As far as I know nothing at all is stated in Theravadin Buddhism regarding vegetarianism. To be thankful that you have food, to be thankful it keeps you healthy and strong and so on. Using similar food reflections to what we teach here, to be content, to be more peaceful. But what's your reason for eating? Hopefully to get on with your spiritual growth as a human being, to be a nicer, kinder human being, to make good Kamma and so on.
Many people have views and opinions about being a vegetarian, about eating meat or fish, etc. Sometimes their views and opinions are based on the fact that they don't like to see meat and fish because they don't like to see an animal that has been killed. Therefore they want to be a vegetarian so they feel they're not involved in the process of killing, which is totally untrue unless it happens they live on coconuts, etc. that fall off the tree, which is impossible to live on for very long.
The minute you plow the fields it's a process of killing. I've been a farmer, I've plowed up the fields using a massive rugged, powered engine. I walked behind this machine. It ripped up the ground, I go forward, I go back, ripping it up, raking it, smoothing it, planting my seeds, and so on. One day I'm walking behind the machine and a three legged frog jumps away. Now the only reason it had three legs was because I just chopped one off. Blood is squirting out the hole where it's leg used to be. It's going to die soon. Behind me or to the side, depending on where I was, on a post would have been a Kookaburra, an Australian bird with a very big head. Kookaburras love worms. The minute I dug up the soil and a worm came out a Kookaburra would go right in and swallow it down, and these are big worms, not little guys.
Killing, death, you want vegetables? You're a part of killing. You want rice? You're a part of killing. You want a new shirt? You're a part of killing. You want to make some paper, you're a part of killing. So Theravadin Buddhism wants us to look deeper at this, and doesn't get involved with the issue of vegetarianism. Eat to keep your body healthy and strong so you can continue on with your spiritual development.