I genuinely believe that a clean fast is the key to successful intermittent fasting, based on everything I understand about IF. After I explain why this is true, you are going to want to read all of the anecdotal stories from members of our intermittent fasting support groups, which are below my explanation of what it means to have a “clean fast”.  Their stories are even more powerful than any of the scientific reasoning I am going to share with you in this blog post.  In fact, you may want to scroll down and start with their stories, and then read the part written by me.  Their stories are that compelling.

Before I get started explaining the science behind a clean fast, you may wonder: where did the term “clean fast” come from?  That’s a great question.  I am not completely sure, but I think we made it up in our Facebook support groups.  I believe that one day, someone (it might have been me, but it could have been someone else) described the ideal fast as “clean”, and the term stuck.  Maybe one of us read it somewhere else, but regardless of where the term originated, I think it is a beautiful way of describing what we are looking for during the fast.

So–what IS a “clean fast”?  To understand that, think about WHY we are actually fasting.  Believe it or not, intermittent fasting is much more powerful than just being a means to eat fewer calories each day, and there’s a lot more to it than the simple fact that we may be eating less food than we used to.  It’s important to understand that during a clean fast, our bodies are able to do many things that don’t happen when we are in the fed state.  We can access our stored body fat more efficiently and we are more likely to experience certain body processes such as ketosis and autophagy, which do many amazing things within our bodies related to health and longevity.

Keeping in mind that we want our bodies to have the optimum conditions for both fat burning and autophagy, it makes sense that we want to limit anything that would disrupt any of those processes.  The question is:  what actually DOES disrupt these processes?

First of all, it is important to understand that we don’t want to spike insulin during the fast,because insulin is a storage hormone.  During the fast, we want to BURN fat from our bodies. To do so, we want insulin to be as low as possible during the fasting time.  Click here for an absolutely brilliant and simple explanation of how this works, with the key being:  LOWER INSULIN=GREATER FAT LOSS.   From that article:  “even small increases in insulin, within the normal range, virtually abolish lipolysis, or the breakdown of fat.”  WOW.  Do you want to “abolish lipolysis?”  Not me!  BRING ON THE LIPOLYSIS!

Dr. Jason Fung also has a great blog post that explains this how insulin works at this link, which will take you to his website Intensive Dietary Management.  In fact, he has several great posts about insulin on his website.  You can search within his blog if you want to read more of the posts that he has written about this topic, and others.

With this concept in mind–excess circulating insulin will hinder or stop fat burning–doesn’t it make sense that we don’t want to risk spiking insulin during the fast?  As soon as I understood that concept, it radically changed my whole approach to fasting.

What spikes insulin release?  Eating, of course; but sweet tastes also can cause your body to release insulin, since the sweet taste primes your body to expect food with calories.  Your brain doesn’t understand that you are actually drinking a zero calorie diet soda.  The sweet taste tells your brain:  CALORIES ARE COMING!  RELEASE INSULIN NOW!  Click here to go to a scientific study abstract that discusses how an insulin spike was caused when human study participants swished around an artificially sweetened beverage in their mouths–they didn’t even fully ingest it–they just swished it around!

Hopefully, you now understand that your goal should be to do whatever you can to avoid causing an insulin release during the fasting time.  This is why I would never drink anything that was sweetened (artificially or naturally), chew gum, or use commercial breath mints or breath sprays.  I avoid any flavored products (teas, sparkling waters) that have added “natural flavors”, particularly if they are fruity (which means my brain may perceive them as sweet).  Bottom line:  I don’t want to risk causing an insulin spike.  (Note:  I do brush my teeth during the fast, because that is of very short duration, and I only do it once in the morning and once before bed.)

