I should add, that with Orthodox fasting, while weakening the body, we strengthen (or try to) the soul through more time in prayer and greater reflection of our soul. Fasting can give great spiritual strength, which is what they would have needed. Whether they thought they were going to die or not, not knowing what would happen to them or their parents may have encouraged them to be more introspective, and delve deeper into prayer. But of course, I don't know - I wasn't there, it mightn't even be the sort of thing they would write in diaries. But fasting along with the stress and anxiety caused by the situation they were in could account for the girls looking like skin and bones.