It's no surprise that Sir Paul McCartney, a longtime vegetarian, banned all meat from staff meals on his current world tour. But when Mario Batali starts to push people to eat their vegetables, you know something is happening.

The famously rotund and infamously gluttonous chef-restaurateur is to pig what the Beatles are to rock-'n'-roll. Batali, a rock-star chef if there ever was one, has changed the way Americans eat pork, introducing us to cured lonza, guanciale and lardo, which he once described to the New Yorker magazine as "the best song sung in the key of pig."

And yet this month, Batali announced that he would join the Meatless Monday campaign, a movement backed by an array of public-health advocates, animal welfare activists and environmentalists that asks carnivores to give up meat one day a week. Each of Batali's 14 restaurants, which include the meatily named Bar Jamon in N.Y. and Carnevino in Vegas, offers two vegetarian entrees on Mondays, with an "MM" logo.

Batali is one of the movement's latest and most high-profile supporters. But on the vegetable front, he is hardly a pioneer. Baltimore City Public Schools launched meatless Mondays for its 82,000 students in October. Thirty-two U.S. hospitals have signed on to the Balanced Menu Challenge, a commitment to reduce meat purchases by 20 percent. This spring, San Francisco approved a resolution calling on schools, restaurants and stores to offer meatless options, and Michigan held a one-day "Meatout" during which residents were encouraged not to eat meat.

This is not the first time the meat industry has faced a meatless-day movement. The concept has its roots in World War I, when Americans were told that "Food Will Win the War" with Meatless Mondays and Wheatless Wednesdays. The N.Y.-based nonprofit group Healthy Monday relaunched the idea in 2003 with Johns Hopkins.

It began to take off in 2009 when institutions and restaurants started to embrace the idea. And now there's Batali, who lost 45 pounds, flying the flag for meatless Mondays. "Mario still loves meat," said Elizabeth Meltz, his director of sustainability. "But even he believes everything should be eaten in moderation."