Fast food is still unhealthy, say researchers who studied the calories, saturated fat and salt levels in meals offered by three major U.S. fast food outlets.

Little change was noted in any of those measurements from 1996 to 2013, say researchers from Tufts University.

They surveyed the nutritional contents of cheeseburgers, French fries, grilled chicken sandwiches and cola drinks -- all extremely popular fast food menu choices.

There was some good news, they report in the journal Preventing Chronic Disease; portion sizes have stopped their trend toward supersize, and calories in about half the items have been reduced over the years.

Trans fat has been greatly reduced in French fries, the researchers found, as many fast food chains have moved away from partially hydrogenated fat as the frying medium.

However, French fries appear to be the only fast food item undergoing any significant changes, they say; salt and saturated fat are still at unhealthy levels in all fast food products.

And while 56 percent of the items studies had a reduction in calories, the reaming 44 percent actually increased their caloric content over the time period studied, they say.

Fast food restaurants -- which might be seen as too easy a target for a nutrition survey -- were an appropriate subject for their study, the researchers said, because Americans on average consume more than a third of their calories outside the home, and fast food chains are the source of about 40 percent of those calories.

The greatest impact when it comes to obesity and other health problems is when several fast food menu items are consumed together as a complete meal, says study leader Alice Lichtenstein.

The levels of calories, saturated fat and salt "are high for most of the individual menu items assessed, particularly for items frequently sold together as a meal," she said, suggesting a fast food meal is "pushing the limits of what we should be eating to maintain a healthy weight and sodium intake."

As recently as 2013, the researchers noted, one meal of a cheeseburger with French fries and soda could rack up 80 percent of a person's daily recommended calories and 139 percent of their recommended salt intake.

"That does not leave much wiggle room for the rest of the day," Lichtenstein said.

There is an opportunity for the fast-food industry -- the particular chains in the study were not identified -- to improve the health of Americans, she said, by offering smaller portion sizes and by reformulating their offerings with less calories, salt and fat.

"From what we hear, some fast food chains are heading in that direction and also introducing new healthier options," she said. "If taken advantage of, these changes should help consumers adhere to the current dietary recommendations."

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