In Matthew's message on the Golden Age he says the following about diet.
Diets will change from meat and seafood to plants as people learn to respect and honor all animal life. The herds of food animals will decrease through the cessation of breeding and natural transition, and as plants become diet staples, any that were harmfully genetically engineered will shed those properties.
Animals in the wild will instinctively know not to overpopulate and those that are carnivorous will turn to the plant kingdom for sustenance.
So while a lot of people would like to become vegetarian to "respect and honor all animal life" as Matthew says, many find it quite difficult to do in practice. In my experience having a daily spiritual practice is key, for then as your vibration rises your desire and need for meat and seafood is gradually eliminated. You will actually find it even difficult to eat meat/seafood no matter how much you may have craved and savored them previously.
The Founders in their book "Earth Changes and 2012: Messages From the Founders", provide a more detailed view of the dietary changes one should pursue/expect as we enter the Golden Age.
For those of you that are committed to remaining with the Earth by entering fourth or fifth density states, it is essential that you reduce your dependence on the things of this world. You can start by simplifying your lives so that you require less resources in order to keep your lives going. As you raise your vibration, you will naturally need to consume less and less of Earth's resources anyway. You might start by reducing your intake of products from mammals (beef, pork, dairy, etc.) since the production of these foods consumes a great deal of natural resources. As you continue to raise your vibration, you will likely reduce your intake of all animal products, becoming what is termed "vegan." Eventually, you will find that only a tiny amount of highly nutritious food will be necessary to keep your body healthy. At some point in your ascension process you will eliminate food altogether and drink only water. Then the water will also become unnecessary and you will become what some call a "breatharian." Finally, as your light body becomes fully activated, you will exist on direct etheric energy and even oxygen will no longer be necessary.
There are many stories of breatharians that can be found, but I'll once again quote "Autobiography of a Yogi". The following is a conversation between Paramahansa Yogananda and Giri Bala.
Chapter 46: The Woman Yogi who Never Eats (Giri Bala)
"Tell me, Mother, from your own lips-do you live without food?"
"That is true." She was silent for a few moments; her next remark showed that she had been struggling with mental arithmetic. "From the age of twelve years four months down to my present age of sixty-eight--a period of over fifty-six years--I have not eaten food or taken liquids."
"Are you never tempted to eat?"
"If I felt a craving for food, I would have to eat." Simply yet regally she stated this axiomatic truth, one known too well by a world revolving around three meals a day!
"But you do eat something!" My tone held a note of remonstrance.
"Of course!" She smiled in swift understanding.
"Your nourishment derives from the finer energies of the air and sunlight, and from the cosmic power which recharges your body through the medulla oblongata."
"Baba knows." Again she acquiesced, her manner soothing and unemphatic.
