the excess into glucose, since amino acids cannot be stored directly.
Food energy is stored in two forms: glycogen and body fat. Excess glucose, whether derived from protein or from carbohydrates, is strung together in long chains to form the molecule glycogen, which is stored in the liver. It can be converted to and from glucose easily and released into the bloodstream for use by any cell in the body. Skeletal muscles also store their own glycogen, but only the muscle cell storing the glycogen can use it for energy.
The liver can only stockpile a limited amount of glycogen. Once it is full, the excess glucose is turned into fat by a process called de novo lipogenesis (DNL). De novo means “from new” and lipogenesis means “making new fat,” so this term means literally “to make new fat.” Insulin triggers the liver to turn excess glucose into new fat in the form of triglyceride molecules. The newly created fat is exported out of the liver to be stored in fat cells to supply the body with energy when it is required. In essence, the body stores excess food energy in the form of sugar (glycogen) or body fat. Insulin is the signal to stop burning sugar and fat and to start storing it instead.
This normal process occurs when we stop eating (and begin fasting), which is when the body needs this source of energy. Although we often use the word fasting to describe periods in which we deliberately limit certain foods or abstain from eating altogether, such as before a medical procedure or in conjunction with a religious holiday, it simply applies to any period between snacks or meals when we are not eating. During periods of fasting, our body relies on its stored energy, meaning that it breaks down glycogen and fat.
Figure 5.1. Storage of food energy as sugar or fat
Several hours after a meal, blood glucose drops and insulin levels begin to fall. To provide energy, the liver starts to break down the stored glycogen into component glucose molecules and releases it into general circulation in the blood. This is merely the glycogen-storage process in reverse. This happens most nights, assuming you don’t eat at night.
Glycogen is easily available but in limited supply. During a short-term fast (twenty-four to thirty-six hours), glycogen will provide all the glucose necessary for normal body functioning. During a prolonged fast, the liver will manufacture new glucose from stored body fat. This process is called gluconeogenesis, meaning literally “the making of new sugar.” In essence, fat is burned to release energy. This is merely the fat-storage process in reverse.
Figure 5.2. Gluconeogenesis: The reverse of the glycogen storage process
This energy storage-and-release process happens every day. Normally this well-designed, balanced system keeps itself in check. We eat, insulin goes up, and we store energy as glycogen and fat. We fast, insulin goes down, and we use our stored glycogen and fat. As long as feeding (insulin high) is balanced with fasting (insulin low), no overall fat is gained.
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