37k Fasted Run – The Limit?

This morning I ran 37k, having not eaten since dinner the night before. I’ve been gradually building on the lengths of these Saturday fasted runs with the theory that it might help develop my metabolic efficiency and the show me what my limits are. And apparently I’m a sucker for punishment.

I got out of the house at 6:30 on a crisp Toronto morning, -3°C, mostly clear skies and was treated to another incredible sunrise over the downtown skyline. It was my usual route up to Dixon Road by way of Scarlett and back down along the Humber River. Today, I would add a couple of laps around High Park to get up to the required mileage.

I started slowly, 5:30/km pace and the plan was to run below 145 bpm and keep to a generally low effort level. The first 15k or so were uneventful, but the first “negative brainwaves” started to happen just before 20k, which was a little earlier than prior. By 25-27k, I was starting to question whether I could/should go the distance, and had to repeatedly remind myself that it was okay to slow to whatever pace I needed to finish, or even to run/walk if necessary. At about 29k, during my extra lap around High Park, I had a major, almost sickening hunger pang, but I realized that its unpleasantness was directly proportional to my effort level, so I eased up. The last 10k or so was a complete unfun grind. My mind was mostly willing and my energy level was relatively high, but my muscular system was staging a minor revolt. Every muscle that had displayed any marginal behaviour over the last weeks was in full tantrum mode – particularly my right glute, quads and right lower back. It was like being jabbed with a bunch of small daggers and the occasional large one. This made moving forward increasingly difficult, but by then, I knew I could finish.

So, the question is, what happened? What happens to the body as runs last longer than two or three hours? And what impact does eating or not eating have on all of my systems as efforts get extended? The run today lasted roughly 3h 30m, and though there was nothing resembling a bonk, my muscular system seemed to be shutting down. Could I have run through 40k or even a little more? Sure, but it would have been extremely unpleasant and today I started to wonder if I was doing more harm than good.

Hard to know anything for sure without a lot more experience, but I suspect that today I reached the limits of benefitting from fasted running and it may be time to start comparing to running not on empty.

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  1. It’s interesting stuff, isn’t it? I always run fasted if I am doing a training run or a for pleasure run first thing in the morning. Never an exception. My body is completely adapted to it and is very efficient at burning fat. Having written that, my limit is a half marathon distance. After that, my muscles start to talk loudly to me.


    1. Thanks for the comment. Yes, it’s all a bit of an experiment – trying to see what the limits are. Interesting to hear that for your situation, it is the muscles that protest first as well.

      Liked by 1 person

      1. Every time. I’m never hungry. In fact the running seems to suppress my appetite. I have to force myself to eat when I get home. I do eat though or my recovery would suffer.


    1. Brave maybe, possibly stupid…

      … but a calculated risk based on what I know and a slow build. But as I said, 35-37k may be the limit!


      1. I’m keen but a touch apprehensive, I eat enough for a marathon even when I’m doing 15km – I think all this running is just a means of managing my carb addiction!


      2. I’ve worked on diet and training for a while (eating a lot less carbs, running long and slow) to try to get fat-adapted and it seems to work (see my post on metabolic efficiency testing). My goal races are ultra-distance (read: slow) so I am trying train myself to require less fuel while running


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