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.