PhotoFiguring out the reasons behind the obesity epidemic may not be so difficult. A new study by researchers at the University of North Carolina finds that, in the last three decades, U.S. adults have been eating larger portions and eating more often.

“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 a professor at UNC's 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.”

The study, appearing in the journal PloS Medicine, 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.

The study 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. It 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.

Americans eat too often

Increases in the number of eating occasions 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.

The study concludes that the key to obesity may be quite simple. The researchers say their 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.

The researchers say they believe their findings also have relevance for developing countries, that have also experienced an obesity problem in recent years.