Given the current rate of obesity in the US, you would be forgiven for thinking that fast-food restaurants have increased their portion sizes in recent years. But according to two new studies, this is not the case, and there has also been little change in the amount of saturated fat, sodium or calories in fast foods.

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Over the past 18 years, the portion sizes and nutrient content of fast foods have seen little change, according to two new studies.

These findings come from an analysis of three national fast-food chains between 1996 and 2013, conducted by researchers from the United States Department of Agriculture (USDA) Human Nutrition Research Center on Aging at Tufts University in Boston, MA.

Lead researcher Alice H. Lichtenstein, director of the Cardiovascular Nutrition Laboratory at the center, and her team publish their findings in Preventing Chronic Disease - a journal of the Centers for Disease Control and Prevention (CDC).

The researchers assessed the average portion size and the nutrient content in four of the most popular items sold in fast-food restaurants in the US: cheeseburgers (including 2 oz and 4 oz), fries (small, medium and large), grilled chicken sandwiches and cola beverages.

The Internet and a public database were used to gather archived nutrition data of each food item over the 18-year period.

Amount of saturated, trans fat in fries has fallen

Between 1996 and 2013, the researchers found there was little change in portion sizes among the three fast-food chains assessed.

"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," says Lichtenstein.

What is more, the team found that the overall amount of calories, sodium and saturated fat in the foods among the three chains remained "relatively consistent" over the 18-year period.

The amount of saturated fat found in fries, however, began to decline in 2001, while the amount of trans fat found in fries fell between 2005 and 2009.

The team says this is likely due to changes in frying fat, after some US states put trans fat regulations in place. In 2007, for example, New York City adopted a regulation banning the use of such fat in restaurants - something Lichtenstein says is beneficial:

"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 foods 'push the limits' of what we should eat for a healthy weight, sodium intake

Although there has been no increase in fast-food portion sizes, and the nutrient content of such foods has remained fairly stable, the researchers say their analysis revealed "high variability" in the nutrient content of fast foods between each chain.

They note, for example, that the calories found in a large cheeseburger meal - with fries and a regular cola - varied from 1,114 to 1,757 among each restaurant over the study period. Such calorie contents account for 57-88% of an individual's recommended calorie intake, which Lichtenstein says "does not leave much wiggle room for the rest of the day."

Furthermore, 2013 data from the studies revealed that a cheeseburger meal over the three fast-food chains accounts for 65-80% of a 2,000 calorie-per-day diet and 63-91% of the 2,300 mg of sodium per day recommended under the US Dietary Guidelines for Americans.

"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," says Lichtenstein, adding:

"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.

A 100 calorie difference per day can mean about a 10 pound weight change per year."

According to the CDC, fast-food consumption has increasingly become a part of the American diet, with many of us reaching for the "quick availability" options as our lives become more hectic. Fast-food consumption has also been shown to contribute to obesity, which currently affects almost 35% of American adults and 17% of children and adolescents.

Lichtenstein notes that some fast-food chains are already moving toward reformulating their foods to reduce calorie and sodium content, with some chains introducing healthier options for consumers. "If taken advantage of, these changes should help consumers adhere to the current dietary recommendations," she adds.

Medical News Today recently reported on a study led by researchers from Ohio State University, which suggested the amount of fast food children eat may influence their academic growth.