Intermittent fasting, cortisol and blood sugar


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.

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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Comments Join the Conversation

  1. Mindi says

    Thank you so much for this posting. I know that I have blood sugar regulation problems and have not known how to resolve them. I appreciate the advice to eat every 2-3 hours- makes sense to me, as I feel like I could eat all day long. The challenge is to figure out how to do this on such a restrictive diet. I am following the Autoimmune Paleo diet, the Candida diet, and also a low-oxalate diet. Based on these restrictions, there are only about 15 foods I can eat. I also have to be careful to rotate my foods because of food sensitivities. Does anyone have ideas how to eat frequently on such a restrictive diet? Recommendations for a nutritionist? I would be grateful for any suggestions. Thank you.

    • sal bisslessi says

      I would advise everyone to get on a low carb, high fat and protein diet. And than after a few moths, begin intermittent fasting. You will find doing both will be tremendous.

      Eat your fruits and vegetables and keep your carbs between 50-100. You will melt away fat and health problems. You will be amazed. EAT WHOLE FOODS. This is a perfect fit for everyone. Try it.

      You see, fasting gives your body a rest period, and releases all kinds of wonderful hormones, fat burning, muscle building hormones that will burn fat in your body. Doing both will keep insulin levels very low.

      • jj says

        That is good advise however doesn’t always work. I have sluggish adrenals and liver detoxification and if ..even after ketogenic for years causes my blood sugar to sky rocket.

      • Michael Alber says

        Sal: I guess you didn’t read this article. Chris Kresser says fasting can cause cortisol levels to increase and affect your blood glucose. Funny, I guess you missed that Chris recommends not to do fasting if you have blood glucose regulation problems and to eat every 2-3 hours.

        • markku says

          But IF has been shown to improve insulin sensitivity.. so if you keep eating ever 2 to 3 hours you are not teaching your body to deal with the problem.

          • Michael Alber says

            I guess it is true to teach your body about dealing with fasting but everyone is unique. First of all we don’t live in the same world as our ancestors so I believe you have to stay on top of what you eat and the chemicals around you. Fasting today is different than our ancestor since anything affects cortisol levels. I’m finding personally cortisol is a problem with my own health problems. I believe stress is a big a factor in our modern lives.

            • musings says

              I sense the problem might have to do with different genetic problems associated with the pathways involved in processing glucose. The glitches may be at different points for different people, with various hormones as potentially flawed. One of the places this is being studied is the Salk Institute. So different genes for different people that control these hormones may be implicated.

              I myself am overweight and I get some activity, although in the recent winter, not nearly as much. My A1c went from 6 to 8 in this period and I have been prescribed Metformin.

              Meanwhile, I know that in the past I did well with intermittent fasting. I believe that my weight gain made me not so much unstable in blood sugar readings, as predictably lousy.

              For now, my solution will be to make sure each meal is calorically less because even if I think I am getting enough activity (say 2 miles of walking per day on average), this is not adequate to deal with the problem until there is significant weight loss. Twenty pounds off, if carefully maintained (not like the last time I did this), will help. Also, probably some vitamin supplements. I know that my family has a lot of type 2 – my grandfather, mother and half my siblings. The grandfather lived to be much older than the average in his day (83) and my mother is alive at 91 (though blind – which is soooo scary for me – it happened in a cascade when she was about 87, perhaps partly to do with a shock to her system with cataract surgery – don’t really know). We may have some “thrifty genes” which allowed ancestors to get through long periods of near-starvation. But like sickle cell anemia being protective against malaria, this kind of diabetes may have saved lives which later were shortened when food was more freely available. However in our having Type 2, my family is not so unusual for Americans since it is rising here.

              Here I am at my laptop, and the level of sitting during a New England winter is far too much to be healthy in any case. There have to be some guidelines – like getting up for several minutes of any hour for all chair-workers. One burst of exercise in the day, as we know, does not help enough.

          • Jer says

            I have IF’d for 3 years – skipping breakfast, fasting 16 hours a day. Every year my cholesterol gets better, and my blood sugar worse. Over 3 years it’s gone from 99 – 107 – 124. I’m going to give breakfast a shot, and maybe even eating every 2-3 hours.

