US Diet Slowly Improving, Study Suggests

Ricki Lewis, PhD

September 25, 2019

The macronutrient diet among US adults improved from 1999 through 2016, but intake of low-quality carbohydrates and saturated fat remained too high, according to findings published online September 24 in JAMA.

Tracking overall trends in what people are eating is essential to improve public health and to guide recommendations that aim to reduce risks for conditions such as cardiovascular disease, type 2 diabetes, and certain types of cancer. To view the state of the diet in the United States, Zhilei Shan, MD, PhD, from the Department of Nutrition, Harvard T. H. Chan School of Public Health, Boston, Massachusetts, and colleagues analyzed data from nine consecutive cycles of the National Health and Nutrition Examination Survey (NHANES).

The improvement in national nutrition was small and of uncertain clinical importance but suggests that recommendations are having a positive effect. The changes appear to reflect a shift from the low-fat diets that dominated the 1990s to consumption of healthy fats and complex carbohydrates and limiting consumption of refined carbohydrates, the researchers suggest.

The authors analyzed data from 43,996 people aged 20 years or older about what as eaten over a 24-hour period. They estimated respondents' diet quality using the Healthy Eating Index (HEI) 2015, which "measures the adherence to the 2015–2020 Dietary Guidelines for Americans and is considered an indicator for overall diet quality," coauthor Fang Fang Zhang, MD, PhD, of the Friedman School of Nutrition Science and Policy at Tufts University, told Medscape Medical News.

HEI values range from 0 to 100, with higher scores indicating a better diet. However, "there are no established standards for clinically meaningful increases in the HEI," Zhang said.

"We found that American adults improved their HEI-2015 by 2 points from 1999 to 2016. The mean score is about 58, which is still far below the maximum score of 100," Zhang said.

The estimated overall HEI-2015 score increased from 55.7 to 57.7 (difference, 2.01; 95% confidence interval [CI], 0.86 – 3.16).

During the study period, the estimated energy from total carbohydrates fell from 52.5% to 50.5% (difference, −2.02%; 95% CI, −2.41% to −1.63%); energy from total protein increased from 15.5% to 16.4% (difference, 0.82%; 95% CI, 0.67% – 0.97%); and energy from total fat increased from 32.0% to 33.2% (difference, 1.20%; 95% CI, 0.84% – 1.55%).

The addition of subcategories to the HEI over the years has revealed trends in the right direction. "Between 1999 and 2016, there were significant declines in estimated percentage of energy intake from low-quality carbohydrates and increases in estimated percentage of energy intake from high-quality carbohydrates, plant and animal protein, and polyunsaturated fatty acids across all population subgroups," the researchers write.

Estimated energy from low-quality carbohydrates decreased by 3.25% (95% CI, 2.74% – 3.75%), from 45.1% to 41.8%, while estimated energy from high-quality carbohydrates increased by 1.23% (95% CI, 0.84% – 1.61%), from 7.42% to 8.65%. The trend largely reflects eating more whole grains and fruits and less added sugars and fruit juice.

Still, "a high proportion of daily calories that Americans consumed, 42%, was from low-quality carbohydrates," Zhang said. Those are from refined grains, fruit juice, potatoes, and added sugars in foods and beverages.

But not all those calories were replaced with high-quality sources. "One of our major findings was that despite a decreasing trend of low-quality carbohydrates, the consumption of refined grains, a major contributor to low-quality carbohydrates, increased in the past 10 to 15 years," Zhang explained.

For plant protein, estimated energy increased by 0.38% (95% CI, 0.28% – 0.49%), from 5.38% to 5.76%. The increase was attributed to people eating more whole grains, soy, and nuts. The increase in animal protein was from increased consumption of poultry and eggs.

Estimated energy from saturated fatty acids increased by 0.36% (95% CI, 0.20% – 0.51%), from 11.5% to 11.9%. Energy from polyunsaturated fatty acids increased by 0.65% (95% CI, 0.56% – 0.74%), from 7.58% to 8.23%.

The major trends in improved diet were observed in all population subgroups, but they were greater for younger individuals and for people at the highest income levels.

The researchers write that the trend toward increasing intake of whole grains and plant proteins reflects the shift in emphasis of both dietary recommendations and pop culture from 1971–2000 to more recent years. The earlier period was the age of the low-fat diet and the concomitant surge in the consumption of products with high-fructose corn syrup. During this period, the government encouraged the population to eat more carbs. Gradually, low-carb diets emerged, evolving from the Atkins and the South Beach diets, to Mediterranean and paleo diets, to recent ketogenic diets.

Current dietary challenges, the researchers point out, are to decrease consumption of low-quality carbohydrates, red meat, and saturated fat.

Linda Van Horn, PhD, RDN, and Marilyn C. Cornelis, PhD, both from the Department of Preventive Medicine at the Northwestern University Feinberg School of Medicine, Chicago, Illinois, agree. They write in an accompanying editorial, "Cooperation from the food industry to encourage the recommended foods including whole grains, fruits, vegetables, and plant-based protein sources while systematically reducing sugar, salt, and saturated fat are key to improving consumer adherence to the US Dietary Guidelines."

The researchers and editorialists suggest further research is needed into why US adults with lower income and educational attainment improved their diets to a lesser degree than others.

Summing up, Zhang said, "The results are not really surprising, but the high proportion of calories from low-quality carbohydrates and the increasing trend in refined grain consumption are definitely worthy of attention for both the general public and for policy makers. We may have not paid enough attention to these in the past." She suggests population-based strategies such as labeling policies, financial incentives and disincentives, and educational campaigns to increase whole grain consumption and decrease refined grain consumption.

Limitations of the study include self-reported, cross-sectional, short-term data. The editorialists point out a more general limitation of developing guidelines: a disconnect in how people categorize foods. Surveys probe macronutrients, whereas people are more likely to view foods in categories such as "desserts," "snacks," or fast foods, such as burgers, tacos, and pizza. These are more meaningful descriptors than fats, carbohydrates, and proteins, they point out.

The study was supported by the National Institutes of Health and the Young Scientists Fund of the National Natural Science Foundation of China. Rehm has received fees from Nestec Ltd and the Dairy Management Institute. Other study authors' relevant financial relationships are listed in the original article. Van Horn is a member of the 2020 US Dietary Guidelines Advisory Committee. Cornelis has disclosed no relevant financial relationships.

JAMA. Published online September 24, 2019. Full text, Editorial

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