Just substitute "plant-based diet" for the word "vegan".
St. Benedict's Rule #39:St. Thomas Aquinas also viewed meat as a food for sick/weak people.
Before original sin, God only gave Adam & Eve fruits and vegetables for food, saying that this food is also for the animals:
Flesh meat, eggs, and milk foods are "are a very great incentive to lust" (Summa Theologica II-II q. 147 a. 8 co.), which is one reason why the Church forbids eating them certain times throughout the year.
When I was taking a college class, we had to study the writings of Christopher Columbus. He mentioned the Black Fast in his diary. Extra provisions had to be taken on the ship to allow for this Black Fast, which was the Great Lenten Fast, during which eggs, milk products, butter, olive oil, lard, fish, and meat were not allowed. After Vatican I, the church lessened the Black Fast as it was considered too harsh for the modern working man and woman.
However, many monasteries still observed the Black Fast for a few more years. Even today, cloistered nuns abstain from meat, but allow cheese, milk, and eggs.