Do probiotics break a fast? 

Good news! Neither probiotics nor prebiotics break a fast. Taking them during your fasting period may also provide health benefits. 

How do I know?

I’m a doctoral and fellowship-trained physical therapist and have done 9 fasts over the last 3 years of 48 hours or longer, including two 5-day fasts. I’ve scoured countless research, analyzed four different books from experts on fasting, and performed testing with a continuous glucose monitor on day 3 of a 5-day fast to answer this question. 

Through all my fasting, which includes one to two 18-24-hour fasts every week, I’ve become extremely in tune with changes in my blood sugar levels. This intuitive sense, along with a continuous glucose monitor, allowed me to analyze the changes in my blood glucose to answer the question – Do probiotics break a fast? 

Keep reading to see screenshots from our continuous glucose monitor, learn why it is beneficial to take prebiotics and probiotics while fasting to improve gut health, and our dietitian-recommended probiotics to use during your fasting window. 

Do Probiotics Break a Fast?

No, probiotic supplements do not break a fast. In testing with a continuous glucose monitor, which measures changes in blood sugar and insulin levels, we were able to confirm that there was no change in blood glucose in the hour after taking probiotic bacteria. 

If you try this at home, be aware that there is one thing that may confuse you. 

Glucose usually naturally rises in the morning. Take a look at the pictures below from first thing in the morning vs in the afternoon. 

The natural rise in glucose happened to us as we took our probiotic in the morning, so we re-performed our testing later in the day, which showed no testing in blood glucose.

Additionally, we confirmed that on other days of fasting, especially if there was a stressor (i.e. kids being fussy when you’re sprinting to get them ready before work), a rise in glucose was often noted.  

So, here is what we saw in the morning when taking our probiotics. Note the rise just before 7:30 about 45 minutes after we took it. This coincided with a fussy moment with our kids too.

Screenshot from Supersapians continuous glucose monitor shot showing probiotics do not break a fast

Here is what we saw later in the day when taking a probiotic:

Screenshot of taking probiotics a 2nd time to answer the question do probiotics break a fast. The continuous glucose monitor reading in this photo shoes it does not.

Note that there would have to be a rise of more than 8-12 points to be considered an “event” that caused a change in blood glucose. So, a small change is normal due to activity, movement, natural hormone fluctuations, etc. If the probiotics were breaking our fast due to changing our blood glucose, a rise of more than 8-12 points would be seen within an hour after the dose was taken. 

For a point of reference, below is what a glucose spike and insulin response look like after a meal. This was not good. Having this spike caused too high of an insulin response leading to my glucose then dropping below normal around 2pm which likely lead to me feeling tired. Smoothing out these spikes improves energy levels and mental focus.

Post meal glucose spike with insulin response.

Is it OK to Take Probiotic Supplements While Fasting?

Absolutely! Taking probiotics on an empty stomach during periods of fasting can be the best time to improve gut and overall health. 

Due to processed foods, high sugar consumption, stress, heavy training loads in endurance athletes, and more, we often end up with abnormally high levels of bad gut bacteria and low levels of good gut bacteria. Consuming probiotics during periods of fasting can encourage the growth of good bacteria in the gut. 

Basically, we always want to have a higher amount of good gut bugs in our digestive tract.

We regularly test the health of our clients’ digestive system to give them personalized recommendations on how to improve the health of their microbiome. 

Is it inflamed? Overpopulated with yeast, which often causes constipation? Too much bad bacteria leading to digestive issues and bloating? We can get a lot of answers from gut testing.

When you take probiotics while in a fasted state, you can starve out some of the bad gut bacteria that thrive on sugar and foods commonly found on the Standard American Diet (appropriately abbreviated SAD), and increase the number of good gut bacteria.

We frequently take this a step further and take both probiotics and prebiotics during fasting. This helps feed and support the good gut bacteria and enhances starving and crowding out the bad gut bacteria.

>>>Read More: The Best Time to Take Probiotics – 5 Crucial Tips

How to Test Your Gut to See if Your Probiotics are Working

We do complex gut testing with our clients that is not available to non-medical professionals. But, there is a really good test that is available to the public, its not too expensive, and it provides easy-to-understand results with actional steps to improve things based on your individual results.

