20 Days Into Intermittent Fasting – First Impressions

For the last 20 days I’ve been on an intermittent fasting challenge. What is intermittent fasting, you asked?

Well, intermittent fasting is not a diet, but an eating routine. It groups all the meals of the day in a certain interval and sticks to that routine. My intermittent fasting is 16/8, which means: I’m fasting for 16 hours, then I’m eating for the next 8. In mundane terms: my first meal is at 10 AM and the last one at 6 PM.

There are other flavors of intermittent fasting, like not eating at all for 24 hours, but I’m not (yet) into that.

Other than being vegetarian (not eating anything that had eyes or a pulse) I don’t have any other dietary restriction. I ate whatever I want, whenever I want and as much as I want.

Why I Do Intermittent Fasting?

There are many reasons for that, all related to improving health, but mines are related to ultra-running. For the last 12 months I did a few things to move my ultra-running training from burning sugar to burning fat. This is also known as “fat adapting” and it changes the body metabolism in such a way that the main energy source changes to fat (which has more energy and we carry more of it, even when we’re lean) instead of sugar (which has less energy and our stores are depleting faster).

Another reason for becoming fat adapted is that burning sugar is more costly, in terms of health, than burning fat. It creates more free radicals, increases acidity and makes us more prone to injury. Burning sugar was designed for “fight or flight” situations, in which we had to escape very fast an imminent danger (like being chased by a tiger). Burning fat, on the other side, is what made us capable of persistence hunting, or chasing other animals for days, until they literally felt down because of exhaustion or overheating.

As an ultra-runner, burning fat translates in running longer, faster and with less damage.

First Impressions

20 days is not enough for creating a lasting change in metabolism, but there are a ew things which are visible enough to be mentioned.

First of all, I lost some weight. This was expected, since, during the fasting interval, my body has to rely primarily on fat resources now. I didn’t measure, but I do know my body well and I estimate I lost between one and two kilos. All from around the belly, by the way.

Second, my sleep quality increased. That was a bit surprising, but not totally unexpected. I wake up more easily before or around 6 AM without feeling tired or feeling that my sleep routine was incomplete. I can also do my morning runs fasted, without any noticeable side effects, no nausea, no fatigue, on the contrary, I feel lighter and more flexible.

Third, my heart rate improved dramatically. This is by far the most visible benefit of this intermittent fasting challenge. By the way, I call it intermittent fasting challenge because initially I intended to do it only for a week and see how I feel. Then I extended it to another week and now I’m sure that I want to integrate it as a lifestyle.

Back to my HR now. When I start my runs, I have this pattern: for the first 10 minutes I have a spike, with HR going as high as 170 bpm, and then, after 10 minutes, dropping suddenly to 125. From there it keeps going high again, but very slowly. On a moderate run of, let’s say, 2 hours, I barely scratch 140-145 at the end of it, which is my MAF (Maximum Aerobic Fitness level, or the max HR at which I can perform while still burning fat, mostly).

Since I started intermittent fasting, I observed a change in this pattern. First, it doesn’t happen all the time now, looks like every other run. When I do have a spike, though, the HR doesn’t go as high as before, the top level is 145. Then, after 10 minutes, it drops to 120 bpm.

The rest of the time, I just start with a HR of 100-110 and then gradually climb t 125-130 at which I can stay for the entire race.

Of course, HR is highly dependent of the speed and of the nature of the terrain. I run on flat and I keep my speed very low, with the clear intent of adapting the body aerobically.

So far, I think this is going great. I’ll keep you posted.

P.S. If you’re here for the first time, you could check out my book, Running For My Life, available on Amazon and Kindle.

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