Whole-body glycogen

Gammarus pseudolimnaeus 770 Significant decreases in free amino acids after 5 days in whole-body glycogen, protein, and caloric content after 15 days and in lipid content after 30 days 23... 

Fig. 6.13 (a) Relation between whole-body glycogen and time redrawn from [93]. The glycogen synthesis rate declines exponentially as the stores are filled up. (b) Relation demonstrating the exponentially decline in glycogen storage rate as the glycogen stores are filled up, for different half times, Ty2. 

RER > 0.9) immediately after feeding. When glycogen reserves are depleted, whole-body fuel utilization shifts almost exclusively to fat (RER = 0.8) before feeding [21, 42],... 

At the molecular level, the rate of fuel oxidation is tightly linked to ATP demand. At the whole body level, fuel intake in the form of food is not linked to the immediate demand for ATP. In other words, we bum food at the same time as we ingest it. Thus there must be fuel reserves, and most of the food that we eat will initially be stored rather than immediately oxidized. The reserves are primarily triglycerides, which are the store of fatty acids, and glycogen, which is the store of glucose (see Chaps. 11 and 12 for the molecular details). Historically it is thought that we evolved with the need to store metabolic fuel for times of famine so that we are inclined to overeat in times of plenty, and store this fuel as triglycerides. 

Whole-body MRS is potentially the most powerful tool available for the study of human metabolism in vivo. The nuclide s low natural abundance (1.1%) allows the study of important biological molecules such as glycogen and lipids, while at the same time allowing the use of C-enriched compounds in dynamic measurements of intermediate metabolism. However, several technical issues need to be addressed before its implementation in vivo, including a second transmit channel, optimum localization and proton decoupling. The latter has been one of the major stumbling blocks for the application of in vivo... 

Now that we know something about the effects of hormones in triggering responses within the cell, we can return to and expand on some earlier points about metabolic control. In Section 18.3, we discussed some points about control mechanisms in carbohydrate metabolism. We saw at that time how glycolysis and gluconeogenesis can be regulated and how glycogen synthesis and breakdown can respond to the body s needs. Phosphorylation and dephosphorylation of the appropriate enzymes played a large role there, and that whole scheme is subject to hormonal action. 

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