There is a sutra titled the "Twelve Dhuta Sutra." It involves release from ties to clothing, food, and dwelling. Here I translate five things about diet below:
1. When a Bhikshu comes to a town for begging, he should control his six sense organs: eye, ear, nose, tongue, body, and mind. He must not let them attach themselves to the six dusts: sight, sound, smell, taste, touch, and idea. He must not distinguish women from men nor gain from no-gain. His mind should be kept in equilibrium. Whether the food is good or bad, he should not be glad or weary. He should not ask for more or less. When he does not receive, he should think, "Even our Lord Gautama Buddha who left His palace and became a Bhikshu and came to the town for begging, sometimes got nothing, much the less, I'm so lacking in merits."
This is the Dharma of begging.
2. A Bhikshu should not be attached to the taste. Nor should he disregard any sentient being. He must have equal mind to pity every sentient being and should not choose only those rich persons. This is the importance of the method of begging door to door without discrimination.
3. A Bhikshu should have such a good thought: "Now, only for one meal shall I beg which still wastes my meditative time. Were I to beg again for snacks, for lunch, and for dinner, I would lose a lot of time! If I don't reduce my eating, I might lose half of my day to begging for food. I would not have enough time to meditate. I beg only to practice the Buddha Dharma--not for my body or life as pigs, dogs, and horses. I must stop all other eating and take only breakfast." It is the rule to beg for only one meal.4. When one gets a meal, he should give one-third to a poor person and wish that he may never be greedy again. Then he should take the food to a silent place and put a little bit of it on a clean stone and leave it for birds or animals. He also should have good thoughts about that poor person eating one-third of the food and leave the rest for the bird or animal who will be satisfied.
Before eating, he should wash his hands. He should think that in his body there are eight myriad insects, "When I attain some realization of the Dharma, I shall again give alms of Dharma to them and they shall also partake of my attainment."
Suppose after this begging he meets no poor person, he should still keep one-third aside. In this way, he will have good health, make his body light, and food will digest easily. His meditation will be very good. It is the rule to restrain eating.
5. After one meal, a Bhikshu should not go searching for drinks, if he does, he will pursue a variety of drinks. He will not diligently practice the Buddha Dharma. It is like the horse who does not hold the bridle and seeks grass on two sides. He will not run the way of the goal. This is the reason for the rule to keep away any kind of drink after noon.