It's important to eat healthful food, but our bodies are very complex and they process food and protect us.
"You are what you eat" probably comes from the kind of people who daydreamed, talked and doodled while their teachers tried to teach how the body works and what the internal organs do.
I once had a friend who was eaten alive with worry because there was a swelling in her underarm. She was afraid it was breast cancer. I told her it was probably a gland which was swollen because it was fighting the infection from a bad tooth. "Didn't you learn about those glands in the fifth grade?" I asked.
She said, "Oh, I never paid any attention to that stuff in school."
What's the name of those glands? I can't recall it right now.