When I attended Novus Ordo church, I would always hear a sermon during or preceding Lent about how fasting doesn't primarily mean fasting from food and drink. This is the natural consequence of Paul VI abolishing fasting from meat on Fridays.

Yet St. Thomas Aquinas answers the question "Whether it is fitting that those who fast should be bidden to abstain from flesh meat, eggs, and milk foods?," saying:

As stated above (Article [6]), fasting was instituted by the Church in order to bridle the concupiscences of the flesh, which regard pleasures of touch in connection with food and sex. Wherefore the Church forbade those who fast to partake of those foods which both afford most pleasure to the palate, and besides are a very great incentive to lust. Such are the flesh of animals that take their rest on the earth, and of those that breathe the air and their products, such as milk from those that walk on the earth, and eggs from birds. For, since such like animals are more like man in body, they afford greater pleasure as food, and greater nourishment to the human body, so that from their consumption there results a greater surplus available for seminal matter, which when abundant becomes a great incentive to lust. Hence the Church has bidden those who fast to abstain especially from these foods.

St. Thomas More, in his Utopia, describes the relationship between food and sex when he contrasts the married and priestly states of his fictional, ideal society:

Of these there are two sorts: some live unmarried and chaste, and abstain from eating any sort of flesh; and thus weaning themselves from all the pleasures of the present life, which they account hurtful, they pursue, even by the hardest and painfullest methods possible, that blessedness which they hope for hereafter; and the nearer they approach to it, they are the more cheerful and earnest in their endeavours after it.  Another sort of them is less willing to put themselves to much toil, and therefore prefer a married state to a single one; and as they do not deny themselves the pleasure of it, so they think the begetting of children is a debt which they owe to human nature, and to their country; nor do they avoid any pleasure that  does not hinder labour; and therefore eat flesh so much the more willingly, as they find that by this means they are the more able to work: the Utopians look upon these as the wiser sect, but they esteem the others as the most holy.  They would indeed laugh at any man who, from the principles of reason, would prefer an unmarried state to a married, or a life of labour to an easy life: but they reverence and admire such as do it from the motives of religion.  There is nothing in which they are more cautious than in giving their opinion positively concerning any sort of religion.  The men that lead those severe lives are called in the language of their country Brutheskas, which answers to those we call Religious Orders.

What do you think of St. Thomas Aquinas's and St. Thomas More's conception of marriage, food, fasting, etc.? I have a feeling most moderns would say they both have a very outdated conception of biology and nutrition, yet, from personal experience, I know what they write is true. Fasting does indeed and very effectively "bridle the concupiscences of the flesh."