The findings should come as a surprise to many people, who, when trying to reduce their sodium intake, focus on cutting down certain kinds of food, like canned soup or pickles. They don't realize that wide disparities can show up in foods that are otherwise comparable.

The study was conducted by the Center for Science in the Public Interest, a Washington-based group that has been a staunch critic of the food industry and has been on an antisalt campaign for years. (See the full text of the study.⁵) For this study, CSPI looked at more than 550 products in the grocery store and in fast-food restaurants and used information gleaned from companies' Web sites and packages to compare the sodium contents of various foods. The Wall Street Journal checked and confirmed data in the CSPI study and did additional research on other food items, using information from food labels and manufacturers' Web sites.

The new research highlights the wide variety in salt content among similar packaged and restaurant foods. Americans get about three-quarters of their salt from packaged food and fast-food restaurants. Only about 11% comes from salt added at the table.

The research also shows that in recent years some foods have been getting saltier. CSPI has been tracking the contents of 100 popular foods since 1983. Of the 69 products still on the market in 2004, the average sodium content fell by 11% from 1983 to 1994, to 533 milligrams from 592 milligrams. But from 1994 to 2004, it jumped 6% to 564 milligrams.

Jason Huff/Wall Street Journal   Some common fast foods that contain sharply different amounts of salt.

A walk through the grocery store or a trip to a fast-food joint shows big differences in the sodium levels of comparable products. A cup of tomato soup from Campbell Soup Co. has twice as much sodium as a cup of Organic Valley tomato soup. Two tablespoons of ConAgra Foods Inc.'s Hunt's tomato paste has 4½ times the sodium of the same amount of Del Monte Foods Co.'s Contadina tomato paste. And a slice of thin crust cheese pizza from Yum Brands Inc.'s Pizza Hut has twice the sodium of a slice from Little Caesar's Inc.

Food companies generally defend the level of sodium in their products by saying that it is what customers want. "Generally, our consumers like a saltier profile," says Lauren Hammann, a spokeswoman for Burger King Corp. McDonald's Corp. and Burger King liberally salt their fries in the restaurant, but the difference in their salt content comes mainly in the amount of salt in the precooked fry. McDonald's unsalted fries have about 30 mg of sodium in a small order (and add 110 mg of salt for taste), whereas Burger King's fries have 240 mg of sodium for a small, unsalted order (and add about 140 mg of salt after cooking). "Salt reduction, says Ms. Hammann, "is not really a competitive advantage for us."

CSPI Director Michael F. Jacobson, says the big differences in sodium levels among similar foods support his group's position that food companies should reduce salt content.

"The fact that sodium levels vary so widely, indeed inexplicably widely, between brands in a given category shows that many companies could lower the sodium contents of their products without seriously sacrificing flavor," Dr. Jacobson says.

In grocery stores, all packaged foods have a sodium content line in the Nutrition Facts panel on the back of the package. Now it's becoming easier to find out the salt content of many fast foods as well. Most fast-food restaurants have Web sites with nutritional information, or brochures in their restaurants that customers can request. Late last year, McDonald's started telling its franchisees to put nutritional information for the company's products on the backs of tray liners.

Stephen Havas, professor of epidemiology and preventative medicine at the University of Maryland School of Medicine, urges people concerned about their salt intake to request salt-free preparation in restaurants. (Indeed, both McDonald's and Burger King say it is possible to request unsalted french fries, and the cooks will whip up a special batch.)

Reducing sodium in products can be a perilous exercise for food companies. Many companies abandoned lower-sodium products in the mid-90s, as consumers defected from low-sodium products to snap up their low-fat replacements. Today, most companies have found sodium to be one of the most difficult ingredients to take out of food. Salt does more than make food taste salty. It extends the shelf life of packaged food and hides bitter tastes that are common in potatoes and vegetables. It helps keep processed meat firm and gives ham and hot dogs their pinkish color. It can keep bread from going stale and helps the "browning" of cereal flakes during the cooking process. Also, sometimes low-carb and low-fat foods need more sodium in order to make them taste better.

Doctors worry about salt because of its link to high blood pressure, a condition that afflicts one in three American adults and is a risk factor for heart attacks and strokes. Increased salt intake causes more fluid to be contained in the blood vessels. The increased volume of blood requires the heart to work harder, and raises blood pressure.

U.S. Dietary Guidelines recommend that healthy adults consume no more than 2,300 milligrams of salt a day. But the recommendations are much lower, 1,500 milligrams, for "middle-aged and older adults," blacks and those with high blood pressure. Last year, the Dietary Guidelines Advisory Committee was considering cutting the recommended daily salt intake for all adults to 1,500 milligrams, but rejected the lower number. At the time, Lawrence Appel, professor of medicine at Johns Hopkins University and a member of the committee that examined sodium in the diet, said, "Because of the extraordinarily low number of low-salt processed foods in the food supply, the committee worried that no one would be able to follow the lower guideline."

Doctors know that reducing salt intake helps lower blood pressure. In a 2001 study published in the New England Journal of Medicine, researchers found that when 412 adults reduced the sodium in their diet from a teaspoon and a third of salt to about a teaspoon, and increased their intake of fruits and vegetables, the mean systolic blood pressure of both healthy people and people with hypertension dropped.

Pat Verduin, a senior vice president of product development at ConAgra, says that most consumers are turned off by big labels that flag sodium reduction in products. "It's a sign to them that something is different," she says. That's why when ConAgra recently reformulated its Chef Boyardee Mini Ravioli to have 950 mg of sodium in half a can instead of 1,180 mg, the company didn't advertise the change on the package. For products like Chef Boyardee, where the sodium level was already high, reductions are easier, says Dr. Verduin. For the company's Healthy Choice products, which have lower sodium levels to start with, "it's harder to go down."

Write to Sarah Ellison at sarah.ellison@wsj.com⁶