The authors found that the share of American adults in or near poverty with poor diets declined from 68 percent in 2003 and 2004 to 61 percent in 2011 and 2012. Yet for affluent adults, the share eating poorly declined even more, from 50 percent to 36 percent over the same period. The researchers defined poor diets using criteria developed by the American Heart Association.
The data is based on a series of national surveys conducted in two-year stages, beginning in 1999, in which participants were asked to catalogue all the food they had eaten in the past 24 hours. Overall, the share of adults with poor diets declined from 56 percent in 1999 and 2000 to 46 percent in the most recent figures from 2011 and 2012.
"Americans do really want to eat better," said Dr. Dariush Mozaffarian of Tufts University, one of the authors of the study. "There’s a long way to go, but that’s really good news."
While the data shows changes for some foods, there is room for improvement. Americans are eating more whole fruit, but the increase in their overall consumption of produce was not statistically significant. Consumption of whole grains nearly doubled, from 0.56 servings a day on average in 1999 and 2000 to 1 serving a day in 2011 and 2012. Yet there was almost no change in the amount of refined grains Americans are eating.
Americans are eating more fish and shellfish — a positive shift for the national diet, but only a small one. The average adult ate 0.16 servings of seafood a day in 1999 and 2000 and 0.19 servings a day in 2011 and 2012.
Meanwhile, Americans are not eating any less sodium, saturated fat or processed meat. There was only an insignificant decline in the consumption of unprocessed red meat.
The new data on beverages sweetened with sugar offer context for a continuing political controversy over the eating habits of America's poor.
Paul LePage, the Republican governor of Maine, has become the most recent conservative politician to argue that recipients of food stamps are wasting public money on sugary junk food. He has demanded that the federal government allow him to ban recipients in his state from using food stamps to purchase soda and candy, threatening to take actions that would result in the end of the food-stamp program in his state if the Obama administration does not accede.
Consumption of sugary drinks among poor American adults already declined 23 percent between 1999 and 2012, according to the data published this week, from 2.16 servings to 1.67 servings daily. Among the wealthy, consumption declined from 1.48 servings to 0.91 servings daily, a change of 39 percent.
Mozaffarian argues the government should do more to discourage Americans, including food-stamp recipients, from drinking sugary beverages and other unhealthy foods. He does not support a complete ban, like the one LePage proposed, but he does support a system in which recipients would have to pay a penalty if using food stamps to buy soda and similar products, while receiving coupons for produce.
Mozaffarian compared the idea of governmental rules for nutrition in food to existing standards for cars, buildings and drugs. "It's like a seatbelt," he said. "We do this for everything else."
This issue is hotly debated among experts on food. Sarah Bowen, a sociologist at North Carolina State University, contends that individual households are often best positioned to make decisions about what to buy at the grocery store, and that even the unhealthiest foods can serve a purpose in certain situations.
Based on her interviews with extremely poor families, Bowen suspects that some who cannot afford three meals a day are relying on sugary drinks as a cheap source of needed calories -- though she does not have conclusive evidence for this theory.
"Instead of having breakfast, they would just have a sweet tea or a soda," Bowen said. "It does point to letting people make their own choices, because you just don’t know. Foods that seem like they’re not very healthy, they may be important filling in the gaps for people."
This post has been updated.