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Rapper Jay Z turns 44 today, but don't expect him to celebrate with cake and ice cream.

The "Holy Grail" musician, whose real name is Shawn Carter, has gone vegan, he revealed.

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On his blog, Life and Times, Carter wrote that for 22 days, he and his wife, singer Beyoncé, will be sticking to an entirely plant-based diet.

"This all began a few months back when a good friend and vegan challenged me to embrace a 'plant-based breakfast' every day," he wrote. "It was surprisingly easier on me than I thought…"

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The timing is especially significant for the couple, for whom four has always been a lucky number. (Not only is Carter's birthday Dec. 4, but Beyoncé's is Sept. 4 and the couple was wed on April 4, 2008.) Plus, Carter explained, it takes 21 days to "make or break a habit," and, he added, "On the 22nd day, you've found the way."

"There's something spiritual to me about it being my 44th birthday and the serendipity behind the number of days in this challenge; 22 (2+2=4) coupled with the fact that the challenge ends on Christmas day," he explained. "It just feels right!"

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Carter concluded by saying he'll continue to keep fans updated about his "spiritual and physical cleanse," and asked for vegan restaurant recommendations too.

"I don't know what happens after Christmas. A semi-vegan, a full plant-based diet? Or just a spiritual and physical challenge?" he wrote. "We'll see…"