Jay Z -- MEATLESS Birthday Lunch ... with Beyonce & Anne Hathaway

It's only Day 1, but Beyonce and Jay Z are serious as a heart attack about adopting a no-meat diet ... because they just pigged out at a classy vegan lunch joint in L.A. to celebrate Jay's 44th birthday.

Bey and Jay ordered a variety of organic vegan menu items at Cafe Gratitude -- a place that touts its food and its people as "a celebration of our aliveness."

B&J have said they're going on a 22-day plant-based diet to become more spiritual ... and they chose today -- Jay Z's birthday -- as the day to start.

They're going whole hog, too ... ordering drinks made of apples, beets, lemon and ginger ... and entrees including marinated kale salad, Indian curried lentils and corn tacos with chipotle butternut squash.

They also discovered they're not alone on their journey ... 'cause Anne Hathaway and Ariana Grande were also at the restaurant at the same time vegging out.

Jay and Bey topped off their lunch with a dessert -- a chocolate truffle -- but there were no birthday candles and no birthday singing. They're probably saving that for the party.