Barry Popkin, a University of North Carolina professor who has studied food data extensively, described the development as a “turning point.

There is no perfect way to measure American calorie consumption. But three large sources of data about diet all point in the same direction. Detailed daily food diaries tracked by government researchers, data from food bar codes and estimates of food production all show reductions in the calories consumed by the average American since the early 2000s. Those signals, along with the flattening of the national obesity rate, have convinced many public health researchers that the changes are meaningful.

The eating changes have been the most substantial in households with children. Becky Lopes-Filho’s 4-year-old son, Sebastian, has always been at the top of the growth chart for weight. Ms. Lopes-Filho, 35, is the operations manager of a pizzeria in Cambridge, Mass., and her son, like her, loves food. As he has gotten older, she has grown more concerned about his cravings for sweets. Instead of a cookie every day now, she said, she has been trying to limit him to one a week. “If he was given access, he would just go nuts,” she said. “He, I think, would tend to be a super obese kid.”

There is no single moment when American attitudes toward eating changed, but researchers point to a 1999 study as a breakthrough. That year, researchers from the Centers for Disease Control and Prevention published a paper in The Journal of the American Medical Association that turned into something of a blockbuster.

The paper included bright blue maps illustrating worsening obesity rates in the 1980s and 1990s in all 50 states. Researchers knew the obesity rate was rising, but Dr. Ali Mokdad, the paper’s lead author, said that when he presented the maps at conferences, even the experts were gasping. A year later, he published another paper, with a similar set of maps, showing a related explosion in diabetes diagnoses. “People became more aware of it in a very visual and impactful way,” said Hank Cardello, a former food industry executive who is now a senior fellow at the Hudson Institute, a conservative policy center. “That created a lot of attention and concern.”

Shortly afterward, the surgeon general, Dr. David Satcher, issued a report — “Call to Action to Prevent and Decrease Overweight and Obesity” — modeled on the famous 1964 surgeon general’s report on tobacco. The 2001 report summarized the increasing evidence that obesity was a risk factor for several chronic diseases, and said controlling children’s weight should be a priority, to prevent the onset of obesity-related illnesses.