Burn fat, not muscle

I’m kind of a statistics geek.  I wear a GPS watch and heart rate monitor when I run, which records route, pace, etc.  Afterward I upload the data from the watch to the computer where I can see my route on a map and see my other stats mile by mile.  I loved seeing the numbers improve over time, but I had no context for it.  My coach suggested I take advantage of the physiology testing offered through our club, so I’d better understand what the numbers mean for me.  What I learned was so interesting and taught me exactly what I needed to know in terms of how to get the most out of my training.

The test told me several things, the most of important of which is the point at which I stop burning fat for fuel, and start burning glucose instead.  Why is that important?  This is going to get a little technical, stick with me.  Fat is good fuel, glucose isn’t.  The #1 factor that limits your physical ability to sustain an intense effort over time is your system’s ability to process lactic acid.  Lactic acid is the waste by-product of glucose and causes muscle fatigue.  It starts to build up in your system as soon as you start burning glucose for fuel.

I stop burning fat for fuel waaaay too soon in my runs.  My cross-over point sits at a pretty low heart rate in terms of my cardio range, and at the moment I cross over from fat to glucose the level of lactic acid in my system skyrockets instead of increasing gradually.  What does this mean for me?  (A) I have to carry a lot of food with me on long runs.  (B) I have to train differently to increase my body’s ability to utilize fat for food.

A little more technical information (we’re almost finished!): your muscles are made up of three types of fibers, one slow-twitch fiber and two fast-twitch fibers.  Slow-twitch fibers burn fat and happen to be really good at cleaning up lactic acid.  Fast-twitch fibers burn glucose and are really bad at cleaning up lactic acid.  So the longer you can keep your slow-twitch muscle fibers firing, the more you’ll get out of your muscles.

What this means for training is keeping my heart rate low enough that I don’t force my system into crossing over to glucose-burning.  That means I do my runs slower than I’m capable of, and I force my slow-twitch muscles to do the work.  Over time the heart rate at which I cross over to glucose will increase, and I will be able to run faster while still burning fat.  It doesn’t take that long to improve — I’ve already had my range increased once, and I’m going back for more testing in April.

What this means for you is that if you’re exercising to lose weight you should not go all out.  You do not need to push yourself to the limit in order to lose weight.  All that will do is cause you to lose muscle and probably make you hate your workouts.  Whatever your workout of choice is, you should exercise at an intensity that gets you somewhat out of breath but still able to talk.  That should be your primary indicator for whether or not you’re exercising at the appropriate level.  If you’re huffing and puffing and can’t get a sentence out without gasping for breath, you’re working too hard.

Ultimately it would be most effective if you got yourself a heart rate monitor and had someone test you for your maximum heart rate.  Then you’d be able to know exactly when you were in the right zone (Zone 2 for anyone who might already have had this done) and maximize your workout efforts.