Bloodglucose Regulation

Blood-glucose regulation is achieved by an intricate balance among many factors, including nutritional status, endocrine and neural mechanisms, and physical activity. However, for the purpose of this discussion, we will just focus on the different biochemical processes of glucose utilization and synthesis in the tissues, which maintain normal blood glucose. In the fed state, blood glucose is kept normal by increased utilization of glucose and storage as glycogen. Glucose produced by the catabolism of amino acids not used for protein synthesis is also stored as glycogen. Fatty acids not resynthesized into triglycerides for storage as lipids are oxidized and, by so doing, generate substrates also used for de novo glucose synthesis (gluconeo-genesis) and storage as glycogen. In the postabsorptive state, normal blood glucose is maintained by a combination of gluconeogenesis and glycogenolysis. During short-term fasting, blood glucose is primarily maintained by hepatic glycogen breakdown. With extended periods of fasting, fatty-acid oxidation and amino-acid catab-olism become predominant sources of energy, and through these processes, substrates are generated for gluconeogenesis.

As noted earlier, glucose is a major fuel that also occupies a central position in the metabolism of other nutrients in the body. It is a precursor molecule that is capable of providing many metabolic intermediates for various biosynthetic reactions (3). Hence, the metabolism of glucose is normally regulated by a well-coordinated system among the different tissues in the body. For instance, in the muscle, glycolytic degradation of glucose produces ATP, and the rate of glycolysis increases as the muscle contracts more intensely, thereby demanding more ATP. On the other hand, as previously noted, the liver and the kidneys serve to keep a constant level of glucose in the blood by producing and exporting glucose when the tissues demand it, while the liver takes up and stores glucose when it is available in excess (3). The turnover of muscle protein occurs slowly with little or no diurnal changes in the size of the protein pool in response to feeding and fasting (4). There are 20 standard amino acids in proteins, with variations in their carbon skeletons. Consequently, there are many different catabolic pathways for the degradation of amino acids for energy production. Altogether, the energy from these pathways accounts for only 10 percent to 15 percent of the body's energy production (3). Although much of the catabolism of amino acids takes place in the liver, six amino acids, namely leucine, isoleucine, valine, asparagine, aspartate, and glutamate, are metabolized in the resting muscle (4). However, the three branched-chain amino acids, leucine, isoleucine, and valine, are only oxidized as metabolic fuels in the muscle, adipose tissue, kidney, and brain. These extra-hepatic tissues have a single aminotransferase that is not present in the liver and acts on all three branched-chain amino acids to produce the corresponding keto-acids (3). The overwhelming majority of amino acids are glucogenic. Hence, their carbon skeleton generates intermediates of tricaboxylic acid (TCA) cycle that are used for de novo glucose synthesis.

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