The diets of low-income Americans have worsened in the past decade, even as the diets of the wealthiest Americans have improved, according to a new study that is among the first to measure changes in diet quality over time by socioeconomic status. Overall diet quality in the United States remains poor, said the lead author of the study, published Monday in the Journal of the American Medical Association Internal Medicine.

Although the study found that the diet of all Americans improved on average between 2005 and 2010, the progress masked a decline in diet quality among the poor. The result: a doubling of the gap in diet quality between the wealthiest Americans and the poorest.

The study attributed the change to the higher cost of convenient and healthy meals, as well as limited access to quality supermarkets in some poorer neighborhoods.

Frank Hu, a study author and co-director of the Program in Obesity Epidemiology and Prevention at the Harvard School of Public Health, cautioned against taking the improvements as a sign that Americans eat well. "This is really almost like an American diet report card," Hu said. "This has the good news that there has been some improvement in overall diet quality, but the report card still doesn't look very good."

View Images NG Staff. Source: Dong D. Wang, et al., Harvard University School of Public Health

Even with the improvements, Americans collectively scored under 50 out of 110 on the Alternative Healthy Eating Index, according to the study.

Most of the improvement in the American diet, researchers said, was due to a steady decline in the consumption of trans fats. Consumption of sugar-sweetened beverages also dropped, giving a minor boost to overall diet scores. American diets otherwise stayed consistent, with low scores for fruits, vegetables, and whole grains.

Trans fats have been a target of public policy efforts to improve diet. Last fall the Food and Drug Administration announced it was considering banning the fats in processed foods, a decade after the agency required that trans fats be clearly listed on packaged foods. Federal efforts to promote consumption of fruits, vegetables, and whole grains, meanwhile, have largely been confined to education and public service announcements.

The study has implications for efforts to improve Americans' diets, particularly among the poor, experts said. "It really speaks to the evidence that if you want to change the American diet, you have to change the policy," said Marlene Schwartz, director for the Rudd Center for Food Policy & Obesity at Yale University.

"Education will only get you so far," she said, noting that education is often most successful for those who can afford to pay for it. Improving diet among the poor, she said, requires "improving the food supply so people can eat what's there and not be exposed to so many dangerous things."

Jessica Caouette, a nutrition and cooking instructor with Cooking Matters—a national nonprofit—who works with low-income families, echoed that reaction. "All parents are interested in feeding their families healthy meals," she said, but "price is a concern for low-income families."

A survey of Cooking Matters students from 2012 found that 85 percent said they wanted to eat healthfully but that only half were able to do so. More recently a survey from the food bank umbrella group Feeding America found that nearly 80 percent of its clients bought the cheapest food available even though they knew it wasn't healthy. (Related "Study Sheds Light on Broadening U.S. Hunger Problem.")

The best bet for improving the American diet, Hu said, lies in a broad approach. "Without changing the food environment and food system," he said, "education alone is not going to be very effective."