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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. 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?

  2. 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?

  3. 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?


  4. 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?

  5. 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.

  6. Chris,

    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.

  7. 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?

    • 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!

      • 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.

        • Add me as a 4th with the same issue. 😛 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.

          • 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…

  8. 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.

  9. 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?

    • 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.



  10. 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.

  11. 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.

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

  12. 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.

  13. 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.

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

  14. 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.

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

      • . so, when some talk about fasting,
        they are referring to the Atkins Induction diet,
        aka, the Ketogenic diet .

    • Fasting blood sugar dropped to Normal for me on strict LCHF = ketogenic diet, but ONLY after intermittent fasting. Water fasting for five days in a row, 3 times, fixed it for me. Before the fasts my FBG rose steadily, on same LCHF as after the fasts, and I leaned toward explanations like yours, gluconeogenesis and cortisol. Now instead I think the fasting cleared up visceral fats around and inside liver and other organs, enabling their full function again. FBG 80-90 now. Around 110 before. Fung hinted me in the right direction!

  15. Dierdre,

    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.

    • 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,

      • 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

        • 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

  16. 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.

  17. 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,

  18. 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.

  19. 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.