On an index of healthy eating where a perfect score is 110, U.S. adults averaged just 40 points in 1999-2000, climbing steadily to 47 points in 2009-10, the study found.
Scores for low-income adults were lower than the average and barely budged during the years studied. They averaged almost four points lower than those for high-income adults at the beginning; the difference increased to more than six points in 2009-10.
Higher scores mean greater intake of heart-healthy foods including vegetables, fruits, whole grains and healthy fats, and a high score means a low risk of obesity and chronic illnesses including heart disease, strokes and diabetes. Low scores mean people face greater chances for developing those ailments.
The widening rich-poor diet gap is disconcerting and "will have important public health implications," said study co-author Dr. Frank Hu of the Harvard School of Public Health. Diet-linked chronic diseases such as diabetes have become more common in Americans in general, and especially in the poor, he noted.
"Declining diet quality over time may actually widen the gap between the poor and the rich," Hu said.
Harvard School of Public Health researchers developed the healthy diet index used for the study. It is similar to federal dietary guidelines but features additional categories including red and processed meats, sugar-sweetened beverages and alcohol.
The study authors used that index along with government estimates on trans fat intake to evaluate information in 1999-2010 national health surveys that included interviews with people about their eating habits. The results are published Monday in JAMA Internal Medicine.
Hu said the widening diet gap reflects an income gap that deepened during the recent financial crisis, which likely made healthy food less affordable for many people. Hu also noted that inexpensive highly processed foods are often widely available in low-income neighborhoods.
The overall diet improvement was largely due to decreased intake of foods containing trans fats but the disappointing results point to a need for policy changes including better nutrition education, Hu said.
In recent years the government and manufacturers have moved to phase out use of artificial trans fats in foods including processed cookies, cakes, frozen pizza and margarines. Trans fats contribute to unhealthy cholesterol levels and can increase heart disease risks. These fats are made by adding hydrogen to vegetable oil to improve texture and shelf life.
The study authors say their results are consistent with an earlier report showing that "nearly the entire U.S. population fell short of meeting federal dietary recommendations."
The federal guidelines are updated every five years and new ones will be issued next year. The current recommendations emphasize limiting intake of trans fats, sodium, processed foods and added sugars. They don't specify amounts but encourage diets high in whole grains, vegetables and fruits.
The Harvard index has a similar emphasis with some specifics; to get a top score would include eating daily more than two cups of vegetables, at least four servings of fruit and at least one ounce of nuts.
A JAMA Internal Medicine editorial says the Harvard diet index isn't perfect because it puts equal emphasis on various foods that may not contribute equally to health. Still, the study highlights a "growing chasm" that is a public health concern, the editorial says. It suggests that government efforts to close the gap with programs including food stamps may be insufficient and that limiting government benefits to cover only healthful foods might be a better strategy.