"Would a skinny person have to go on a diet?" Well, yes. From my ignorance of medicine, a few come to mind:   Celiacs must avoid anything including gluten.   Lactose intolerants.   People with an allergy to nuts can die or get easily intoxicated.   I have a friend with a rare condition whose body doesn't burn fat properly and has a tendency to get high cholesterol. She's very slim, small and delicate, and unlike the rest of the population, she cannot eat fats (not even healthy ones), and must live on carbs, which is what her body mainly uses.    People I cannot think about, out of me not being a doctor.   So, yes, I'm pretty sure there are at least dozens of reasons why thin people would go on a diet.