You might benefit from some sort of therapy, if you feel like your obsessions are truly stopping you from socializing or interfering with your homework. If you're in college, you might be able to get cheap or free therapy or counseling through your university health services.

Personally speaking, what help me deal with invasive thoughts--while not necessarily food related-- is:

(A) recognizing them and allowing myself to think them, rather than trying to fight them ("Stop thinking about Food! Stop! Do homework! What do I want to eat for dinner! Stop thinking about food! But I'm hungry" is a self-defeating cycle).

(B) Writing thought out and "dialoguing" with myself. This is basically a CBT technique. Allow yourself to think the thought,vand write it down. "My friends want to go get food now. It's not what I planned!" --> "Why is it important to stick to my plan?" --> "To make calorie counting easier" --> "Can I manage my calorie count if I go with them now?" --> Yes/No, and so on. Basically, slow down, look at your thoughts critically, without the emotional component that goes with the them when they're rattling around inside your head, and make a confident decision about what you want, one way or the other.