Shouldn't all doctors specialize in nutrition since that is what is essential for our bodies? how can i trust a doctor who isn't trained in nutrition?

Let me be frank: I appreciate your concern. All physicians learn basic nutrition. However, the best scientific studies show that the impact of food choices on most disease is limited -- physical fitness has much more overall impact. You're 19 and probably as good-hearted & naive as i was at that age. There's lots of money to be made by claiming "doctors don't understand nutrition" and selling stuff you don't need.

I agree: in each field doctors do learn about nutrition. For example, as a pediatrician, I learned about infant formulas and starting solids and food allergies. The training can be different depend on where you train and how much nutrition is emphasized and how interested the individual is in learning about nutrition