Free of charge, here is my summation of exercise fuel sources, their uses, availability. Different intensities of exercise use different fuels.
First, the low intensity fuel source, what I call ‘the whale oil lamp’. Think of the way an old whale oil lamp works. It is unpressurized, smoky, inefficient, but cheap to operate. It does provide light, but it isn’t a terribly bright light, and it can’t light more than a small area. This means, then, in exercise physiology theory, you can only use this energy source to perform low-intensity exercises. Hiking, walking, easy swimming, easy long distance running are good examples of this.
This first source is your body’s stored fat. It will burn, but it only burns slowly. You can go a long time on this energy source, because most of us have at least some stored fat. This source has severe limitations. It can ONLY provide a slow, but steady trickle of fuel for low-intensity aerobic demands on the body.
The beauty of the human body is that it has an automatic cut-over switch to a more efficient fuel system if the physical demand outstrips the energy supply from fat cells.
This high intensity source is what I call ‘the Coleman lantern’. It burns brightly, and will light a large area, but there is only a small tankful of fuel to use. Pump the pressurization system, and the light will get brighter. This second source is glycogen, which is energy stored in muscles. This source is perfect for high demand aerobic activities, and also anaerobic activities. Sprinting, the middle section of most spinning/aerobics classes, mixed martial arts, boxing are good examples.
As you attempt to gain fitness at your anaerobic threshold, you are almost exclusively using the ‘Coleman lantern’ energy system. The more efficiently this Coleman burns, the quicker you can compete and the quicker you recover. The downside is you burn very little fat during this type of training.
If you need/wish to lose some weight that you consider fat cells, you must use the ‘whale oil’ system for the majority of your training. This system, remember, is for use in low-demand but endurance situations. You must try to keep your heart rate below 55% of maximum (or maybe 60%, but that’s pushing it). Anything higher, and the body automatically switches to the ‘Coleman fuel’ system of energy supply.
By the way, once you’ve turned on the Coleman lantern, it will stay on for the rest of that exercise session. If you want to go back to whale oil, stop that workout, rest for an hour or two, and start a new workout. EASY, this time.