A study of heart patients has shown just how difficult it can be to eat a low salt diet. The patients thought they were following doctor's orders, but they weren't. It's all because of the hidden salt that's present in many — perhaps way too many — foods.

About 70% of the salt in the American diet comes from salt that is in processed foods when they're purchased, rather than in what consumers add themselves.

It's particularly important for people who have experienced heart failure to lower their sodium intake to a recommended intake of 2,000 milligrams a day. The 116 patients in the study were asked to write down everything they ate for three days. From this information, doctors calculated that they were taking in 2,671 milligrams, on average. Only one−third of the subjects in the study were meeting the 2,000 milligram guideline.

The reason for this is that the patients were focused on how much salt they were adding to their food from the shaker. What they weren't considering was the salt that was already in the food they purchased. About 70% of the salt in the American diet comes from salt that is in processed foods when they're purchased, rather than in what consumers add themselves.

Why is there so much salt in processed foods? Basically, it is because the American food industry finds it a convenient way to make food palatable.

The study authors single out about a dozen different types of food as prime offenders. While most people are aware that fast food burgers and luncheon meats contain lots of sodium, the presence of bread and canned soup on this list may be surprising. Some of the highest−sodium foods eaten by the study participants included hot dogs, sausage and bacon, canned soups, salad dressings, condiments, fast food, lunch meat, bread, pizza, processed entrees, prepared grits and cornbread.

Commercially sold bread often contains 150−200 milligrams of sodium per slice. Canned soups are even worse. They generally contain 500−1,000 milligrams of sodium per serving. This includes innocuous sounding soups like vegetable. So a lunch of soup and a sandwich could have as much as 1,400 milligrams of sodium. That's if the sandwich happens to be an air sandwich. If it has meat in it, the number can get much higher. And that's just lunch.

Carolyn M. Reilly, a co−author of the study and a postdoctoral fellow at Emory, recommends that heart failure patients all receive individualized diet plans to help deal with the sodium issue. While this certainly makes sense, how can the rest of the population lower their sodium intake?

The American Heart Association recommends that healthy people aim to eat less than 2,300 mg of salt per day. They also recommend that some people, those likely to be salt sensitive — African Americans, middle−aged and older adults and people with high blood pressure — should aim for less than 1,500 mg per day.

Reading labels helps. Even people who shudder at the thought of numbers can tell that 500 milligrams of salt is a lot more than 100 milligrams. It's really the only way to know the enemy.

The best strategy is probably to have more control over how the food you eat is prepared. Almost all fast food is loaded with sodium. Ideally, you'd be growing, processing and cooking your own. This won't work for most people, but you can help lower your intake by making an effort to prepare more of your own food and going to restaurants where you know the food is prepared from scratch, rather than packages.

Hidden salt. It's out to get you.

The research was presented April 25, 2009, at the American Heart Association's 10th Scientific Forum on Quality of Care and Outcomes Research in Cardiovascular Disease and Stroke, which was held in Washington, D.C.