Say hello to the Biohm Gut Test.

Biohm is an amazing company based in Cleveland, Ohio, and directed by one of the top gut experts in the country.

Here is an example of the results from my test:

Probiotic gut test results looking at digestive issues and if probiotics supplements are effective.

If you would like to purchase the BIohm Gut Test for yourself, 👉use code FWDFUEL for 20% off👈 your test.

Do Prebiotics Break a Fast?

No, taking a small amount of prebiotics does not break a fast. It will not affect weight loss or cause an insulin response, which would lessen then benefits of your fasting period. 

How do we know? 

We also tested this with a continuous glucose monitor. Twice and with two different kinds of prebiotics. 

Screenshot showing how prebiotics do not break a fast.

While there are 4-5 calories consumed with prebiotics, most of it is nondigestible fiber that will not be metabolized like a calorie used for energy.

Thus, it will not break your fast.

Please don’t listen to anyone that preaches this is a “dirty fast”. Being this dogmatic about your fasting limits you from achieving better results from your fasting over time.

Fasting glucose analysis after taking 5g of inulin in an attempt to answer - do prebiotics break a fast.

Additionally, doing a “dirty fast,” i.e., one that includes anything other than water, coffee, or tea, can enhance autophagy, improve gut health, and lead to greater results in a shorter amount of time. 

Do Probiotics Stop Autophagy?

No, probiotic supplementation will not stop autophagy because it will not break your fast. 

If you want to enhance autophagy, in addition to fasting, consume proteolytic enzymes, which help with the recycling of cells and tissues. 

Research has proven the effectiveness of proteolytic enzymes in multiple studies, including a 2021 study on people affected with pulmonary fibrosis. This study showed improved quality of life and subjective report of improved function when taking proteolytic enzymes.[1] 

For this reason, I take 2-4 capsules of proteolytic enzymes morning and night when I’m fasting.  

What are the Best Probiotics, Prebiotics, and Proteolytic Enzymes to Take While Fasting?

There are a lot of great probiotics, prebiotics, and proteolytic enzymes. Here we’re going to give you a few of our favorites, the ones we use, and a way to buy them at a massive discount.

Best Probiotics

MegaSporeBiotic by Microbiome Labs 

MegaSporebiotic probiotic supplement by Microbiome Labs which is a great probiotic when fasting if you have digestive issues.
  • 100% spore-based probiotic that helps with gut barrier function
  • Great choice for endurance athletes or those who have battled brain fog and leaky gut

Being someone who has battled severe brain fog due to a combination of mold illness, leaky gut, and endurance training, I go with our dietitian’s top pick for improving gut barrier function, MegaSporeBiotic by Microbiome Labs. 

You may buy the supplements on this list from that place that sounds like a rainforest, but, counterfeit supplements are a problem, and the ones that aren’t are often stored in hot warehouses that ruin their potency. 

A better option is to save $6 👍 and buy them through our favorite affiliate, Fullscript.  They’ll ship your supplements direct from the manufacturer and use freezer packs when necessary to maintain your product quality. When you buy through Fullscript, we receive a small commission that support our business, and you save big vs if you bought it elsewhere.  It’s a win-win.👌 

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RestoreFlora by Microbiome Labs

  • Combines strains to reduce yeast overgrowth and improve gut health
  • Great for those with candida or yeast overgrowth due to the yeast-eating saccharomyces boulardii

If you’ve battled candida or yeast overgrowth, then you know how tough it can be to overcome. Our dietitian’s go-to pick in this area is the RestoreFlora by Microbiome Labs, which is not only helpful with decreasing yeast overgrowth but also great for your overall gut health.

Many supplements that aim to reduce yeast overgrowth include saccharomyces boulardii, which is a healthy yeast that thrives on consuming the bad yeast in your gut. However, this supplement sets itself apart from others by also including probiotics to help your overall gut health.

Ther-Biotic Complete by SFI

  • The best option for those without major issues and looking to maintain optimal health
  • Combines 12 different broad-spectrum gut-loving probiotic species in one capsule

If you’re someone without any health issues and are simply looking to optimize your health, the Therbiotic Complete by SFI is our top recommendation. 

