Face it, during intense exercise you burn a lot of a calories. Which, for most of us, is a good thing. It's one of the reasons I prefer cross-country skiing to golfing (the other being that I am just horrible at golfing!). But you may have also noticed that during intense exercise, you're not able to absorb a lot of calories. You can eat as many as you want but you aren't going to be able to process them. Why? Because you need to send blood to your arms and legs to make them go which means you aren't sending as much blood to your stomach to help it do it's job. (And yes, biology/anatomy folks - I know that I've just made a horrible oversimplification there, but it's the basic concept.) So, while you might be burning 700+ calories an hour on your bike ride, if you eat 700+ calories an hour, you are probably not going to feel good.
You can only absorb about 250 to 300 calories per hour - less if you are smaller or working with greater intensity - which means that extra 400 calories has to come from somewhere. Where it comes from is your energy stores. If you are a Sprint or International Distance triathlete, that energy is primarily going to come from your muscle glycogen stores. Once you start doing longer races, you won't have enough stored glycogen to fuel your effort but you have more than enough stored fat. That's not an insult - even the most fit, elite athletes have enough fat to fuel an Ironman Distance effort. I personally probably have enough stored fat to run across the United States.
How does heart rate zone training play into this? Through training, you can more efficiently access your stored energy - there's no point in having the energy equivalent of a nuclear reactor if you can't access more than a 9 Volt battery's worth. First, if you increase your VO2 Max and Anaerobic Threshold (AT), you can improve your ability to burn fat at a higher pace. A structured program including efforts just above your AT can help to increase your AT and therefore the pace at which you can burn fat. Second, combining training effort and nutrition can help to train metabolic efficiency. I'll return to that topic in a future post but for now check out the work of Bob Seebohar to learn more about that subject http://www.fuel4mance.com/
This leads us to reason three. So return 'soon' to learn more.