Greenwald is a vegan and has appeared in at least two ads for PETA. He told PETA his reasons for going vegan: "In college I stopped eating red meat on a bet with my girlfriend at the time. She bet she could stop smoking, and I bet that I could stop eating red meat. She started smoking again, so I won, which I'm always proud of.…I had been a vegetarian as a child for whatever reason. I guess kids sometimes follow their instincts…My friend dared me to go vegan as I was reading this book "Fast Food Nation", which opened my eyes to a lot of cruelties.…Ever since then, I've been vegan and enjoyed it daily."