The 42-year-old dad-to-be is supporting Beyonce by dieting with her. A source tells the new Us Weekly -- on stands Friday -- that the duo are maintaining a "partially vegan diet."

"Jay's trying to be disciplined, and he needs the energy to support her," a source adds.

Beyonce, 30, isn't going to let herself go just because she's expecting! "I've been really conscious [about food]," she told 20/20. "I've been trying my best not to lose control!"

Jay and Bey's commitment to healthy living seems to be paying off. A source tells Us that Monday night the duo were "dancing up a storm" at Hollywood's Spare Room club. "They were loved up and sweet together [and] partied until 4 a.m.!

"The next night, Beyonce and Jay-Z partied once again! This time, the couple hung out with Serena Williams, Kanye West and friends at Watch The Throne's after party.