10 In the first place then, till you have passed the years of early womanhood, take only water to drink, for this is by nature of all drinks the most cooling. This, if your stomach is strong enough to bear it; but if your digestion is weak, hear what the apostle says to Timothy: “use a little wine for your stomach's sake and your frequent infirmities.” Then as regards your food you must avoid all heating dishes. I do not speak of flesh dishes only (although of these the chosen vessel declares his mind thus: “it is good neither to eat flesh nor to drink wine”) but of vegetables as well.
Everything provocative or indigestible is to be refused. Be assured that nothing is so good for young Christians as the eating of herbs. Accordingly in another place he says: “another who is weak eats herbs.” Thus the heat of the body must be tempered with cold food. Daniel and the three children lived on pulse. They were still boys and had not come yet to that frying-pan on which the King of Babylon fried the elders who were judges. Moreover, by an express privilege of God's own giving their bodily condition was improved by their regimen.
We do not expect that it will be so with us, but we look for increased vigour of soul which becomes stronger as the flesh grows weaker. Some persons who aspire to the life of chastity fall midway in their journey from supposing that they need only abstain from flesh. They load their stomachs with vegetables which are only harmless when taken sparingly and in moderation. If I am to say what I think, there is nothing which so much heats the body and inflames the passions as undigested food and breathing broken with hiccoughs.
As for you, my daughter, I would rather wound your modesty than endanger my case by understatement. Regard everything as poison which bears within it the seeds of sensual pleasure. A meagre diet which leaves the appetite always unsatisfied is to be preferred to fasts three days long. It is much better to take a little every day than some days to abstain wholly and on others to surfeit oneself. That rain is best which falls slowly to the ground. Showers that come down suddenly and with violence wash away the soil.
Source: Letters (New Advent)