The Holy Fathers and Canons of the Church clearly say that fasting has to be done according to your own strenght, and personal circumstances.
You made me laugh a lot with your comment about cave men. My blood type is 0 negative, and according to some researches, my racial group has the purest genome in the world, and our languages, most of them extinct, can be traced as far as the cave men period.
All that of cave men, and the evolutionist tales are not true. The truth is we all come from Adam and Eve, from the tribes, and according to oral tradition, I am part of the lost tribe.
It's foolish to think you can't fast because of your blood type, I, who according to the tale of evolution am the closest to a cave man, with blood type O- have no problem at all fasting.
The Holy (Church) Fathers were not ignorant, the Holy Spirit guided them, and their teachings are based on the Truth, and not on human fallacies, and vain philosophy.
Be not corrupted by vain philosophy of men, don't believe in the heresies of psychiatry, psychology, evolution, and other pernicious heresies tailored by men, based on assumptions, and not the Truth.
If you can, fast, if you can't, don't fast, what's important is during fasting period, you redouble your efforts to repent, and come closer to God, fasting from pride, evil thoughts, anger, schadenfreude, hatred, loath, lewdness, sloth, and everything evil, is the core of fasting.
Fasting from food is just an aid, an external help, which should be done according to our own measure.
A friend who knew Saint John of Shanghai told me that once, the then Archbishop John, told a young man to have some meat for lunch. Amazed, my friend told him "Vladiko, Vladiko, it's Great Lent", Saint John looked at him, and said "This man needs to learn how to fast from his pride, and a little meat will teach him humility. God will not scrutinize us and see how much meat, or eggs, or milk we had during fasting, God will see our repentance in the treasury of our hearts".