I do not want to concentrate too much on the scientific aspects of vegetarian food, but the spiritual ones as has been in the life of man since Adam.
It is true that principal amino acids abound more in animal than in vegetable protein. However vegetable protein has sustained and kept healthy monks, vegetarians and those mentioned above.
We should also not forget that the Church allows fish in some fasts, which of course, contains animal protein. Moreover, there are long non-fast periods. Therefore, do not be afraid of fasting, for it is of benefit to the body.