"We recognize customers want to know more about the nutrition content of the food and beverages they order," Jan Fields, president of McDonald's USA, said in a statement.
McDonald's has been experimenting this summer with placards listing "Favorites Under 400," the items on its menu that have 400 calories or fewer. This fall, some 750,000 of its employees will be able to participate in an e-learning program about nutrition, calories and the company's menu to help them answer customers' questions.
McDonald's plans to introduce an app this year that will provide customers with information about calories in its products and help them build a customized meal plan.
Under the health care bill upheld by the Supreme Court this summer, all restaurants with more than 20 locations will have to post calorie counts on their menus, though the precise regulations and timetable for doing so have not yet been detailed.
McDonald's, which has 14,000 locations around the country, is by far the largest chain to post calorie counts nationwide. Panera Bread, which is much smaller with more than 1,500 bakery-cafes across the country, also posts calorie counts.
McDonald's decided to act before the federal requirement takes effect, even though the industry at large has resisted listing calories on menus.
"They are such a huge restaurant and there are so many people that eat their food, so this is a really positive step," said Margo Wootan, director of nutrition policy at the Center for Science in the Public Interest. "It will help their customers get more familiar with calorie counts and make decisions about what they eat based on them, and it will probably improve McDonald's menu over time."
Other signs indicate that major restaurant chains are working to improve the nutritional quality and reduce the caloric content of their food ahead of the health care law's menu requirements. Just last week, for instance, Dunkin' Donuts and IHOP announced that they were adding Quaker Oats oatmeal to their menus.
New York City and Philadelphia already require chain restaurants to post calorie counts, but so far, research seems to show minimal impact on consumer behavior.
A Stanford University study found a 6 percent reduction in the number of calories purchased by consumers buying food in Starbucks stores in New York City after the company began posting calorie counts in April 2008. But for customers who averaged more than 250 calories per transaction, calories declined by 26 percent.
Ms. Fields said that although posting calorie counts did not seem to make a marked difference in customer behavior, consumers liked having the information.
Ms. Wootan said that calorie counters might start to become a bigger factor as consumers gain more familiarity with them.
"People may start ordering small fries, say, instead of large ones, or a regular hamburger instead of a quarter-pounder," she said. "This is all still very new."
Not everyone applauded McDonald's announcement. Sara Deon, who works on campaigns to reduce junk food marketing and build food sustainability at Corporate Accountability International, a watchdog group, said it had more to do with public relations than with improving public health, particularly as it related to children's consumption of McDonald's food.
"Offering a healthier option in the Happy Meal doesn't put an end to the marketing that's directed at children," Ms. Deon said. "The healthier options over all are little more than a vehicle for selling more of McDonald's bread and butter - burgers, fries and soda."