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Intermittent Fasting, Cortisol and Blood Sugar


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There’s been a lot of discussion about the benefits of intermittent fasting (IF) in the paleo community lately. Paul Jaminet mentions it’s role in boosting the immune system in his book, The Perfect Health Diet, and IF can also be helpful for those trying to lose weight and tune their metabolism.

From an evolutionary perspective, intermittent fasting was probably the normal state of affairs. There were no grocery stores, restaurants or convenience stores, and food was not nearly as readily available or easy to come by as it is today. Nor were there watches, schedules, lunch breaks or the kind of structure and routine we have in the modern world. This means it’s likely that our paleo ancestors often did go 12-16 hours between meals on a regular basis, and perhaps had full days when they ate lightly or didn’t eat at all.

So, while I agree that IF is part of our heritage, and that it can be helpful in certain situations, I don’t believe it’s an appropriate strategy for everyone.

Why? Because fasting can elevate cortisol levels.

One of cortisol’s effects is that it raises blood sugar. So, in someone with blood sugar regulation issues, fasting can actually make them worse.

I’ve seen this time and time again with my patients. Almost all of my patients have blood sugar imbalances. And it’s usually not as simple as “high blood sugar” or “low blood sugar”. They often have a combination of both (reactive hypoglycemia), or strange blood sugar patterns that, on the surface, don’t make much sense. These folks aren’t eating a Standard American Diet. Most of them are already on a paleo-type or low-carb diet. Yet they still have blood sugar issues.

In these cases, cortisol dysregulation is almost always the culprit. When these patients try intermittent fasting, their blood sugar control gets worse. I will see fasting blood sugar readings in the 90s and even low 100s, in spite of the fact that they are eating a low-carb, paleo-type diet.

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That’s why I don’t recommend intermittent fasting for people with blood sugar regulation problems. Instead, I suggest that they eat every 2-3 hours. This helps to maintain stable blood sugar throughout the day and prevents cortisol and other stress hormones like epinephrine and norepinephrine from getting involved. When my patients that have been fasting and experiencing high blood sugar readings switch to eating this way, their blood sugar numbers almost always normalize.

I don’t think eating every 2-3 hours is “normal” from an evolutionary perspective. But neither is driving in traffic, worrying about your 401k, or staying up until 2:00am on Facebook. The paleo template is there to guide us, but it’s not a set of rules to be followed blindly. This should also be a reminder that there’s no “one size fits all” approach when it comes to healthcare. Successful treatment depends on identifying the underlying mechanisms for each individual and addressing them accordingly.

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  1. Can your blood sugar be higher when you eat a large late dinner meal when you check your blood sugar the next morning?

  2. In 2007 I was diagnosed with diabetes Type 2, cholesterol of 340, blood pressure 210 over 179. & a liver enzyme count that was headed straight for cancer. I was 57 years old, 5’8 & weighed 223 pounds.

    Luckily for me I spent a lot of time in hippie communes in the 60’s & i already knew a lot about diet & herbal medicine although I had obviously not been applying such knowledge.

    The day I was diagnosed I quit smoking & drinking. I refused all the poisonous meds that the doctors offered me & put myself on a basically vege diet & cut out all sugars hidden or otherwise. This eventually evolved into the Paleo diet which I have been on very strictly for the last 4 years.

    I also started exercising heavily. I had been an amateur boxer & long distance runner in my youth before everything start falling apart in my early 30’s. I started an intense program that has evolved to 2000 crunches 5 days a week, weight lifting twice a week & hitting the heavy bag for 3 sessions of 3 – 15 minute rounds a week. Plus a lot of floor exercise & stretching

    Lastly just under a year ago I took up a heavy intermittent fasting protocol: I eat once every day after fasting between 20 & 24 hours. The results of all this is that I’ve gone from 223 pounds to 142 (what I weighed at 21, I’m 64 now). My diabetes has been gone for over 5 years, I now have low blood pressure & cholesterol of 167.

    I have never felt stronger or had more endurance in my life. The point of writing all this is to let people know that it’s never too late to improve your health. Also, I was never a paragon of virtue & will power. If I can do this as messed up as I was, anybody can.

    Reading the article posted here & the comments got me to write all this as I have never posted on someone’s blog before but this is a terrific site with some great information!

