An "unhealthy fixation" means being overly obsessed or preoccupied with something, to an extent that it becomes harmful to a person's well-being or affects their daily life negatively. Full definition
It's not necessarily abnormal for a child to stash a favorite snack or some sneakily acquired candy in a bedroom drawer or backpack, but if you notice your child hoarding large amounts of food, binging on hoarded food, stealing food to hoard, or otherwise showing an unhealthy fixation on food, it's time to mention it to a mental health professional or your child's doctor. (verywellfamily.com)
Her unhealthy fixation with healthy food was something else, and it was years before she realized it had a name: orthorexia. (health.com)
You need to know: Theres a difference between a healthy diet and an unhealthy fixation on a healthy diet. (health.com)