Besides spiking insulin, we want to avoid protein during the fast, because protein has been shown to stop autophagy.  Click here for a link to a blog post about that topic, also written by Dr. Jason Fung.  From that post:  What turns off autophagy? Eating. Glucose, insulin (or decreased glucagon) and proteins all turn off this self-cleaning process. And it doesn’t take much. Even a small amount of amino acid (leucine) could stop autophagy cold.  Once you understand that protein stops autophagy, you would never want to include anything with protein during the fast.  This includes supplements that are often recommended by bodybuilders, such as BCAA’s (Branched Chain Amino Acids).  What are BCAA’s?  They are made up of leucine, isoleucine, and valine.  Wait, didn’t we just read the leucine can “stop autophagy cold”?  Based on that information, if you are interested in autophagy, it’s clear that you don’t want BCAA’s (or any protein source) during your fast.  This also includes bone broth, which some intermittent fasters want to include during the fasting time.  Remember:  bone broth contains protein. As I already mentioned, if your goal is autophagy, you don’t want to ingest protein, so avoid the bone broth.  (Note:  if you are following a multi-day fasting protocol, bone broth may have benefits.  That is not what I am discussing here–I am focusing on intermittent fasting rather than extended fasts.)

What about fat?  Can we have fat during a clean fast?  That is often a sticking point for many people, because even Dr. Jason Fung “allows” his patients to have a small amount of heavy cream during the fast.  If he “allows” it, isn’t it okay? Yes, Dr. Fung (who I consider to be an intermittent fasting superstar) allows a SMALL AMOUNT of cream in coffee.  Why?  I have heard him say that it is for the purpose of “increased compliance.”  I don’t know about you, but I am more interested in the benefits of a clean fast than anything else.  Also, even if cream doesn’t spike your insulin very much, or have much protein (it does have some protein, by the way), it’s still a source of energy that your body needs to utilize before accessing your stored fat.  Do you want to burn the fat from your body, or the fat from your coffee cup?  I know my answer.  Plus, heavy cream makes me ravenous.  Clearly, it makes my body think that it’s time to eat, and I have heard others report this, as well.

So:  what can you have during a clean fast?  A clean fast includes plain (unflavored) water, unflavored sparkling water and mineral water, black coffee, and plain (unflavored) teas.  There are some things (such as a slice of lemon in your water, cinnamon in your coffee, apple cider vinegar, etc.) that fall into what we call the “grey area,” and these are addressed here: can-i-have-_______-while-fasting.html.

In a clean fast, stick to plain and boring beverages.  The fasting time isn’t supposed to be a flavor adventure.

During the fast, avoid any artificially or naturally sweetened products, including all beverages, water or coffee additives, gum, mints, or breath sprays.  Avoid anything that is “naturally flavored”.  Avoid flavored coffee.  Also, avoid anything with sweet or fruity flavors.  This includes anything that may be marketed as “zero calorie” or “doesn’t raise blood glucose” or “low glycemic.”  Don’t forget:  we aren’t worried about the glycemic or blood glucose response here;  we are working to prevent INSULIN release.   Don’t use any protein-containing supplements such as BCAAs, or drink bone broth. Also, avoid adding fat during the fast (particularly to your coffee), since your goal is to burn the fat from your body.   

Bottom line:  the clean fast is much more important than some people in the intermittent fasting community realize.  Whenever you hear someone say that sweet tastes don’t spike insulin, or that diet sodas “worked for them”, or when someone says that they say that they use heavy cream every day and have great results, ask yourself this:  WHY are you fasting?

If you are after maximum health benefits and fat loss results, you want a clean fast.  Period.

Don’t just take it from me.  I posted a question in one of my Facebook support groups a few  days ago, asking if anyone had anything to share regarding the importance of a clean fast.  Even though I understand the importance of a clean fast, the comments still BLEW ME AWAY.  So, with no further ado, I am going to share these comments with you.

If these amazing stories don’t convince you to make sure you have a clean fast, then nothing will, and you’ll just have to stay dirty. 


RENNAE:  Clean fasting was a revelation for me. Previously, I had used cream & stevia in coffee,
dilute fruit juices, and breath mints. I felt restless and thought of food constantly, and just had to “push through” the fasting period. Clean fasting made all the difference in a hunger free and mentally peaceful fast, with accelerated weight loss! Now my fasting times flow in an unforced rhythm. I had to get out of my old comfort zone to achieve IF’s full benefit; I have a new comfort zone!

SUSAN:  Hi, I was one of the “believers” that Bullet Proof Coffee was ok during the fast… and finally went clean to black coffee with just cinnamon, I have my coconut oil in my green tea when I open my window and the butter on my corn…I definitely have noticed a difference now in my work clothes getting larger. Gave up my scale 6/4/17 (also clung to that old notion) so can’t quote weight difference. Thanks Gin! Sorry it took me so long to “listen”.