    • Stephen Gordon. Have you ever had a fasting blood sugar problem? I mean high fasting blood sugar?

      I agree with you, intermittent fasting is the best way to fix a diabetes/obesity problem.

  3. I am newly diagnosed with diabetes type 2. I also have high blood pressure. I take nothing for the high blood pressure…and currently on metformin 500mg 1 time daily. When i had a random test two months ago my sugars were 256. Since then i have gone on a low carb diet/high protein. I eat about 25-30 grams of carbs a day. I also walk every day and ride my bike 5 miles every other day. I am 50 pounds overweight. The weight is coming off and the sugars are down dramatically. But not as far down as i would like to see them. My morning is usually around 106, eating walnuts before bedtime helps that. My after meal sugars are usually around 117 to 122 except for dinner which is usually under 140 unless i eat anything with starch- then it zooms near 160. I just cant seem to stop thinking that my morning sugars should be much lower and that i should be seeing more readings in the 90’s. But anytime im in the 90’s which is rare i feel really horrible. I have also noticed that when my period comes thdn my sugars go up no matter what i do and even if i have a simple cokd they also seem to rise a bit. I sleep 5-6 hours a night then nap in the daytime after work. Even on the weekends i cant sleep more than 7 hours. Im so worried that I’m killing myself and that something is very abnormal that even eating a half cup of low carb pasta would send my sugars soaring to 155! I’ve been told that things will get better after i lose more weight…but I’m truly scared about my fasting sugars and the fact that starches of any kind send me soaring. Im 47 and other than being overweight much of my life i dont have any serious health conditions…til now. Any suggestions or info would be really appreciated. Im thinking maybe fasting would help…but of course Im scared to try it.

  4. Hello Mr. Kresser –

    The patients you mention are clearly clinically abnormal, and their results or experiences are not at all representative of those achievable by the general population.

    If conventional dieting strategies do not work for this particular set of patients, its hardly fair to malign IF for not working either.

  5. Can you recommend some instructions re: how to use a glucometer to assess potential blood sugar issues? I’d like to know when I should test, and basic reference ranges.

    I recently began IFing as per PHD suggestions, but I’ve been eating low-carb for nearly four years and have also just begun introducing more “safe starches.” I am trying to understand whether these changes are making things better or worse!

    Much appreciation for this article, and the podcast I listened to today where Chris/Laura discussed various low-carb challenges and possible treatments!

  6. after reading all the comments i have a question: if i have become insulin resistant due to long term fasting and low carb diets- but am not in the pre diabetes stage as yet- is there any way that i can reverse this condition? because frankly, the thought of diabetes really scares me…

    • I believe the answer is “yes, it is reversible”. From reading this post from Peter at Hyperlipid, there are different mechanisms behind what he refers to as physiological insulin resistance (what you experience on a low-carb diet) and pathological changes that causes diabetes.

      On a very low carb diet, your body makes your muscles and other tissues less insulin sensitive to spare glucose for those tissues that absolutely require it (brain, red blood cells, etc.). Once you start eating more carbs, your body will sense the higher availability of glucose and allow your muscles and other tissues to take it up.


  7. okay, this is wierd. i have never had sugar issues before. infact, it almost always was on the lower edge. now suddenly my fasting blood sugar is in 90s. my bmi is good…. so is my blood pressure. my diet is fine as well…. no high carbs or high fats. my h1ac came back 5. last night before going to bed my sugar was 79. yet in the morning it was 96. i dont get it. my peak comes at about 90 minutes after a meal and it is in 120s. so i think my only problem is fasting blood sugar. any suggestions on what i should do now?

    • Hello Ambreen. I recently started IF and I am getting similar results you are. Post-prandial BS is below 85, I eat nothing of course since I am on a fast and take my BS in the morning and it’s 99. I don’t get it. Are you still having this issue or have you found a way out of this by manipulating your macros? Would really like to know. Thank you. — Michael, Tampa FL