  2. says

    I have a feeling that one of the answers to the increase in blood sugar might lie in Stephen Gordon’s post further up the page.

    We are broadly applying the thinking that ‘paleo’ or ‘IF’ etc diets may be helpful because they are more in line with the nutritional environment that we evolved for. It would then make sense to modify other aspects of our life in the same manner.

    Perhaps the increased blood sugar that results from these diets is actually a natural healthy response if we look at the big picture. Maybe our body is moving back towards its natural apex physiology which suits a much higher/intense activity level (hunting, gathering, fighting predators etc) than we are exposed to in modern sedentary society?

    I think Boyd-Eaton(?) had some papers on the activity levels of indigenous/paleo peoples compared to today’s society. Showing that the old “30 minutes of moderate exercise 5 days a week” was nowhere near the levels of activity required to trigger healthy cell function.

    In fact the 30 minutes thing has only ever been the bare minimum required to avoid a diagnosed sickness – which is actually a totally different goal to creating health.

    I wonder how many of the people here have introduced a lifestyle change to a fairly high/intense activity level such as Stephen Gordon’s boxing training etc alongside the IF nutritional changes?

    Also, what about the mental-emotional component of our lifestyle. If this is not also changed in a manner that honours health it can easily contribute to amplified/misplaced endocrine responses. This could also make an otherwise healthy diet appear to be unsuitable?

    Everything is connected – the way we eat, the way we move and the way we think are all interrelated when it comes to non-linear endocrine and neurological feedback loops. Helping the body make its way from high allostatic load back to homeostasis requires an orchestration application of lifestyle change.

    Hence, I wonder if it is not so much a failing of the IF diet, but an erroneous expectation that a nutritional change alone is a fix-all solution a highly complex problem… Seeking to make food a natural ‘medicine’ to fix a diagnosis, rather than seeing it as an integral aspect of a living environment which facilitates for the creation of health.

    Without requisite changes to the interdependent physical and mental-emotional aspects of life we may be compromising or confounding the IF/paleo diets elgance or effectiveness.


  3. Larry says

    Yes doctor, you’ve got the answer why my blood glucose is much more higher at the end of the day than when I get up during fasting.

    I’m so confused whether to take my medicine or eat my low-carb diet or what? Now I’ve got it to to eat every 2-3 hours but moderate amount of food.

    Is there some way to control or reduce cortisol during fasting?

  4. Laura says

    TL;DR: I’m an archaeologist. The paleo-diet is not “one size fits all.”

    I’m an archaeologist who specializes in the studies of diets of hunter-gatherers. I’d say eating every few hours is actually pretty normal. Hunter-gatherers spend a large amount of time foraging, during this time they’re snacking more or less constantly. Like, if you go out picking berries, you’re not going to snack on a few? Also, hunters would usually take food with them.

    What we have to remember is that the hunter-gatherers you see today are disadvantaged, fourth-world peoples who have been forced into marginal environments. Many of our ancestors lived in much more accommodating environments.

    Furthermore, hunter-gatherers live (and lived) in wildly variable conditions, with wildly varying diets. The Inuit eat 90% meat whereas the San eat 90% plants (give or take). If your body tells you to eat every few hours, then do it. It might have something to do with hundreds of thousands of years of your ancestry.

  5. Al says

    Hey thanks for the post. It was very helpful.

    Over the past 3years, I have lost about 30lbs by just eating meat and vegetables with regular exercise. I am a 5’8″ male and weigh about 210lbs.

    I believe I hit a plateau. I am looking into intermittent fasting. I want to start very slow. I find that I have more bowel movement when I fast, once/week. I came to use fasting since I found that after about two weeks of intense/cardio exercise (5x/wk) and eating 3x/day, I was tired and my weight fluctuated between 205-210lbs.

    What are your thoughts?

    thanks so much again !

  6. Melissa says

    I just switched over to a 30 day paleo meal plan from “Practically Paleo” from a typical diet high in carbs and sugars. I have PCOS and to help prevent insulin resistance I am on metformin 500mg twice a day. My question is this: I am 4 days in and seeing spikes in my blood glucose levels. I monitor with a meter as you suggested in one of your articles. I typically have very normal non-diabetic numbers for blood glucose. Now I am seeing my baseline at 93-95 and my fasting at 100. Is this typical? should I see it level out and return to my old levels if I just stick with the paleo diet?

