At least once a week, on an intake call with a new client, the topic comes up. Sometimes they've tried it. Sometimes they're doing it right now. Sometimes they just want to know what I think about it. Intermittent fasting isn't the trend it was a few years ago, but it hasn't gone anywhere either. A lot of people still believe it's the magic bullet behind a healthy life, and I think it's worth talking about, especially for those of us trying to live this out as Catholics.
So let me say clearly up front: I'm not opposed to intermittent fasting in principle. I've had clients use it to lose or maintain weight with great success. It's especially helpful for the person who needs a black and white rule for when they can and can't eat, because they don't want to track macros, don't have the mental bandwidth to track macros, or find that even when they do track, they still end up overeating. A clear window solves this problem for a lot of people.
But here's what I want to stress, and it's the whole reason I'm writing this: intermittent fasting is not the same thing as the fasting we practice as Catholics, or the fasting Jesus assumes his disciples will do when he teaches on it in the Sermon on the Mount. "When you fast," he says, not if (Matthew 6:16). He takes it for granted that his followers will fast. But the fasting he has in mind has almost nothing in common with an eight hour eating window.
What fasting has always meant to the Church
Go back to the Old Testament and you'll find fasting used first and foremost as an act of penance. Think of Jonah and the king of Nineveh, who fasted and dressed in sackcloth and ashes the moment Jonah told him to repent. He likely ate and drank nothing at all. Or think of David, fasting after the prophet Nathan confronted him over his sin against Bathsheba and then finding out that his child was sick. In both cases, fasting was the body's way of saying what the heart meant: I have sinned, and I am sorry.
Over time, the Church also came to see fasting as a way of making reparation for sins committed against the Sacred Heart of Jesus, and as a tool for detachment, denying ourselves in small ways now so we become less enslaved to the things of this world and more open to the Holy Spirit. The Desert Fathers built their whole way of life around this second point. As one of them put it, "the ultimate goal of fasting is to help lead us, or to open us, to the fear of God." Every meal skipped, every comfort denied, was aimed at loosening the world's grip on the soul.
The Catechism gathers all of this up simply: "Scripture and the Fathers insist above all on three forms, fasting, prayer, and almsgiving, which express conversion in relation to oneself, to God, and to others" (CCC 1434). Fasting, in other words, is never just about the body. It is about turning the whole self back toward God.
That's why the Church still marks out specific times for it. "The seasons and days of penance in the course of the liturgical year (Lent, and each Friday in memory of the death of the Lord) are intense moments of the Church's penitential practice," intended for "voluntary self-denial such as fasting and almsgiving" (CCC 1438). And the fourth precept of the Church exists precisely to protect this: to "observe the days of fasting and abstinence established by the Church," which "ensures the times of ascesis and penance which prepare us for the liturgical feasts and help us acquire mastery over our instincts and freedom of heart" (CCC 2043). Notice what that mastery is for. Not a smaller waistline. A freer heart.
St. John Chrysostom pushed this even further, warning against a fast that's only physical. "What good is it," he asked, "if we abstain from birds and fish, and yet bite and devour our brothers?" A true fast, he taught, isn't just the mouth abstaining from food. It's the eye fasting from envy, the tongue fasting from gossip, the feet fasting from the paths that lead to sin. St. Francis de Sales made a nearly identical point centuries later in Introduction to the Devout Life, describing the man who "sets great value on fasting, and believes himself to be leading a very devout life, so long as he fasts rigorously, although the while his heart is full of bitterness." Rigor without charity, both saints agree, isn't holiness. It's just hunger.
So the fasting we practice as Catholics has nothing to do with weight loss or health management and everything to do with our interior disposition: growing more like Jesus, making reparation for sin, and mortifying disordered desires so grace has more room to work.
What intermittent fasting actually is
Intermittent fasting is a secular approach built around a window of time in which you're allowed to eat. The most popular is 16:8, eight hours of eating followed by sixteen hours of water only. More extreme versions push twelve to twenty-four hours without food, usually for medical reasons or fringe cases. Most people I coach who practice it land somewhere in a five to eight hour eating window.
If you've enjoyed fasting like this in the past, or you're doing it right now, this isn't me telling you to stop. If it works, it works. But I do want to flag a few pitfalls so you can be sure you're actually optimizing what you're doing.
The first is protein. Most people need at least 0.7 grams of protein per kilogram of body weight, and you can absolutely hit that inside a shorter window, but it takes more intention with how much protein you're getting at each meal.
The second is fiber. Most people need roughly 15 grams of fiber per 1,000 calories, which works out to about 20 to 30 grams a day for most adults. Again, doable in a shorter window, but you'll be pairing a lot of fiber with a lot of protein in a short amount of time, and you're going to feel it.
The third pitfall is the one I actually worry about most, and it's not nutritional. It's the guilt. I've watched people treat eating outside their window like a moral failure, as though breaking the fast on vacation or at a birthday dinner were some kind of sin. Research on dietary restraint backs up what I see in coaching: rigid, all-or-nothing food rules are the ones most tightly linked to guilt after eating and to eventual binge episodes, more so than flexible approaches to eating. When a rule about food starts functioning like a moral law, something has gone sideways, and that's worth naming before it becomes a bigger problem than the diet itself was ever meant to fix.
It's also worth being honest about what the science actually shows on the metabolic side, since this is where a lot of the "magic bullet" language comes from. A large meta-analysis comparing alternate day fasting, the 5:2 diet, and time-restricted eating found that intermittent fasting regimens produced weight loss essentially comparable to plain old calorie restriction. Another review of time-restricted feeding trials came to the same conclusion: nine of eleven studies found no real difference in weight or fat loss between intermittent fasting and simply eating less throughout the day. There is no secret metabolic switch. There's no window that unlocks something calorie counting can't. What these approaches do offer, for the right person, is a simpler set of rules to follow, and for some people that structure is worth a great deal. But it's structure, not magic.
Bringing it together
As Catholics, we're bound to fast and abstain on Ash Wednesday and Good Friday, and to abstain every Friday during Lent. That's the floor, not the ceiling. I'd encourage you to find ways of fasting throughout the year, especially when you're wrestling with a particular sin, or fasting on behalf of someone else who is. That kind of fasting has a purpose no eating window can touch.
So here's how I'd land this. If intermittent fasting helps you manage your weight and your relationship with food, use it, with your eyes open to the protein, the fiber, and the guilt trap above. But don't let it wear the costume of Catholic fasting, and don't ask it to do a job it was never built for. One is a tool for your body. The other is a discipline for your soul. Know which one you're reaching for, and reach for both, on purpose, for the right reasons.
Laudem Gloriae