Should Endurance Athletes do Fasting? | 80/20 Endurance

Should Endurance Athletes do Fasting?

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    German De La Rosa

    This question seems to be on a lot of people’s minds with the increasing popularity of Intermittent Fasting and Keto diets, so I decided to use myself as a Guinea Pig and see what all the fuss is about.

    **DISCLAIMER** This is just my personal experience with this diet or “eating pattern” as some prefer to call it. Every person is different have different nutritional needs and preferences.

    According to the experts, the human body is like a hybrid engine. It can either run on carbs (glucose) or ketones (fats), we just have to teach it to do so and become “fat adapted”. What does being “fat adapted” mean? it means our bodies have to get used to burning fat for fuel instead of glucose.

    This “fat adaptation” process can be a little painful at first, because if we’ve always been used to using glucose as fuel, and we run out of that glucose, we feel awful. The good old “bonk”.

    That’s where fasting comes in. After about 16 hours of not eating anything, we’ve consumed most of the glucose in our system and our insulin levels are low enough that our bodies start using stored fat for fuel in lieu of glucose.

    I started doing my workouts in this fasted state. At first I couldn’t sustain more than one hour of Zone 2 training and my heart rate went through the roof.
    But after some time of getting used to it, I could sustain longer and more intense efforts. I no longer needed to be eating gels or bananas every hour to keep my glucose levels during my long endurance rides because I didn’t have any glucose to begin with; all my energy was coming from stored fat (and most of us, no matter how lean we are, have more than plenty).

    As a result, I dropped almost 20 lbs in 6 months and went from 5:20 in my last 70.3 to a 4:30 on a virtual 70.3 (IRONMAN VR20).

    I started my Virtual 70.3 on a Sunday morning (around 10 am), having fasted since the previous night (around 7 pm). I didn’t eat anything before or during the race. Just water and electrolytes and I finished with plenty of energy left. I didn’t feel weak or woozy or anything. It’s very liberating not having to cary a bunch of food or snacks and gels in your jersey pockets or your bento box. Unless you’re an elite athlete with under 5% body fat, you have more than enough fuel to sustain you for hours on end without the need to refuel every 3 or 4 hours.

    Again; this is just MY experience and I wanted to share it with you.




    I always run in the morning in a fasted state. I don’t feel any limitations and I run in Z2 (and even long intervals at LT) pretty comfortable.

    But, wow, a Half Ironman is a different feat: congrats! Do you do it in Z2 or scale up to Z3?


    German De La Rosa


    For racing, I try to stick to the 80/20 Race Pacing Guide
    That magic spot between the end of Zone 2 and Zone X for the bike leg, and Zone X for the run leg. Maybe some Zone 3 in the last Km or two if I want to finish strong.


    Hi German, do you follow a specific keto diet or just wing it ? I normally train fasted but have yet to trust racing (IM) without nutrition.

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