    • says

      . getting too much protein can work against the metformin
      by raising glucagon that increases liver-made sugar;
      remember, the magic of the Atkins diet
      in the way it lowers blood sugar and cholesterol
      is not just from being low in carbs,
      but also by being high in fat
      rather than high in protein .
      . try getting less than 50g protein per day;
      and get more gentle, low-glycemic carbs
      like greens, cabbage, broccoli … .
      — peas are a low-toxicity, nearly-Paleo legume
      and hence a good carb according to the Perfect Health Diet .

  7. Gloria Cole says

    I have been intermittent fasting for several years now. I stop eating at 4 pm and start again around 9 or 10. I would not eat every 2 or 3 hours — that would only run my bood sugar up. For me eating is a problem, I feel much better when I dont eat. I eat 2 or 3 small meals a day. More ketogenic than paleo. I am diabetic and hyperthyroid. I am now off insulin. A1c 5.4. So Im still working on diabetes. This summer I started having unreasonable fasting glucose readings–to 180. It turned out to be due to deydration — and a faulty blood glucose monitor. I can recommend IF as a means to deal with diabetes.

    • Janet Ruffin says

      Hi Gloria,
      Thanks for your post. Can you clarify your IF for me? You stated that you stop eating at 4pm and you start eating at 9-10. I assume that is 9-10 AM but don’t want to assume….

      I have been doing IF from 8p-about Noon…. having my first meal at lunchtime. The morning I am drinking some bulletproof coffee. Gets me through the morning fast…

      Trying to get my morning BS’s below high 90’s. Some days it is and others it isn’t. Having quite figured out the trick yet….
      I’ll just keep on working at it.

  8. anna says

    Sigh! I eat a low-carb, ‘clean, wholefood’ type of diet (spend quite a bit of time in nutritional ketosis), and I fast daily for about 18 hours, plus a 24 hour fast weekly.

    I LOVE fasting – it gives me a sense of control, and wonderfully level energy, and for the first time in my life, I am free of cravings.

    But my blood glucose sux – fasting can be as high as 7, and I spent most of the day at around 6. It drops to 4-5 soon after eating (the strange patterns you mentioned). HBA1c has gone 34mmol/mol -> 35 -> 37 over the last year (which my Dr says it completely normal, as is my BG).

    I am very frustrated – I desperately don’t want to stop fasting and go back to feeling out of control, hungry, and like I am on an energy rollercoaster.

      • anna says

        Except I’m pretty sure I am not high protein – I am a pescetarian, but my diet is largely planted-based. I worked out I am eating about 50g/day protein.

        I had a look at Zone, and why it may cause high BG – but if it was a cortisol issue then I think I would be experiencing loss or muscle mass, possible weight gain etc, but I’m 14% body fat, and pretty strong, so I’m not convinced it is that. Interesting trying to work it out.

        • says

          I wasn’t referring to your cortisol;
          the zone diet says dietary protein raises your glucogon hormone levels
          which cause your liver to produce more glucose;
          you don’t need 50grams per day unless you’re doing major weightlifting work .

          • Janet Ruffin says

            PhT. – I believe that this is what occurs with me. If I eat more protein than 50, my morning FBS seems to be elevated (high 90’s to low 100’s). If I eat limted protein the day before, I have been getting FBS’s in the 80’s. I am finally getting to the bottom of this. I think I am very sensitive to carbs and protein. Too much protein is not what is needed for me.

    • sal bisslessi says

      Fasting and consuming high fat only gets very old. If I fasted as much as you did, I would be ripped to shreds.

      You never want to totally get rid of wholesome foods, like vegetables and fruits. I understand fruits have sugar, but eat small amounts.

      I seldom use ketosis. I like staying between 50-100 carbs per day.

      I like ketosis in theory but giving up my fruits, can’t do it. Wholesome nutritional foods have too many benefits for the human body.

