In short, no.  And what really matters most is how much energy (or calories) you burn.

Lots of cardiovascular machines have heart rate charts on them or programs labeled “Fat Burning Zone”.  I’ve always wanted to tear them off in a rage, because they end up making most people not work hard enough.

Here’s the scoop:  You use a combination of both carbohydrates and fats for energy, but predominately more fat the majority of the day.  Again, you’re using BOTH, but a higher percentage of fat.  

When you’re exercising you’re still using more fat than carbohydrate until you get closer to your lactate threshold (known as the transition from working aerobically to anaerobically – or “workin’ it ” to “Holy crap I’m dying”).  As you creep up toward your lactate threshold, the contribution from fat begins to decrease while the energy from carbohydrates increases.  When you are just below your lactate threshold you’re still using a higher percentage of fats, and when you are working above your lactate threshold (sprinting, for instance) carbohydrates become the only fuel your body can use.  

Now before you think “Well there’s no way I’m going to even get close to my lactate threshold because I want to burn as much fat as possible”, STOP.  Have you ever seen an overweight sprinter?

WHAT’S MORE IMPORTANT IS THE TOTAL NUMBER OF CALORIES BURNED.  When you are exercising at a high intensity – say just below your lactate threshold – the total number of calories being burned is much greater than when exercising at a lower intensity, SO THE TOAL AMOUNT OF FAT USED IS ALSO GREATER.  “What matters is the rate of energy expenditure, rather than simply the percentage of energy expenditure derived from fat.” (IDEA Fitness Journal, Oct 2009)

Think about this:  you’ve just won a five minute shopping spree in your favorite store.  The timer starts and you walk over to a coat.  “Hmmmm… this is kind of cute.  But I don’t know if it’s going to be warm enough.  I’m also not sure about the length – can I wear it with a dress?  I wonder if it comes in other colors?  I guess I’ll try it on and see how it looks…”  WHAT?  No way!  You’re going to tear into that store and just start grabbing things.  Who cares what it is, you’ll figure it out later.  The point is to get as much as you can in the time you have.

It’s the same way with exercise: it’s an energy shopping spree.  The point is to get as much as you can in the time you have.  How much energy can you burn in 20 minutes?  If you’re working hard, you can burn a lot.  If you’re only working moderately, you’re going to burn less.  

From now on, you’re not “working out”.  You’re “Going on a shopping spree”.  How fun.