Her vita reports that when she was forced to eat enough for nourishment, her body was weakened, but when she fasted, she was more healthy, upending the normal responses to depriving oneself of food.

on Syncletica of Alexandria

And I think that one should sparingly eat all kinds of food which cause heat to the limbs, for flesh drags down even eagles as they fly. But within you let that bird of which we read, Your youth shall be renewed like the eagle’s, holding its course on high, swift in its virgin flight, be ignorant of the desire for unnecessary food.

—St. Ambrose, Concerning Virginity Book III

Once some monks of Scetis came to visit Abbess Sarah. As she offered them a small basket of fruit, they ate the bad fruit and left the good. So she said to them, “You are true monks of Scetis.”

EARLY MONASTICISM AMONG WOMEN IN THE COPTIC CHURCH
Father Tadros Y. Malaty

Coptic Church Review
Vol. 1 Num. 4

Many, however, slide into the very opposite kind of excess, and unconsciously to themselves, in their over-preciseness, laboriously thwart their own design; they let their soul fall down the other side from the heights of Divine elevation to the level of dull thoughts and occupations, where their minds are so bent upon regulations which merely affect the body, that they can no longer walk in their heavenly freedom and gaze above; their only inclination is to this tormenting and afflicting of the flesh. It would be well, then, to give this also careful thought, so as to be equally on our guard against either over-amount, neither stifling the mind beneath the wound of the flesh, nor, on the other hand, by gratuitously inflicted weakenings sapping and lowering the powers, so that it can have no thought but of the body’s pain; and let everyone remember that wise precept, which warns us from turning to the right hand or to the left.

—Saint Gregory of Nyssa, On Virginity

To prevent this, then, we want to apply to our own lives that rule of all temperance, never to let the mind dwell on anything wherein pleasure’s bait is hid; but above all to be specially watchful against the pleasure of taste. For that seems in a way the most deeply rooted, and to be the mother as it were of all forbidden enjoyment. The pleasures of eating and drinking, leading to boundless excess, inflict upon the body the doom of the most dreadful sufferings; for over-indulgence is the parent of most of the painful diseases. To secure for the body a continuous tranquility, unstirred by the pains of surfeit, we must make up our minds to a more sparing regimen, and constitute the need of it on each occasion not the pleasure of it, as the measure and limit of our indulgence. If the sweetness will nevertheless mingle itself with the satisfaction of the need (for hunger knows how to sweeten everything, and by the vehemence of appetite she gives the zest of pleasure to every discoverable supply of the need), we must not because of the resulting enjoyment reject the satisfaction, nor yet make this latter our leading aim. In everything we must select the expedient quantity, and leave untouched what merely feasts the senses.

—Saint Gregory of Nyssa, On Virginity

Their food consisted of only a small amount of bread and salt that they ate after sunset.

Saint John Chrysostom, On the Vanity of Riches