      Ketosis is overrated. 100 grams and below should do you well. You need some carbs to keep your body to trigger bother energy sources fat and glycogen. Have the best for both worlds just do not blow out you cabs consumption. Works for me.

  9. Ben says

    Cortisol is a hormone that’s released when we are physically or mentally under stress (the body can not tell the difference between the two) the reason that blood sugar increases is because your body thinks you are about to be physically active. So it provides glucose for your muscles… Even if you aren’t going to use them

  10. Janet Ruffin says

    I find this very interesting. I believe that I am similar. If I don’t eat for a long period of time, my fasting BS is high 90’s to 110…. When I eat prior to going to bed, my fasting BS is better. I think I am going to have to do some more reading on this to get a better handle on this. I had no idea. Thanks everyone for sharing your stories.

  11. Aimee says

    Thanks for these fabulous posts! It is refreshing to hear some data from a practitioner who is NOT just pushing pharmaceutical solutions. I am attempting to lose weight low-carb style. I know I’ve been insulin resistant for years. After a week and a half averaging 14g of net carbs per day, I’m only losing a small amount of weight, and my blood sugar is hovering between 95-100 (fasting and after meals). Why would blood sugar remain so high, and why am I not seeming to stay in ketosis with heavy carb restriction? Thanks!

    • Janet Ruffin says

      I’m anxious to hear some responses as I am in the same boat as you. I don’t think I am as low as 14 for carbs each day…. that’s pretty incredible.

    • says

      . if 14g of carb hinder ketosis,
      consider less-glycemic carbs, eg, for 2 weeks
      try your only carbs are a soluble fiber supplement
      and remove conditions causing insulin resistance
      like caffeine, alchohol, other meds .
      if stress or lack of sleep is causing
      too much cortisol and not enough DHEA,
      do weightlifting or peak resistance isotonic exercises .

    • jh says

      moderately educated theory here:

      protein, especially in excess(high) amounts as in many low carb diets can undergo the gluconeogenesis pathway. If you were eating a traditional/standard american high carb and relatively frequent meal schedule prior to switching to low carb but high protein, perhaps the body is trying to do what it’s used to – using the enzymatic pathways that are ramped up – burn glucose for fuel. To get the body shifted to fat burning, we have to eat fat and moderate/adequate protein (approx under 100 g perday) and fasting should help with that, but it takes some time for the body to build up the fat/ketone burning enzymes. Apparently, I believe I heard in an interview with Tim Noakes, full ketoadaptation can take years.

  12. Kerrie Dolan says

    Hi Chris:
    This sounds like me! I’ve had reactive hypoglycemia for years. Now I’m dealing with post-menopause while on bio-identical hormones. Meanwhile, my fasting blood sugar is hovering around 100, my cholesterol is over 200 even on a statin & I just feel old & gross. I’m 51 & weigh 170. Yuck! I know eating my husband’s BLT, tuna melt or chicken parm w/ garlic bread along w/ several glasses of red wine on a Saturday night aren’t helping! I know I need to exercise but I feel so wiped out after work & a 40 minute drive. I also have a spouse & an 11 year old (which is great) but does take some time. Sorry this is so whiny but it’s been hard to find sound realistic diet & exercise advice for women my age! I’m having a hard time with sustained motivation! Thanks!

  13. c says

    hey everyone- i have a question. can anyone tell me why i get horrible hypoglycemia, weight gain, and just feel horrible when my cortisol is LOW? i have had cushings disease, and when my cortisol is high i immediately lose weight, feel wonderful, and my blood sugars normalize. this is the opposite of what should be happening. any advice is greatly appreciated. i recently had to have my adrenals removed because of the high cortisol- and the cure from cushings and the “low cortisol” has brought on serious weight gain, blood sugar issues, and pre diabetes. it is insane. any a dice is greatly appreciated.!

    • says

      if you felt great, why remove your adrenals? just kidding ;)
      too much cortisol is like too many pain killers …
      you feel great but you die young .
      . usually people who feel bad (as from low cortisol)
      abuse food — which would be the likely cause of your
      “serious weight gain, blood sugar issues, and pre diabetes.”
      . it’s not high cortisol that causes blood sugar instability;
      rather, insulin rebound from a glycemic (grainy sugary) diet
      (which mean having too high then too low of blood sugar)
      causes cortisol to be raised in order to
      protect the brain from low blood sugar
      (cortisol causes insulin resistance in the body
      so there is a guaranteed supply of sugar for the brain).

