From Coyle EF, a few posts back...Muscle glycogen and blood glucose contribute equally to carbohydrate energy production over 2-3 h of moderate-intensity exercise; fatigue develops when these substrates are depleted.
This doesn't address your question about the rate of blood glucose consumption by muscle tissue being relatively limited and whether or not greater capillary density would increase muscle uptake of glucose and spare glycogen. I understood that as long as activity was easy enough, fatty acids were consumed. Then, as activity increased, blood glucose was mostly consumed (fatty acids are still being consumed as before...they don't shut off just because glucose is being utilized), as activity increased even more, glycogen was mostly consumed due to a lack of sufficient readily-available oxygen (and none of this is all-or-none, but just relative terms, you may still burn some glycogen at very easy efforts, just very small amounts; sort of like aerobic and anaerobic pathways actually occuring simultaneously much of the time)...and lack of sufficient readily available oxygen is often directly equated with blood flow...this may be a chicken-or-egg thing, which is it, blood flow too low to deliver enough oxygen or not enough glucose, or both?
Another layman's view...I don't know if it is correct.
I think you are also right that muscle glycogen doesn't contribute to blood sugar levels. Blood glucose is used to form (directly or indirectly) liver and muscle glycogen. But, it's my understanding that if you either increase intensity or drop blood glucose too much, the muscle then must mostly use glycogen (much less efficient) instead of glucose. The muscle can experience low glucose availability by having too little blood carrying a normal level of glucose to the muscle, or if there were a drop in blood normal glucose concentration.
Again, so many studies look at VO2 max intensity exercise, you hear much more about glycogen and lactate being used my working muscle. It's the sub-VO2 max state when glucose-utilization plays a larger role in working muscle.
Quid quid latine dictum sit altum videtur
(That which is said in Latin sounds profound)
This doesn't address your question about the rate of blood glucose consumption by muscle tissue being relatively limited and whether or not greater capillary density would increase muscle uptake of glucose and spare glycogen. I understood that as long as activity was easy enough, fatty acids were consumed. Then, as activity increased, blood glucose was mostly consumed (fatty acids are still being consumed as before...they don't shut off just because glucose is being utilized), as activity increased even more, glycogen was mostly consumed due to a lack of sufficient readily-available oxygen (and none of this is all-or-none, but just relative terms, you may still burn some glycogen at very easy efforts, just very small amounts; sort of like aerobic and anaerobic pathways actually occuring simultaneously much of the time)...and lack of sufficient readily available oxygen is often directly equated with blood flow...this may be a chicken-or-egg thing, which is it, blood flow too low to deliver enough oxygen or not enough glucose, or both?
Another layman's view...I don't know if it is correct.
I think you are also right that muscle glycogen doesn't contribute to blood sugar levels. Blood glucose is used to form (directly or indirectly) liver and muscle glycogen. But, it's my understanding that if you either increase intensity or drop blood glucose too much, the muscle then must mostly use glycogen (much less efficient) instead of glucose. The muscle can experience low glucose availability by having too little blood carrying a normal level of glucose to the muscle, or if there were a drop in blood normal glucose concentration.
Again, so many studies look at VO2 max intensity exercise, you hear much more about glycogen and lactate being used my working muscle. It's the sub-VO2 max state when glucose-utilization plays a larger role in working muscle.
Quid quid latine dictum sit altum videtur
(That which is said in Latin sounds profound)
Last edited by:
yaquicarbo: May 23, 04 13:30