Starvation fuels used

Figure 1-16-5. Fuel Use in the Brain During Fasting and Starvation...

In the options above, each graph depicts the primary source of fuel used by the brain during fasting/starvation. For each condition listed below, select the most closely matched graph. 

Table 2.2 Approximate content of glycogen, triacylglycerol and glutamine in a normal adult male and the estimated time for which they would last, if they were the only fuel used, during two forms of physical activity and during starvation...

Figure 3.18 Oxidation of glucose and ketone bodies by the brain. Glucose is the sole fuel used by the brain, except in prolonged starvation in adults or relatively short-term starvation in children. In both cases, ketone bodies plus glucose are used.

In the fed state, the only fuel used by the brain is glucose, derived from the blood. In prolonged starvation or chronic hypoglycaemia, ketone bodies are nsed which rednce the rate of utilisation of glucose by the brain bnt, even so, glucose still provides about 50% of the energy. Consequently, under all conditions, maintenance of the blood glucose concentration is essential for the function of the brain the mechanisms that are responsible for this are discnssed in Chapters 6, 12 and 16. 

The primary fate of acetyl CoA under normal metabolic conditions is degradation in the citric acid cycle to yield C02. When the body is stressed by prolonged starvation, however, acetyl CoA is converted into compounds called ketone bodies, which can be used by the brain as a temporary fuel. Fill in the missing information indicated by the four question marks in the following biochemical pathway for the synthesis of ketone bodies from acetyl CoA ... 

Skeletal muscle utilizes glucose as a fuel, forming both lactate and CO2. It stores glycogen as a fuel for its use in muscular contraction and synthesizes muscle protein from plasma amino acids. Muscle accounts for approximately 50% of body mass and consequently represents a considerable store of protein that can be drawn upon to supply amino acids for gluconeogenesis in starvation. 

Diabetes results from a lack of insulin secretion by the pancreas. Without insulin, cells take up glucose very slowly. The lack of insulin results in an inability to use blood glucose for fuel. Consequently, the body behaves as if it were starving even though food is available. The metabolic responses of the untreated insulin-dependent diabetic are essentially the metabolic responses of starvation. 

Brain Brain does not burn fat as an energy source however, after adapting to long-term starvation, brain can use ketone bodies for fuel. 

The calorific capacity of amino acids is comparable to that of carbohydrates so despite their prime importance in maintaining structural integrity of cells as proteins, amino acids may be used as fuels especially during times when carbohydrate metabolism is compromised, for example, starvation or prolonged vigorous exercise. Muscle and liver are particularly important in the metabolism of amino acids as both have transaminase enzymes (see Figures 6.2 and 6.3 and Section 6.4.2) which convert the carbon skeletons of several different amino acids into intermediates of glycolysis (e.g. pyruvate) or the TCA cycle (e.g. oxaloacetate). Not all amino acids are catabolized to the same extent... 

The extract from Colin Blakemore s book provides a vivid account of the behavionral consequences of a low blood glucose level. This is because the brain uses glucose as the only fuel, except in prolonged starvation. Problems also arise if the blood glucose level increases well above the normal. An increase of only about twofold above the... 

Ketone bodies are oxidised by most aerobic tissues including skeletal muscle, heart, kidney, lung, intestine and brain. Since the last two cannot oxidise fatty acids, their ability to oxidise ketone bodies is very important, because they provide another fuel in addition to, or as an alternative to, glucose. Hence, they can be used to replace some of the glucose to maintain the blood glucose concentration (e.g. in prolonged starvation or hypoglycaemia). 

The major role of skeletal muscle is movement, which is described and discussed in Chapter 13). Nevertheless, since muscle comprises 40% of the body it is large enough to play a part in control of the blood concentrations of the major fuels glucose, fatty acids, triacylglycerol and some amino acids. Skeletal muscle contains the largest quantity of protein in the body, which is used as a source of amino acids under various conditions (e.g. starvation, trauma, cancer see above). It plays an important part in the metabolism, in particular, of branched-chain amino acids, glutamine and alanine, which are important in the overall metabolism of amino acids in the body (discussed below). 

Air from the compressor is split into two streams primary air is premixed with the fuel and then fed to the catalyst, which is operated under O2 defect conditions secondary air is used first as a catalyst cooling stream and then mixed with the partially converted stream from the catalyst in a downstream homogeneous section where ignition of gas-phase combustion occurs and complete fuel burnout is readily achieved. The control of the catalyst temperature below 1000 Cis achieved by means of O2 starvation to the catalyst surface, which leads to the release of reaction heat controlled by the mass transfer rate of O2 in the fuel-rich stream and of backside cooling of the catalyst with secondary air. To handle both processes, a catalyst/heat exchanger module has been developed, which consists of a bundle of small tubes externally coated with an active catalyst layer, with cooling air and fuel-rich stream flowing in the tube and in the shell side, respectively [24]. 

A. If fasting extends past 1-2 days, which is considered to be a long-term fast or starvation, further changes in fuel synthesis and use by several organs can occur, principally a conversion from a glucose economy to one dominated by ketone bodies as fuel (Figure 5—6). 

During starvation or in uncontrolled diabetes mel-litus, when carbohydrates are either unavailable or not properly utilized, cellular proteins are used as fuel. 

In this chapter, we will review the fundamental models that we developed to predict cathode carbon-support corrosion induced by local H2 starvation and start-stop in a PEM fuel cell, and show how we used them to understand experiments and provide guidelines for developing strategies to mitigate carbon corrosion. We will discuss the kinetic model,12 coupled kinetic and transport model,14 and pseudo-capacitance model15 sequentially in the three sections that follow. Given the measured electrode kinetics for the electrochemical reactions appearing in Fig. 1, we will describe a model, compare the model results with available experimental data, and then present... 

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