While I don't think it's literal, I've always liked the idea of it as a bit of divine punishment. It's a religion heavily based on giving to the poor. They're ending up suffering due to their own excesses poisoning them. If those fat christians had actually followed their faith than they'd be in great shape thanks to having given all of that excess food to the poor. But they didn't so they get to experience the hell on earth that is being fat.