The N-Zone

  • Creator
  • #52014

    I would like to propose a series of hypothetical scenarios for discussion. Some may have definitive answers, many may not. I’ll post my opinion, but in most cases I probably don’t know the answer, even though it may sound like I do–I want to be educated. Guesses and hunches are welcome. No judgements—just opinions. Anyone who has what they believe is the answer please feel free to explain the science, or not, its up to you. My goal is to stimulate discussion and learn physiology at the same time.

    I’ll post a new scenario, often just a variation of an earlier one, on this thread every Sunday or so for the next few weeks and see how it goes. If you post, and are interested, please click on “Notify me of follow-up replies via email” so we can maybe maintain some momentum.

    Welcome to The Nerd-Zone!

  • Participant
    russes011 on #52015


    Fred and Chad are twins. They always train together, have the same body composition, and equal anaerobic thresholds.

    They both eat the same diet, with an equal number of calories from carbs and fats.

    Fred only drinks water during training, while Chad consumes carbs every 30 minutes.

    Similar to training, Fred only drinks water during races, while Chad consumes some carbs every 30 minutes.

    They compete in a 6hr race on a flat course. There is an aid station every 2 miles with water, a drink that supplies both simple and complex carbs, and energy gels.

    Who will win the race? Who has the advantage and why?

    (What I’m asking is: are carbs or fats the most efficient fuel for aerobic metabolism?)

    Thomas Summer, MD on #52033

    Hey Steve!

    First of all: I like the idea of the N-Zone!

    interesting scenario. I think Chad would win. If he gets the carbs during the race, he will have an additional fuel source. His aerobic system and fat metabolism will be trained almost like the one of Fred. Probably even better, because he was able to run with better intensity and recover better with the carbs. I could argue in the other direction too;-) but in the scenario, where he gets the carbs during the race, he will win (when he doesn’t get sick from the sugar). In a scenario where there are no carbs at the race, then he will have no chance.

    I think it’s not a question of carbs or fat. It’s about how well you can use both.
    Maybe we should bring ketones into the discussion!?;-)

    AshRick on #52038

    Train to burn more fat, sure. But on race day…you’re short of carbs, not fat. No matter how much better your fat metabolism pathways, you need carbs on race day.

    Rachel on #52043

    I think if they have the same nutrition during the race Fred will win (assuming his GI system can handle carbs since he didn’t train with them). But if Fred is fasted during the race my money is on Chad.

    russes011 on #52048

    In order to win, Chad and Fred will run the race at AeT. Since they ate the same diet and have performed the same work-outs, for simplicity sake let’s assume their AeTs are equal. They have full and equal glycogen and fat stores before the race begins. In summary, they run most of the race at the same AeT, and therefore at the same HR and speed. Aerobic metabolism supplies the ATP, and since they’re at AeT, by definition 50% is from carbs and 50% is from fat (they are at the crossover point). From a biochemical point of view, carbs and fats are equally efficient at producing ATP, per unit of oxygen: they both produce about 5 ATP per O2 molecule. Both Chad and Fred have unlimited fat stores. At the start line, Chad and Fred both have about 2000cal worth of glycogen.

    Let’s say they burn 4800cal during the 6hr race (~800cal/hr@10min/mi), which is about average or so. Since they have the same AeT and are both running at their AeT, by definition they’re using the same amount of carbohydrates per hour (400cal), at least until Fred runs out. Fred, with 2000cal of glycogen, runs out of carbs at ~5hrs. Chad, in essence, has unlimited access to carbs during the race, thus preserving his glycogen stores. Overall, Chad was best able to spare carbohydrates during the race, which is perhaps the main factor that determines onset of fatigue. Note, however, that without carb supplementation Chad’s ability to spare carbs would still be equal to Fred’s, since they both have the same AeT. Chad wins the race because he could maintain his AeT, while Fred has to slow down around 5hrs because he can no longer maintain AeT, which by definition requires 50% carbs as a fuel source. If the race was say ~5hr long, they would have tied.

    Carbs are always important on race day—ie, the carbs you already have within you. Carb supplementation, per se, may or may not be important on race day, depending on the distance (and intensity level). Chad’s supplementation would not have helped him win the race if it was say ~2-5hrs long—assuming they raced solely at AeT, which on average is the fastest one can race at these durations, and neurological factors are excluded. To maintain AeT beyond about 5hr, a carb fueling strategy is required. That’s not to say one can’t run way beyond ~5hr without carbs, but that they will be doing this at a pace below AeT. This underscores the importance of improving one’s AeT.

