One of the most common questions I get is “Does [x] break a fast?”
What they’re really inquiring about is: “Does this interfere with, negate, or nullify the benefits of fasting?”
These benefits include:
Ketosis: Fasting is the quickest way to get into ketosis, an metabolic state characterized by increasing fat burning, fat adaptation, and—in some people—improved cognitive function.
Fat Loss: When you’re fasting, you’re not eating, and not eating is the best way to force your body to burn the fat it already possesses. Fasting also means no additional calories are coming in, and many people find that fasting is a great way to control their calorie intake.
Autophagy: Autophagy, or “self-eating,” is the process by which our cells prune damaged components, maintain proper function, and keep aging at bay. Fasting triggers autophagy. Breaking the fast will stop autophagy.
Let’s go through the most popular queries one by one and figure out how each one affects an intermittent fast. (For questions about what supplements break a fast, check out my post, “What Breaks a Fast: Supplements Edition.”)
Depends on who you ask. Some say the fact that coffee triggers a metabolic response means it breaks the fast. I say that coffee increases fat mobilization and burning, independently triggers autophagy (something we’re looking for when we fast), and makes it easier to stave off hunger. For my full treatment, check out this post on coffee and fasting.
I’m going to say “no.”
Does tea break a fast?
Tea contains no calories, improves metabolic health, and can aid fat burning. All signs point to it being great during a fast. Of course, if you had a tablespoon of sugar and a half cup of milk, you’re breaking the fast. But tea itself is a great addition.
I’m going to say “no.”
Does Yerba mate break a fast?
Yerba mate is essentially non-caloric, like tea or black coffee. It also has beneficial effects on glucose tolerance, which is a big plus.
I’m going to say “no.”
Does bone broth break a fast?
I covered this in full a few months ago. Go read that post. In short, a bit is probably okay. Just keep in mind that the more gelatinous your broth is, the more collagen protein it will contain and the greater its potential to inhibit autophagy. This isn’t established in humans yet (see the collagen section below), but it’s worth considering. A nice salty broth has gotten many a faster through a tough fast, especially if they’re still learning the ropes and need some electrolytes.
I’m going to say “technically yes” but “realistically no.”
Does lemon water break a fast?
A tablespoon of fresh squeezed lemon juice has just a couple calories and a decent amount of potassium. Combined with salt, lemon water is actually a nice way to hydrate during a fast without breaking it.
No, lemon water does not break a fast.
Does diet soda break a fast?
Diet soda may mess with your gut. It’s linked to weight issues, though not conclusively and certainly not in a causative manner; it’s just as likely that the relationship can be explained by overweight and unhealthy people using diet sodas in a bid to lose weight. I don’t like them myself, and I’ve witnessed people fail to ever kick the sweet tooth as long as they drank diet sodas. But many people find they do improve dietary adherence and do improve fasting tolerance. If that’s the case, they are very pro-fasting.
I’m going to say “no.” Diet soda doesn’t break a fast. Can you drink diet soda while fasting? Yes. Should you? Maybe not.
Does juice break a fast?
A juice fast isn’t really a fast. You’re consuming fewer calories than you might eating normal food, but you’re still consuming a good number of calories—most of them carbohydrate, no less.
I’m going to say “yes” unless you’re specifically engaging in “juice fasting,” in which case it’s still not fasting despite what you call it.
Does heavy cream break a fast?
Technically, as a source of calories, cream breaks a fast. But it doesn’t provoke an insulin response when consumed in isolation, it doesn’t impact ketosis, and many people find it makes sticking to the fast easier.
I’m going to say “technically yes, but realistically no—just keep it to a couple teaspoons or less.”
Does almond milk break a fast?
It depends on the almond milk. A full cup of the standard sugar-free almond milk has just 36 calories, about a gram of carbs, 2 grams of fat, and a gram of protein. That’s almost nothing. You could probably get away with a quarter or third cup and have minimal impact on your fast, but why not just drink some water or coffee?
I’m going to say “technically yes,” but you can get away with a little bit.
Does oat milk break a fast?
Oat milk is high in carbs and has a decent amount of fat (from canola oil). It is full of calories and will definitely break your fast.
Yes, oat milk breaks a fast.
Does butter break a fast?
Like cream, butter doesn’t provoke an insulin response in isolation. It’s more calorically dense than cream, though, so watch how much you eat.
I’m going to say “technically yes, but realistically no as long as you’re not using more than a teaspoon.”
Do coconut oil or MCT oil break a fast?
MCT oil is pure fat and thus calorically dense, but it has three benefits going for it. First, it doesn’t provoke an insulin response in isolation. Two, it increases energy expenditure. Three, it converts directly to ketones. People new to fasting can often speed up the fat adaptation process by incorporating a little MCT oil. Coconut oil is the main source of MCT oil, so it’ll have similar effects, though not as pronounced.
I’m going to say “technically yes, but realistically no—and it may even enhance your fasting experience when consumed in moderation.”
