Yup, I've had this all my life from parents, friends, teachers, work colleagues, you name it - and I find it incredibly irritating!
I have always been very thin and lanky, but I don't have any diet related health problems or eating disorder that I know of, and I rarely lack the energy to do whatever I want to do - and that often means walking ten miles or more in a day in a hilly part of the country (usually very briskly too!)
When people comment on the quality of the food I eat, I don't mind quite so much, as I recognise that I do have problems with shopping, planning meals, preparing food etc. and I do eat at odd times of day sometimes. But I've never felt as if I'm wasting away to nothing, or that I'm about to keel over, and I loathe being talked about as if I'm a turkey that needs fattening up for Xmas!!
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When you are fighting an invisible monster, first throw a bucket of paint over it.