MARY:  I was a diet soda addict for years and I was always hungry. I actually thought I was just the type of person that would never experience appetite suppression. I tried intermittent fasting using the Fast 5 method back in 2012 and I was drinking diet soda during the fast because it was calorie free. I couldn’t stick with the fasting because the hunger never settled down. I tried to give up diet soda but I never lasted long enough (just a few days before I would break down and have one) for me to see any benefits. Finally when I read about clean fasting in DDD, I gave up the diet soda but only in my fasting hours. I noticed a slight difference in hunger. My fitness trainer challenged me to give it up entirely for 30 days. Two weeks in, I couldn’t believe the drop in hunger. I wouldn’t have believed it if I hadn’t experienced it myself. I no longer feel like someone with a broken appetite. I get the normal waves of hunger and am not in a constant state of waiting for the next meal.

STEPHANIE:  I was addicted to flavored La Croix! I had tried fasting, with some success, but I was always hungry, and gave up after a month or two. After reading DDD, I ditched the flavors, and it has been much smoother sailing. I was so hesitant to believe my favorite non sweetened beverage was hurting me so much, but now I enjoy it even more during my window. Three months in, and still going stronger than ever.

JENNY:  I can’t believe I drank coffee with milk for years when right under my nose my mum was drinking it black. She is 85 this year and perfect health and ideal weight, sharp as a tack and my goal. I now also drink black coffee. I’m a slow learner but I feel so good now and am hardly ever hungry.

SARAH:  If I break fast or fast incorrectly, my stomach lets me know immediately and my energy levels plummet. Immediately after I started IF, only drinking water or black coffee, my energy levels improved. This was the first thing I noticed. My body now lets me know what it prefers, and truly it prefers to fast all day.

LILY:  Initially, I struggled BIG time (hated black coffee with a passion) and did not give up heavy cream in my coffee. Though I gave up my bulletproof coffee (butter, coconut oil, MCT oil, heavy cream and Splenda… which was VERRRY difficult to do…) I kept the heavy cream (a little black coffee in my heavy cream was more my roll)… I chewed sugar free gum like a chain smoker does cigarettes… I lost weight… BUT my eczema, cravings and appetite went nowhere… and though my weight dropped… my measurements, exhaustion and inflammation didn’t improve much. I quit the heavy cream, embraced black coffee one careful swig at a time and replaced gum with organic peppermint oil… and VOILA… the weight AND inches dropped. No more crazy cravings or struggling with waiting to eat… I started to enjoy my coffee black (hot and iced). I started to feel and look lighter. I had less pain – inflammation was very obviously reduced… and (drum roll, please…) my eczema went away… even my rosacea calmed down significantly. My body is changing in ways I love. My weight and measurements are dropping. My energy is great and I am not obsessing on food. Clean fasting 20-24 hours (occasionally 16-8 or 18-6) is what works for me.

ANGIE:  I understood a clean fast, or so I thought! The 2nd week I developed a cold and didn’t think twice about using cough drops to help, I was so hungry and couldn’t figure out why until I saw someone mention them as breaking your fast! It’s true, I stopped and things returned to normal. I also on a couple of occasions…getting my haircut and teaching at my co-op, felt I needed a mint to cut the nasty breath and both times I struggled fasting and barely made it to my window. I don’t have those problems when I do a completely clean fast. It’s the only thing I worry about and it hasn’t let me down yet!

KELLI:  For nearly three weeks, I was fasting “dirty” (before reading DDD) with my diet soda with no success. The same week I quit my addiction I lost about 4 lbs. Since fasting clean I have consistently lost weight and haven’t felt better. I haven’t missed my diet soda nor those first 4lbs.

MARILYN:  Gin said apple cider vinegar is in the gray area of a clean fast. Since I started IF I only drink black coffee during fasting period and it works- no hunger pain until my eating window. Out of curiosity since I miss taking apple cider vinegar, especially in the morning, I tried it during my fast..and after a while it only makes me feel like eating earlier before my window opens because of “hunger pain” like before when I haven’t discovered clean fast yet.

