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To (Intermittent) Fast or Not to Fast; That Is the Question


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Let me start by saying that I’m a fan of intermittent fasting. I think going for extended periods without eating was probably normal for humans throughout most of our evolution, and I’ve seen enough evidence to convince me that it can help with everything from weight loss to improving insulin sensitivity to boosting the immune system.

Martin Berkhan at Leangains and Paul Jaminet at Perfect Health Diet both write extensively about the benefits of intermittent fasting, so I’m not going to cover that here.

Instead, I want to talk briefly about when intermittent fasting might not be a good idea. I haven’t arrived at this notion through combing the research literature, but through direct experience with patients in my clinical practice.

In the last few months I’ve had a few patients come to me with several months of low-carb, paleo nutrition and intermittent fasting experience behind them. When I had them test their blood sugar with a glucometer, we discovered some very erratic patterns. They didn’t just have high fasting blood sugar in the morning, as is typical with low-carb dieters, but they had high levels or a “yo yo” pattern throughout the day.

I tested the cortisol/melatonin rhythm in two of these patients, and it was off in both of them. Why would this be?

It’s possible that the blood sugar ups and downs they’re experiencing are exhausting their adrenals. Or, that their adrenals were already under stress and the repeated cortisol secretions necessary to bring their blood sugar back up when it drops created further stress.

As an experiment we decided to have them try eating regular meals, and even more frequently than normal (i.e. every 2-3 hours). Guess what? Their blood sugar normalized within a few days, they started sleeping better and other symptoms improved. I’ve only re-tested cortisol rhythm in one of these patients, but it was back in the normal range after three weeks of eating more regularly.

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So how could intermittent fasting normalize blood sugar in some people, but throw it off in others? I’m not sure, but as I said above, my guess is that it has a lot to do with their adrenal status when they begin fasting.

There’s a complex relationship between blood sugar and hormones like cortisol, glucagon, epinephrine, norepinephrine and growth hormone. While it’s true that balancing blood sugar can bring the hormones back into line, it’s also true that hormonal imbalances can throw blood sugar out of whack.

I’ve heard anecdotal reports from some that intermittent fasting was rough on them when they first started, but after sticking with it for quite a while it had the desired effect. It’s possible, of course, that if these patients of mine had simply kept going their blood sugar and insulin sensitivity would have normalized. But three of them, at least, had been doing it for six months or longer before they came to me – so I think it’s unlikely that another few months would have made the difference.

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  1. Hi I low-carb diet and IF for 16/8 hours, everything ok! no hunger, no problems with sleep and greater strength, but after a month I had gout resulted from sugar, so here in Italy I did not understand how to solve the problem .. high blood sugar and gout for more than a month, but the doctors can not figure out the cause, help me and sorry for my english

  2. Hi, I am writing because my ability to ‘listen’ to my body has been completely destroyed. I was overweight as a child (170lbs at 13) then began to lose weight in spurts until i weighed in at 140& 6 feet tall at 18. I certaintly felt hunger and fulness at that point. Since then I struggled to maintain the weight and lost/gained/lost/gained yo-yoing in between 140-155. I am at 154 now, but I have completely lost any connection to my stomach. I havent eaten for almost 24 hours and I am not hungry, conversely I can eat MASSIVE amounts of food without feeling the slightest fulness. Although I havent called it IT, it can describe my eating patterns for the last few weeks. I dont see any improvement. Can you tell me what is wrong with me? Its as if I have taken a numbing pill or if the ‘connection’ between my brain and my stomach has been severed. Any suggestions would be very appreciated. Thanks!

  3. I have noticed several things about my blood sugar and the paleo/primal diet. I’ve been gluten free for 6 weeks and grain free for 4. My carb intake is less than 50g per day and I eat a high fat, moderate protein diet. After 4 weeks, my fasting blood sugar went from the 110s and peaked at the 140s, and is on it’s way down, but still it was 130 this am. I have no trouble controlling it and keeping post-prandial levels below 120. In fact, throughout the day they are close to 100, and have dropped into the 90s. At bedtime, my numbers are in the 90s. But in the AM, I have a terrible time. I was diagnosed with metabolic syndrome, PCOS and infertility 2 years ago, and with some stops and starts, I feel like I’m on the right track now. But my fasting numbers are a mystery to me. One thing that I noticed in my tracking, is that my FBG numbers correlated directly with my progesterone levels, and that both peaked on the same day, and my usual migraine followed shortly after. I think that a lot of IF sites, especially lean gains, kind of recognize that women react differently, but they don’t put much thought or effort into figuring out HOW to customize to them. We’re 50% of the population, but somehow our hormone fluctuations aren’t worth the effort? Frustrating.