  8. I was going crazy with worry before finding this article. I eat low-carb and have been getting blood sugar readings in the 190’s post meals!
    I have been on a low-carb diet for 4 months (less than 25 grams per day). I work-out 45 minutes to an hour 6 days a week (alternating between kettlebells and recumbent bike).
    After a month of this diet and exercise routine (just recumbent bike at first), I bought a glucose meter (ReliOn) and started tracking my sugar levels. After a dinner of spinach salad and chicken and lime vinaigrette, I would consistently see readings of 135 mg/dl an hour after eating, but I wasn’t too worried and would lower the levels with a post meal workout. My morning fasting blood sugar has always been good (mid 80’s). After a 3 egg and cheese breakfast, my levels generally would rise to 115 and fall quickly. For lunch I’d have chicken and broccoli and see 135 an hour after eating. This was consistent for 3 months. I had couple of post-dinner readings in the 150’s, so I had an A1C test done at this point, and it came back normal. My fasting was also normal; my doctor wouldn’t perform the OGTT.
    Then, I started adding weight training to my workout routine via kettebells (love them). Additionally, I started intermittent fasting by skipping lunch at work. My afternoon blood sugar levels are in the low 70’s during this period. I drink one cup of coffee in the morning and another at noon. I eat my normal dinner of chicken and spinach, but now my blood sugar levels are in 190’s a half hour after eating and 170’s an hour after eating. This is not normal. I immediately go and workout to lower them back down, but I really am not eating carbs!
    After reading this, I think it may be attributed to cortisol imbalances from either overtraining or fasting or both. I have a heart condition (LQTS) that requires that I take a beta blocker, which can cause blood sugars to increase, too. But, I really think my issue may be cortisol. As much as I feel good on the afternoon fast between breakfast and dinner, I am going to try to have more frequent small meals or snacks (low-carb and high-protein) to see if I can fix these high blood sugar readings.

  9. THANK YOU! I have been low carb for many years, and after listening to the mainstream of reducing calories, and intermittent fasting gospel, it is good for someone to come out and say hey that doesn’t work for everyone. I even gained weight by eating less and fasting. I am one of those people, the longer I go without eating the higher my numbers go. The problem was also that being I was eating low carb I really wasn’t hungry. Now I have to force myself to eat regularly, this does work for me and keeps my blood sugars below the 100 mark. It would be nice if there was more information out there on this. It is hard after years of being engrained on the the eat less theory that maybe we actually need to eat more.

  10. Thanks for the useful information you are giving people who need to keep their blood glucose level normal. diabetes is a serious health condition and proper dieting is one crucial solution to keep it t bay.

  11. Prolonged fasting or severe calorie restriction causes elevated baseline levels of cortisol. This occurs in conjunction with depletion of liver glycogen, as cortisol speeds up DNG, which is necessary to maintain blood sugar in absence of dietary carbs, protein, or stored glycogen. It seems someone looked at what happens during starvation and took that to mean that short-term fasting is bad.

  12. shredded you are misinformed. It is not a false reading. You have a high fasted glucose level because the low carb has made you insulin resistant. Low carb is bad news for a lot of people and you are one of them!

  13. I did IF and low carb for about 6 months. Prior to that, my fasting glucose was always in the low ’80s. I went for a test in December and it was 96. I freaked out, thinking I was becoming diabetic. Then I read on several sources that prolonged low carb can give you a high fasted glucose level. I’ve heard it’s not dangerous, it’s just a false reading, but it scared me enough to cut out fasting and eat more carbs. I might also add that I gained nearly 20 pounds doing IF and low carb…

  14. I really need someone’s opinion on these hormonal isssues, eating low carb, and IF because I have had some very confusing contradictory experiences, and now I don’t know how and what to eat anymore.
    When I try eating high fat low carb diet tohether with IF (but even without), it always leads me to immediate fat loss (i am a woman with 21% body fat), high energy, glowing skin, no blood sugar issues, and just everything seems great. But after just a week or 10 days, I always encounter some unpleasant effects. Namely: hypoglicemia (for example the other day, after skipping dinner, I woke up with fasting glucose reading of 58, and so I ran for a sugar fix), i get very angry or anxious very often(as if my body is either trying to raise cortisol levels or maybe they re too high already…i dont really know how this works…), I d say it happens especially after a meal of protein and fat (I always eat vegetables, and i have often a glass of wine, so I am far from zero carbs). but it happens also in between meals. Another issue: night sweats and waking up around 3/4am each night (yet it doesnt make me tired). But I feel just very uncalm, I become obsessive, and it is rather a hell. Yet, when I complain, no believes I feel so crappy because it is during these periods that I look the fittest. So then I usually go back to eating pasta and bread, having breakfast. The symptoms disappear immediately. I become calm, I sleep normally, I always start to gain weight. I must absolutely eat 5 times a day food with carbs and protein to avoid shakiness from hypoglicemia. But overall, I certainly feel less energy burning all those carbs then when i have very little (I play tennis competitevily- so I notice immediately whrere my energy levels are). So if it wasnt for all those side effects, I would happily continue eating high fat-moderate protein-low carb. But it just doesnt seem sustainable for me. And having glucose of 58 is even rather dangerous I think. It is probably hormonal issues. When i eat paleo, i become so insulin sensitive that I think I realease too much insulin even just with a portion of fruit and some vegetables. And then the cortisol is obviosly there to raise the glucose to make up for thw catbs I didn’t eat….so I do t know what to do? Should I go back to my standard “balanced” diet of 3 meals (with starches), and snacks inbetween?

