Re: Fuel use in aerobic and anaerobic metabolism

[Trishmare@aol.com]

Dear Beth,
   Great post!!!  You explained this really clearly, so that even a layperson
such as myself understood it.  But I've got a question.  How can my horse
David be doing all this stuff, and STILL have time to notice a horse-eating
pinebranch lying in the road?

Trish & pretty David, (who says, "but really mom, I KNOW it was gonna get
me!")
Grand Blanc, Michigan

<< Hi Shannon,
 Well, this is essentially true.  There are a few important points 
 that need to be understood.   If this is too basic a review, please 
 pardon me, but I'll try to back up a little and give a bit more 
 biochemistry.  Glycolysis is the process where glucose is converted 
 through a series of steps to pyruvate.  During the process 2 ATPs 
 [which yield energy] are produced per molecule of glucose and no 
 oxygen is needed, therefore this metabolism is called "anaerobic".  
 At the end of glycolysis 2 molecules of pyruvate are formed.  [Now, 
 here comes the "aerobic" part], IF oxygen is readily available and if 
 this metabolism is occuring in a muscle fiber that prefers aerobic 
 metabolism, the pyruvate molecules will continue on through the Krebs 
 cycle, producing 36 more ATPs.  However, if the work is occuring so 
 rapidly that oxygen can't be provided to cover all the energy needs, 
 or if the exercise is very forceful and therefore stresses the fast 
 twitch fibers, an excess of pyruvate will be formed which cannot be 
 cycled through the Krebs cycle.  This pyruvate then is converted to 
 lactate.  
 
 A small amount of the lactate diffuses out of the cell intact, & is 
 sent to the liver where it can be reformed into glucose [about 25% 
 of the lactate produced is disposed of in this way].  This glucose is 
 then used to maintain blood sugar and can be used for energy.  Some 
 of the lactate that remains within the cell will be used by the 
 mitochondria oxidatively to produce energy. So, some of the lactate 
 can be recylced for anaerobic metabolism [by reforming sugar] and 
 some will go to oxidative metabolism.   Lactate is a preferred fuel 
 for highly oxidative tissue, such as heart muscle.    In this way, 
 even submaximal exercise actually is producing quite a bit of lactic 
 acid, but it is being metabolized rapidly.  As workload increases so 
 will lactic acid production and eventually the body's ability to 
 metabolize it will be outstripped.  At that point lactate will 
 accumulate increasingly in the cell and in the blood, causing an 
 acidic environment that does not allow the enzyme systems to work 
 well, and exercise will become more difficult. If doing true 
 intervals, where there is a significant lactate accumulation, it is 
 essential that low intensity exercise be used between hard intervals, 
 such as walking or jogging,  rather than complete rest between 
 intervals:  the contracting muscles will use up some of the lactate, 
 and will allow for faster recovery for the next interval.  
 
 Have a good weekend,
 
 Beth Glace, MS
 Sports Nutritionist >>