INGRID:  For the first month of IF I was doing 16:8 but having a bit of milk in my coffee. I was always so hungry. Since I began fasting clean a month ago I’m often able to go much longer without eating and any hunger pains pass quickly.

VINUTHA:  I am from India, and I love my tea with milk and sugar. I thought how much harm could a little cup of tea do, since I wasn’t eating anything else and limited myself to one cup of milk tea in the morning. But with this I used to be ravenous entire day and I would stuff myself with food once the window opens. But one fine day I thought of giving that up, and now I can sail through my fast easily . My fast is of at least 20 hours and I eat because I was missing food. Not because I am ravenous.

KELA:  When I was only a few weeks in to IF, I attended a professional development that included a boxed lunch. I sat with my colleagues but didn’t eat the lunch, intending to save it for later. During the very last work session, my swamp breath got out of control. I’d not finished reading Gin’s book yet and didn’t know that gum was a big no no. Not 5 seconds after I started chewing the gum, I went from easily delaying/clean fasting to dizzy, lightheaded, and ravenous…which I now know was because of the insulin response caused by the minty sugar-free gum. It was so bad I broke open my box of food right there in the session and ate like it was my last meal. That I night I read the section on gum and never again have I fasted unclean.

AMY:  I started out drinking lime sparkling water during my fast because I was sure that unflavored would be unbearable. It made me ravenously hungry and I was sure sparkling water just made me hungry! I couldn’t get why and nearly gave up on it. There was NO WAY the lime was making any difference! Lime isn’t sweet at all! One day I tried an unflavored sparkling water and magic happened! Hunger quenched completely. Easiest Fast Ever. I’ve never looked back.

MALLORY:  I used to HATE plain water, and I do mean HATE it. Diet soda and water enhancers were my go-to beverages. Finally, enough was enough. After 4 days of 20:4 followed by a 41 hour fast (all of them clean) I woke up and had a glass of water. It was so good. I literally thought to myself, “So this is what water is supposed to taste like!”

LAURIE:  I used to drink diet soda (1978 – 2015) thinking it was OK, zero calories. When I started doing zero calorie down days, diet soda suddenly tasted sickeningly sweet with a strong chemical aftertaste. Revolting. Ugh. So I bought some Perrier and it tasted perfect and quenched my desire for “intake” as well as my thirst. It also torpedoed a plateau, so I realized that aspartame was interfering with my Kreb’s cycle and tripping my blood sugar settings in those little mitochondrial factories in there. I have dispensed with Perrier. Too expensive. Now I drink the cheap stuff, just as good: Canada Dry Seltzer Water.

ROSALIND:  I started IF with a 18/6 window and drank my two usual morning coffees with half and half and stevia instead of sugar. Basically skipped breakfast. I ate lunch and dinner. No snacking and no problems, including hunger, with this regimen. After 3 months I wanted to go to OMAD (One Meal a Day) with a 22/2 window, but found my hunger too extreme to wait so long to eat. I was really hungry by 4 pm when my window opened and I over ate. I was determined to do 22/2 OMAD, so started experimenting. I cut out the stevia in the two morning coffees, but continued to use the cream. Rest of the day I only drink water. This seemed easy enough, but still no real change in the hunger I experienced. Mid-June, after reading Gin’s book DDD, I knew I had to try going to black coffee only. I don’t like tea. This was the key to stopping the extreme hunger issue. Black coffee only resulted in NO cravings, NO extreme hunger or hangry (hangry=so hungry you become angry). I sailed into my 4 pm window opening and ate a meal comfortably without eating so fast, like I had been or over eating. I could relax and enjoy my food. Some days I’m not very hungry and other days I am, but either way, I eat until I’m full and enjoy meal time.

CLAIRE:  Clean fasting is a complete game changer.  I’ve been diligently measuring milk to be under 50 calories for my coffee.  I struggled to fast past 16 hours and lost no weight. Started clean fasting and lost 2 lb the first day and now have no issues with hunger pangs at all.

