Eating too much leads to many diseases, causes laziness, making one feel it heavy and burdensome to do deeds of obedience to Allah (swt) and hardens a persons heart – and we seek refuge in Allah (swt) from all of that. The opposite way, eating too little, also weakens a persons body, making him too weak to do acts of obedience to Allah (swt) . The best cure and prevention is what is prescribed by the Prophet (saas) and were we to follow the Prophets way, we would not, in many instances, have to pay a visit to the doctor. Miqdaam Ibn Madee Karib (ra) reported that he heard the Messenger of Allah (saas) say,

 A human being has never filled a container that is worse than his stomach. It is sufficient for the son of Adam to have just that amount of morsels that cause his back to remain upright. But if there is no excape (i.e., he wants more), then one third for his food, one third for his drink, and one third for his breathing. 1

We would do well to heed what some of our pious predecessors had to say about this subject. Ibn Abdul-Barr and others related that, one day, when Umar Ibn Al-Khattab (ra) delivered a sermon, he (ra) said, Beware of gluttony, for it makes one lazy from prayer and it is harmful to the body. Upon you is moderation in your food, for that will keep you further away from ungratefulness, healthier in your body, and stronger to do worship. And a person does not become destroyed until he prefers his desire over his religion. Alee (ra) said, The stomach is the tank of the body: liquids come into it and flow out of it. If it is healthy, liquids will flow out of it in a healthy manner, but if it is sick, liquids will flow out of it with sickness. Fudail Ibn Iyaadh said, Two actions harden the heart: much talk and much eating. And it was said to Imam Ahmad,  These are the people who eat very little food. He said, That does not impress me. I heard Abdur-Rahmaan Ibn Mahdee say, A group of people did the same and it resulted in them being cut off from obligatory duties (i.e., their bodies became so weak that they became negligent in performing obligatory duties).

1) Related by At-Tirmidhee (2380), who said, Hasan Saheeh; by Ibn Maajah (3349), and Al-Albaane gave a Saheeh grade to his narration; and by Ahamad (16735)