdisciplinary Obesity Center, analyzed individuals' dietary intake over a 24-hour period, based on surveys of U.S. adults taken between 1977-78, 1989-91, 1994-98 and 2003-06.

They found that the average daily total energy intake, measured in calories, increased from about 1,803 kcal in 1977-78 to 2,374 kcal in 2003-06, an increase of 570 kcal.

Increases in the number of eating occasions (meals and snacks) and portion sizes of foods and beverages over the past 30 years accounted for most of the increase. Energy density (the number of calories in a specific amount of food) also accounted for some of the change, but may have decreased slightly in recent years, the researchers reported.

Looking at the changes between each survey period, portion size accounted for an annual increase in the daily total energy intake of nearly 15 kcal between 1977-78 and 1989-91, whereas changes in the number of eating occasions accounted for an increase of just 4 kcal per year.

Then, between 1994-98 and 2003-06, changes in the number of eating occasions accounted for an annual increase in daily total energy intake of 39 kcal, whereas changes in portion size accounted for an annual decrease of 1 kcal.

Changes in the energy density of food and drink accounted for a slight decrease in daily total energy intake over the 30-year study period.

As participants in the surveys may have under- or over-reported the amount of food consumed, the findings may not be completely accurate, Popkin said.

"Still, these findings suggest that efforts to prevent obesity among adults in the U.S should focus on reducing the number of meals and snacks people consume during the day and reducing portion size as a way to reduce the energy imbalance caused by recent increases in energy intake," he said. "I would speculate that the same advice would apply to other developed countries."