The average ancient Egyptian's diet consisted almost entirely of grain.
This was sustaining, but only barely. Compared to contemporary societies in every direction, they lived shorter lives, had smaller stature, and poorer health. The coarse millstones left grit in the flour that wrecked their teeth over the few decades of their lives.
It was, of course, all they had.
My college football soul is being nourished entirely by the grain of FSU's sucking and it is sustaining, but only barely.