People in the United States have been eating healthier — in fact, a new study finds that improved diets have prevented 1.1 million premature deaths over a 14-year period.

However, the overall quality of the American diet remains poor, the researchers said.

"Our findings provide further justification for promoting healthful diets as a national priority for chronic disease prevention, as well as for legislative and regulatory actions to improve the food supply more broadly," study author Dong Wang, a doctoral candidate in the nutrition and epidemiology departments at the Harvard T.H. Chan School of Public Health, said in a statement.

In the study, the researchers looked at trends in people's diets, pulling data from another study of about 34,000 U.S. adults who were each surveyed twice between 1999 and 2012.The researchers applied a scoring system to the participants' diets called the Alternate Healthy Eating Index 2010, which takes into account people's intake of fruit, vegetables and whole grains, as well as their consumption of unhealthy foods such as sugar-sweetened beverages and processed meats.

To see what effect any changes in the quality of people's diets over this time period may have had, the researchers looked at the rates of premature deaths and disease from two separate studies that involved a total of about 173,000 people in the U.S. [10 New Ways to Eat Well]

The researchers found that the average healthy-eating score increased from 39.9 to 48.2 over the study period. (The highest possible score was 110.) A reduction in trans-fat intake over the course of the study accounted for about half of the improvement in people's dietary scores, the researchers said.

Moreover, the researchers linked improvements in people's diets during the 14 years of the study to a 13 percent reduction in type 2 diabetes cases, an almost 9 percent reduction in cardiovascular disease rates and an approximately 1 percent reduction in cancer cases.

"The most surprising finding is that even very small amounts of dietary quality improvement actually led to substantial reduction in disease burden," Wang told Live Science.

However, the overall quality of diet in the U.S. remained poor, the researchers said. In fact, the average score in the study never even reached 50 (out of the highest possible score of 110), they noted.

And while the consumption of healthy foods such as fruits and whole grains increased during the study, the consumption of vegetables and healthy omega-3 fats did not, according to the study, published Nov. 2 in the journal Health Affairs. At the same time, the intake of sodium increased over time, the researchers said.

The two most significant changes in the U.S. diet that the researchers observed during the study period were a decrease in the consumption of trans fat (by nearly 72 percent) and a decrease in the consumption of sugar-sweetened beverages and juice (by about 36 percent), the researchers said.

The reduction in the consumption of trans fat was driven primarily by regulatory actions, such as the mandatory disclosure of trans fat on nutrition labels that took effect in 2006, the researchers said. The reduction in the consumption of sugar-sweetened beverages and juice was likely due to a combination of education, voluntary changes in people's dietary choices and regulations such as soda bans in schools, the researchers said.

They recommended additional policies, such as expanding taxation on sugar-sweetened beverages and mandating restrictions on the use of salt in food products, that could help to maintain and even accelerate improvements in the quality of the American diet.

Copyright 2015 LiveScience, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.