It's what's in your Heart that dictates how open or closed you are to Spirit. And how you train your thoughts to meet up to what's in your Heart.
Eating some junk food now and again won't hurt you unless you are allergic or have food sensitivities.
So long as you don't live on it. Don't eat junk food instead of a proper meal, healthy food. Eating a little now and again is fine!
Not getting enough nutrients can dull your body and mind and make it very hard for your Soul to communicate with your brain.