Two new studies found fast food has not significantly improved health-wise since 1996 except for a slight decrease in trans fat in fries.

To make their findings, researchers from the USDA Human Nutrition Research Center on Aging at Tufts University looked at the "calorie, sodium, saturated fat and trans fat content" in popular menu items served at three fast food chains in the U.S.

"There is a perception that restaurants have significantly expanded their portion sizes over the years, but the fast food we assessed does not appear to be part of that trend," said Alice H. Lichtenstein, director of the Cardiovascular Nutrition Laboratory at the USDA HNRCA. "Our analysis indicates relative consistency in the quantities of calories, saturated fat, and sodium. However, the variability among chains is considerable and the levels are high for most of the individual menu items assessed, particularly for items frequently sold together as a meal, pushing the limits of what we should be eating to maintain a healthy weight and sodium intake."

The researchers focused on the most popular menu items including "fries, cheeseburgers, grilled chicken sandwiches, and regular cola" in order to look for trends in nutritional content and portion size over the past 18 years. The team found only minimal fluctuations in levels of saturated fat and sodium over this period. The researchers did notice a decline in saturated and trans fats in fries that occurred around 2001.

"The decline in trans fat we saw between 2005 and 2009 appears to be related to legislative efforts," Lichtenstein said. "The success of New York City's trans fat ban and others like it, suggest it is worth pursuing these types of approaches because they make the default option the healthier option. Of course, it is important to note that the healthier option in terms of fat does not translate into lower calories or less salt."

Fast food sales are still going strong, but the researchers noted many of these establishments are heading in the right direction by introducing healthier options. The team also noted nutritional content varied among the chains, and a small order of fries could differ by as much as 110 calories and 320 milligrams of salt.

"For this reason our findings strongly suggest that public health efforts promoting reduction of calories and over-consumed nutrients need to shift from emphasizing small, medium and large portion sizes, to additional factors such as actual number of calories and the nutrient content of the items, as is increasingly becoming available at point of purchase," Lichtenstein said. "A 100 calorie difference per day can mean about a 10 pound weight change per year."

The findings were published in a recent edition of the journal Preventing Chronic Disease.