Salt does not break a fast. Actually, adding a pinch or two of salt to your water during a fast can increase your tolerance of the fasting process and improve hydration status.
I’m going to say “no.”
Does stevia break a fast?
Stevia contains no calories and has no effect on insulin secretion (if anything, it increases insulin sensitivity).12 However, it’s often used to sweeten foods that do contain calories, so be mindful of how you’re using it.
I’m going to say “no.”
Does monk fruit break a fast?
For a good overview of monk fruit, read this. Suffice it to say, monk fruit is similar to stevia in that it’s a non-caloric, naturally-occurring sweetener with unique health effects. It will not break your fast.
Xylitol only has a strong metabolic effect in excess of 30 grams in a sitting. Keep the dose low and it does not break the fast..
Low amounts of xylitol do not break a fast.
Does Splenda break a fast?
Sucralose (Splenda) does not provoke an insulin response or increase blood glucose—great news for fasters who want to use it—but it does seem to impair whole body insulin sensitivity.3 That’s bad for everyone.
Splenda doesn’t break a fast, but there are other downsides.
Does aspartame break a fast?
Those same studies on monk fruit and stevia also tested aspartame, finding similar results. Aspartame does not provoke an insulin or glucose response. I’m no fan of the stuff, but I don’t see any evidence that it will break a fast.
Aspartame does not break a fast.
Does CBD oil break a fast?
Assuming you’re doing the kind of hemp oil that comes in droppers and not the kind that you pour from a culinary oil bottle, the caloric content can’t possibly impact your fast. There are no studies examining the metabolic effects of CBD in the fasted state, but I don’t see any reason why it would impact ketosis, autophagy, or fat-burning—and without psychoactive THC involved, you won’t be getting the munchies.
Small (non-caloric) doses of CBD oil do not break a fast.
Does protein powder break a fast?
Protein powder provokes an insulin response, which opposes autophagy, which means you’re breaking your fast. Plus, protein powder contains calories.
I’m going to say “yes.”
Does collagen break a fast?
If you’re strict and technical, then yes, collagen breaks a fast. There’s evidence that glycine—the most prominent amino acid in collagen—can inhibit autophagy, but it was a convoluted animal study where inhibiting autophagy with large doses of glycine after brain injury actually improved outcomes. It probably doesn’t apply to someone adding a scoop of collagen to their coffee. Besides, even if it slightly reduces autophagy, a little collagen won’t negatively impact ketosis, fat-burning, or energy intake.
I’m going to say “technically yes,” but “realistically no.” Avoid if your main focus is autophagy, though.
Do BCAAs break a fast?
BCAAs trigger an insulin response and thus stop autophagy and the fast. That said, many proponents of fasted training recommend using BCAAs before a workout to help preserve muscle and improve the post-workout anabolic response.
I’m going to say “yes.”
Does apple cider vinegar break a fast?
Apple cider vinegar is made by double fermenting the sugars present in apple juice. First, yeast convert the sugars to alcohol. Next, the alcohol converts to acetic acid. The result is a liquid that’s virtually calorie-free, and almost certainly sugar-free. Studies showing that consuming vinegar lowers the blood glucose response to a subsequent meal aren’t really relevant if you’re fasting, but they don’t hurt.
Apple cider vinegar doesn’t break a fast.
Do electrolytes break a fast?
Electrolyte powders/tabs used to come festooned with sucrose, making them decidedly anti-fasting. These days, most of them are sweetened with stevia or some other natural non-caloric sweetener. LMNT is a great brand that doesn’t break a fast. Even the ones that have a little bit of sugar (1-2 g) are probably okay to consume without much negative effect. Best of all, electrolytes can really help you tolerate a fast.
Electrolytes don’t break a fast as long as they are sugar-free.
Can I chew gum while fasting?
If we’re talking sugar-rich gum, the answer is no. Those definitely break a fast. If we’re talking xylitol gum, the answer is more mixed. In healthy individuals, 30 grams of pure xylitol triggers a small but significant rise in glucose and insulin.4 That might sound scary to a prospective IFer, but most people aren’t chewing gum made with 30 grams of xylitol. The average piece of xylitol gum barely weighs a gram.
Yes, you can chew sugar-free gum while fasting.
Can I brush my teeth while fasting?
I always consume my toothpaste (around a tablespoon of the good stuff per brushing) and I’ve never had it knock me out of ketosis, autophagy, or in any way shape or form break my fast. I’m kidding. I don’t consume my toothpaste, but brushing your teeth doesn’t break a fast.
I’m going to say “yes.” Don’t eat it though.
Does mouthwash break a fast?
Pretty much the same as toothpaste. Look for a brand that doesn’t contain sugar or one of the artificial sweeteners above that trips insulin. As the instructions (and common sense) suggest, don’t drink it.
That’s it, folks. If you have additional questions about what does or doesn’t break a fast, leave them down below. Thanks for reading, and I hope you found the post helpful. Forward it on if you did.
Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.