Fat and protein can both be converted into glucose if necessary through a process called gluconeogenesis. The use of proteins or fat for gluconeogenesis requires more energy than the more straightforward metabolism of starches and sugar into glucose.
Identification
Gluconeogenesis occurs when additional glucose is needed by your body, such as after a bout of intense exercise. Glucose obtained from the breakdown of fats and proteins is the only kind of energy that the brain, testes and kidney medulla can use. It is also the only form of energy that your erythrocytes, or red blood cells, can use for their own source of energy.
Function
Glucose is stored in your liver as glycogen. During periods of intense physical activity, or during starvation scenarios such as fasting, this glycogen is converted into glucose and released into your bloodstream. If this situation continues, the stores of glycogen in your liver become depleted. When this happens, your body turns to its storage of fat, converting adipose triacylglycerols into fatty acids.
Glycerol's Role
During this process, glycerol is released. It is used for further gluconeogenesis, first by activating another trigger that releases amino acids stored in your muscles for conversion into further supplies of glucose. The trigger for gluconeogenesis is the release of amino acids from your muscles. The amino acids combine with other precursors to begin the process of making more glucose available.
Protein Conversion
Once all the protein has been converted into glucose, your liver turns its attention to the fats from your diet. The carbon left behind in your liver from the breakdown of protein is the basis for gluconeogenesis using fatty acids. One of the lesser-known functions of insulin is to regulate this fatty acid synthesis. When your body has more glucose available than it has immediate needs for, the excess glucose gets stored in your fat cells as fatty acids.
Fat Conversion
Insulin is a protein itself. When your body needs to raid your fat cells for glucose stores, insulin regulates how much fat is converted to fatty acids and then transports these fatty acids around your body to your cells. Once it reaches your cells, the insulin releases the fatty acids, which are then converted into glucose in another biochemical reaction that also releases energy.
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