MICHELLE:  I was popping tic tacs all day at work and experiencing increased hunger…stopped and all’s good, fasting at least 20 hours a day, very little (if any) hunger. I also stopped the flavored, artificially sweetened water except occasionally in my eating window. I’m still barely slogging down the black coffee after 2 weeks though. (Note from Gin:  the black coffee is an acquired taste!)

DEBBIE:  I thought gum chewing and sipping diet soda throughout the day would save me from the feeling of deprivation I thought fasting would bring. But I found out, in reality, it was just stimulating my hunger all day. By tasting the sweetness of the artificial flavors, my body was anticipating digestion. I believe this inhibited appetite correction. The act of chewing gum continually released enzymes and saliva, preparing my body for eating. My digestive tract was in a perpetual state of anticipating digestion. It wasn’t until I gave up gum and diet soda and went to clean fasting with black coffee, unsweetened tea and water that I achieved appetite correction and was no longer reacting like Pavlov’s theory. I then entered a different world of clean fasting. One I never thought possible where I was no longer hungry all day long.

MARY BETH:  Clean fasting for me meant giving up diet soda. I was committed and determined to try this and was stunned at the results. Did it help with weight loss, yes. But the thing it surprisingly had a huge impact on were my migraines. They became almost non existent. No longer having them every week to having them only rarely. It was freeing in ways I never dreamed possible.

MINDY:  I was fasting clean (black coffee, tea, plain sparkling water) for a couple weeks and felt really good, but missed cream in my coffee. Last week, knowing that some other groups say it’s fine to add full cream, I added it to my coffee and loved it. ….until I didn’t. That day was so much harder! I was so much hungrier than I am when I fast clean. That little bit of cream tasted great but my fast was 10 times harder. I am sure now that fasting clean is what enables me to fast with any pleasure at all.

HEATHER:  I did LCHF (low carb high fat) since June 2015. Started IF in April 2016, but always with heavy whipping cream in my coffee. I lost 16 lb and stalled June 2016. I went dairy free, nut free, paleo, still with something in my coffee. Nothing moved. Started seeing a functional medicine doctor in February 2017. I saw her June 10, 2017 and she had no clue why I was not losing:  labs good, diet on point. I even gained inches in my hips and thighs, so she gave my a prescription appetite suppressant. It made me hungry 30 min after taking it.  I read DDD on July 1st. I started clean fasting. 14 days out of 25, I did 19+ hours CLEAN fasts. I lost 5 lbs of fat and no muscle. I measured on July 10th, and on July 25th before my doctor’s appointment. I measured and lost 2.5 total inches in 15 days. Fasting clean now and I eat what I want….no more low carb!!! Loving this!!!

BARBARA:  I have always fasted clean but have since learned that in addition to that, you have to learn to listen to your particular body. For example, I bought some cold-brewed coffee concentrate. Coffee was the only ingredient so I thought it was fine. It was delicious! Drank it in the mornings three mornings in a row and for three days I felt like I was starving, restless, unable to focus. FINALLY made the connection and skipped it the 4th morning– and I was back in Fasting Nirvana. Easy, satisfied, clean energy. Apparently the coffee was so smooth and so good, almost sweet that it trigger insulin spikes for me. The differences between trying to fast with low insulin and trying to fast with higher insulin are like night and day. If everyone would take the time to experience that then NO ONE would choose to fast dirty. Its just not worth it.

MELIA:  I didn’t think using my “go to” calorie free flavor packets would hurt.  I also would drink fruit flavored calorie free sparkling water too during my fasting window.  I was still in the “DIET MINDSET”.  I thought I was doing it right, until I posted about it and to my surprise I was so wrong. That is when I couldn’t wait for Gin’s first book to be done so I could do it right.  I had already read the Obesity Code and kind of got lost in it but understood the concept.  Then there was my coffee. NO CREAMER, are you mad? It took me a good week or so to get to drinking it black.  I tried it with creamer other day in my window and I don’t even like it anymore, it was too sweet.  It is truly amazing that these little changes could make such a difference.

SHERI:  I lost and maintained without clean fasting. I was using stevia in my coffee and stevia sweetened drink mix in my sparkling water. However I struggled to fast for longer than 16 or 17 hours without being ravenously hungry after 14 hours or so. Since fasting clean I can easily fast 19 or 20 hours without feeling hunger pains and I’ve lost 2 lbs in 2 weeks. I have changed my goal weight as a result. Before I took the highest end of normal and went with it. Now I’m motivated to move fully into the “normal zone.”

