TODO: add alt text

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

From the desk of Robb Wolf

<p><span style="font-weight: 400">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.</span></p><p><span style="font-weight: 400">I wrote this article to enshrine my thoughts online. That way, I can refer all future queries to one place.</span></p><p><span style="font-weight: 400">To be honest, I think questions of this nature get too much airplay. When I saw “</span><i><span style="font-weight: 400">does Diet Coke break a fast?”</span></i><span style="font-weight: 400"> on a recent podcast headline, I was unable to suppress an eye roll.</span></p><p><span style="font-weight: 400">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.</span></p><p><span style="font-weight: 400">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.</span></p><p><span style="font-weight: 400">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.</span></p><p><span style="font-weight: 400">And the benefits are why folks fast in the first place, right? People fast to…</span></p><ul><li><b>Lose fat.</b><span> When you limit calories, your body starts consuming its own calories as body fat.</span></li><li><span style="font-weight: bold">Enter </span><a target="_blank" rel="noopener noreferrer" href="https://drinklmnt.com/blogs/health/how-to-get-into-ketosis" style="font-weight: bold">ketosis</a><span style="font-weight: bold">. </span><span>During a fast, your liver burns body fat and produces ketones—important molecules that serve as clean, efficient fuel for your brain and body.</span></li><li><span style="font-weight: bold">Stimulate autophagy. </span><span>Triggered by nutrient deprivation, autophagy is a cellular recycling program </span><a target="_blank" rel="noopener noreferrer" href="https://pubmed.ncbi.nlm.nih.gov/31144030/" style="font-weight: bold"><span style="font-weight: 400">linked</span></a><span> to enhanced longevity. Out with the old cellular parts, in with the new.</span></li><li><b>Lower inflammation.</b><span> Fasting is a promising intervention for a host of inflammatory conditions like </span><a target="_blank" rel="noopener noreferrer" href="https://pubmed.ncbi.nlm.nih.gov/19405258/"><span>IBD</span></a><span>, neurodegenerative disease, and many others.</span></li><li><b>Be more productive.</b><span> Less eating time means more time to get stuff done.</span></li></ul><p><span style="font-weight: 400">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.</span></p><p><span style="font-weight: 400">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.</span></p><h2><strong>What Is Fasting?</strong></h2><p><span style="font-weight: 400">The quick answer is that </span><a target="_blank" rel="noopener noreferrer" href="https://drinklmnt.com/blogs/health/fasting-what-you-need-to-know"><span style="font-weight: 400">fasting</span></a><span style="font-weight: 400"> 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.</span></p><p><span style="font-weight: 400">But this definition is incomplete. For instance, how long need one abstain from calories to be officially in a fasted state?</span></p><p><span style="font-weight: 400">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).</span></p><p><span style="font-weight: 400">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.</span></p><p><span style="font-weight: 400">Things get cloudier from here. Certain forms of IF—like 5:2 and alternate-day fasting—don’t entail </span><i><span style="font-weight: 400">total </span></i><span style="font-weight: 400">calorie restriction, but </span><i><span style="font-weight: 400">partial </span></i><span style="font-weight: 400">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 </span><a target="_blank" rel="noopener noreferrer" href="https://www.cureus.com/articles/12903-intermittent-fasting-the-choice-for-a-healthier-lifestyle"><span style="font-weight: 400">shown</span></a><span style="font-weight: 400">.</span></p><p><span style="font-weight: 400">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 </span><a target="_blank" rel="noopener noreferrer" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6816332/"><span style="font-weight: 400">clinical trial</span></a><span style="font-weight: 400"> to promote similar metabolic benefits as zero-calorie fasting.</span></p><p><span style="font-weight: 400">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.</span></p><p><span style="font-weight: 400">Certain macronutrients, however, disrupt the benefits of fasting more than others.</span></p><h2><strong>What Breaks a Fast?</strong></h2><p><span style="font-weight: 400">Again, I’ll start with the short answer: consuming calories breaks a fast.</span></p><p><span style="font-weight: 400">Why does eating break a fast? A big reason is that digesting food raises blood sugar, which signals your pancreas to release insulin.</span></p><p><a target="_blank" rel="noopener noreferrer" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1204764/"><span style="font-weight: 400">Insulin is a growth hormone</span></a><span style="font-weight: 400">. It’s a hormone that helps you build muscle and store fat with the nutrients (calories) available. It’s an anti-fasting hormone.