Jay Z, Beyonce Have Vegan Lunch on His Birthday, Run Into Ariana Grande, Anne Hathaway

Credit: Kevin Mazur/WireImage.com; inset: Stefanie Keenan/WireImage.com

No cheeseburgers or milkshakes for them! Jay Z and wife Beyonce are making good on their vow to go vegan for the next 22 days -- hitting organic, vegan eatery Cafe Gratitude in Hollywood for his 44th birthday lunch on Wednesday, Dec. 4.

TMZ reports that the rapper and his wife, 32, sipped on pressed juices and noshed on marinated kale salad, Indian curried lentils, corn tacos with chipotle butternut squash -- polishing it off with a chocolate truffle (sans birthday candles or birthday song).

And, an insider tells Us Weekly that other vegan-friendly celebs were at the same Cafe Gratitude during the superstar lunch: Ariana Grande (who couldn't help but introduce herself to the couple) plus Anne Hathaway and her husband, Adam Shulman.

"So you can call it a spiritual and physical cleanse," Jay Z explained in a Dec. 3 blog post. "I will post my progress...Any professional vegans out there that have any great food spots, please help out!"