    I track everything I eat, the timing and test several times a day. Even with fruits or starch my numbers seem okay if I eat them after noon. I’ve now moved to cutting out fruits and may eventually do dairy as well if my fasting numbers don’t start to improve. I did my first IF yesterday and my second today. For me, my FBG when I take it in the morning is still in the 130s, but within 2-3 hours it is below 120. If I eat immediately, no matter the meal, my PP numbers spike to 160 and take even longer to normalize. With IF, if I wait until my numbers are below 120, my PP numbers are below 120. I’ve been using that as a cue for when to break the fast.

    I love the blog, I’ve gotten so much information and it’s helped me stay committed to making a change. Thanks so much!!

    • It seems you are experiencing dawn pehnominum. Also known as liver dumping. Try look for dr Jason Fung’s videos. My fasting blood sugar continued to drop on low carb as my fatty liver used up all my sugar surplus kept as fat. It took me 3 weeks for it to go bellow 100. Intermittent fasting helps speed this recovery i think. Hope this helps.

  4. I have been IF-ing 16h a day just like Leangains prescribed, for the past 2 months. My aim going into it was to lower my bodyfat %, mixed in with some strength training coupled with Crossfit-style metabolic conditioning.

    Some observations:
    – I haven’t dropped much fat weight.
    – No noticeable increase in muscle mass despite eating plenty of protein from animal sources (I shoot for 1.5g/Kg bodyweight
    – Although, my lifts have continued to increase. And my metcon times are consistent.
    – Some days I am very cranky during the mornings. Cranky, weak and sleepy. This could do with a reduction from 8h to 6-7h of sleep. Some days I caught insomnia too.
    – Some days the hunger were so bad I can’t do anything else but focus on that hunger.

    That being said, some positives and some negatives. Given all of Martin Berkham’s positive client updates, I’m sure IF works. But hey, I didn’t sign up as his client, and I must have only covered the surface of his protocol based on what I follow from his blog. And after reading tis article and the comments, it makes me think that maybe I should go back to cleaning out my diet for a while (again) before jumping back into IF.

    Thank you for such an honest article Chris.

  5. While it is good wisdom to “clean up” your diet before trying IF, speaking for myself, doing IF first was necessary. IF gave me the tools to be able to clean up my diet by teaching me how to cope with hunger and by managing my sugar cravings. I tried – and failed – several times to be low-carb, and I wasn’t able to thrive on it until I’d been doing IF for several months.

    And I definitely fall under the category of “rough for a few weeks then it got better”. Shaky, light-headed, cold, etc. I suspect that I did subject my body to a heckuva lot of cortisol. But if it’s a choice between cortisol and super-strong sugar/carb cravings, I’ll take the cortisol! (And heck, I lost 6″ from my waist and 32 lbs, not including water weight, so even the cortisol couldn’t stop the fat from leaving.)

  6. Tested fasting blood sugar this a.m. (88-seemed kind of high, as had less than 50 g CHO the day before and less than 1000 cals). Did 1 hour treadmill (3.0 mph at 5% incline) with sprint bursts 4-5 times of about 1 min each). I was surprised because 2.5 hours later, my blood sugar was the same (light exercise for 10 minutes will often lower my BS by from 100ish to 80ish post prandial).

    Am wondering if this is glucagon “kicking in” to keep my BS at a high enough level to sustain semi-strenous exercise? And same thing for the overnight BS levels. Based on my diet, I would think my blood sugar would be in the high 70s tops. These numbers are via Bayer Contour (and are routinely 10-15 points lower than Accu-check taken at the same time). Something about Accu-check just doesn’t work correctly for me. Wonder if the strips detect Stevia metabolites and register them as a “sugar”

    I’m 15% body fat and 19.5 BMI (female). Any ideas/suggestions would be appreciated.

  7. Hi Chris,

    How often, and for how long (how many hrs), were these patients fasting?

    Can there be a reduction or increase in thyroid hormone levels thanks to intermittent fasting?

  8. One caveat here is that very low-carb diets will produce elevated fasting blood glucose levels. Why? Because low-carb diets induce insulin resistance. Restricting carbohydrates produces a natural drop in insulin levels, which in turn activates hormone sensitive lipase. Fat tissue is then broken down, and non-esterified fatty acids (a.k.a. “free fatty acids” or NEFA) are released into the bloodstream. These NEFA are taken up by the muscles, which use them as fuel. And since the muscle’s needs for fuel has been met, it decreases sensitivity to insulin. You can read more about this at Hyperlipid.

    From: http://chriskresser.com/when-your-“normal”-blood-sugar-isn’t-normal-part-2

  9. Wondering why low-carb diets would cause high fasting glucose? You might have explained this somewhere – can you point me to the answer? Thanks!

  10. Have not, but I will mention it next time I chat with a doc. Thanks for the heads up and a good read.

  11. Hi Suzan,

    It’s worth a try to do a period where you eat more frequently. You’ll have to eat 30-min to 1-hour after rising, whether hungry or not. If you’re like most other people, you’ll begin to get hungry again in the morning after a while of doing this. A lot of people with blood sugar issues don’t get hungry in the morning.

