The American diet is slightly healthier than it was 20 years ago, but low-quality carbohydrates such as sugary foods, refined grains, and starchy vegetables continue to make up more than 40% of U.S. adults' daily calorie intake, according to a study published last week in JAMA.

Study details

For the study, a team of researchers analyzed data from nine National Health and Nutrition Examination Surveys conducted from 1999 to 2016, which asked nearly 44,000 U.S. adults to report everything they ate in the previous 24 hours. About 70% of respondents participated in at least two different surveys. 

US adults are eating a little healthier

Overall, the researchers found the average score on the 100-point Healthy Eating Index rose slightly from 55.7 to 57.7.

The researchers said the change was largely driven by a decrease in U.S. adults' total carbohydrate intake, which fell from 52.5% of daily calorie intake to 50.5%. According to the study, U.S. adults' estimated energy intake from low-quality carbs—such as sweets, white bread, and French fries—fell by 3.3% from 1999 to 41.8% in 2016. Meanwhile, energy intake from high-quality carbs, such as whole grains, whole fruits, non-starchy vegetables, and legumes, rose by 1.2% to 8.7% in 2016.

However, the researchers noted that daily carbohydrate intake in 2016 was still high, and largely consisted of low-quality carbs.

As U.S. adults consumed fewer carbs, they increased their daily intake of protein and fats, the researchers found. According to the study, the percentage of U.S adults' daily energy intake from protein increased from 15.5% in 1999 to 16.4% in 2016, while fat intake rose from 32% to 33.2%.

The researchers noted that increases in plant protein consumption, such as nuts, helped drive the increases in protein and fats. For instance, the percentage of U.S adults' daily energy intake from plant-based protein rose from less than 1% in 1999 to nearly 6% in 2016.

The researchers noted one key limitation to their findings is their reliance on 24-hour recall questionnaires, which do not always accurately portray the types of foods people really eat.

What's driving the trend toward healthier diets?

While the researchers acknowledged that the United States still has a long way to go to improve daily dietary intake, the shift toward healthier foods is positive.

While the study did not assess whether specific factors, such as underlying health conditions, affected the shifts in U.S. adults' eating habits, they suggested the shift in U.S. adults' diet could have been driven in part by changes to U.S. dietary guidelines.

"These opposing trends could be due to dietary guidelines prior to 2000 that recommended low-fat diets, which was associated with decreased intake of total fat and increased intake of refined grains and added sugar," the authors wrote. "After 2000, shifts in scientific evidence and dietary guidelines promoted health benefits of healthy fats and plant sources of protein and the harms of low-quality carbohydrates."

They also speculated that the popularity of new diets, like Atkins and paleo, and a recent wave in vegan and vegetarian diets may have prompted some of the recent dietary changes.

In an accompanying editorial, Linda Van Horn and Marilyn Cornelis, both of Northwestern University Feinberg School of Medicine, wrote that despite the statistically significant increase in the Healthy Eating Index, there is more work to be done to improve Americans' diets.

"If all Americans would include more fruits, vegetables and whole grains they would greatly improve their overall diet quality while hopefully reducing intake of sugar, salt and saturated fats that are detrimental to health," Van Horn said.

The study authors agreed. Shilpa Bhupathiraju, senior author of the study and a nutrition researcher at Harvard Medical School and Brigham and Women's Hospital in Boston, said that despite the positive trends, "[o]ur findings show that we still have a long way to go to meet dietary recommendations" (Monaco, MedPage Today, 9/24; Rapaport, Reuters, 9/24).