Intermittent fasting and insulin – Part 3, Conclusions

There were several surprises when I stopped eating dinner for 3 weeks and did my endurance training about 17 hours after my last meal. The biggest surprise was how great it felt. 

a41ed09cf41077bcd910d2a4ffe2923c--double-chin-the-doubleWe all think we “need” more food than we actually do, but endurance athletes take the collective American delusion to a whole new level. So many recreational runners, cyclists, triathletes, and even group fitness enthusiasts think that their daily workout requires vast amounts of food for proper “recovery.” There is a tendency to inflate the number of calories we think we burn in a workout, and underestimate the number of calories in the food we eat. For example, in a recent half marathon I burned about 1085 calories. That is almost exactly the same as the number of calories in a Chipotle burrito (1060 calories, the way I order it), and that doesn’t even count the chips! While a burrito is a lot of food, it is only one of the 3-6 meals most people eat in a day. That’s 3-6 opportunities to overshoot the number of calories you burned in your morning workout, and very few people are running 13 miles a day. My point is that our sense of how much a workout “depletes” us is all out of whack, and you can very easily overcompensate and eat your way out of the calorie deficit from pretty much any workout.

And yet we tend to think we “need” a 300-calorie smoothie to recover from a 300-calorie 30-minute run. When you think about activity levels through most of human history, whether a caveman in hunter-gatherer days, as a farmer after the agricultural revolution, as a factory worker after the industrial revolution, or even just as a mail carrier before the snacking revolution, people used to do far more activity between meals than simply carrying their body weight 3 miles. And it was no big deal. People were just smaller. They had less muscle and less fat than people do today, not just because they weren’t constantly eating (snacking wasn’t really a thing until the 1980’s), but also because they didn’t constantly have anabolic (storage) hormones like insulin floating through their system from being in a constant post-feeding state. They had periods of more than 4 hours when they weren’t, and so their pancreases got to rest between spurts of insulin, and there was time to burn fat for fuel, rather than just burning glucose and storing the excess all day long.

The thing that I was most apprehensive about when I decided to try doing fasted workouts was whether the quality of my workouts would suffer, whether I would bonk (“hit the wall”), and whether my recovery would be impaired between workouts. I absolutely did not expect to feel less effort at a given pace, feel improved mobility and range of motion, experience improved recovery, and be able to run for hours in a fasted state without a bite of food. I absolutely did not expect to feel better. But what about the numbers? Could I quantify that “better” feeling?



While appearance is not quantifiable, it is useful to see all the things that the data can’t measure. I wasn’t trying to lose weight in this phase, and so didn’t restrict my calories during my eating window. I’ll build up to that someday… but for now body composition changes weren’t what I was after. Other than the very critical things I am saying to myself in my head (give me your worst, I’m sure the comments in my own head are far harsher than whatever you’re thinking), the only real observable change that I see is that I look a little less puffy around the middle. Maybe. Or maybe it’s the lighting.

Pic Jointer (3)
Top images from January 1, 2018, bottom images from January 28, 2018. I’m still working out the issues of camera placement and lighting in my bathroom.

Another thing that I noticed over the course of the first month was that my skin cleared up considerably. Before the experiment, I noticed that the scabs from particularly deep and pernicious zits tended to last for weeks, and leave shadows once they finally healed. I am forever battling chafe from my heart rate monitor, and the chafe from the half marathon in Disney was with me for nearly two weeks. However, the chafe from the 16-mile run that I did a few weeks later flaked off after only a couple of days. My face looked a lot less puffy too.

Much of this improved healing was probably due to the quality of my diet rather than the time that I spent not eating. I had not only cut out sugar, but I was also eating a lot more vegetables, whole grains and whole foods than during the Halloween-to-New Year binge on sweets, candies and processed foods. But in addition to all that, as counterintuitive as it may seem, science shows that fasting actually improves recovery. Part of it has to do with dedicating less energy to digesting food, but also because fasting promotes autophagy or the break-down of damaged cells and other junk. When you go through periods of caloric restriction, your body goes through and identifies the cells that it doesn’t need first. These cells might be scar tissue, cancer cells, infections, or just old beat-up cells that are due for a trip to the junk yard.

I’ve already mentioned how my recovery from workouts felt better than it had in years once I started skipping dinner, but that reduced inflammation and better overall health was also visible in my face and body. You probably can’t tell this from the 4am bathroom pictures, but people who see me under the florescent lights at work every day did comment that my skin looked much better.


Hemoglobin A1C

This all began a1c-graph-216x241with the hemoglobin A1C test that is part of my annual physical. For 3 years in a row I had come in at 5.7, which is the limit between a “healthy” and “prediabetic.” I did not get a test at the very beginning of this experiment, but based on how often I was engaging in cookie therapy, I am confident that had I been tested on January 1 it would have been as bad if not worse than the previous 3 tests.

