Total fast versus carb fast? | Uphill Athlete

Total fast versus carb fast?

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  • #41735

    I was watching a triathalon youtube video the other day where the host mentioned a carb-specific fast before a long endurance workout instead of a total fast (i.e. some fat and protein is OK before the workout).

    This piqued my interest because I’ve not been able to overcome problems with nausea and lethargy on fasted runs and have stopped doing them (tried 1x week for two months), so the idea of being able to eat e.g. a couple of hard boiled eggs, smoked salmon, cottage cheese, before heading out sounds pretty good to me. FWIW, I don’t follow a low carb diet, but my carb intake is pretty low nonetheless.

    So, my question: is this a legitimate approach to fasted training? Will this still accomplish some of the fat adaptation effects of a total fasted session, but maybe to a lesser degree? Is it nonsense?

    Thanks in advance and apologies if this question has been asked before, nothing showed up in my search.

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    Anonymous on #41740

    Rebecca is the one to comment, but I’ll offer some anecdotes (which may therefore be nonsense…)

    The best time for me to do anything fasted is first thing in the morning right after I get up. Coffee helps postpone any hunger. Once I’m moving, I feel fine for quite a while.

    However, if I try a fasted session later in the day, I usually bonk much quicker, and it feels worse. I assume that this is because of glycogen depletion during the day perhaps.

    So if there’s a delay in the morning (driving to a far trailhead, etc) or if I’m training later in the day, I’ve used hard-boiled eggs to stave off the bonky feeling.

    So as far as making the sessions feel better, that works for me. But whether or not they have the same effect as a purely fasted session, I don’t know.

    Rebecca Dent on #41848

    Hi DaveB,

    Thanks for your question and thanks Scott for sharing your experience.

    The term for theses fasted style workouts for endurance training carried out in zones 1 & 2 was called ‘Train Low’ strategies and more recently the terminology has changed to ‘fuel for the work required’ nutrition strategies, to encourage periodisation of nutrition depending on the specific phase of training.

    Research has shown that by carrying out ‘fasted’ or ‘train low in carbohydrate’ training, this positively influences some of the cellular pathways involved in endurance training and enhances both the physiological adaptations to endurance training and fat adaptation. It is the low carbohydrate availability that has been demonstrated to initiate these positive adaptive changes in training response. Research has also shown that including a source of protein before your ‘fasted’ workouts does not impact this adaptive response to the low carbohydrate intake so you can indeed ingest some protein prior to your ‘fasted’ training which then coins the phrase you used a ‘carb specific fast’.

    When working with uphill athletes I would recommend eating some carbohydrate free protein supplement before to help offset the increase in muscle damage that occurs during fasted workouts of longer than 90mins. So eating boiled eggs or smoked salmon would be ok but not cottage cheese, as it contains a source of carbohydrate from the milk sugars present in dairy products. As Scott has experienced it may help with your feelings of nausea and lethargy you can certainly give it a try.

    In relation to your nausea and lethargy, are you training in the zone 1&2 during these fasted sessions? And at what time point during your fasted session are you starting to feel nauseas/lethargic? Additionally when introducing fasted sessions they do feel hard to do, you need to give it time and build up slowly the length of time you are carrying out a fasted session.

    I hope that clears up your question?

    Rebecca Uphill Athlete Dietitian

    DaveB on #41875

    Thanks for the thorough response, Rebecca! This is helpful.

    To answer your questions, yes my fasted training is primarily in Z1 with about 30% in Z2. I feel lethargic from the start, but the nausea usually kicks in at about 30 minutes (I have a 500 ft hill outside my front door, it’s usually on the way down that I start getting nauseous).

    I’ll try eating an egg or some salmon first, if the nauseau persists, would it be worth it to carry a small carb source e.g. Gu? I.e. is 30 minutes fasted going to accomplish that much?

    Thomas Summer, MD on #41894

    Hi Dave!

    I would also recommend salt tablets. Or something salty before. And drinking enough.
    In regards to fat-adaptation, I wouldn’t eat too much protein, because that could also be gluconeogenic. I really like essential amino acids (EAA), while my long fasted runs. They are not insulinogenic and can even delay mental fatigue.
    Are you using any caffeine?
    For me, it helps, when I only use a little bit of sugar – like sucking on a gummy bear – when I start to feel sluggish, into my long runs. You get the carbs through your mucous membrane in the mouth and probably without any effect on fat burning.


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