What's the difference?  What does it matter?  Why should I care?  For some of you out there looking to lose a few pounds or training lingo is a foreign language, this is an important concept to understand.  The two ideas were introduced many many years ago to help differentiate workouts.  The Fat burning zone was meant to be for those looking to lose weight.  The Carb burning zone was meant for those looking to maintain a healthy weight.  

   What's the difference?

     First,  the Fat burning zone is a work rate that requires a lower heart rate, 60-70% of heart rate max, slow and steady.  Where fat stores in the body are broken down into glucose in order to fuel the activity.  The slower pace of the work allows the body to breakdown energy stores outside the muscle in order to maintain the work rate.  When possible the body prefers to maintain glycogen stores in the muscle.  When its a slower intensity workout the body will call on fat stores from other parts of the body, adipose tissue mostly found in the abdominal area, instead of using up the local energy stores in the muscles themselves.  This is the main reason its considered the "fat burning zone".

      The Carb burning zone requires a higher level of intensity, +70% of heart rate max, where the energy is taken from glycogen muscle stores.  All muscles maintain energy stores called glycogen, long chains of glucose molecules strung together for storage purposes.  When these local stores are full the remaining glucose in the blood will be stashed away into adipose tissue.  When the intensity of the work exceeds the speed at which the body can breakdown, glycolysis, the fat reserves it is forced to use the local supply until it is exhausted.  The local supply includes the local muscle glycogen reserves as well as glucose already present in the blood.  This threshold is around 70-80% of heart rate max or higher.  

     On a side note, yet very important, if you're curious about heart rate max....... take 220 and subtract your age.  For me, my max heart rate would be 220-39= 181 beats per minute.  This means, for me I'd need to get my heart rate up above (181 x .7=127) 127 beats per minute to maintain a good cardio/carb burning workout.

      The important concept to understand is that your burning calories REGARDLESS.  The biggest difference is how much time do you want to spend, or even better, how much time do you have!?  Not many people these days have 4 hours to spend on the treadmill to burn the calories they need, NOT THIS GUY!!

    Why should it matter?  It matters because you should be aware of what your goals are and how you can be both effective and efficient in your workouts.  For those folks that are high risk of a cardiac event (older, more obese, previous history, etc.) the high intensity workouts should be done for less time and be closely monitored.  No point in hitting the gym hard just to keel over with a heart attack a couple weeks into the new workout regimen.  Checking with your primary physician and using a heart rate monitor are both good starts.  Consulting with a dietician or nutrition expert are also good choices.  
   

    You should care because if all you ever do is the slow and steady burn, you're missing out on much needed cardio.  The Carb burning zone (higher intensity) gets 2 birds with one stone in that it burns calories faster and it also pushes the heart to work hard!  Making the heart push to its upper limits, based on age and ability, are what keeps it healthy.  Keeping active (fit bit, pedometers etc.) is good but it doesn't ever really push the system to get stronger.  High intensity workouts, which varies from person to person, is where you get stronger in the heart and cardiovascular system.  So don't be satisfied with walking 10,000 steps each day where your heart rate never gets above 50% max, push for more!  Push your body and you'll be pleasantly surprised with how it reacts!