After 3 weeks off sweets and 2 weeks of skipping dinner, my A1C had come down by a single tenth of a point (to 5.6), but that tiny change brought me into the “normal” range. Since A1C is a reflection of your blood sugar for the past 3-4 months, I will continue to track this number going forward. If there is an A+ in A1C, I want to figure out how to earn it.


giphy15I had numbers for this saved in my Apple Health app… but then I got a new phone, and the transition didn’t go well, and there were some settings changes… and… well… the data is gone. So I’ll have to go from memory. I make no claims that this experiment could hold up to scientific peer review. My minimum acceptable standard is “science-ish.” So with disclaimers out of the way…

I got the hang of keeping my blood sugar down pretty quickly. The fact that my pancreas still works properly and therefore am not actually diabetic helped a lot with this. While a bunch of carbs of any kind, and in combination with any other foods will make my type 1 diabetic coworker’s blood glucose will climb and climb and climb until he takes some extra insulin, my pancreas does the job of injecting me with the insulin I need to keep my blood sugar down. My pancreas is also far more sensitive than my coworker’s insulin pump. However, it’s hard to know when my glucose peaks are low because I’m doing a “good job” at not overtaxing the system, and when my pancreas is doing a “good job” of coming in to save the day.

I did notice that the most reliable way to bring on a high glucose peak was to just eat a really big meal. I expected to see really high numbers with a lunch that was really high in “healthy” carbohydrates like brown rice and beans. However, probably because those foods take a long time to break down, they gave up their glucose more slowly and my body was able to regulate. But remember, just because my glucose levels weren’t too high, that doesn’t necessarily mean that my insulin levels weren’t raised higher than they would have been if I’d had a meal with a similar number of calories but mostly from fat and protein such as an egg and cheese omelette with a small piece of whole wheat avocado toast. In my experience, the omelette produced a similar glucose profile, even though it contained many fewer carbs. In other words, there are 2 ways to keep your blood glucose profile flat: either don’t eat that many carbs to begin with, or eat a bunch of slow-burning carbs and then douse ’em to hell with insulin.

IMG_8273Just like it says in the text books, it took about 2 hours for my blood glucose to peak after a meal, and by about the 4th hour I was right back to my baseline. I did see a change in my baseline over time, but in the beginning my sensor was giving me wonky measurements, so it’s hard to tell how much of that was an improvement in my body and how much was sensor error. In the IMG_0002beginning of the experiment, my overnight glucose was usually in the 60’s (the point where my diabetic coworker starts getting worried, but I just feel like me), and during workouts it would hang out in the 40’s and interrupt my headphones every couple of minutes to tell me it was unhappy about the situation. By the fourth week, my overnight levels were up to the 70’s, I was no longer being woken up by low glucose alarms, and my exercising glucose would actually go up. The morning that I’m writing this, I came off of a 10-mile fasted run with a blood glucose level of 112.

The other interesting thing that I learned about my blood glucose was its response to stress. I know that when you’re stressed your body responds by dumping tons of glucose into your system so that you can run from a saber-toothed tiger or whatever neolithic thing your body thinks a stressor is. It was interesting to see it in action though. One day during the experiment I had a very traumatic experience. It was the kind of day that only comes around once every few years: the day when you have to lock yourself in the bathroom to cry, and even when you compose yourself you burst into tears again at your desk. It’s the sort of day that makes your boss very uncomfortable, and makes all your coworkers agree that you’re going to be useless for the rest of the day and send you home early. This was my glucose profile for that afternoon:

Note that my post-meal BG high is usually between 130 and 150. I had barely broken the surface of 200 only a handful of times in a month. However, once I got the phone call at 1:46pm, my glucose shot up to the high 240s where it stayed until I was calmed down enough to come sniffling out of the bathroom.

Body composition:


As mentioned earlier, my weight didn’t do anything dramatic throughout the experiment. Through the whole thing I only lost 1.1lb. But I wasn’t trying to lose weight. I ate well during my eating window, and didn’t try to restrict my calories in any way.


Fat mass

My body fat — as measured by my incredibly inaccurate scale — did go up, but I attribute that more to my scale’s inaccuracy and having run a marathon the day before the first reading. Looking at the graph below, the first measurement is clearly an outlier. If we discount the outlier, my starting body fat was higher in the first week, and then leveled off over time.



