This exact thing happened to a buddy of mine. Was a fatass, went vegan, lost a shit ton of weight, which was obviously a good thing. But then after a couple years of it, he had some health issues. Went to the doctor, they diagnosed him with kidney stones and pretty severe anemia. Basically told him if he doesn't eat meat he might die. He's not vegan anymore. He kept the weight off though and he's since become a huge jacked bodybuilder so it might've all been worth it in the end. He probably could have lost the weight in a less dangerous way, though. I still remember his face when talking about the kidney stones. Said the pain was worse than anything he'd ever experienced.