Popkin and his researchers say that packaged foods have long been the hardest to monitor because of the sheer volume and rapid change in the marketplace.
The Healthy Weight Commitment Foundation, an industry group representing the 16 companies that made the pledge to reduce 1.5 trillion calories, says it will report this summer on how successful they've been, according to Lisa Gable, the group's president. The first results from Popkin's study aren't expected until later this year.
Marion Nestle, a New York University professor of nutrition, food studies and public health, says the data could be useful in pressuring companies to make more changes for the better. Companies often use "the research isn't there" as a defense against making changes recommended by public health groups, she notes, and it can be hard to prove them wrong.
"What people eat is the great mystery of nutrition," Nestle says. "It would be wonderful to have a handle on it."