What breaks a fast and what doesn't? A science-backed guide

I’m frequently asked what breaks a fast. People want to know if coffee breaks a fast, if MCT oil breaks a fast, and (groan) if toothpaste breaks a fast.

I wrote this article to enshrine my thoughts online. That way, I can refer all future queries to one place.

To be honest, I think questions of this nature get too much airplay. When I saw “does Diet Coke break a fast?” on a recent podcast headline, I was unable to suppress an eye roll.

Does Diet Coke have calories? Does it have amino acids? Does it contain anything to significantly raise insulin levels and interfere with fat-burning? No. I do understand that artificial sweeteners can enhance insulin secretion or alter appetite regulation in unfavorable ways, but fasting is not the context in which you should be worried about all this.

Allow me to give some context here: Folks are shifting from consuming 2,000-3,000 calories per day (or more) to damn near nothing. This is a stunning change, yet some worry that just smelling food will kick them out of ketosis, halt autophagy, or ramp up mTOR… While I appreciate the desire to get the best results out of your investment, folks are likely making all this far harder than it needs to be.

I’m not endorsing Diet Coke. What I’m saying is: when someone asks if x, y, or z breaks a fast, they’re really asking if it interferes with the potential benefits of fasting.

And the benefits are why folks fast in the first place, right? People fast to…

• Lose fat. When you limit calories, your body starts consuming its own calories as body fat.
• Enter ketosis. During a fast, your liver burns body fat and produces ketones—important molecules that serve as clean, efficient fuel for your brain and body.
• Stimulate autophagy. Triggered by nutrient deprivation, autophagy is a cellular recycling program linked to enhanced longevity. Out with the old cellular parts, in with the new.
• Lower inflammation. Fasting is a promising intervention for a host of inflammatory conditions like IBD, neurodegenerative disease, and many others.
• Be more productive. Less eating time means more time to get stuff done.

And many of the foods and drinks people worry about don’t meaningfully interfere with the potential benefits above. I remind you: fasting is a tool. But like with anything, more is not always better and not all tools are appropriate for all jobs. So, although I appreciate the various possible benefits of fasting, I think some folks are doing too much. More on that in a separate installment.

For the sake of this piece, I’m operating under the assumption folks are using fasting in a smart, productive way… With that frame in mind, let’s lay some groundwork.

What Is Fasting?

The quick answer is that fasting is a period in which you don’t consume calories. During a strict fast, you consume zero calories (stored food energy) from carbohydrates, protein, fat, or alcohol.

But this definition is incomplete. For instance, how long need one abstain from calories to be officially in a fasted state?

There’s no consensus figure, but I consider any food break over 12 hours to be a fast. These shorter fasts are known as time-restricted feeding (TRF)—or more commonly, intermittent fasting (IF).

Intermittent fasts range from 12 to 36 hours, while extended fasts are 36 hours plus. Going 36 hours or more without food is not without risks, so I generally recommend folks stick to IF unless they have medical supervision.

Things get cloudier from here. Certain forms of IF—like 5:2 and alternate-day fasting—don’t entail total calorie restriction, but partial calorie restriction on fasting days. (500 calories per day or so). This is an important point to keep in mind when thinking through the effects of stevia and artificial sweeteners. There are well-studied protocols that show benefits resulting from non-trivial calorie consumption so long as it’s still a significant deficit. The interesting part is: these partial-calorie regimens have similar benefits to other IF regimens. This has been shown.

Also germane to this discussion is Dr. Valter Longo’s Fasting-Mimicking Diet (FMD). This program—which entails 5 consecutive days of calorie restriction per month—has been shown in a clinical trial to promote similar metabolic benefits as zero-calorie fasting.

Practically speaking, this means you shouldn’t stress if a few calories invade your fast. It’s not like the benefits will vanish. If you WANT to run with a zero-calorie fasting protocol, that’s fantastic. But there is not strong evidence that this will be remarkably better than a FMD.

Certain macronutrients, however, disrupt the benefits of fasting more than others.

What Breaks a Fast?

Again, I’ll start with the short answer: consuming calories breaks a fast.

Why does eating break a fast? A big reason is that digesting food raises blood sugar, which signals your pancreas to release insulin.

Insulin is a growth hormone. It’s a hormone that helps you build muscle and store fat with the nutrients (calories) available. It’s an anti-fasting hormone.

During a fast, insulin levels fall. This signals your body to burn fat and produce ketones.

When you eat (or drink) calories again, insulin levels rise. This shuts down fat burning and ketone production. It breaks your fast.

Not all calories stimulate insulin equally. Fat just a little bit, protein a medium amount, and carbs a lot.

This means that, from a fat-burning perspective, carb calories are most likely to derail your fast.

Protein calories can also derail your fast, and not just from the insulin bump. Eating protein (especially protein high in the amino acid leucine) activates a growth pathway called mTOR. When mTOR goes up, autophagy and fat burning go down. Like insulin, mTor activation breaks your fast.

Does This Break a Fast?

With the last section in mind, let’s talk about what drinks, supplements, and sweeteners may or not break a fast.

Drinks

Water

Some people practice “dry fasting” without water. This concerns me because fasting has a diuretic effect. You need to replace those fluids or you risk dehydration.

Verdict? Water contains zero calories and won’t interfere with a fast.

