Will Ketones Kick You Out Of A Fast?


You’ve been on a short fast, but your mental energy is lagging, your physical performance is struggling and you feel like you need a pick me up. You reach for some exogenous ketones as this is what the benefits of the products claim, but wait! Won’t that kick you out of your fast? Won’t you just have to start all over since you just consumed something? 

Being as I run a company that makes exogenous ketones and I write a lot about fasting, this question comes up pretty frequently. Both fasting and exogenous ketones are rightfully getting more popular. People are smart enough to combine the two. Kudos. No reason to stress out my little ketos.


The short answer here is no. You will not “break your fast” with using exogenous ketones.You are supplying your body with the fuel it needs, and is already using, to operate while on a fast. You aren’t supplying it with anything else (save for some mineral salts that the exogenous ketones are bound to). 

When your body enters a state of fasting, you end up mobilizing fat, which turns into ketones. Then you end up burning ketones for energy. This is known as fasting ketosis and a huge reason why a lot of people get the benefit that they do from fasting. Beta-hydroxybutyrate (the form of ketone your body most realdily uses) actually acts as a signaling molecule and is responsible for way more than just supplying energy to cells.

As I’ve written before, one of the reasons I often fast is to get into ketosis easily. If this didn’t happen, your simple little human body would perish because you wouldn’t be able to get any energy from your stored fat. Hooray, body!

So, if you fast for any considerable amount of time (24-36 hours) you are breaking down fat tissue to ketone molecules for energy. When you take exogenous ketones, you are just adding the same biochemically identically molecule to your bloodstream that is already there. There is no change metabolically here.  

Your body is also has a bunch of water in it and you probably drink that when you are fasting, right? You’re just adding water to whatever is there already. Exogenous ketones won’t kick you out of fasting anymore than water will. This is under the caveat that there’s not a ton of garbage in the products, however. One of the reasons I make our exogenous ketone products so clean and don’t add tons of trash you don’t need is so you can use them for whatever application you want. If you consume tons of artificial colors, sweeteners, filling agents and additives, then yes, you will be kicked out of fasting more or less. 


The most important thing with fasting for an overall health benefit is to measure your glucose ketone index (GKI). This is just the ratio of glucose to ketones in your blood. You want more ketones than glucose. 

For example, research has shown that higher levels of ketones relative to glucose (above 1.0 ratio) lead to significant amount of cancer cell death. Interested in more about the glucose ketone index? Click that link and you can read more. Click this link to see my fasting results and the calculations I did. 

Tip: To calculate your GKI, just take your blood glucose measurement (in mg/dL) and divide by 18. Then divide by amount of ketones (in mmol). 


If your main goal with fasting is to maximize the benefits of autophagy and cellular recycling, there is some debate to whether you will achieve the full benefits if you consume anything but water. The verdict is still out on this and I assume you’ll get the same benefits as there is no change in energy metabolism. There are still fasting purists out there that are going to scoff at anything you put in your body while on a fast (besides water) so be careful so these people don’t go crazy on you. Don’t worry if they do you can just fast from fasting police. If you’re concerned about this, just don’t have the ketones. You’ll live either way. 


If you aren’t convinced, that’s fine. Enjoy a water fast. Or be an intelligent person and test these things side by side with measuring certain biomarkers so you know exactly how a fast with ketones versus without impacts you. Then you’ll get fast results that look a lot like this. 

Ultimately the answer to this question is “no” and for almost all cases an exogenous ketone is the same molecule your body will be breaking down for energy in fasting anyway. Fast away!