    • I agree with everything you say!
      Why don’t you just eat good carbs like veggies. but still follow the protein/fat/carbs diet (but good carbs).
      I think cortisol is a big factor in all this and cortisol regulation is the factor in weather or not your body can handle fasting and low carb. I just don’t think the anxiety and side effects we get are worth it. It is too much stress on the body not eating any carbs. I think everything is so linked… it all gets so confusing!

  15. Also I think it would be worth me using a blood sugar monitor a testing which foods and what amounts keep my sugar levels in the healthy range. As I will do anything to keep those panic attacks away!! They are absolutely dreadful.

  16. Thank you for the article.
    I have hypoglycemia and pcos (hormone issues) and for a period of about 6 months I did fasting for about 20 hours 3-5 days a week. I felt great while doing it, lost a lot of weight! Although one day I started getting horrible panic attacks which I am still suffering from here and there to this day. I believe that it was the fasting, that put such a strain on my body and elevated cortisol levels. Do you agree with this? Right now I kind of follow a paleo, low carb, High protein kind of lifestyle… Thinking this is the best way to keep my hypoglycemia and panic attacks at bay- as they both go hand in hand. My naturopath/homeopath seems to think this is the best way.. Do you agree and what are your thoughts?

    I think fasting is great for those with no sugar imbalances- but I also know that fasting can lead to sugar imbalances! My hypoglycemia was unnoticeable till I started fasting… It was just something the doctor told me I had but I felt no affect from it prior to those months of fasting. So I think people should be careful before entering into fasting as it must elevate cortisol and put a lot of strain on the adrenals and other organs. Now if I feel I need to shed a kilo or two I just eat a very small protein breakfast such as an egg or a small can of tuna, and this really seems to work as eating less in the morning seems to make me less hungry for the rest of the day.
    Sorry for rambling but I would love your opinion!

    P.s just read your article on stress making you fat.. This couldn’t be truer. My body is in such harmony when I have inner peace. The weight seems to fall off no matter what I eat and food digests so easily!

  17. My blood sugar ranges from 84 to 125. Usually anytime I test it, it is around 115 to 125. I haven’t eaten for two days. My sugar began at 135 the evening I began fasting. It was 115 in the morning and 96 that night. It was 114 the next morning and 84 that night. What’s going on is that my liver dumps sugar into my blood in the early morning to fuel my body. I suspect my sugar will be in the 90s in the morning. I think it is important to get one’s sugar down below 85 part of each day – whether it’s before bedtime or when waking up in the morning. Below 85 is when the pancreas stops producing insulin constantly. If you don’t give your pancreas a rest it wears out over the years, and your diabetes gets worse. I need to lose about 15 pounds. I gained 10 over the past two months, and my blood sugar rose slightly as a result. I am beginning to believe that a low fat vegan diet is the way to go.

    • I agree that a vegan diet is the way to go, since I myself have been mostly vegan for some 40 years. Vegan, low fat–no. Vegan plus moderate amounts of healthy fats–yes.

      I still get very high fasting blood sugars, though.

  18. Thank you so much for this article. I started monitoring my glucose two weeks ago and my fasting blood sugar upon waking has never been under 100 – usually in the 110’s. Is this really a problem though? By lunch time, my first meal of the day, it’s back around the high 80s/low 90s.

  19. Would this also explain why glucose readings before going to bed can be lower or the same as when one rises (morning fasting numbers)? I have that problem. (note: I am still learning about this diet and have not implemented it; was following a dietician).