    • Rachel says

      From what I’ve learnt on my MSc in personalised Nutrition, when your blood sugar levels are low, your body needs to pump cortisol & adrenaline from your adrenals to raise your blood sugar levels, so if you can’t produce enough cortisol to do this, your BS will remain low. It also uses glucagon from your liver to turn glycogen into glucose but if glycogen is depleted through low carb diet then you won’t be able to increase your blood sugar that way either…just a thought

  14. Sonya says

    I didn’t hear any mention of time frame. Yes, glucose may temporarily be worse, but is blood pressure when you exercise. What matters is the long term benefit.

  15. Sky says

    Hi thee, Just got diabetes, eat no carb or sugar for 3 weeks now. My FBS was 283, now when I test myself in the morn it’s 220-240. As soon as I eat something it drops to like 178. & steady or drops more throughout the day to never as low as 173 so far. Still working on that. But my post meal never spike more then 20 points. Sometimes it only spike 2 points. I try to always snack & morning readings is still high. Fasting about 12 hrs. Please help. Any ideas? Or do I need to be patient, it’s been only 3 weeks. And am I lowering the numbers fast enough? Is hardly any post meal spike more important then the over all high number.? HELP!

    • says

      I believe waking up with high glucose
      is a sign of too much protein;
      get most calories from monounsaturates or mct oil,
      get a little fish oil, and cut back on vegetable oils (omega-6).
      . get most protein from raw yolks
      (Eggland’s Best or pastured eggs).
      . get carbs that increase insulin sensitivity
      like greens and brocolli . puree the hot greens in olive oil .
      take supplements that enhance metabolism:
      carnitine, alpha lipoic acid, Q10, mct oil .
      . if that wasn’t working for me,
      I would see my doctor for metformin (Glucophage).
      google this: metformin

      • Amy says

        PhT, who are you and what is your background/what are your credentials?

        You’re giving out a lot of advice here and IME not all of it is good.

        Be careful what you say to people.

        • says

          . I get the thread is about
          hyperglycemia due to high-stress cortisol;
          but if a person is still not getting results,
          then according to the zone diet’s Dr.Sears,
          high protein will raise your blood sugar levels .

  16. Stephen Gordon says

    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!

  17. jen says

    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.

  18. Joe Smith says

    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.

  19. says

    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!

  20. Julie says

    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…

    • Ryan says

      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.

  21. ambreen says

    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?

    • Michael says

      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

  22. Lynn says

    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.

  23. rdzins says

    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.

  24. says

    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.

  25. Peter says

    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.

  26. Margaret says

    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!

  27. shredded says

    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…

  28. jana topinkova says

    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?

    • stephanie says

      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!

  29. Stephanie says

    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.

  30. Stephanie says

    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!

  31. Andy says

    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.

  32. Eggie says

    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.

  33. Susan says

    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).

  34. Dave says

    Very informative article. I am following IF with pretty low carbs for weight-loss and despite having pretty low bodyfat, around 13%, I find my fasting blood sugar to be in the 90’s no matter when I test it, after 8 hours, 16 hours, or 16 hours + light cardio. The strange thing is, that even after a high-carb meal, blood sugar goes down to 82-83 or so, and stays there for a couple of hours.
    I have diabetes in my family and am worried I could make things worse with the current pattern of low carb IF + high carb refeeds 2x/weekly.
    I’d like to add in to Ryan’s question about carb/calorie levels during the normal meal pattern test period. Should I also switch to a moderate carb diet and near-maintenance level calories before I check for improvement?

  35. Ryan says

    Interesting post. This sounds like it could be the cause for my high fasting glucose. In addition to my high fasting glucose, my total cholesterol has been creeping up, to where it is now over 300 last time I had it checked. Could the high cholesterol level also be caused by excessive IF? I have been using daily IF for well over a year, sometimes one meal a day, along with heavy weight training 3-4 times a week.

