Physical ailments attack and harm the body and alter its normal functions, because of an excess amount of a substance. This type constitutes the majority of diseases and occurs because of overeating or consuming more than what the body needs, that which brings about little benefit or is not digested easily, or due to complex meals. When the son of Adam habitually fills his stomach with these types of foods, he will end up with various types of illnesses, some of which take a long time to remedy. On the other hand. when one consumes moderate amounts of food and eats sensibly, the body will get the maximum benefit from this diet, as opposed to when one overeats. The foods we eat are either for necessity, sufficiency or excessiveness. The Prophet peace be upon him told us that one only needs a few bites to sustain him, sot that his strength does not fail him. When one wishes to exceed what is barely enough, he should reserve a third of his stomach for his food, another third of the water or drink and the last third for breathing. This is the best method of eating, both for the body and the heart. When the stomach is full of food, there will not be enough space for drinking. When one consumes something to drink on a full stomach, one’s breathing will be difficult, thus bringing about laziness and fatigue. One will feel heavy, as if carrying a load on his stomach. Consequently, one will be lazy fulfilling his obligations and will seek other desires now that his stomach is full! Eating until one is full harms the body and the heart, when it becomes a habit. There is no harm if one occasionally eats his fill.
Excerpt from the book Prophetic medicine by Ibn Qayyim al Jawziyy