Workout data:

Heart Rate

What happened to my heart rate was absolutely the most surprising part of the experiment. Like I mentioned before, my heart rate strap chafes me something fierce, so I try not to wear it unless I have a good reason for it. However, since we tend to burn more fat at lower heart rates, I was curious to see if there would be any change in my running heart rate with fasting. I wasn’t really sure if the logic would bear out. I thought that probably the reason we burn more fat at lower heart rates is because our energy demands aren’t as high. In other words, maybe fat burning is like the battery in a Prius: the battery (your fat) covers more of your energy needs in slow city driving, but you need your gas tank (carbohydrates) to feed your engine at freeway speeds. If the Prius model was correct, then I wouldn’t see a difference in my heart rate at a given running intensity, whether I was fasted or not.

But that’s not what I found. When I ran fasted, my heart rate was 10-20 bpm lower at any given speed than it was when I ran fasted. See the heart rate graphs below for a half marathon where I was eating carbohydrate gummies every 2 miles, versus a fasted run:

Heart rate and pace data for a fed half marathon and eating every 2 miles. Average heart rate 168 bpm (max 177), average pace 9:07 min/mi*
8 mile fasted run after 3 weeks of skipping dinner and fasted workouts. Average heart rate 151 bpm (max 164), average pace 9:24 min/mi*

*My pride insists that I point out that I wasn’t racing the half marathon (my best time is sub-1:40), and the 8 mile run’s pace was dictated by the dogs and their shenanigans

In the second workout, despite only running 17s/mi slower, my heart rate was a tremendous 17 bpm lower. I have tried to reduce my running heart rate in the past with no success. Several years ago, I spent months trying to run with my heart rate below 156 bpm, and found that I was running so slow that it was messing with my gait and making my feet hurt. Even then couldn’t keep my heart rate below the limit for more than a couple of minutes. I dutifully took walking breaks when my watch beeped, but over time the walking breaks didn’t get any less frequent, and eventually I gave up. I just accepted that all my runs would be between 168 and 179bpm regardless of pace and fitness. But here I had just run 8 miles and kept my average heart rate 5 bpm below the beep zone!!! I was flabbergasted.

I’m really not sure what was happening here. Maybe the Prius theory of fat burning wasn’t correct, and it’s your metabolism (i.e. whether you’re burning mostly fats or glucose for fuel) that determines your working heart rate. Or maybe I always had so much circulating insulin on my runs that I was unable to burn fat, and the high heart rates were a signal that my body was working overtime to release the glucose to make my run happen with my busted engine. Maybe my cells were getting so resistant to the glucose-delivering insulin that they couldn’t get enough fuel to make the exercise happen. Maybe it was witchcraft. Who knows… But you can bet that once I find something that can explain this, I’m going to experiment the hell out of it. Whatever this is, I think it’s the key to how I Salazared myself a few years ago (this is a post I need to write).


I debated whether to include this in this post because no matter how many articles I read, I can’t make myself understand more than the basics. If you want to learn more than the simplistic summary below, you’re going to have to train to be a cardiologist.

Heart rate variability (HRV) is a measurement of the variability of the time between your heart beats. Your heart rate is never exactly the same; the spaces between heart beats vary based on a variety of things, like breathing, stress and fatigue levels, whether you need to pee, etc. Having a more irregular heart beat (on this tiny scale) is actually a good thing, and the less variation there is in the amount of time between heart beats the more stressed your system is. I have no earthly idea why. I imagine it like someone who doggedly sticks to a plan even when it is no longer working for them, compared to someone who is relaxed enough to see all the options. But maybe it’s nothing like that. I have no idea… What I do know is that HRV is a better indicator than resting heart rate of whether your body is overtaxed, whether from overtraining, stress, or because you’re fighting illness or an infection.

I don’t really know how to analyze these graphs other than that higher is probably better. Below are the graphs: the left one-third of the graph I had a beastly cold and was not working out, the middle one-third I was working out only very lightly and still a little sick, and the right one-third I was back to my regular routine.

What is rMSSD? No idea, but it likes rest as long as you don’t have a cold.
What is pNN50? Haven’t the foggiest, but it likes rest, yet tolerates exercise a lot better than the last chart.


What is LF? Low something. It goes way up with exercise.
What is HF? High something. It also goes way up with exercise.

What do all of these charts mean? ::Shrug emoji:: They all seem to agree that being sick is bad, recovery is good, and exercise does something to the heart. If you find out what these charts mean, please report back to the group because I would love to be able to quantify my recovery. Thanks, internet!


What I learned from 3 weeks of skipping dinner and working out fasted is that:

  • Fasting does not necessarily mean weight loss
  • It may or may not help with fat loss
  • It does seem to help with A1C levels, even on a high carbohydrate vegetarian diet
  • All the stuff they say about blood glucose in the text books is true
  • Anyone who doesn’t have diabetes is a lucky duck, because poor glucose regulation can be very disruptive to a busy life
  • Running fasted produced a lower heart rate at a given running intensity
  • And what HRV actually measures is a mystery, but it makes some pretty science-y looking charts

Next up: I play around with gluten.

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