I think proper nutrition is essential for many reasons and the point made by Flash is a good one regarding if you have not been diligent with your nutrtional needs you will likely notice positive differences quickly.
An example of this was when I first began training seriously with Charlie and Mauro DiPasquali gave me a list of nutritional benefits for various supplements. Not to make too fine a point about this but it was almost like I turned a switch on. There were problems for sure like taking too much iron. Bad plan for me but I solved it over time and learned as I went along. Now I prefer foods like ( raw juicing), liquid supplements, powders especially those condensed super foods and digestive aids to assist the decreased levels of our hydrochloric acid. ( essential for proper absorption