Theravadin Buddhism does not teach about what to eat and what not to eat in this way. The monks and nuns have a training rule that they eat what is given, unless they are sick, unless there is some physical problem. It is quite ok to request certain foods for their diet if they have a physical problem. Otherwise whatever is offered to them, they accept it and eat it knowing that it is basically to keep the body alive and healthy, amongst other things.
Now when you think about why people don't eat meat, in particular, some people's views on the matter relate to not wanting to kill, which is a Buddhist precept, a Buddhist encouragement to not kill other living beings. The Buddha did stress not to kill. The question comes, "Why didn't he say not to eat meat because that is a killed animal?" When you ask that question and you look deeper at how I teach food reflection, you see that plowing the fields to grow rice causes living beings to be being killed.
I have been a farmer in a past life. We had a machine, a big rotary hoe (we weren't rich enough to have a big tractor so we had this big machine). I would walk behind it as it dug up the soil. So I was real close to watching what was going on. One day I was going along, digging up the soil in order to plant strawberries. I didn't want to harm anybody, I just wanted to grow strawberries. I was walking behind this machine when all of the sudden a frog jumps away, and the frog had three legs. If you cut off one of a frog's legs, you will see a frog with three legs. And the blood was squirting out the back place where the leg used to be, The frog was not going to live. I just wanted to grow strawberries, but I knew what I was also doing was killing. So, when you think about whether or not to eat meat and whatever, if the reason is just not to be involved in killing, then let's look deeper at what it means to be part of the process of killing.
Want a new coat? How many beings die for the coat? Want a piece of paper? Ever watch a tree come down? In order to clear land for farming you usually need to cut down trees. When a tree falls it hits the ground pretty hard, so any beings that are on the ground under there, they are dead instantly. And any beings that live in the bark of the tree on that side of the tree, they are dead instantly. And a lot of beings that live in the bark elsewhere die quickly. There is a huge amount of animals living in the tree, animals, insects, whatever, a huge amount of living beings living in every tree. When you chop them down and you just want to get paper, beings are dying. Or clearing land to grow cotton to make clothing, many beings are dying. Or mining it to get metal or oil for machines or heating, many beings are dying.
So, when we think about what the Buddha is actually teaching us on this level, can we limit how much we are part of the process which creates deaths for others? Can we actually limit it, can we reduce it, can we reduce our role in this chain of killing? Do we really need 42 pairs of shoes? How many did the Imelda Marcos have in the Philippines 20 years ago? Was it over a 1,000? I don't know. Can we limit our material acquisitions, our material possessions so we are not so much part of the killing? Can we limit how many newspapers we buy, how many magazines we buy? Be more simple in our desires and lifestyle. All sorts of things. Can we utilize this thought of wanting to create less harm on the planet, and in specific, actually reduce our excessive desires for and consumption of material things?
Of course, that is the Buddhist practice of renunciation arising from compassionate understanding. By definition, a fully enlightened person doesn't want any more. Doesn't actually want the paper, doesn't actually want the food. So, they are actually not involved in the process of killing anymore because their thoughts are not wanting anything. So, the more we reduce thoughts of wanting then the more we are going to not be part of this process.