    (I hope this reasoning isn’t too reductive to the point where it makes my argument false)

    Thomas Summer, MD on #52061

    Hey Steve!
    I think it’s really a bit too reductive;-)
    Especially the assumption that they have equal AeT. If we assume that, then we also assume that the whole point about training without carbs is useless. Train low, race high…
    But if we assume that they have the same AeT, then we could also assume that Fred trained and races with ketone esters. So also an additional fuel source for him.
    Who wins now?

    russes011 on #52078


    I agree with you that my above analysis is too reductive to be truly valid or predictive. That said, I think it’s still conceptually sound, at least from the level of a freshman physiology course. One key question, however, that you and others astutely bring up, is whether Fred actually had a higher AeT than Chad at the start line due to his fasted training—or, more generally, did Fred have an advantage because he was more fat adapted? For example, if his AeT was 20% higher due to his fasted training he could have at least tied Chad, even without any carbs during the race.

    IN GENERAL, I do not think Fred was more fat adapted, nor do I think Fred experienced any significant ketosis prior to the race.

    I understand training increases your AeT regardless of diet. I also understand that a ketogenic diet, or even a high fat diet without ketosis, could shift your respiratory quotient downward, and presumably your AeT upward. But Fred (like Chad) was on a 50/50 (by calorie) carb/fat diet, which is twice as many carbs than fats by gram. Was Fred’s training without carbs, in and of itself, enough to raise his AeT higher than Chad’s? My hunch is no, since his overall diet had enough carbs to replenish most of his glycogen stores, and most of his training was likely in a time range (1-4hrs) where his internal carbs were still mostly available for use—in effect his muscles never really experienced a true low carb state, and didn’t really experience much ketosis, if any.

    Let’s say instead Fred fasted for the 12hrs prior to every training session, the currently popular, “rest low, train high” approach to ramping up your AeT via fat adaptation (as you mentioned). I, unfortunately, don’t think this would have made a real difference either, at least from a glycogen point of view. Fred’s consumption of Chad’s 50/50 diet the rest of his day should have been enough carbs to (mostly) refill his glycogen stores, at least to the same degree as Chad, and not eating breakfast prior to training, eg, is just not enough of a low carb state (given his decent glycogen stores and that most of his training would only be in the few hour range and at low intensity). I presume this approach is too homeopathic to be effective. I understand that the fasted training approach is on a spectrum of efficacy—say his dinners were no carb, or he performed doubles off carbs—but this is too far from Scenario 1A.


    russes011 on #52079

    My hunch is that the result of Scenario 1A would be that Fred and Chad would finish at the same time. I think this because the differences mentioned in the scenario are just not enough to have an effect on the outcome. Shooting from the hip here, but I suppose AeT is not a static metric (it collapses after a certain distance), and that after 3-4hr of running (say 20-25 miles) the factors that determine success and performance are mostly unknown, or at least misunderstood. To summarize, I think exercise physiology turns to exercise pathophysiology after a certain distance or time running.

    Thomas Summer, MD on #52089

    I like that! The longer you run the more you have to deal with exercise pathophysiology. But that’s what we enjoy, isn’t it!?;-)

    Rachel on #52096

    I also doubt if they would have the same AeT. In theory Chad could be an ADS-afflicted carb-burner with a low AeT since he can fuel every 30 minutes for every training session. One question would be how much volume are they doing? If they are working out at 12+ hours a week they will probably both improve their fat oxidation a lot. But if they are only training say 6 hours a week, Chad might not be nudging his AeT up a lot.

    And also can I say I’m super impressed with Fred — he is doing the exact same workouts as Chad, same pace, but totally fasted. 😉

    Aaron on #52181

    If I recall correctly from the recent podcast on metabolic testing AeT and the crossover point may or may not be similar.

    russes011 on #52183


    I have recently learned what you say is true. I have been learning quite a bit from the other participants like yourself by putting my understanding, or mis-understanding, of concepts out there for commentary.

    AeT may coincide with CP, but they aren’t necessarily directly related to each other. Scott says its his experience, however, that for most well trained athletes their CPs and AeT are usually similar. Nevertheless, this represents a major flaw in my analysis original analysis. That said, since Chad and Fred has identical diets and exercise programs (aside from fueling during training) their respective crossover points should be similar, I’m guessing. Therefore, my overall conclusions may remain somewhat accurate–who knows.

    — Steve

    russes011 on #52272

    Alas, being identical twins, no one could actually tell who won the race

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