BOSTON — U.S. adults over an 18-year period decreased their intake of lower quality carbohydrates, primarily added sugars, and increased their intake of higher quality carbohydrates, plant protein and healthy polyunsaturated fat, according to a study published Sept. 24 in JAMA.

Diets still could improve, however. Intake of what JAMA called lower quality carbohydrates such as refined grains, starchy vegetables and added sugars accounted for 42% of the typical American’s daily calories. Carbohydrates from sources such as whole grains and whole fruit accounted for 9%. Total saturated fat intake represented 12% of daily calories, which was above a recommended daily amount of 10%.

“Although there are some encouraging signs that the American diet improved slightly over time, we are still a long way from getting an ‘A’ on this report card,” said Fang Fang Zhang, Ph.D., an author of the study and a nutrition epidemiologist at the Friedman School of Nutrition Science and Policy at Tufts University in Boston.

The study examined the diets of 43,996 adults using data from the National Health and Nutrition Examination Survey (NHANES). Participants completed at least one valid 24-hour dietary recall from nine consecutive cycles of the NHANES (1999 to 2016).

Total carbohydrate intake declined to 50.5% from 52.5% of the total diet intake. Higher-income adults reduced their intake of lower quality carbohydrates by 4%. Those living below the poverty level reduced their intake by 2%.

Total protein intake increased to 16.4% from 15.5%, and total fat intake increased to 33.2% from 32%. Plant protein intake increased to 5.8% from 5.4%. Intake of polyunsaturated fat increased to 8.2% from 7.6%.

Most protein that Americans consume comes from meat, including red and processed meat, said Shilpa Bhupathiraju, Ph.D., research scientist at the Harvard T.H. Chan School of Public Health in Boston. Proteins from seafood and plant sources like whole grains, nuts and legumes make up a smaller proportion.

“Our research suggests that Americans have an opportunity to diversify their sources of protein to include more seafood, beans, soy products, nuts and seeds,” Dr. Bhupathiraju said.

The estimated overall Healthy Eating Index increased to 57.7 from 55.7 over the 18-year period. Adherence to dietary guidelines improved for most Americans, but no improvement was seen for people over age 50, people with less than a high school education and people living below the poverty line.