Coffee

“Will coffee break my fast?” is perhaps the most common fasting question I hear. Since coffee has no calories and enhances autophagy in mice—I say it won’t.

Verdict? Coffee won’t break a fast. It may even enhance it.

Tea

Tea is like coffee. No calories, a bit of caffeine, a bunch of antioxidants. Consume at your pleasure during a fast.

Verdict? Tea won’t break a fast.

Diet soda

Diet soda contains neither calories nor any compounds with measurable effects on insulin. It won’t break a fast, but that doesn’t mean I’m a fan. Try putting a no-sugar drink mix like LMNT in some sparkling water. Your health will thank you.

Verdict? If you must have a diet soda during your fast, it probably won’t interfere with it.

Bone broth

Bone broth is made from simmering bones and connective tissue. From this connective tissue comes collagen, a type of protein with a unique amino acid profile. The upshot is that collagen activates mTOR to a minimal degree.

Verdict? Bone broth contains calories, but a mug or two shouldn’t hinder your fast. I’m not being sloppy here, I’m just acknowledging that if the only calories you consume all day come from a few cups of bone broth… this is not going to derail most efforts.

Foods and Supplements

Butter, cream, and milk

If you take cream, milk, or butter in your coffee, does that break your fast? Technically, yes—but realistically no.

Fat, if you recall, is the least insulinogenic of the macronutrients. So if you’re going to have a few calories, you want them to be fat. It also has virtually no impact on mTOR signaling due to its scarcity of carbs and amino acids.

Verdict? Technically, yes. But assuming it’s high-fat dairy without added sugar, a few calories of butter or cream is fine.

MCT Oil

My thoughts on MCT oil are similar to my thoughts on butter. A little fat in your coffee won’t hamper your fast, but it CAN give you disaster pants—so go easy on the MCT oil!

MCT oil is probably the most fasting-friendly fat. When you consume MCTs, they travel rapidly to your liver for ketone production. It deepens ketosis, in other words.

Verdict? A tablespoon or two of MCT oil can enhance ketosis without derailing your fast.

Electrolyte drinks and powders

My answer here depends on the product. Electrolytes alone won’t break a fast.

In fact, taking electrolytes (especially sodium) can help prevent the muscle cramps, energy dips, and headaches that often plague folks while fasting. These are symptoms of electrolyte deficiencies that manifest because fasting regimens cause enhanced electrolyte loss through urine.

Verdict? A zero-sugar electrolyte product won’t break your fast. I humbly recommend Drink LMNT. For years, we supported our fasts with homemade electrolyte drinks because we couldn’t recommend anything on the market. That’s why we created Drink LMNT.

Branched-chain amino acids

Leucine, isoleucine, and valine are known as branched-chain amino acids (BCAAs). These amino acids—found in whey protein, meat, fish, and supplements—activate mTOR, stimulate insulin release, and promote muscle synthesis. These aren’t functions you want to maximize during a fast.

Verdict? BCAAs break a fast.

Protein powder

Most protein powders are high in BCAAs (and calories) and therefore not a friend to fasting. The exception is collagen powder, which is low in growth-promoting aminos.

Verdict? Protein powders break a fast.

Mouthwash or toothpaste

Believe it or not, people ask me if it’s okay to brush their teeth while fasting.

Allow me to make an obvious remark. Provided you aren’t swallowing the toothpaste or mouthwash, it won’t break your fast. If you ARE swallowing your toothpaste and mouthwash… I’m not sure I’ll convince you to do otherwise.

Verdict? Please continue to practice oral hygiene while fasting.

Sweeteners

Stevia and monk fruit

Stevia and monk fruit are plant-based sweeteners with similar sweetening powers, a bunch of antioxidants, and zero calories. They may have a tiny insulin effect, but they’re unlikely to interfere with a fast.

I know folks will push back on this one, but hear me out: even if we get a small bump in insulin release, what happens? Blood glucose decreases. So, although you may have a tiny decrease in fasting activity (mTOR signaling, in theory) the subsequent decrease in blood glucose should only “deepen” the fast.

Verdict? Stevia and monk fruit won’t break a fast.

Allulose

Like stevia and monk fruit, allulose is a non-caloric sweetener. In one small study, allulose reduced blood glucose without raising insulin.

Verdict? Allulose won’t break a fast.

Aspartame

Aspartame gets a lot of flak, but the health risks are largely exaggerated. In my view, the worst consequence of aspartame is that it might increase compensatory eating.

Aspartame contains zero calories and doesn’t provoke an insulin response.

Verdict? Aspartame won’t break a fast

The Right Mindset for Fasting

One of my goals today was to help folks chill out about fasting. You don’t need to be a water-only person to benefit.

There are a couple of principles to keep in mind. First, if it doesn’t have calories, it won’t break a fast. That means coffee, tea, and non-caloric sweeteners are fair game.

Also, calories aren’t the “kryptonite” of fasting. Intermittent calorie restriction, or even just a significant calorie restriction, is a perfectly acceptable form of fasting. Keep in mind, the goal here should be to reap the benefits of fasting, not win some kind of hypothetical “fasting olympics.”

If you do decide to consume calories during your fast, you should favor fat or collagen calories. This prevents your body from releasing insulin, activating mTOR, and switching into growth mode.

You’ll be in growth mode soon enough, once you officially break your fast. And that, folks, is my cue to go eat something.