    How long does it usually take to see improvement when switching from IF to several meals a day?

  36. jackie says

    is IF not recommended for people with adrenal fatigue? and what if you have cravings from neurotransmitter imbalances. i also have hormonal imbalances. in other words, should people who do not have optimal health be IFing at all?


  37. Sharon says

    Chris..thanks for this. Very interesting!
    I’ve been intermittent fasting since July 2011 using the Warrior Diet concepts. Big meal at night, undereating during the day, using whey concentrate and water before and after workouts, etc. I was only 116 lbs when I started and got to about 108.
    My post prandial numbers were fine and my AM Blood fasting numbers were low to mid 80s since I got my glucometer in November.
    For the past week, I have tested randomly in the mornings. I have been getting readings of 70, 73, 78 and today I got 65. This scared me a little.
    I’m wondering if it’s a good idea to go back to 3 meals a day..with some snacks.
    Also, I eat pretty low carb and have been for quite a long time. I’m adding a lot more fat than usual lately since the high fat/mod protein theory has come about.
    Any thoughts? Should I be worried about these low AM fasting numbers and what can I do to correct it?

  38. Mike Ellwood says

    This makes sense. I doubt if our ancestors would choose to go 12-14 hours without food if they didn’t have to. Fasting is a stressor, and cortisol is a stress relief hormone.

    How do we know that our ancestors didn’t take “snack” food with them on hunting expeditions? (e.g. dried meat or pemmican). I’m sure the more intelligent groups (i.e. the more adaptable and more resilient and the ones more likely to survive and propagate) would have done.

  39. says


    I also wanted to add something to my last post. Sometimes at night my blood sugar will feel low. I will check it and it will be in the 90’s. If I don’t eat anything, I will continue to wake up off and on and feel bad in the morning. If I do eat something then my numbers will be up higher when I check them in the morning. I really want to maintain good blood sugar control but short of not eating at all, I am not sure what to do.

  40. says

    I have been hypoglycemic since I was young. I had borderline gestational diabetes with one pregnancy and had gestational diabetes with another. My endocrinologist told me I was fine after my baby was born when I had a follow up appt. She said I wouldn’t have any problem as long as I continued to stay thin, which I am. A few years ago, I had a blood test for life insurance and saw that my A1c was 6.1 and my fasting was 78 so I knew there was a problem. I have been low carb since then but my fbs now is usually around 90-100. After a low carb meal, it usually stays under 120 but hangs around 98-105 2-3 hours after eating. It takes several hours for it to come down into the low 90’s or 80’s. Do you think eating every 2-3 hours would be helpful? I recently tried pgx for blood sugar control which kept my bs low but I think I had an allergic reaction because I was very itchy. Do you have any suggestions?

    • Amy says

      Cristie – Hi! I wonder what has transpired for you over the last few years since your post. I have had basically the exact same presentation of dysfunctional blood sugar regulation. I rarely run across someone whose story fits mine so closely. Would love to hear what has transpired and what you’ve learned. I wasn’t tested in my first pregnancy at age 31, and ended up very, very sick with HELLP Syndrome. I was borderline gestationally diabetic in my second and third pregnancies. I’ve considered myself “diet-controlled” pre-diabetic ever since. Basically, anyone who has gestational issues has underlying dysfunction and it’s just a matter of time, diet and lifestyle. My numbers have been on a gradual decline over the last 7 years since my first pregnancy despite my fitness, thinness and low carb high protein diet. These days I can’t get my fasting under low 100s no matter what I do. This morning it was 131 and I cried. Anyway, hope you are still getting the email follow-up for this post! Hope you are well!

      • Tara says

        Christie, Amy, are we all the same person? :) I don’t know if either of you are still reading this, but I have a very similar story. After gestational diabetes with my second baby, my blood sugars never returned to normal. I have a BMI of 20, I am fit, I have great cholesterol numbers, I eat a strict low-carb diet. It doesn’t make sense and my doctor has no explanation. For two years now I have been diet-controlled, with my a1c right around 5.6. But my fasting blood sugar, which always used to be normal, is creeping up now into the 90s. I’m just not sure what else I’m supposed to do.

