Which should you practice: Fasting or calorie restriction?
I’m not a big fan of this question. It’s like asking whether you should avoid carbs or sugar. There’s a lot of overlap.
When you think about it, fasting (of any kind) entails calorie restriction. You aren’t eating any food for a set period of time. That counts.
More nuance is required when talking about intermittent fasting, which is more about restricting when you eat than how much you eat. Still, you almost ALWAYS see a drop in caloric intake with a compressed feeding window. Less time to eat means less snacks go down your gaping maw.
This intermittent calorie restriction is often beneficial. It helps you burn body fat and enter ketosis. It increases your metabolic flexibility. It can promote weight loss, if that’s your goal.
But there’s a fine line between mild, intermittent calorie restriction and chronic calorie restriction. You’ll recognize chronic calorie restriction by its other name: Starvation.
When starvation mode gets activated for long enough, people start to look like Christian Bale in The Machinist. But even before all the body fat and muscle disappears, there can be long-term metabolic consequences.
Today I’ll peel intermittent fasting and calorie restriction apart. You’ll learn where they overlap, where they don’t, and what the science says.
What Is Intermittent Fasting?
When you practice intermittent fasting (IF), you take regular breaks from calories. That’s it.
These breaks can last anywhere from 12 to 36 hours, depending who you talk to. Fasts that exceed 36 hours are generally considered “extended fasts”.
An intermittent fast needn’t be a zero calorie fast. Some protocols—like alternate day fasting—allow up to 25% calories on “fasting” days. Alternate day fasting, as the name suggests, entails fasting every other day.
Other protocols—like 16/8 and OMAD—are practiced on a daily basis. With these daily fasting methods, you simply eat all your calories within a set time window. (Note: Here’s our deep dive into fasting methods).
I’m a fan of daily fasting. I eat breakfast around 8, a late lunch around 3, then I’m done for the day. It’s more or less 16/8. I find this protocol works best for my sleep, energy levels, appetite, and weight maintenance.
Probably the biggest benefit of IF is metabolic flexibility. When you take a break from calories, the hormone insulin stays low. (Insulin is normally released in response to a meal). And keeping insulin low trains your liver to burn fat and produce ketones.
What Is Calorie Restriction?
Calorie restriction (CR) is an umbrella term. Let’s break it into two chunks:
- Mild calorie restriction
- Chronic calorie restriction
If you have body fat to lose, a mild calorie deficit of 5-20% per day can be a sustainable way to lose it. This mild energy deficit is generally the kind promoted by IF.
Chronic calorie restriction, on the other hand, refers to a chronically insufficient diet—usually 20-50% fewer calories per day than your metabolism demands. This is starvation.
If you’ve ever watched History Channel’s hit show Alone, you’re witnessing starvation up close. In the show, contestants try to outlast one another in brutal environments (like the arctic), and never get enough to eat.
Everybody loses weight on Alone, much of it from muscle. No surprise there.
If you haven’t seen Alone, you might have watched The Biggest Loser—a show in which severe calorie restriction spurs rapid and significant weight loss in overweight participants. And it works, at least while the cameras are rolling.
But when the contestants return home and resume normal eating patterns, they invariably regain the lost weight. Why? Because their metabolisms have permanently slowed down, even beyond what would be predicted by weight loss. In other words, they burn fewer calories at rest. Not ideal for weight loss maintenance.
IF vs. CR: Similarities
Recall from earlier that intermittent fasting is a form of calorie restriction. When you compress your feeding window, you almost always consume fewer calories.
So there’s overlap there. Here’s how that overlap can be beneficial.
#1: Weight loss
If you want to lose weight, it makes sense to maintain a mild caloric deficit. You should consume slightly fewer calories than you expend.
You might be thinking: Wait, a calorie isn’t a calorie, right? That’s true. It’s why a ketogenic diet works better for weight loss and appetite management than the modern, sugary, high-carb diet.
The problem with the modern diet? It’s hyperpalatable. Once you start, you can’t stop. Good luck with that mild caloric deficit.
But whatever your baseline diet, intermittent fasting is a solid strategy for cutting back on calories. Eliminating that midnight snack makes a big difference. I suspect this is why IF has shown promise for weight loss.
When you fast or calorie restrict, your insulin levels stay low. This, in turn, activates your backup energy system: Ketosis.