Keeping in mind that optimal gut health improves food breakdown and absorption, workout recovery, boosts immune function, and much more, taking care of your gut is a great idea, even if you don’t have health issues.  If this is you, go with the Ther-Biotic Complete. 

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Best Prebiotics

Ther-Biotic Biotagen by SFI

  • Blends 3 different prebiotics in one product to feed a variety of good-gut bugs
  • Tastes like cotton candy

When it comes to a prebiotic, it is best to have a blend of prebiotics that will feed the wide variety of beneficial bacteria strains in our gut. Not every strain of healthy bacteria thrives on the same non-digestible fiber so when possible, go with a blend like this one. 

It is always best to get most of your prebiotics from the top prebiotic-rich foods, but prebiotic supplements are a great idea to optimize gut health, especially. when you’re fasting.

Organic Inulin Powder by NOW Foods

  • Ultimate budget pick💰
  • Organic and made from agave

If you’re looking to stretch a dollar but still keep a healthy gut microbiome, its hard to beat the Organic Inulin by NOW Foods. 

NOW Foods is a quality company that has 3rd party testing of its supplements and uses GMP-certified facilities to produce them. We highly recommend them as a company, and they’re often a great choice if you’re looking for something that is budget-friendly.  

👉Save $$$ now by buying on Fullscript here.👈 

Best Proteolytic Enzymes

Davinci Nattokinase Plus proteolytic enzyme / fibrinolytic enzyme which can help boost autopaghy when fasting.

Nattokinase Plus by DaVinci Labs

  • Combines two different proteolytic (fibrinolytic) enzymes
  • Includes Rutin, an antioxidant

We’re fans of the Nattokinase Plus by DaVinci Labs because it combines two different types of proteolytic enzymes, which are sometimes also referred to as fibrinolytic enzymes. 

This formula also adds a flavonoid that is a similar antioxidant to the super-powerful quercetin phytosome that we recommend, which can further support vascular and heart health. 

👉Available for $8 less than most retailers only on Fullscript.👈 

What Supplements Will Break a Fast?

You want to avoid some dietary supplements because they will either break your fast, lessen its benefits, or not be absorbed properly. 

At the top of the list is Fat-soluble vitamins. These are vitamins D, E, A, and K.  These are absorbed best with food, or at a minimum, they need healthy fats to be absorbed properly.  So, without having any food, it is best to skip these supplements when you’re fasting. 

If you enjoy starting your fast with some MCT oil or bulletproof coffee to get into ketosis faster, you choose to take these supplements on day 1 of a prolonged fast. 

Be sure to avoid gummy vitamins as well as these often have sugar and calories that could slightly effect your blood glucose levels and hinder your ability to stay in ketosis or trigger fasting benefits.  

Other supplements to avoid are those that may cause your blood sugar or blood pressure to drop. Some people notice magnesium can drop their blood pressure, and berberine, apple cider vinegar, and cinnamon can lower their blood sugar. If you’re fasting, it may be best to avoid these.  

Lastly, outside of prebiotics, try to consume zero calories even if you do some light physical activity. 

Remember that protein powder and supplements with amino acids such as branch chained amino acids (BCAA’s) or essential amino acids (EAA’s) will signal to turn off some of the cellular cleanup or autophagy processes.  Because amino acids are used for building, they trigger the body to stop breakdown and focus on building, the opposite of what we want during a fast. 

What Can You Have While Fasting?

We recommend sticking with the items below while fasting:

  • Water with electrolytes
  • Plan water
  • Black Coffee or tea
  • Proteolytic enzymes
  • Probiotics
  • Prebiotics
  • Water-soluble vitamins such as vitamin c
  • Trace minerals


Probiotics and prebiotics do not break a fast; including them in your fasting routine can improve gut health and support your immune system. 

While some may wrongly label taking supplements during fasting as a “dirty fast”, they can improve the effectiveness of your fasting, and we personally always consume supplements during our prolonged fasting as the positive effects far outweigh the risk. 

If you have gut issues that you’re looking to overcome or are trying to figure out how to optimize your health, check out our virtual nutrition coaching packages and book a free 15-minute call to get started on optimizing your health now.