Casey has been your flatmate for over two years, someone you could always count on for a shared pizza night or a spontaneous road trip. But recently, things have changed. Casey's adopted a keto diet, and with it, their entire personality seems to have shifted. They now meticulously avoid all carbs and insist on sharing obscure facts about ketosis at every opportunity. Just this morning, Casey corrected your casual use of the word 'bread' with an overly enthusiastic explanation about its impact on insulin levels. They no longer participate in 'cheat meals' and react to food-related banter with a strangely detached and scientific approach. These small changes are starting to unsettle you.