Vegetarian diet

Sattvika-ahara

"One must eat appropriately and not take after food as hogs take after stool. For a human being there are eatables described in Bhagavad-gita (17.8) as sattvika-ahara, or food in the mode of goodness. One should not indulge in eating food in the modes of passion and ignorance. This is called ucitahara, or appropriate eating. One who is always eating meat or drinking liquor, which is eating and drinking in passion and ignorance, must give these things up so that his real consciousness may be awakened. In this way one may become peaceful and refreshed. If one is restless or fatigued, one cannot understand the science of God. As stated in Srimad-Bhagavatam (1.2.20):

evam prasanna-manaso
bhagavad-bhakti-yogatah
bhagavat-tattva-vijnanam
mukta-sangasya jayate

Unless one can become free from the influence of passion and ignorance, he cannot be pacified, and without being pacified, one cannot understand the science of God."
(Srimad-Bhagavatam 4:26:11)

"Foods in the mode of goodness increase the duration of life, purify one's existence and give strength, health, happiness and satisfaction. Such nourishing foods are sweet, juicy, fatty and palatable.

Foods that are too bitter, too sour, salty, pungent, dry and hot, are liked by people in the mode of passion. Such foods cause pain, distress, and disease.

Food cooked more than three hours before being eaten, which is tasteless, stale, putrid, decomposed and unclean, is food liked by people in the mode of ignorance.

Purport: The purpose of food is to increase the duration of life, purify the mind and aid bodily strength. This is its only purpose. In the past, great authorities selected those foods that best aid health and increase life's duration, such as milk products, sugar, rice, wheat, fruits and vegetables. These foods are very dear to those in the mode of goodness. Some other foods, such as baked corn and molasses, while not very palatable in themselves, can be made pleasant when mixed with milk or other foods. They are then in the mode of goodness. All these foods are pure by nature. They are quite distinct from untouchable things like meat and liquor. Fatty foods, as mentioned in the eighth verse, have no connection with animal fat obtained by slaughter. Animal fat is available in the form of milk, which is the most wonderful of all foods. Milk, butter, cheese and similar products give animal fat in a form which rules out any need for the killing of innocent creatures. It is only through brute mentality that this killing goes on. The civilized method of obtaining needed fat is by milk. Slaughter is the way of subhumans. Protein is amply available through split peas, dal, whole wheat, etc.

Foods in the mode of passion, which are bitter, too salty, or too hot or overly mixed with red pepper, cause misery by producing mucus in the stomach, leading to disease. Foods in the mode of ignorance or darkness are essentially those that are not fresh. Any food cooked more than three hours before it is eaten (except prasadam, food offered to the Lord) is considered to be in the mode of darkness. Because they are decomposing, such foods give a bad odor, which often attracts people in this mode but repulses those in the mode of goodness.

Bhagavad-gita as it is 17:8-10 Text and Purport