3 Reasons We Fast, From St Thomas Aquinas

Well, since Ash Wednesday is tomorrow I thought it might be a good time to talk about fasting—and not the trendy intermittent fasting stuff you see on podcasts and health blogs (as good as that might be). I mean good, old-fashioned fasting. The kind the Church has been encouraging for the last two thousand years.

What’s the point of fasting and how does it help us?

St Thomas Aquinas took this up in his Summa Theologiae, where he asks a very straightforward question: Is fasting a virtuous act? In other words, does fasting actually make you more virtuous—more the kind of person God wants you to be?

Spoiler: He says yes. And then, in classic Aquinas fashion, he gives us not one, not two, but three solid reasons why fasting is good for you.

Man, imagine if St Thomas was alive today. That man would love himself a good spreadsheet.

Photo by Michael Vasilkovsky on Unsplash

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1. Fasting Tames Our Desires

First, fasting helps us get a handle on our desires. Helps subordinate our lower passions to the rule of reason.

Here’s how he puts it:

"Fasting is practiced... in order to bridle the lusts of the flesh, wherefore the Apostle says: 'In fasting, in chastity,' since fasting is the guardian of chastity." (ST II-II, Q. 147, A. 1)

The logic is simple—when we deny ourself food and drink (or social media, video games, etc. etc.) we’re training ourselves to say no to disordered desires. I like the way my friend Christopher West puts it, “if you can’t say no to a chocolate chip cookie, how will you say no to the temptation to look at pornography.” St Thomas quotes St Jerome to drive the point home, but it can be confusing to modern readers. Here’s the quote and then I’ll break it down.

“Venus is cold when Ceres and Bacchus are not there.”

Translation: Lust (that’s Venus) cools off when we’re not stuffing ourselves with food (Ceres) and drink (Bacchus). There’s something about moderating our physical appetites that helps all our other appetites fall in line.

2. Fasting Clears the Mind for Prayer

The second reason St Thomas gives is that fasting frees up the mind for spiritual things.

He writes:

"We have recourse to fasting in order that the mind may arise more freely to the contemplation of heavenly things." (ST II-II, Q. 147, A. 1)

There’s something about an empty stomach that sharpens the mind and makes it easier to turn our attention to God. St Thomas points to the prophet Daniel, who fasted for three weeks before receiving a vision from God. It’s as if fasting clears out the clutter, making space for deeper prayer, clearer thought, and a more receptive heart.

Actually, the more I’ve reflected on this, the more I’m convinced that part of the benefit comes from stepping off the roller coaster of sugar spikes, crashes, and all the chaos that comes with junk food. When we fast from that kind of food—or cut it out entirely—we’re not just improving our physical health, we’re also creating a more stable, calm interior life. Our bodies aren’t constantly lurching from one extreme to the next. At least, that’s been my experience.

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3. Fasting Cleanses the Soul

The third reason St Thomas gives has to do with the penitential side of fasting—fasting is a way to express sorrow for sin and do something about it.

He quotes the prophet Joel:

“Be converted to Me with all your heart, in fasting and in weeping and in mourning.” (Joel 2:12)

And he follows up with St Augustine, who says:

“Fasting cleanses the soul, raises the mind, subjects one’s flesh to the spirit, renders the heart contrite and humble, scatters the clouds of concupiscence, quenches the fire of lust, and kindles the true light of chastity.”

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Fasting Isn’t About Starving Yourself

One final thing. St Thomas is clear that fasting should be guided by reason. Going too far—to the point of ruining your health or making yourself incapable of fulfilling your duties—isn’t virtuous, it’s foolish.

He quotes St Jerome again:

“It matters not whether thou art a long or a short time in destroying thyself, since to afflict the body immoderately, whether by excessive lack of nourishment, or by eating or sleeping too little, is to offer a sacrifice of stolen goods.”

In other words, your body and your health are gifts from God, and you don’t honor Him by wrecking them. St Thomas insists that true fasting always respects the right balance, where your body gets enough to keep going—you’re aiming for self-control, not self-destruction.