The N-Zone

SCENARIO 1A:

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?)

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,

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.

Steve

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