The analysis also found that although the size of meal portions has stabilized in recent years, but the number of total calories consumed is rising.

By 2006, the end of the period studied, Americans were consuming 570 more calories per day than they did in the late 1970s.

A chief culprit behind the calorie gain: Americans now consume 220 more calories daily from sugar-sweetened soft drinks than they did in the 1960s, the study found.

The study is thought to be the first to examine the combined contribution of changes in portion sizes, the caloric level of foods, and eating frequency on people's total calorie consumption.

The study was funded by the National Institutes of Health, and the findings appear in the June 2011 issue of the journal PLoS Medicine.

Kiyah Duffey, a postdoctoral fellow at the UNC Interdisciplinary Obesity Center and one of the study's authors, said large portion sizes drove the rise in calories during the early part of the study period.

"Around the time people became aware of the portion sizes, we see a decline in the portion sizes they are consuming," Duffey told Reuters.

"It really seems that in the last couple of decades, it is the number of eating occasions that is driving this change."

A proliferation of food availability and a decline in regular mealtimes may be fueling the pattern, Duffey said.

"People aren't sitting down to three meals anymore," she said. "We sort of think about eating all through the day."

Some sources of dieting and health advice say frequent eating in small doses revs up the metabolism and controls hunger, and is a healthier way of eating than three big meals.

Duffey said what matters is what and how much you eat over the course of the day rather than how often you eat.

"Don't eat seven times a day if what you're eating is a salty snack or a pizza," she said. "If you're going to do that, reach for an apple instead."

From the June 30, 2011, Prepared Foods' Daily News.