</span></p><p><span style="font-weight: 400">During a fast, insulin levels fall. This signals your body to burn fat and produce ketones.</span></p><p><span style="font-weight: 400">When you eat (or drink) calories again, insulin levels rise. This shuts down fat burning and ketone production. It breaks your fast.</span></p><p><span style="font-weight: 400">Not all calories stimulate insulin equally. Fat just a little bit, protein a medium amount, and carbs a lot.</span></p><p><span style="font-weight: 400">This means that, from a fat-burning perspective, carb calories are most likely to derail your fast.</span></p><p><span style="font-weight: 400">Protein calories can also derail your fast, and not just from the insulin bump. Eating protein (especially protein high in the </span><a target="_blank" rel="noopener noreferrer" href="https://pubmed.ncbi.nlm.nih.gov/11238775/"><span style="font-weight: 400">amino acid leucine</span></a><span style="font-weight: 400">) activates a growth pathway called mTOR. When mTOR goes up, autophagy and fat burning go down. Like insulin, mTor activation breaks your fast.</span></p><h2><strong>Does This Break a Fast?</strong></h2><p><span style="font-weight: 400">With the last section in mind, let’s talk about what drinks, supplements, and sweeteners may or not break a fast.</span></p><h3><span style="text-decoration: underline"><b>Drinks</b></span></h3><h3><span style="font-weight: 400">Water</span></h3><p><span style="font-weight: 400">Some people practice “dry fasting” without water. This concerns me because fasting has a </span><a target="_blank" rel="noopener noreferrer" href="https://www.amjmed.com/article/0002-9343(71)90152-5/fulltext"><span style="font-weight: 400">diuretic effect</span></a><span style="font-weight: 400">. You need to replace those fluids or you risk </span><a target="_blank" rel="noopener noreferrer" href="https://drinklmnt.com/blogs/health/how-to-know-youre-dehydrated"><span style="font-weight: 400">dehydration</span></a><span style="font-weight: 400">.</span></p><p><b>Verdict?</b><span style="font-weight: 400"> Water contains zero calories and won’t interfere with a fast.</span></p><h3><span style="font-weight: 400">Coffee</span></h3><p><span style="font-weight: 400">“Will coffee break my fast?” is perhaps the most common fasting question I hear. Since coffee has no calories and </span><a target="_blank" rel="noopener noreferrer" href="https://pubmed.ncbi.nlm.nih.gov/24769862/"><span style="font-weight: 400">enhances autophagy in mice</span></a><span style="font-weight: 400">—I say it won’t.</span></p><p><b>Verdict? </b><span style="font-weight: 400">Coffee won’t break a fast. It may even enhance it.</span></p><h3><span style="font-weight: 400">Tea</span></h3><p><span style="font-weight: 400">Tea is like coffee. No calories, a bit of caffeine, a bunch of antioxidants. Consume at your pleasure during a fast.</span></p><p><b>Verdict? </b><span style="font-weight: 400">Tea won’t break a fast.</span></p><h3><span style="font-weight: 400">Diet soda</span></h3><p><span style="font-weight: 400">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 </span><a target="_blank" rel="noopener noreferrer" href="https://drinklmnt.com/products/lmnt-recharge-electrolyte-drink"><span style="font-weight: 400">LMNT</span></a><span style="font-weight: 400"> in some sparkling water. Your health will thank you.</span></p><p><b>Verdict? </b><span style="font-weight: 400">If you must have a diet soda during your fast, it probably won’t interfere with it.</span></p><h3><span style="font-weight: 400">Bone broth</span></h3><p><span style="font-weight: 400">Bone broth is made from simmering bones and connective tissue. From this connective tissue comes collagen, a type of protein with a </span><a target="_blank" rel="noopener noreferrer" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6566836/"><span style="font-weight: 400">unique amino acid profile</span></a><span style="font-weight: 400">. The upshot is that collagen activates mTOR to a minimal degree.</span></p><p><b>Verdict? </b><span style="font-weight: 400">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.</span></p><h3><span style="text-decoration: underline"><strong>Foods and Supplements </strong></span></h3><h3><span style="font-weight: 400">Butter, cream, and milk</span></h3><p><span style="font-weight: 400">If you take cream, milk, or butter in your coffee, does that break your fast? Technically, yes—but realistically no.</span></p><p><span style="font-weight: 400">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.</span></p><p><b>Verdict? </b><span style="font-weight: 400">Technically, yes. But assuming it’s high-fat dairy without added sugar, a few calories of butter or cream is fine.</span></p><h3><span style="font-weight: 400">MCT Oil</span></h3><p><span style="font-weight: 400">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!