From: mazur@inmet.camb.inmet.com (Beth Mazur)
Subject: Re: Aerobic Workout Length
Organization: Intermetrics Inc, Cambridge MA
Date: Mon, 28 Sep 1992 18:13:23 GMT

In article <1992Sep28.134324.9611@wam.umd.edu> sfjr@wam.umd.edu (Steve Russell) writes:
>Thanks for all the tips I've gotten from last post about aerobic exercise.
>  I have started to read the referrences that people have informed about.
>  Which means I now have more questions:)  Sorry *chuckle*
>
>  For fat burning purposes some expersts on aerobic exercise say a 40 minuet
>workout should be strived for.  Their reasoning is that your body doesn't
>start using fat as a primary fuel source until after 20 minuets have
>elapsed.

According to other experts, fat is your primary fuel source while you are
at rest and up until the point that you begin doing anaerobic exercise.  But
I know think I know where this 20 minute number is coming from.  The
nutrition textbook I just picked up claims that it takes 20 minutes of
aerobic exercise before the body releases additional fat into the blood
stream.  This is a bit different from 20 minutes before you burn
fat.  It also is another possible reason why fit people may burn fat 
better than non-fit people--you can only burn fat if it is in your
blood stream.  Perhaps fat people are much better at storing fat than
they are using it, especially if they are sedentary. 
>
>   Another expert (Bailey of Fit Or Fit) claims that the actual exercise
>itself burns little,  the benefit comes from forcing your metabolism to change
>which according to him happens at 12 minuets in target zone.  Anything after
>this this magic 12 minuets is a "dimminished return".

Actually, what Bailey claims is that 12 minutes of exercise at the target
heart rate causes your body to generate more "fat-burning" enzymes which
will enable your muscles to burn fat (if it's in the blood stream) even
after you exercise.  Bailey's theory is that sedentary fat people do not
have as many of these enzymes as fit people do, and so they do not burn
as much fat.

>   So?  Who is correct?  Is it worth it for a person interested in fat loss
>to stay working out after 12 minuets in the heart rate target zone or would
>they be wasting their time by burning only a small amount of calories?

15 minutes of the activities you list might add up to around 500 calories
burned over the course of your week.  In either case, Bailey's 12 minutes,
or the "fat burning after 20", 500 calories is an extra Big Mac a week. 
Certainly the extra 15 minutes is not going to hurt, and as Bailey says,
it will get you to your goal faster.  

>So how about it?  Will that extra 15 min rev up my fat burning metabolism
>more or will I just use up some paltry number of calories that will be
>quickly replaced?

According to Bailey, you would be better off tacking that 15 minutes at
a different part of the day to achieve that "magic 12 minutes" twice.
Adding it to your current routine is only using a paltry number of calories.

Of course, there is no universal consensus on the net (or anywhere else)
one way or another (except that 15 minutes is pretty low-cal unless you
are a 5-minute miler).

Beth Mazur                   "...life is more than a vision.  The sweetest
mazur@inmet.inmet.com         part is acting after making a decision."
...!uunet!inmet!mazur                                  -- The Indigo Girls