Chain restaurants found companies have reduced sodium by an average of six percent between 2009 and 2013, but the progress has been characterized as "slow and inconsistent."

Researchers at the Center for Science in the Public Interest reviewed 136 meals from 17 top restaurants to make their findings.

The biggest sodium reductions were seen in Subway, McDonalds, and Burger King; but KFC and Jack in the Box increased the amount of sodium in their food by 12.4 percent and 7.2 percent, respectively, the Center reported. The average amount of sodium in 55 kids' meals dropped by only 2.6 percent since 2009.

Despite the slight improvement, 79 percent of the 81 adult meals included in the study contained over 1,500 milligrams of sodium. The Centers for Disease Control and Prevention (CDC) suggests people over 50, those with high blood pressure, and African Americans, should try to limit themselves to 1,500 milligrams of sodium per day.

Sodium is linked to health risks such as "high blood pressure, stroke, heart attacks, kidney disease, and other health problems," the Center for Public Interest said.

Meals at restaurants such as Red Lobster, Chili's, and Olive Garden, can reach as high as 5,000 calories, which is more than most people should consume over the course of three days.

"For far too long, the FDA has relied on a voluntary, wait-and-see approach when it comes to reducing sodium in packaged and restaurant food," said CSPI executive director Michael F. Jacobson. "If chains like KFC, Jack in the Box, and Red Lobster are actually raising sodium levels in some meals, FDA's current approach clearly isn't working."

Some of these chains are making an effort to reduce sodium content in their food. Sunway reduced the sodium in every one of its 10 meals over the course of the study.

Reducing sodium is just one of many parts of our commitment to continuously improve our menu," Lanette Kovachi, senior dietitian for the Subway restaurant chain said. "So far we have reduced sodium 30 percent in our sandwiches, including the sandwiches in the 10 meals cited by CSPI, and 35 percent in our Fresh Fit offerings, meeting the National Salt Reduction Initiative targets for sandwiches. We are always working to find new ways to serve our customers a wide variety of menu offerings that taste great and are better-for-you."