Sub-questions:
X "What is the definition of obesity?"
Sub-questions are not about formulating definitions. The sub-question should always be needed to answer your main question.
X "Does nutritional supply influence obesity?"
That is a closed question.
Better sub-questions are:
V "To what extent does the supply of unhealthy food in canteens affect students' diets?"
V "What measures are taken in school canteens to reduce obesity among students?"
V "What influences students' food choices?"