Ketosis is the body’s way of surviving in times of carbohydrate scarcity. Instead of relying primarily on glucose for energy, the burden shifts to fatty acids and ketones.
Despite what many confused practitioners believe, ketosis is a safe and normal physiologic state. Humans have cycled in and out of ketosis for thousands of years. It’s helped our species stay lean, metabolically healthy, and—most importantly—alive.
#3: Autophagy and apoptosis
As the years slide by, wear and tear accumulates within our cells. Sometimes, damaged cells self-destruct. Sometimes they’re recycled and repaired. But other times, these cells become senescent, increasing their potential to turn cancerous.
Both fasting and CR activate programs that prevent and destroy senescent cells.
- Autophagy—cellular clean up and recycling (to prevent senescent cells)
- Apoptosis—programmed cell death (to destroy senescent cells)
One caveat though. Autophagy and apoptosis are super hard to measure in humans, so it pains me to see folks pushing their fasting limits (basically wasting away with big bags under their eyes) to chase the “benefit” of autophagy.
IF vs. CR: Differences
Calorie restriction often verges into starvation territory. If you eat 50% of what your metabolism requires on a continuous basis, it won’t end well.
Let's zoom in on that theme to see where IF and CR diverge:
#1: Metabolic rate
Your resting metabolic rate (RMR) is the amount of energy needed to power basic bodily functions. These functions include heart beat, breathing, brain power, and everything else that keeps you alive.
Depending on your lifestyle, about 60% to 70% of daily energy expenditure goes towards RMR, and the rest towards physical activity. Spontaneous physical activity (which includes shifting nervously in your seat as you read this article), accounts for about 10% of your total energy burn.
Your total energy burn is a crucial input in weight regulation. If it goes down, you’ll gain weight at a lower food intake, all things equal.
Chronic calorie restriction permanently slows RMR. You learned this earlier, when we talked about The Biggest Loser. (This phenomenon has been shown in peer reviewed research, by the way).
What about intermittent fasting? Does fasting slow your metabolism? Two studies provide clues:
- Sixteen nonobese people doing alternate day fasting lost 2.5% of their bodyweight (mostly fat) over 22 days. Their resting metabolisms did not significantly change.
- Four days of fasting increased resting metabolic rate in lean subjects, probably driven by elevations in norepinephrine, your “get-up-and-go” hormone.
At the very least, it seems like intermittent fasting does not slow your metabolism. And periodic bouts of extended fasting appear to raise it.
But if you’re constantly underfed—aka, chronically calorie restricted—your metabolism adapts to that sluggish state. The key is to alternate fasting and feeding, without underdoing calories overall. That’s how you stay metabolically supple.
Muscle mass is a strong predictor of healthy aging. The more someone has, the longer they tend to live.
And that’s leaving aside the aesthetic benefits. If you want to look good with or without clothes, muscle is kinda important. Call me vain, but I care about that sort of thing.
The problem is, chronic calorie restriction will shrink your muscles faster than your favorite sweater in a clothes dryer. (Okay, not that fast, but I like the analogy). Yep. If you don’t eat enough calories (especially protein), your body will pillage muscle for those amino acids. It’s not a good situation.
Intermittent fasting is different. Done correctly, intermittent fasting is perfectly compatible with happy muscles. But there are three rules to follow.
3 Rules for Keeping Muscle While Intermittent Fasting
- Lift weights at least once per week.
- Eat at least 1 gram protein per pound lean mass.
- Only restrict calories during feeding windows, not overall.
The research suggests that following these rules is sufficient for muscle maintenance. Just realize that you’ll have to work harder to cram in the necessary calories.
Starvation mode isn’t sustainable. Not if you want to look, feel, and perform your best.
A better approach is to align your calorie intake with your daily calorie burn. If you want to lose weight, you eat a little less than you burn. Five or ten percent less. If you want to gain muscle, you eat a little more than you burn. You get the idea.
Unlike chronic CR, intermittent fasting can be sustainable. (Especially if you want to lose or maintain weight). It’s how our ancestors stayed lean and metabolically flexible. Though, as a very important note of caution, I’d never recommend any form of fasting to folks who have a history of disordered eating—there are obvious issues with this regimen in that case.
But we have a leg up on the old timers. We can control how, what, and when we eat. They didn’t have a choice, but we do. I hope this article helps you choose wisely, in a way that benefits your body.