        • says

          Add me as a 4th with the same issue. :P I also discovered prediabetes during pregnancy, have had to take out more and more carbs to keep blood sugars low and stable. And lately, they’ve crept up, too, where fasting is ~90 in the morning. It sometimes used to be 70! I’ve read about physiological insulin resistance from a low carb diet, but I don’t think that’s it. When I add carbs back, I feel awful, on that blood sugar roller coaster. I do only eat 3 low carb meals a day–meat, veggies, and coconut oil.

          • christina says

            Hi, me too. I had gestational diabetes with my second baby, however, it was gone after baby delivering. Last month my a1c test was 5.7! I am prediabetic, but fasting was 68! Then, since i cut almost carbs (sweets, noodle,rice…), my fasting increasing to 100!!!! two hours after one cup of soy milk in the moring, raising to 190!!!
            I have no idea…

  41. says

    you mentioned at front of the article that some IF’ers had high fast glucose, then, listed 90-100 as high? I always thought that was “acceptable”? I’d love to have a fasting glucose of under 100– haven’t had that since I was born.., I think? I was 120 fasting at one time, since have gotten down into the hundreds with IF, and just loosing wt. and healthy life.

  42. Cg says

    I have Crohn’s disease and have been interested in giving my gut more of a rest by intermittent fasting, just having an 8 hour feeding window, the Lean Gains approach. However, I’m flaring and am currently on prednisone, which I know raises your blood sugar and cortisol. Am I making things worse by intermittent fasting while on prednisone? Also, do you recommend IF for people with autoimmune diseases like Crohn’s, or are our adrenals probably too taxed to be doing something like this? What are your thoughts?

    • TV says

      Hey Cg,

      I, like you, are a crohn’s sufferer, and contemplating intermittent fasting for the reported healing benefits. Currently undergoing a flare as you were and i’d be very interested to know how you got on if you ultimately decided to go ahead with IF.



  43. Trevor Edmond says

    I’ve been searching long for a discussion on fasting and cortisol/cortisone levels.
    Several years ago I came across published results on the effects of fasting. This was publicised on (ABC Radio National – Health Report) and the Sydney Morning Herald.
    18 hours of fasting each day caused
    production of glucocorticosteroids sufficient to counteract inflammatory responses
    stimulates production of growth hormone
    strengthens the immune system
    With this regime I have managed to control (poly) arthritis in my body. I have a gouty arthritis condition. Stopping this regime brings on severe joint aches and pains within two or three weeks.
    Unfortunately I have developed type 2 diabetes. Together with a previous condition of sleep apnoea and a sleep requirement of 5 hours or less per day has precipitated a complex metabolic outcome for me.
    I continue to fast 16 to 18 hours a day. I feel all the better for it.

  44. Warthog says

    I have had “reactive hypoglycemia” for years, and had kept it under control by very careful staging of meals and snacks. My experience is that “intermittent fasting” works BETTER than such staging. I do IF three days a week (20 hours with an eating window of 4 hours). My blood sugar is MORE stable, I have MORE energy, and my mental clarity is better than on the days I eat normally.

    • chriskresser says

      That’s interesting. I suspect cortisol dysregulation is the x-factor which determines whether people do well or poorly on IF.

  45. Samantha Price says

    Perhaps the reason for the continued blood-sugar control issues in patients going paleo is rooted in under functioning adrenals. I have been following your blog for a awhile and I do not recall you ever addressing adrenal fatigue issues which are rampant in our super fast paced culture. I know that thyroid dysfunction is an area you specialize in and there are many hypothyroid patients that potentially have adrenal fatigue as well.

  46. chriskresser says

    Thanks for your comment, Paul.

    I think your last suggestion of including fiber and ensuring enough coconut oil during the fast is key. I can see that helping to balance the blood sugars and prevent cortisol – and thus glucose – spikes during the fast.

    It’s certainly an interesting topic and one well-worth exploring further.

    • mark says

      How is “taking coconut oil and fiber-rich foods like berries and leafy green vegetables during the fast” considered a fast?

  47. says

    Hi Chris,

    It’s a very interesting and complex topic, and great to hear your clinical experience.

