Consumption of trans fats fell, but red and processed meat and alcohol consumption stayed the same, the researchers found.
The study, conducted by a research team from the Harvard School of Public Health (HSPH) and published in JAMA Internal Medicine, found a modest improvement in diet quality alongside a significant reduction in trans fat consumption between 1999 and 2010.
Diet and nutrition are key health matters. A poor diet is recognized as a cause of many chronic diseases, including heart disease and certain cancers - the two leading causes of death in the US. Maintaining a healthful diet is an important part of optimizing long-term health.
Since the late 1990s, frequent changes have occurred to the economy, policies regarding food and nutrition, public health guidelines and food processing, all of which may have impacted upon the diets of the American public.
Prompted by these changes, the researchers utilized data from the National Health and Nutrition Examination Survey (NHANES) in order to assess what impact there was on the quality of the American public's diets.
The researchers utilized data from a nationally representative sample of 29,124 adults from 1999-2010. The participants were aged 20-85 years and were evaluated for the duration of the study with two different dietary quality indexes: the Alternate Healthy Eating Index 2010 and the Healthy Eating Index 2010.
Assessing American diets
The Alternate Healthy Eating Index 2010 (AHEI-2010) rates overall dietary quality on a score of 0 to 110. A total of 11 components - consumption of whole grains or trans fat, for instance - are assigned scores out of 10, and the sum of these scores equals the overall dietary quality. The higher the score, the more healthful the diet.
In 1999-2000, the average AHEI-2010 of the participants was 39.9. This score increased to 46.8 in 2009-2010, indicating an overall improvement in dietary quality.
Over half of this gain was attributable to reduced consumption of trans fats - fat that raises levels of bad cholesterol and reduces levels of good cholesterol in the body. Trans fats are commonly found in baked goods, chips and fried foods.
Dietary quality was also improved by raised consumption of whole fruit, whole grains, legumes, nuts and polyunsaturated fats and lowered consumption of sugar-sweetened beverages. In contrast, the consumption of vegetables, red and processed meat and alcohol remained consistent during the study period.
More worryingly, salt intake was found to have increased "despite efforts to reduce this by the Dietary Guidelines for Americans as well as initiatives by the American Heart Association and other public health organizations," say the authors.
The study also found a gap in diet quality between people with higher socioeconomic status (SES) and people with lower SES. This gap increased across the duration of the study.
Among ethnic and racial groups, non-Hispanic black people had the lowest quality of diet. The authors attributed this to lower rates of income and education within the group. Mexican Americans had the best dietary quality, which the authors say could be due to dietary traditions and culture.
Prof. Walter Willett, senior author of the study, believes that despite overall improvement in dietary quality, "the widening gap related to income and education presents a serious challenge to our society as a whole."
The authors suggest that the reduction in trans fat consumption signifies that actions affecting society as a whole - such as legislation, taxation and nutrition education - are most effective in supporting people's healthy choices. Lead author Dong Wang says:
"The study provides the most direct evidence to date that the extensive efforts by many groups and individuals to improve US dietary quality are having some payoff, but it also indicates that these efforts need to be expanded."
The findings suggest that overall dietary quality is still poor despite improvement, and in particular within groups with low levels of income and education. A related commentary in JAMA Internal Medicine states that people of lower SES have less access to healthful foods due to costs and limited knowledge of nutrition.
Dr. Takehiro Sugiyama and Dr. Martin Shapiro suggest that benefits programs such as the Supplemental Nutrition Assistance Program (SNAP) could be restricted to just healthful foods, or that students and residents of underserved areas could be provided with healthful foods.
The authors of the study hope that the evaluation of the trends identified within their study leads to future public health policy aiming to prevent nutrition-related chronic diseases.
Previously, Medical News Today reported that in the UK, salt consumption is highest within low socioeconomic groups.