"If I'm spiritual can I eat unhealthily and not be affected?"
I have been having a difficult time reconciling the physical laws of our planet with the boundless nature of our universal spiritual existence. If we truly are spiritual beings made up of pure energy, then why should any physical indigestible substance like junk food and alcohol be harmful to us. If our physical experience is determined by our thoughts and concepts, it can be argued that I can drink like a fish and pig out on hot dogs every day and it will not have any detrimental effects as long as I believe it won't. I'm aware that this line of thought can be a dangerous rationalization to engage in destructive behavior (which has been the case to a certain degree), but I still believe that it's a valid question that needs to be addressed.
To answer your first question, everything has a vibrational signature. In other words, each thing comes in varying vibrational rates, or energy, from low vibrations to higher vibrations. Some energy is denser, or slower, than others. So, if, for instance, alcohol, or fermented foods carry a vibrational signature that is low, and it doesn't match well with the human vibrational frequency, then there is a disharmonious effect/conflict produced. What you said about being able to pig out on junk food and not be affected by it, if you don't BELIEVE it will affect you negatively, is ultimately true. But, in order not to be negatively affected by junk food, which carries a low vibration, you must have evolved to the point where you have mastered the ability to transform energy, as the masters do. If you have evolved to the point where you have mastered energy, then you can change the vibrational frequency of anything. That takes a lot of advanced evolution. We can fool ourselves into thinking we are more advanced than we are, and in that case, if we pig out on junk food, it will still affect us negatively.
Just saying, "well, if I believe it, it shouldn't hurt me" can be dangerous if your level of consciousness, thought, clarity and intention are not pure yet. If there is anger there, or resentment, or any unresolved issues, then it's just more like being a pissed off child saying, "well, she said it was so, so I can do it!" That's still coming from the ego, not the higher consciousness. If we're actually in third grade on the self realization scale, but we put on a show and try to be something we're not yet, say a twelfth grader, then there can be confusion. That's called having a false self image. Part of evolving is being perfectly honest with ourselves, facing and accepting where we're at on the journey, with Love.
There's a great story about a man in India who had heard that if you are a spiritual being,which he thought he was, then nothing would harm you. So, one day, he's outside the village, and a man riding on an elephant comes rushing toward him. He stands in the middle of the path, secure in his belief that he is so advanced that nothing will harm him. The man on the elephant shouts and shouts at him to get out of the way, but he remains standing in the path, thinking, "I am on the spiritual path and nothing can harm me." Well, the elephant tramples him, of course. God, in the form of the man on the elephant was shouting at him, telling him he wasn't ready for that yet, but he didn't listen. It's just a form of egoistic thinking, still - a form of arrogance. As long as we're still coming from the ego, and not the higher self, we can create trouble for ourselves.
Hope this clarifies things a bit.