    Fasting is a stress on the body, but it is normally a healing stress, because it stimulates cells to exercise metabolic pathways that are damaged in people with blood sugar control issues.

    Like resistance exercise, which is a stress that builds a stronger body, IF can be a stress that makes cells function better.

    So the fact that people don’t deal well with fasting doesn’t necessarily mean they shouldn’t do it. But it’s good to fix diet and nutrition first. A well-nourished body will get the most benefit and the least stress from fasting. (Thus, in our book, fasting comes in Step Four, the last step toward health.)

    In regard to high fasting blood glucose, one should also look at other causes — like very low carb intake. If people eat sufficient starches during their feeding period, fasting blood glucose shouldn’t be elevated by fasting.

    Also, taking coconut oil and fiber-rich foods like berries and leafy green vegetables during the fast can help reduce the stress of the fast, provide some benefits from ketosis, and improve gut flora.

    • mark says

      How is “taking coconut oil and fiber-rich foods like berries and leafy green vegetables during the fast” considered a fast?

  48. chriskresser says


    If you limit your snacks to high-fat, low-to-moderate protein choices, they shouldn’t spike your blood sugar/insulin so much. Eat just enough between meals to keep your blood sugar stable.

    Mike: sounds like a reasonable approach. Let us know how it goes.

    • says

      Hi Chris

      Great post. You mention “blood sugar issues” and irregular patterns, but what are the negative health effects of these? It seems to me that may people learn to live with that without ill effects.

      Look forward to your insights on this.

      Much appreciated,

      • says

        I have had issues with blood sugars for a year now. they drop low, 50-60 and I feel shaky and anxious.
        My fasting Insulin level was 12. I switched over to a paleo diet and now my fasting insulin is <2 (in june) and 4 now.
        My fasting blood sugar is between 83-86.
        My morning cortisol is 10.6
        I have gained 15lbs!
        Oh and my fructosamine is 283 (borderline high)
        my A1C is 5.
        something is not adding up.
        anyone have any ideas?

        I am eating 3-4 times a day and have a snack when I feel I need it, avacado or yogurt.

        typical day
        eggs with greens and pancetta
        decaf coffee with coconut oil/butter and tsp coconut sugar, oh and a bit of whole cream
        steak/chicken and vegetables and olives
        yogurt or goat cheese
        meat and veggies

        • armando says

          I am pretty sure you are not even counting calories it doesn’t matter whether you fast or not,it is all calories in calories out. Intermittent fasting is only good if you are trying to build muscle while gaining fat

  49. Deidre says

    Thanks. I may try it. I find eating to 3 times a day to be so much easier, plus grazing tends to get me into trouble. I definitely have reactive hypoglycemia, and I hated to see them up, so I figured if I minimized eating, even though my net carbs are usually under 30 per day, it would help. This is all very interesting, and there are so many differing points of view. I know that as long as I am paying attention and not following any advice from the ADA, I am much better off. Love your advice on this blog! Thanks again.

  50. Mike says

    Good points! I tried IF for muscular definition, etc, and it did not work initially. If anything, I felt like I had more stubborn belly fat. I am trying IF again now (“leangains” approach). I am careful to keep my (subjectively measured) cortisol levels down and to only IF when I feel like I’m not stressed. I minimize coffee, get adequate sleep, eat properly, then IF. The results seem to be better so far.

    Looking forward to future posts on the topic,

  51. chriskresser says

    It depends on your pattern. For people with reactive hypoglycemia, they have an overactive insulin response after eating a larger meal, and their blood sugar plummets. Then cortisol kicks in and raises it back up – but higher than it needs to go. This creates a “yo-yo” pattern throughout the day.

    You might try eating every 2-3 hours (small snacks between relatively smaller meals) and test your blood sugar throughout the day to see how that changes things.

  52. Deidre says

    Thanks for this post. This is me exactly. Paleo and very low carb, yet higher than I would like blood sugars. I have been trying to do 3 meals a day, thinking that if I didn’t eat snacks, I wouldn’t increase blood sugar post eating and release insulin and would be able to shed that last 10-15 pounds. Is my thinking wrong? I also recently started taking some herbs and cinnamon to try to bring it down.

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