Fasted workouts and carb restriction — how much is enough?

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  • #6309
    Peter Hamel

    I’ve experimented twice with the fasted exercise thing, but I’m not really sure how extreme I should be with fasting or carbohydrate restriction.

    In my first experiment, I did 3 months of training, where I did a fasted morning workout 2 days a week, and I tried to do my longest workouts with minimal food (maybe one energy bar halfway through a 6 hour hike/run). I added some more fat to my diet but didn’t do anything drastic. I did a lab test before and after these 3 months, and found no improvement in my fat metabolism.

    In the second experiment, I tried going on an extremely carb restricted diet, and doing almost all of my workouts fasted. I found that my exercise performance was seriously restricted. My heart rate while exercising was throttled to about 120 bpm (with carbs in my diet, I can sustain 140 bpm as an all day long, conversational pace). It also became much harder to go out for a long hike/run — I had trouble finishing a workout with 4,000 feet of vertical gain (with carbs, my biggest days are over 10,000 gain).

    So, my question is: how do I get the right parameters, for this stuff? If I end up so glycogen depleted that I can’t do as much volume of exercise, or can’t complete a long run, is that hurting my training? Or is it helping me because it’s triggering fat adaptation?

    Also, how should I monitor progress? I know that lab testing would work, but is there a simpler way? Should I expect to see my fasted exercise heart rate improve, as my fat/carb crossover point improves? Should I expect to be able to do longer runs while still in a glycogen depleted state? I tried my heavily carb restricted experiment for about a month and saw no obvious changes in either of those two things. I can try again for longer, but want to make sure I’m doing the right thing, before committing to that.

Posted In: Nutrition

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    Anonymous on #6325


    Thanks for writing into the forums with this valuable question. Based on what you are saying here’s what I recommend you try.

    Since in both “studies” you were struggling to complete the longer workouts it is clear your fat adaptation was not sufficiently well developed to support those efforts. From a metabolic standpoint this means that glycogen was still contributing enough to the overall energy needs that your glycogen tank ran low during these long workouts. Your reduced HR and overall work capacity 4000ft vs 10,000ft days indicates exactly this same thing. You are just not at the level to support that much work on on only fat.

    Add more midweek fasted aerobic workouts and use shorter fasted workouts on the weekends instead of trying to recreate a 6 hour day that you can do on carbs the bonking and need to eat mid workout. Try 2-3 hours and build on that. While a full keto diet will have a dramatic effect on fat adaptation, as you found out it can also have a dramatic effect on your training until you are fully fat adapted AND well trained That is why we propose a more moderate diet modification and training mixture to produce the desired effects for both fitness and fat adaptation over the several months. We’ve seen big shifts in as few in 4 months using this method (Adrian Ballinger training 20+ hours/week) but normally we smaller changes just starting to occur 2-3 months in and significant changes 6 months along.

    I hope this helps,

    Mariner_9 on #6928

    I was wondering whether the long Z1 workouts should be done fasted. Historically, I’ve done shorter Z1/Z2 workouts (generally 1-1.5 hours) fasted but not the long workouts (3-4 hours). Is the idea simply to build up the fat adaptation until all aerobic training can be done fasted?

    Anonymous on #6965


    If you can manage longer workouts fasted, that is a GOOD thing. So yes start increasing the lengths of these gradually.


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