</span></p><p><span style="font-weight: 400">MCT oil is probably the most fasting-friendly fat. When you consume MCTs, they </span><a target="_blank" rel="noopener noreferrer" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7175812/"><span style="font-weight: 400">travel</span></a><span style="font-weight: 400"> rapidly to your liver for ketone production. It </span><i><span style="font-weight: 400">deepens</span></i><span style="font-weight: 400"> ketosis, in other words.</span></p><p><b>Verdict? </b><span style="font-weight: 400">A tablespoon or two of MCT oil can enhance ketosis without derailing your fast.</span></p><h3><span style="font-weight: 400">Electrolyte drinks and powders</span></h3><p><span style="font-weight: 400">My answer here depends on the product. Electrolytes alone won’t break a fast.</span></p><p><span style="font-weight: 400">In fact, </span><a target="_blank" rel="noopener noreferrer" href="https://drinklmnt.com/blogs/health/electrolytes-while-fasting-benefits-and-best-sources"><span style="font-weight: 400">taking electrolytes</span></a><span style="font-weight: 400"> (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 </span><a target="_blank" rel="noopener noreferrer" href="https://www.amjmed.com/article/0002-9343(71)90152-5/fulltext"><span style="font-weight: 400">enhanced electrolyte loss</span></a><span style="font-weight: 400"> through urine.</span></p><p><b>Verdict?</b><span style="font-weight: 400"> A zero-sugar electrolyte product won’t break your fast. I humbly recommend </span><a target="_blank" rel="noopener noreferrer" href="https://drinklmnt.com/products/lmnt-recharge-electrolyte-drink"><span style="font-weight: 400">Drink LMNT</span></a><span style="font-weight: 400">. 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.</span></p><h3><span style="font-weight: 400">Branched-chain amino acids</span></h3><p><span style="font-weight: 400">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.</span></p><p><b>Verdict? </b><span style="font-weight: 400">BCAAs break a fast.</span></p><h3><span style="font-weight: 400">Protein powder</span></h3><p><span style="font-weight: 400">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.</span></p><p><b>Verdict? </b><span style="font-weight: 400">Protein powders break a fast.</span></p><h3><span style="font-weight: 400">Mouthwash or toothpaste</span></h3><p><span style="font-weight: 400">Believe it or not, people ask me if it’s okay to brush their teeth while fasting. </span></p><p><span style="font-weight: 400">Allow me to make an obvious remark. Provided you aren’t </span><i><span style="font-weight: 400">swallowing</span></i><span style="font-weight: 400"> 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.</span></p><p><b>Verdict? </b><span style="font-weight: 400">Please continue to practice oral hygiene while fasting.</span></p><h3><span style="text-decoration: underline"><strong>Sweeteners</strong></span></h3><h3><span style="font-weight: 400">Stevia and monk fruit</span></h3><p><a target="_blank" rel="noopener noreferrer" href="https://drinklmnt.com/blogs/health/what-is-stevia-and-is-it-healthy"><span style="font-weight: 400">Stevia</span></a><span style="font-weight: 400"> 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.</span></p><p><span style="font-weight: 400">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.</span></p><p><b>Verdict? </b><span style="font-weight: 400">Stevia and monk fruit won’t break a fast.</span></p><h3><span style="font-weight: 400">Allulose</span></h3><p><span style="font-weight: 400">Like stevia and monk fruit, allulose is a non-caloric sweetener. In one </span><a target="_blank" rel="noopener noreferrer" href="https://pubmed.ncbi.nlm.nih.gov/28935140/"><span style="font-weight: 400">small study</span></a><span style="font-weight: 400">, allulose reduced blood glucose without raising insulin.</span></p><p><b>Verdict? </b><span style="font-weight: 400">Allulose won’t break a fast.</span></p><h3><span style="font-weight: 400">Aspartame</span></h3><p><span style="font-weight: 400">Aspartame gets a lot of flak, but the health risks are largely exaggerated. In my view, the worst consequence of aspartame is that it </span><i><span style="font-weight: 400">might </span></i><span style="font-weight: 400">increase compensatory eating.</span></p><p><span style="font-weight: 400">Aspartame contains zero calories and doesn’t provoke an insulin response.</span></p><p><b>Verdict? </b><span style="font-weight: 400">Aspartame won’t break a fast</span></p><h2><strong>The Right Mindset for Fasting</strong></h2><p><span style="font-weight: 400">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.</span></p><p><span style="font-weight: 400">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.</span></p><p><span style="font-weight: 400">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.”</span></p><p><span style="font-weight: 400">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.</span></p><p><span style="font-weight: 400">You’ll be in growth mode soon enough, once you officially break your fast. And that, folks, is my cue to go eat something.</span></p>