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

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.

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.

As usual, I’d love to hear your thoughts on this.

  1. Hi Chris,
    Been doing a keto type diet but been on prilosec for 20years for GERD/hiatus hernia and paxil as well, Id eat a snack at 11pm latest and then eat my breakfast at around 4pm and then the second meal about 10pm, the higher than usual protein and fats made me feel sick, with food feeling like its stuck there in the stomach, undigested cause too little acid I suppose to digest the protein, I wonder if IF or having a short feeding window puts too much stress on me seeing as my stomach has to go without food for so many hours, acid starts being produced in the am as this would be the time of my meds doses, I wonder if I could just take the meds but still not eat and see, or maybe I should avoid IF until I get off the meds.
    Really don’t know what to do maybe just get back onto normal feeding, only been doing this 3 days but already feel not so good AT ALL, lost some weight cause I felt too sick to eat.

    • I struggled for YEARS with prilosec and digestive issues. I now do intermittent fasting and eat a high fat moderate protein and carb diet. Look into HCL and ACV and probiotics and enzymes plus things like saurkraut!!! If you want help please let me know (I love helping people get over prilosec)

  2. This year was tough. Missing meals, not sleeping, being addicted to stress as a means of pushing through a physically exhaustive job, eating sugary things to make sure my stomach didn’t cause me fatigue (just keep hauling away, keep your heartrate up).
    And I was trapped in the repercussions for months. I’m learning to chill out and take (literally) a mouthful of supplements, improving my diet (though it kills my wallet and ll the time spent cooking, end up eating junk food).
    Missing meals made my brain fog worse, obviously.

    But now that so many things are getting better and I’m emerging from that vicious cycle, I’m on my PC and skipped lunch. After sitting down and my brain slowing to a crawl (nearly ready for a nap), I’m just sooo chill right now.
    Just wanted to add another voice in in case it helps correlate any data. I’m chilled out sitting here and the comfortably empty stomach and numb mind (no racing thoughts) is pretty chill. Mellow.

  3. Hi Chris,
    I would appreciate if you could give me your opinion on something I’ve been dealing with. Please. Doctors where I live don’t really have an explanation for what was happening (same doctors who adviced me eating low-fat and whole grains 6 times a day so I’m not surprised.)
    I’m 20 and I’ve had severe reactive hypoglycemia after being without my period for 2 years. (I think there’s a connection). My fasting blood sugar was always normal (80). I did a 72h fast in the hospital and they didn’t found anything wrong (they were suspecting an insulinoma, which I always knew it wasn’t the case). Later in that day when I got home I ate dinner and my blood sugar crashed to 35… (like it used to). But if that happened after being 3 days without any food I believe (from what I’ve been studying) maybe cortisol is involved. I also have hashimoto’s since I was 14 years old but my thyroid labs are normal and I never had any issues since I started taking levothyroxine, so I believe that’s not the main culprit. What I did was to eat only fat and protein for a couple of days and every 2h… that really helped me. Now I don’t have hypoglycemia one hour after eating but if I eat something with more than 10 sugar my blood sugar spikes and still crashes. (insulin resistance) but to 60… I’m not in a life threatning situation like I was 3 months ago.. and the fact that I managed to avoid such intense and stressful hypoglycemic episodes made me regain my awareness of them but I still would like to understand what was going on with me and what tests do I think I should ask my doctor. This is a very important issue for me because I am going to college next week and praying that I don’t have to go through that again and that I can finally live without having to measure my blood sugar all the time. (since I was not aware of when I got values like 35 or 40 and I am not diabetic).

    If you could answer me or write an article that could help me I would be thankful for the rest of my life.

  4. What about not eating breakfast and not having adrenal or cortisol issues? Is there a difference in fasting in the early AM for breakfast versus later in the day?

  5. Hi Chris!

    Thanks for a great article. I do 16h fasting 1-2 days/wk and 14h the rest of them (8pm-12am and 8pm-10am) I have been doing this for about 3-4 weeks. I feel great, energetic and satisfied. But I do have tired adrenals in my past, so I get a bit worried reading this. Can I just go with what I feel and continue, or should I assume that my adrenals don’t like this? I’ve been on no-fruit-honey-low-carb GAPS-diet for 8 months, being pregant 4 of those and now I’m nursing a 4 month old. So lack of sleep is kind of normal right now as well. Before I started the fasting I had issues with portion size and could eat ALL the time throughout the day, now it’s a lot easier to control this, so I really like IF. Do you think it puts to much strain on my adrenals?

    Thanks!
    //Malin

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