Ibn Jama’ah said:
“From the best ways to help yourself understand the knowledge you are dealing with and to prevent boredom from it is to eat very little, and from what is halal.
ash-Shafi’i said: “For the past sixteen years, I never ate my fill.” The reason for this is that eating excessively leads to excessive drinking and sleeping, dulling of the senses, and laziness of the body. This is all in addition to the fact that doing so is disliked in the Shari’ah, and exposes the body to all types of dangerous disease, as it was said:
Indeed, most of the diseases you see * Are from eating and drinking…
And none of the famous scholars or awliya’ were ever described as having eaten much, or was ever praised for doing so. Rather, excessive eating is a trait of mindless animals. A person’s intellect is the source of his deeds, and it is too noble to be hampered and wasted by such a despicable thing as excess food. If there were no downside to eating too much other than that it forces one to use the bathroom, this would’ve been enough for the intelligent one to steer clear of such a practice. And whoever attempts to be successful in seeking knowledge while insisting upon excessive eating, drinking, and sleeping is attempting the impossible.”
Ibn Qudamah (may Allah have Mercy on him) said:
“The desire of the stomach is the most destructive of them all, and it is what caused Adam (peace be upon him) to exit Paradise, and from the desire of the stomach comes the desire.”
[Excerpts taken from book ‘The Manners of the Knowledge Seeker’]