Although the typical adult diet in the United States has improved, it still contains too much low quality carbohydrate and saturated fat, recent research concludes.

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New research looks at changes in the nutritional content of a typical U.S. diet.

During 1999–2016, the average proportion of daily calorie intake that refined grains, added sugar, and starchy vegetables represented decreased by 3% in the U.S., according to the new JAMA study.

However, these low quality carbohydrates still account for 42% of daily calories, while high quality carbohydrates — such as whole grains and fruits — only account for 9%.

Over the same period, total fat intake went up by 1%. Half of this increase was due to saturated fat, which now accounts for 12% of daily calories. This figure is above the 10% maximum in the U.S. dietary guidelines.

"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," says co-senior study author Dr. Fang Fang Zhang, a nutrition epidemiologist at the Friedman School of Nutrition Science and Policy at Tufts University in Boston, MA.

For the study, the researchers drew on the records of the nationally representative National Health and Nutrition Examination Survey (NHANES).

Their analysis included dietary data from nearly 44,000 adults who had reported what they had consumed in a 24-hour period at least once between 1999 and 2016. Their average age was 47 years, and 52% were female.

Carbs, proteins, and fats in U.S. diet

The researchers estimated nutrient intake with the help of a U.S. Department of Agriculture (USDA) database.

They assessed dietary quality using the USDA's Healthy Eating Index (HEI), which measures how well a diet aligns with the U.S. dietary guidelines.

The results showed that during 1999–2016, the estimated calorie intake from carbohydrates, fat, and proteins in the U.S. diet altered as follows:

  • Total carbohydrates fell from 52.5% to 50.5%.
  • Total protein increased from 15.5% to 16.4%.
  • Total fat increased from 32.0% to 33.2%.
  • Low quality carbohydrates fell from 45.1% to 41.8%.
  • High quality carbohydrates increased from 7.42% to 8.65%.
  • Plant protein increased from 5.38% to 5.76%.
  • Saturated fat increased from 11.5% to 11.9%.
  • Polyunsaturated fat increased from 7.58% to 8.23%.

The increase in high quality carbohydrate consumption came mostly from whole grains, while the reduction in low quality carbohydrate consumption was primarily due to lower intake of added sugar.

"Because low quality carbs are associated with disease risk, taking in higher quality carbs could mean better health for Americans in the future," says first study author Zhilei Shan, Ph.D.

Shan is a nutritional epidemiology fellow in the Harvard T.H. Chan School of Public Health in Boston, MA. During the study, he was also working at Tongji Medical College at Huazhong University of Science and Technology in China.

Most protein still comes from meat

The modest increase in plant protein intake also came from higher whole grain consumption, together with a slight increase in the consumption of nuts.

There was a small but significant increase in the HEI, which went up from 55.7 to 57.7.

The analysis reveals that most of the protein in the typical U.S. diet still comes from meat, including processed and red meat.

"Proteins consumed from seafood and healthy plant sources, such as whole grains, nuts, and legumes, remained a much smaller proportion," says co-senior study author Shilpa Bhupathiraju, Ph.D., a research scientist at Harvard T.H. Chan School of Public Health.

"Our research suggests," she adds, "that Americans have an opportunity to diversify their sources of protein to include more seafood, beans, soy products, nuts, and seeds."

Food industry cooperation is key

In an editorial article, Linda Van Horn, Ph.D., and Marilyn C. Cornelis, Ph.D., both from Northwestern University Feinberg School of Medicine in Chicago, IL, comment on the new findings.

"Despite some improvement in these results, the overall HEI score remains woefully shy of the recommended guidelines," they write.

They suggest that a key factor in helping consumers follow the U.S. dietary guidelines is getting the food industry to promote whole grains, fruits, plant based protein, and vegetables while reducing sugar, salt, and saturated fat.

Despite the researchers' "efforts to focus on food groups of similar quality," the study only offers "a 'macro' view," they argue. There is a need to unravel the more "specific intricacies of diet."

"Snacks, desserts, pizza, fast food sandwiches, and sugar sweetened beverages are currently major contributors to the population energy intake and confer dubious contributions to diet quality as measured by HEI."

Linda Van Horn, Ph.D., and Marilyn C. Cornelis, Ph.D.