KRISTAL:  I started 16/8 and would have zero calorie diet drinks. I was always sooooooo ready for my window to open. I would lose a little weight, but more non-scale victories. After reading DDD, I switched to only black coffee (totally sucked at first), tea and water. The very next day, I looked at the clock and had gone 20 hours. I now do 22 or 23 hours and OMAD (One Meal a Day). I have lost 10 pounds since the beginning of July, thanks to giving up my zero calorie diet drinks. Currently I’m on hour 23, sipping my black coffee and getting ready to hike 4 miles in my fasted state.

VICTORIA:  Clean fasting has allowed me to fast so much longer with much more ease. It severely lessened hunger pains and the overall constant thoughts of food.

TRICIA:  I thought I was fasting clean. I thought I was perfect, ha ha. But my weight loss had stalled for months. I was frustrated. I started reading more on clean fasting and I came across an article that suggested that even zero calorie sweeteners in soft drink could break a fast.  Well, I was perfect– I was only drinking water (don’t like coffee). Then the next line terrified me, also chewing sugar free gum could have the same affect. What! I chewed that all day, everyday. That was how I succeeded to fast so well. I thought, I’ll change nothing this week except stop chewing the gum and I’ll see if it affects me. I secretly hoped it wouldn’t have an effect, as the gum was my safety scaffolding. So I stopped chewing the gum, changed absolutely nothing else, nothing. I stepped on the scales, the same scales that had been going up and down by about 500 grams for the last few months and they were down 2.1 kg in a week. WOW. I can’t believe that all that time I’d been chewing gum so as to keep me on track, it was actually the thing that was secretly derailing me. I actually look daggers at chewing gum in the supermarket aisles.  I think I’ll hold a forever grudge against it.

TRES:  When I was drinking diet soda I was hungry all the time, when I started to fast clean not only did the hunger subside so did the bloating.

CYNDI:  When I started drinking my coffee black it was gross but eventually I got to where I preferred it black. It kept me from being hungry which I didn’t know it would do. Made fasting a lot easier.

RONDA:  While I was drinking diet soda, I’d get hungry 20 to 30 minutes later. I never connected the hunger pains to drinking diet soda until I read the books!

BEV:  I started drinking diet soda during my fast, also coffee with cream and sweetener, I read DDD and decided to clean fast.  I did that about 3-4 weeks and decided it was too hard, so I got sloppy again, and went back to my other habits again. I was looking at some comments the other day, and something clicked. I am clean fasting again, and also not that interested in food. Doing 17-18 hr. fasts now. It seems easier this time, not sure what else is happening.

ASHLEY:  Reading your book and identifying “clean fast” was a complete game changer for me. I have never met anyone who drank as much aspartame as I did… diet soda, sugar free coffee flavors, only flavored waters… at one point I was randomly dumping a sweetener packet in my mouth. Ashamed but honest… it was that bad. Clean fasting was a game changer for me and made the entire experience different. Black coffee and water for me.

MARYELLEN:  I was doing 16:8 when I decided to do OMAD (One Meal a Day) after much research on OMAD. Found Gin’s book Delay, Don’t Deny and read it which brought me to her Facebook groups. I always wondered why if I”m eating 2 small meals (possibly less than my mother who doesn’t have a weight problem) I was gaining and she wasn’t. It was the fact that I was drinking my coffee with cream and the fasting time before OMAD was not clean. 2 days after I came to these groups I decided to leave the coffee with cream for my window and it was life changing. I didn’t feel the hunger or the pains of GERD. It was only after the 3rd week of nothing but water and sparking water during my fast that I introduced black coffee into my fast. I drank my coffee black for 30 years and gave it up in my 40s by adding cream to neutralize my GERD. So going back to black wasn’t that difficult. At first I thought the coffee was making me hungry but it was the added cream that was causing the hunger, not the coffee itself.


Aren’t these stories incredible?

Does a “Clean Fast” REALLY matter?  

I believe that the answer is absolutely, undoubtedly, YES.