The dietary quality of low-income Americans has deteriorated in the last decade despite improvement in the diet of their wealthy counterparts, according to a new study by the Harvard School of Public Health.

As a result, the gap in the diet quality has doubled between the richest and the poorest Americans. The reasons for the widening gap can be attributed to costly healthy meals and lack of quality supermarkets in poorer neighborhoods.

This is the first study to measure diet quality over time based on socioeconomic status.

"This is really almost like an American diet report card. 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," said Frank Hu, a study author and co-director of the Program in Obesity Epidemiology and Prevention, National Geographic reports.

According to the Food Research and Action Center, two-thirds of Americans are overweight or obese, majority of them belonging to poorer backgrounds.

The study also found that despite wealthy Americans consuming relatively healthy diets, they overall scored under 50 out of 110 on the Alternative Healthy Eating Index.

The researchers said that much of the improvement in the American diet was due to drop in the consumption of trans fats and sugar-sweetened beverages. At the same time, they ate fruits, vegetables and whole grains in lower quantities.

"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.

Schwartz said that improving the food supply is necessary to encourage healthy diets among poor people.

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

A 2012 survey of Cooking Matters found that 85 percent wanted to eat healthy food, but only 50 percent of them could actually afford the same. Another survey from Feeding America found that nearly 80 percent of its clients purchased cheapest food despite being aware of its quality.  

The finding is published in the Journal of the American Medical Association Internal Medicine.