How to ignore my cocky know-it-all roommate?

My roommate has really been pissing me off. He thinks he knows everything about everything and will make sure you know it. Some background, we are both in college. One thing that has really been making me mad is how he thinks he is the most healthy person on the planet. First off, he is not anywhere close to healthy. He eats 50 times a day. He gives me crap when I come home from a long days work with fast food and claims I have an addiction and tells people that behind my back. I'm way more fit than him and I weigh less than him and I keep continuing to lose weight while he still has a belly despite his "healthy eating". I honestly feel he needs to get his *** beat one good time but I know the best thing to do is ignore him and tell myself I'm better than that. I was also thinking I should start giving him crap for not losing weight then maybe he will shut up. Any pointers?