Again, you're confusing principle with quantitative absolute; the difference here is providing you with an exact list of things you should and shouldn't eat, versus saying "Don't eat poison, it'll hurt you." You can infer that chugging rat poison is probably not in the interest of a long, healthy and fulfilling existence. As such, Objectivism deductively and inductively proves that certain actions are an
objective good for the soul as a condition of being human. Soul-tending, satisfying your values, being productive, etc.
No honest philosophy claims to supplant the special sciences, only serve as a foundation, nor will it tell you exactly what you must do to apply those principles to your life. If rigid dogma is what you expect or seek, I suggest investigating religion instead.