What is the study of food and the body's use of it? which doctors specialize in this?

That's nutrition: Unfortunately, presently most physicians have very limited knowledge in regards to nutrition(though this is slowly changing at the medical school level). In reality, you may be best served working with a nutritionist or finding a doctor who has a specialist interest in nutrition(this could be an internal medicine doctor, endocrinologist, obesity specialist, cardiologist, etc!)