Major fast food chains have failed to make any healthy adjustments to their menus over the past 14 years, a new study released Tuesday says.

According to the study published in the American Journal of Preventive Medicine, over a quarter of American adults eat fast food two or more times a week.

These meals are only furthering the obesity epidemic in the US as they are packed with huge amounts of fat, sugar, and salt, contributing to America's poor diets and increased risk of diet-related chronic diseases, like heart disease and diabetes.

"Despite qualitative evidence that the fast-food industry is making improvements to the nutritional quality of at least some of their menu items, a quantitative evaluation of trends in the nutritional quality of fast food available in the marketplace was lacking," says lead investigator Mary Hearst, PhD, MPH, Associate Professor of Public Health at St. Catherine University in St. Paul, Minnesota.

In the study, the researchers examined menu trends at eight fast-food restaurants using data from 1997 to 2010 for Wendy's, Arby's, McDonald's, Jack in the Box, Dairy Queen, Burger King, Taco Bell and Kentucky Fried Chicken (KFC).

In a commentary in the journal, Margo Wootan of the Center for Science in the Public Interest in Washington DC, says the restaurants score of 48 is lower even than the average American's diet of 55, which government agencies brands "far from optimal".

"Americans now spend 41% of their food dollars on foods eaten outside the home, up from 26% in 1970. Adults and children consume an average of one-third of their calories from eating out," she writes, adding that restaurants need to change their thinking about the role their food plays in their customers' diets and impact on their health.

Wootan suggests a range of measures, incuding reducing portion sizes, offering more fruit (not just juice) and vegetables, and making them default side dishes with meals. She also suggests using containers no bigger than 16oz for sugar-sweetened beverages and removing them from children's menus altogether.