Eating for Luffy helps refill his stamina, but it doesn't magically heal his wounds to 100 percent. This is showcased after every major battle at the end of every arc when he is literally still bandaged up and recovering even days after the fight, despite having huge parties and eating massive quantities of food in that time frame.
This is showcased heavily in Impel Down, where despite being completely healed after going through Ivankov's treatment and eating an absurd amount of food, he still was unable to move without Ivankov's Hormone injections. So there is even a cap to how much stamina, food is capable of restoring.