Fasting and Blood Glucose

Brad Pilon, author of Eat Stop Eat, raised an interesting issue having to do with blood glucose during a fast. Coincidentally, I read that while near the end of a fast, having just come from the gym and a heavy workout. I tested myself at 114 gm/dL, which is higher that I was testing before at any old time during the day (100-105, usually). Hmm… 28 hours into a fast, and after a series of heavy weight lifting. Isn’t the conventional "wisdom" that fasting is bad because your blood sugar lowers too much; and by all means, "you don’t want to exercise." After all, Zeus didn’t make us like he made virtually all the other animals, who engage in their most strenuous activity of hunting when they’re really, really hungry. Right?

So, on my next 30-hr fast a few days later, I decided to check BG right before I left for the gym, at about 25 hours into the fast. I measured 77, which is a very predictable result during a fast. I had an intense routine, but not nearly so much as the previous time. So, at hour 27, and about an hour after completing the workout: 101 gm/dL. How can that be? And without that candy marketed as "power" that they try to sell you so "your blood sugar doesn’t drop" when you exercise. Clever guy that Zeus, eh?

Here’s the comment I posted at Art’s blog about it, and be sure to read Art’s response as to the the underlying chemistry. Also, Brad did another test — measuring before workout, at intervals during the workout, and after. Similar results. Same with John Barbin. Note: they are both using the mmol/L scale, normal range being 3.9 – 7.8. I’m 22 hrs into my first of two fasts this week, and I’ll go through the routine again, just to see if I can reproduce the results. I’m dropping around 2 pounds per week net, and it’s all fat. It’s amazing where all you carry that stuff, which doesn’t seem apparent until you begin to notice voids in all kinds of places.

I really need to update the photo to the right. My face isn’t nearly so fat, anymore; and 5-6" are gone off the waist with at least 6" to go.

Richard Nikoley

I'm Richard Nikoley. Free The Animal began in 2003 and as of 2021, contains 5,000 posts. I blog what I wish...from health, diet, and food to travel and lifestyle; to politics, social antagonism, expat-living location and time independent—while you sleep—income. I celebrate the audacity and hubris to live by your own exclusive authority and take your own chances. Read More


  1. Chris H on January 22, 2008 at 12:53

    Nice post Richard.

    you might be interested in some other research that Matt Metzgar pointed me to

    calorie restriction promoted muscle growth

  2. Chris H on January 22, 2008 at 13:56

    🙂 I have no idea either Richard! I am also just learning about all this IF stuff but am increasingly impressed by all the research out there about the benefits of this way of eating. It works as well. I am getting abs for the first time in my life – and I will be 40 in a couple of weeks. I don't think it is down to calorie restriction either.

    I'm wondering how long it will be before IF is the next mainstream diet trend, although as Pillon points out there is not much money in telling people not to eat occasionally.

  3. Richard Nikoley on January 22, 2008 at 13:25


    I have been noticing something that is happening when I awake in the mornings, but even more so when I awake during a fast, where my last meal was lunch the day before. I have a "pumped" feeling in my muslces, but particularly in the legs. This is even three or more days since the last workout, and isn't really associated with the sore-pumped feeling you get a couple days after a big push.

    I read somewhere that both fasting and deep sleep promote GH release, so I wonder if the combination of both (not to mention the workouts themselves), are producing a noticeable combination in me.

    Just idle speculation (I have no idea, really). Perhaps there's an established reason for such a "pumped" feeling.

  4. Jolly on March 5, 2010 at 13:35

    Whats your thoughts on Dr. Davis’s post on exercise and blood sugar levels? He states that exercise *decreases* blood glucose.

    • Richard Nikoley on March 6, 2010 at 10:18

      I think he’s talking about something far different, non fasted steady aerobic kind of exercise and normal glycogen levels.

  5. David on April 3, 2010 at 19:06

    Very interesting, Richard, as I just had something similar happen to me.

    To set the scene, it should be understood that I’m young (26) and in excellent shape. My fasting BG is in the 80s (and often lower), and my postprandial BG rarely exceeds 100 — even with white potatoes.

    I fast once every week, usually for ~24 hours. I’ve been fasting since last night, and a couple hours ago (21 hours in) I decided to do some jogging followed by some interval sprints. I don’t usually jog, but I do it now and then if I need a mood booster. Anyway, so I was sweating pretty good by the end, and felt I had a pretty good workout. Curious as to what my BG was, I checked my glucometer about 15 minutes after the workout: 143 mg/dl. I couldn’t believe it, so I washed my hands with soap and water and tried again. And then again. All results fell within 5 points of the original. Though I’m back down to normal now, a couple hours later, it seemed excessive to me. Have you heard of glucose going this high after fasting exercise before?

    • Richard Nikoley on April 4, 2010 at 12:15

      Haven’t heard of that high, but I certainly don’t think it’s a problem.

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