During the absorptive state, the liver creates and stores glycogen in its own cells from the carbohydrate calories you have eaten. The liver can hold about 100 grams worth, or about 400 calories of glycogen at four calories per gram (though the true figure may be a little lower as water also figures into the equation). All the body's tissues can draw on glycogen in the liver. But once its stores are full, it passes extra carbohydrates into the bloodstream. Muscle tissue readily takes them up and stores them as muscle glycogen. Depending on your physique, your muscles can store between about 200 and 400 grams of glycogen. Once your liver and muscle glycogen stores are full, the liver removes the excess carbohydrates from the blood and turns them into fat.
At the same time all of this is going on, the liver is also building triglycerides, or fat molecules, from absorbed dietary fat and delivering them to other tissues, in particular to fat and muscle cells which store them for later use as fuel in the right conditions.
What are the right conditions for fat to turn into fuel?
When the body gets enough calories, glucose broken down from muscle glycogen is the primary fuel for muscles to do their work. As long as you eat enough, your muscle energy will come from this source. Of course, the more you move your body, the more you use up your glycogen. And once you deplete the glycogen, the muscles begin using fat.
So for muscle tissue to burn fat, you must first exhaust the glucose circulating in the blood and the glycogen stores in muscle. That’s why one of the best times to perform aerobic exercise is in the morning on an empty stomach. The body has been fasting for hours, glycogen and glucose levels are low, and the potential to burn fat is high. The benefits rise even more if you stop eating carbohydrate-rich
insulin into the bloodstream. Insulin promotes the synthesis of glucose (a sugar that is the body’s most abundant energy source), amino acids (the building blocks of proteins), and triglycerides (the body’s storage form of fat).
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