From: page@ficus.cs.ucla.edu (Thomas Page)
Subject: Re: Fat Burning Primer
Organization: UCLA Computer Science Department
Date: Tue, 29 Sep 92 15:57:47 GMT

In article <1992Sep29.062604.10132@Princeton.EDU>, rks@phoenix.Princeton.EDU (Ramesh K. Sitaraman) writes:
|> There seems to be a lot of debate and a few misconceptions about
|> Fat burning, Stamina etc. Here are a few basics. They are based on
|> standard scientific sources.

Ramesh:  Thanks for raising the scientific level of the discussion.  This is
not a flame, just a few additions.


|> 
|> 1. There are 3 sources of energy : Muscle glycogen, Liver Glycogen,
|> Fat (triglycerides).

There is also blood glycogen.  This is what the brain runs off of.

|> 
|> 2. During excercise the glucose and fatty acid in the blood is utilised.
|> Fatty acid in blood stream is utilised in preference to glucose 
|> for various reasons. (There are organs like the brain that can use only
|> one ene{gy source : glucose)

Muscle glycogen is used before blood or liver glycogen.  When you carbo load
before a race, you are packing in the muscle glycogen.  You only have a few
minutes supply of blood glycogen once you start using that for running. 

|> 
|> 3. Then why can't we run on fat FOREVER ??? Because the fatty acid 
|> concentration in blood can never be much. In fact, it seldom
|> exceeds (in Molars) the concentration of Glucose !!! Why this
|> "transportation problem" for fats ?? There ain't much Albumin to
|> transport the fat and fat in excess in blood stream will deposit on
|> viens constrict and more directly cause blood clots and kill you !!!
|> 

Noakes' book "Lore of Running" phrases it as "fat burns in a glycogen flame."
Even a lean marathoner has a virtually unlimited supply of fat.  But you are
limited in your ability to mobilize the fat from the stores (fat deposits you
know where), and you have the limited amount of glycogen needed to burn along
with the fat.


|> 4. As you workout aerobically, the fat acids start increasing in concentration
|> in the blood stream. SO LONGER YOU WORK OUT THE GREATER PERCENTAGE OF WHAT
|> YOU BURN IS FAT ! But remmeber that the fat concentration cannot 
|> go up too much. Only about 50 percent of your energy can come from fat.
|> 
|> 5. So what happens if I run out of glucose ? Can I run on fat ?
|> Very Very painfully perhaps. Genrally your glucose will last about
|> 17 to 18 miles. Then you run out causing the famous WALL in the marathon.

The "17 to 18 miles" part is too simplistic.  Many of us have run marathons
without hitting the wall.  The elite marathoners almost never hit the wall.
Ultra runners run 30, 50 or 100 miles without hitting the wall.  Your rate
of glycogen usage is a complex function of your running efficiency, what
percentage of VO2max you run (typically 85% for an all out marathon), what
your VO2max is, how much glycogen you are able to store in your muscles, your
state of fitness, etc.

|> 
|> 6. Only long sustained exercise ( 30 -40 mins) is guaranteed to burn
|> fat (see fat mobilisation etc above). I have seen it work on me and 
|> others. I dont know what zip code these other guys are from.

I'm not sure what you are talking about concerning zip codes.  During the first
10 minutes of exercise, you burn almost exclusively glycogen.  From 10-30
minutes, you gradually increase the percentage of energy from fat, and hit a
plateau between 30 and 40.  So, keep running beyond 30-40 and you will keep
burning a maximum amount of fat.  The real magic of the 30-40 minute range is
that if you stop running there, you will continue to burn a large percentage of
fat, for several hours.

|> 
|> 7. Some person said doing exercise with breaks as being better
|> for burning fat. Not true. Neither does it build stamina.
|> (Doing intervals to build stamina is fine. But it should not constitute
|> more than small fraction of your total excercise duration. Most
|> of it should be of the long continous endurance building type.)

For the reason stated above, taking short breaks will not cause you to stop
burning fat.  If by taking walking breaks in the middle of your long runs you
can keep going significantly longer, then taking walking breaks will increase
the fat you burn.  Also, if you are training for a marathon, you need to get in
those 3+ hour runs.  If you need a couple of walking breaks to keep going that
long, you are better off taking them than quitting at 16 miles.  Of course, if
you can't run continuously for 3+ hours what you are concerned about is finishing
the marathon, not racing it fast, so by all means take walking breaks.  If you
want to get fast, train until you don't have to walk.
|>
|> 
|> 		-Ramesh 
|> 
|> -- 
|> ARPA:  rks@cs.princeton.edu  
|>        rks@phoenix.princeton.edu 
|> AT&T:  (609) 683 5394 (Home)  
|>        (609) 258 1794 (Off) 

Tom
page@cs.ucla.edu
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