While current food labels report the total sugar in a product, there is no way for the consumer to differentiate between naturally occurring sugar and sweeteners that have been added in manufacturing, said Michael Jacobson of the Center for Science in the Public Interest.
His group and others petitioned the Food and Drug Administration to change the current food label requirements to add a line listing "added sugar" and show what percentage that is of a recommended daily amount.
FDA spokesman Emil Corwin said the agency had not yet received the petition for label change.
The Sugar Association issued a statement calling the proposal unnecessary.
"American consumers already have all the tools they need to evaluate the nutritional quality of foods," Richard O. Keelor, president of the Sugar Association, said in a statement.
The Grocery Manufacturers of America said the battle against obesity cannot be won by "obsessively targeting" sugar, and the National Food Processors Association said the label change would merely confuse consumers.
Jacobson said the amount of added sugars in the American diet has been increasing steadily.
"Twenty years ago, Americans were getting about 12 percent of their calories from refined sugars, or added sugars," he said. "Today we're averaging about 16 percent. Teen-agers get 20 percent of their calories from added sugars."
While he agreed that the body can't tell the difference between natural sugars and added sugars, Jacobson said the natural sugars come with other nutrients, including vitamins and fiber, while added sugars add only calories to the diet.
Marion Nestle, a nutritionist at New York University, said the increased consumption of soft drinks and other sweetened beverages replaces more healthful items such as milk and fruit juice in the diet.
"I like sweet foods and I do not for a minute believe that sugars are poison," she said. But, she added, "rates of obesity are rising among children as well as adults."
"People would be healthier if they replaced foods high in added sugar with more nutritious foods," she said. "Food labels are a good way to help people do this."
Dr. Mohammad N. Akhter of the American Public Health Association said that "Americans have done wonderful work in modifying their lifestyles. More people exercise today, more watch their diet. Once they have the information they will act on it."
Jacobson said the Agriculture Department recommends that the average person on a 2,000-calorie daily diet include no more than 40 grams of added sugar. That's about 10 teaspoonfuls, or the amount of sugar in one 12-ounce soft drink.
Under his proposal, for example, a product listing total sugars of 15 grams of which grams occur naturally in the food would include a second line saying "added sugar 10 grams, 25 percent" of daily recommended intake.
The proposal was sent to the FDA on Tuesday, and Jacobson said the normal process of studying such recommendations, seeking public comment and changing the rule would probably take up to five years.