Everyone agrees that you can't do work without burning some fuel, but people have all sorts of ideas and working theories that explain how their bodies work. Here's how I think about my running energy supply, in a nutshell.
Fuel that provides the energy for your muscles to work can be stored in (and close to) your muscles. New fuel to replace what was used (and waste products from using that fuel) is carried by your respiratory and circulatory systems. This replacement fuel comes from food that you eat (your digestive system) and from metabolizing fat and protein stored in your body. That's pretty much it. Let's look at some examples of using fuel while running.
In a short, high-intensity effort, the muscle's immediate need for fuel is provided by that fuel most available to the muscle cell. It's really the only very near-term possibility for immediate use. The good news is it's readily available for immediate use, the bad news is it gets used up pretty fast. I'm thinking Usain Bolt primarily uses this source of fuel in 200 meters, and when that source is gone, a fuel shortage contributes to his starting to slow down.
In a longer, and necessarily lower intensity effort (like when running longer distances) new fuel can be supplied to the muscle via blood supply, and that fuel can in turn be resupplied by what the runner is eating, or from metabolizing fat and even protein stored in the body.
Since one goal of running is often to change your body's makeup (in terms of fat and muscle) we'd like to burn fat. Fat contains over 3,000 calories of energy per pound, and we can travel a mile on a little over 100 calories. The good news would seem to be that we can go amazing distances considering the amount of fat we're packing! The bad news is that while metabolizing fat can eventually provide us with a lot of energy, it is a fairly slow process - if in fact you burn through (use up) all the stored energy available for immediate use and haven't eaten anything, the energy you get from burning fat is only going to support relatively slow movement. I know I'm making broad statements here ... the optimal endurance runner is lower weight and very efficient, able to move faster and further while using less energy ... but ultimately he's dealing with the same metabolic processes you and I are.
We've all seen charts posted on exercise machines and gym walls showing lower intensity levels of effort as being "fat burning" exercise, and, after working long enough that's going to be true. On the other hand, calorie consumption is all about how hard you work for how long - a shorter high intensity workout will burn as much as a much longer low intensity workout. There are also benefits associated with the kinds of changes in your body (muscle development) that higher intensity efforts bring too.
Running long while trying to create a calorie shortfall (to lose weight) is not going to be pleasant. A constant challenge for me is coming to grips with the fact that running long creates a demand for replacement fuel. At some level of weekly mileage (>50 miles, which is pretty high for me) I do see a slow but steady weekly weight loss. The bad news is that I can only sustain the higher mileage for about 16 weeks, and then I always see some weight gain in the weeks after a 'goal-race'. It's all about finding a balancing point, and that's something I haven't learned to do very well ...
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