TRENTON, N.J. – Show a group of nutritionists a 10-ounce restaurant hamburger and a side order of onion rings and have them calculate the number of calories in the meal. Easy, right?

Well, not exactly. The food experts consistently underestimated the 1,550-calorie meal – by an average of 685 calories – in a study of 200 dietitians by the Center for Science in the Public Interest and New York University.

“If well-trained food professionals can’t accurately estimate calorie content in a plate of food, you can be sure the average consumer is at a loss when looking at a menu,” said Claudia Malloy of CSPI in Washington, D.C., which co-sponsored the study and is lobbying to force restaurants to provide nutrition information on the foods they serve.

New Jersey is among a half-dozen states considering food labeling laws for restaurants. Under a recent proposal in the New Jersey Assembly and pending in committee, chain and fast-food restaurants with 20 or more franchises would have to post calorie counts on menu boards – Big Mac, 590 calories; Starbucks’ grande cafe mocha, 400 calories – and more extensive nutrition information, such as trans and saturated fats, salt and cholesterol, on menus.

Similar legislation is pending in Connecticut, Maine, Massachusetts, Illinois and Hawaii, and is expected to be reintroduced in the U.S. Senate and House of Representatives, though no state currently has such a law, Malloy said.

Many chain restaurants, from McDonald’s to Subway, already make nutrition information available, although it’s not always publicly posted.

Nutrition labeling bills may have gained momentum with the recent success of “Supersize Me,” an Oscar-nominated documentary exposing the health risks of a fast-food diet, and publicity over a lawsuit blaming McDonald’s for the obesity of teenage customers.

To the New Jersey Restaurant Association, such proposals amount to over-regulation for an industry already struggling to turn a profit.

“Our position is that the individual who is concerned about obesity should emphasize healthy lifestyle, personal responsibility, regular exercise and moderation,” said Dale Florio, legislative counsel for the state restaurateurs. “Seventy-six percent of all meals are prepared at home. That’s where nutrition has to start.”

No one on either side of the issue believes posting calorie and fat content will produce drastic changes in what people eat. But proponents say such information could lead diners to make healthier selections and would provide sorely needed information to those with health problems who need special diets.

“When they purchase foods at restaurants, most people are using their instincts and their desire,” said Daniel J. Hoffman, assistant professor of nutritional sciences at Cook College in New Brunswick. “This is a nice compromise between telling people what they should eat when they’re out at a restaurant and giving them information so they have the power to choose.

“It’s not going to drive them out of the ice cream store, but it may change what they buy,” he said.