I used to do strict fast for lent when I was younger. I identify as an orthodox Christian (even though I don't really practice anymore...still don't know how I feel about things...even though I "don't know how I feel about things" going on two years now lol).
Strict fast for us meant abstaining from meat, dairy, eggs, fish, olive oil, and alcohol. On certain "feast days" you could have fish and/or olive oil and beer/wine. It took quite a bit of discipline to do this. Many people made vegan desserts, but that's kind of cheating. You could also eat shellfish, just not fish. Of course, if a friend invites you to dinner, you don't say, "oh I'm fasting, I can't eat what you made," you eat it out of politeness, and those who are old or cant do the fast don't need to.
The restricted foods were thought to be indulgences, and the purpose of eliminating them from your diet was to keep yourself from focusing on pleasures like food. This way your mind was clear and you could focus on prayer.
I can't help but think there was some farming-related reasoning behind abstaining from some of these foods, too.
