Exercise on an empty stomach

From chapter "riding burns calories and makes you eat more," of Just Ride. 

 On casual rides of up to four hours, don't eat before or during the ride, and drink only water.  You won't starve or die. At that effort level, and without insulin present, your body will bum your own fat as fuel. If you need to ride hard for a while, or race up a hill, that's OK. Your muscles have enough glycogen (a form of glucose) for at least an hour of intense riding.

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Once or twice a week, exercise to exhaustion on an empty stomach. Exert maximum effort through muscle-burning intervals or weight lifting. You're more likely to have low insulin levels on an empty stomach, and that allows the release of growth hormone to help you build muscle and burn fat. It's not fun, but it can be over in five to fifteen minutes, depending on how long you rest between efforts. This is the best way to train muscles to use more oxygen, to be able to work harder without the burning pain of oxygen debt. Then don't eat for an hour afterward. This keeps your level of growth hormone higher longer, helping you build muscle and burn fat. If you want to lose fat, you've got to gain muscle. Despite what you may have heard, muscles at rest don't burn up appreciably more calories than fat does. Muscles help you stay lean by making you insulin sensitive-mean-ins you require less insulin to lower your blood sugar. Since insulin makes you fat, the less of it you have in your blood, the harder it is to gain fat. Losing fat is largely about lowering insulin levels, and increasing muscle does just that.