Fattens the body and brain, I believe the argument is. Just like how meats are to lead to temptations of the flesh.
Over eating in general makes one tend to be lazy and tired and a prime target for temptation which lowers ones's ability to fight off the near occassions of sinning.
Eastern Orthodox fast approximately 176 days out of the year depending upon when Pascha falls. Sometimes its a little less. But you can see that fasting is a very large part of our spiritual life and the more one comes closer to the strickness of fasting the more one can better fight off sin.
Fasting is not the same for everyone although everyone should be fasting. Those sick, infirmed, or on medications are the exception. Those who have strenuous occupations cannot keep all the fast all the time. But we all must strive to do the best fast we can in order to be more Christlike because Christ Himself fasted for 40 days prior to his Passion. We have four (4) fasting periods in the year.
1.Nativity Fast which we are on now 40 days
2.Dormition Fast 14 days.
3.Cheesefare Sunday, Abstaining from dairy products 7 days prior to Meatfare
Meatfare Sunday, Start of Great and Holy Lent ( All days in Lent are Fast days)
Great and Holy Lent Fast which last 40 days(Saturdays and Sundays included)
(sometimes fish and oil are allowed on weekends. Yes, I said fish, which is considered meat because it has backbone and blood) However, shrimp, clams, mussels, scallops and LOBSTER are allowed but you need to keep the spirit of Lent as well as the letter.
Holy Week (Strick fast) 7 days (All days of this week up until receiving Paschal Liturgy Eucharist) = 54 days
4.Sts. Peter and Paul Fast 26 days this year ( but varies according to when Pascha date falls)
ALL Wednesdays and Fridays throughout the year. (I believe we get three (3) free weeks a year where meat can be eaten on Wednesdays and Fridays and these usually preceed or follow major fasting periods such as Great Lent.
You can see it can become pretty intense.