WASHINGTON - Movie theater chains are fighting a federal requirement to disclose that their popcorn contains as many as 1,460 calories, or equal to almost three Big Macs.

Chain restaurants with at least 20 U.S. locations will have to post the calorie content of menu items under a provision in the health care law. Regulators will propose rules by March 23 and can include food sold at concession stands and grocery stores, according to guidance that came out last year.

"It's easy enough to blow your whole diet for a week from one snack at the movie theater," said Margo Wootan, the director of nutrition policy at the Center for Science in the Public Interest in Washington. "Just because you happen to be watching a movie while you're eating doesn't mean you aren't eating out."

Movie theaters and grocery stores are lobbying the Food and Drug Administration to avoid the proposed regulation. Theater chains led by Knoxville, Tenn.-based Regal Entertainment Group generate as much as one-third of their revenue from concessions. Congress didn't mention theaters in the law and the idea of regulating them never came up at legislative hearings, said Patrick Corcoran, a spokesman for the National Association of Theatre Owners, a Washington-based trade group.

"In the basic history of the bill there is no real intent to include movie theaters that we could discern," Corcoran said in a telephone interview. His trade group is recommending that the FDA exempt companies that get less than 35 percent of gross revenue from food sales.

But Rep. Rosa DeLauro, D-Conn., who sponsored a food-labeling bill in the House that was incorporated into the health care law, said movie theater chains were supposed to be targeted by the mandate. The requirement "is meant to let people know what it is that they're consuming," she said in an interview.

The Center for Science in the Public Interest analyzed popcorn sold by Regal Entertainment, Cinemark and AMC Entertainment Inc. of Kansas City, Mo., in 2009 and found it contained from 370 calories to 1,460 calories depending on the serving size and whether butter-flavored toppings were added. A "moderately active" man who is 26 to 45 years old should consume 2,600 calories in a day, according to guidelines published by the Department of Agriculture.