  12. Hi Chris,

    I’ve been doing intermittent fasting for about a year now. I eat two meals per day, within an 8 hour window (12 noon and 7 p.m.) I’m never ravenously hungry, and I usually don’t have an appetite until 11 am or later. I definitely have trouble sleeping, and I often take Melatonin. I find myself extremely sleepy each day between 2-3 p.m., but I don’t like to nap. I don’t know about my blood sugar levels – I don’t measure. I’m overweight, and I’ve been on a Primal-type diet for about 18 months, with a couple of lapses during travel/holidays. I’m about to switch over to Perfect Health Diet (keeping carbs on the low-end) and take their recommended supplements. You’ve got me wondering if I should continue eating 2 meals a day, or bump it up to 3? It’s hard for me to eat 3 meals a day, as I am never hungry in the morning. However, if that is what it will take to get the weight loss going again, then I’ll do it! Any thoughts? Thanks very much. I always enjoy your posts.

  13. Hi Chris,

    You know I find these experiences of your patients fascinating. These are very interesting questions.

    I started looking into the literature about this, and I believe that disrupted rhythms of cortisol, insulin, and glucose release are very important. Cells not only have circadian rhythms but also “ultradian” (~2 hr) and shorter (~6 min) rhythms. These rhythms are disrupted in people with poor glycemic control.

    The people who had the most trouble on intermittent fasting seem to have disrupted ultradian and shorter rhythms. Their blood glucose would be low for 2 hours, then have a sudden burst. Similar time scales for insulin. It is as if the 6 min cycles are suppressed, and the longer ultradian cycle occasionally misses a beat, then recovers.

    I haven’t yet delved far enough to get ideas as to what causes this or how it may be fixed, but as I get time I will look further.

    I suspect Misty may be right that some healing may need to occur first before stressful activities like fasting are undertaken. A low-toxicity, nourishing diet and time may heal these wounds.

    Best, Paul

  14. Adam: symptoms were energy & mood swings, insomnia, and poor stress tolerance. I’ve never heard of shortness of breath with IF. Have you been tested for anemia?

    MAS: I agree. On top of that, IF can free people up to really listen to their bodies and eat only when truly hungry – and even then, only what will nourish them. I think this is probably the biggest benefit of IF.

  15. One of the biggest benefits that I got from IF was the ability to be comfortable with hunger when no good food options were present. The old me would eat bad food instead of dealing with hunger. Living with hunger also gives one the patience to prepare and cook for oneself.

  16. Two questions: first, what were the patients’ symptoms that led you to run the tests aside from sleep problems? Second, have you had any patients experience shortness of breath at 20+ hours fasting? I’ve been using IF for about 7 months and have shortness of breath every time. Doesn’t impair my ability to exercise and I only notice it when I’m -not- exercising, but I’ve been curious and couldn’t find anything on PubMed or anywhere else.

  17. I have found that the low carber/paleo folks generally begin their IF experimentation far too soon in their journey’s. I believe from what I have witnessed in my 11 year journey/recommendations is the need for most to heal the damage done as a result of the SAD prior to beginning any fasting.

    I know for myself had I begun with an IF experiment prior to really understanding my biochemical individuality, I would have been a wreck and potentially, this could have spelled disaster for me.

    Thank you for your contributions Chris! They are always enjoyed.

  18. hey Chris,
    Big fan of your blog. The information here is not only awesome but super well presented and very easy to understand. I live in the bay are and might hit you up sometime!

    I’m a 28 yr old male and have been IF-ing for about a year now with no noticeable negative effects. I follow the leangains approach for the most part, but on rest day I do extend the fast to ~ 18-20 hours depending on how I feel and how accessible good food is. While I eat low carb (<50 gm) on rest days I do eat starch (rice/yams/sweet potatoes) on my workout days… but only immediately post workout. I always train fasted and from a body comp stand point and a strength stand point I've seen exceptional results. My fasting glucose is ~ 85. Another thing worth mentioning is that I have a good number of my clients do IF and no one has whined or complained… not yet! They're all enjoying it and seeing awesome results (fat loss and strength gain).

    That said, I am having my cortisol levels tested soon and will keep you informed.

  19. I envy those who can IF and see benefits right away. It took me some time (2 years of occasional experimentation) to otherwise clean up my nutrition and see tangible benefits from IF.

    Recently, I have been focusing on having a certain mental image/attitude during my fast. I think of the feeling of fasting (slight hunger, increased energy) as being invigorating and energizing, and focus on not feeling stressed or worried about not eating for that brief period. Maybe this imagery helps with my hormonal responses? ? I also limit IF to one day a week.