Metabolic fuels reserves

In the Fed State, Metabolic Fuel Reserves Are Laid Down... 

Metabolic Fuel Reserves Are Mobilized in the Starving State... 

Most of the reactions involved in energy metabolism involve the oxidation of metabolic fuels, while many of the biosynthetic reactions involved in the formation of metabolic fuel reserves and the synthesis of body components are reductions. 

Acetoacetate is chemically unstable, and undergoes a non-enzymic reaction to yield acetone, which is only poorly metabolized. Most of it is excreted in the urine and in exhaled air — a waste of valuable metabolic fuel reserves in the fasting state. To avoid this, much of the acetoacetate is reduced to (3-hydroxybutyrate before being released from the liver. 

Adipose Tissue Maintains Vast Fuel Reserves in the Form of Triacylglycerols The Liver Is the Central Clearing House for All Energy-Related Metabolism ... 

Muscle has an additional energy reserve in creatine phosphate, which generates ATP without the need for metabolizing fuels (see here). This reserve is exhausted early in a period of exertion and must be replenished, along with glycogen stores, as muscle rests after prolonged exertion. 

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. 

Under normal conditions, the processes shown in Figure 3-2 are tightly coupled, so that the oxidation of metabolic fuels is controlled by the availability of ADI which, in turn is controlled by the rate at which ATP is being utilized in performing physical and chemical work. Work output, or energy expenditure, thus controls the rate at which metabolic fuels are oxidized, and hence the amount of food that must be eaten to meet energy requirements. As discussed in section 5.3.1, metabolic fuels in excess of immediate requirements are stored as reserves of glycogen in muscle and liver and as fat in adipose tissue. 

By contrast, if the intake of metabolic fuels is greater than is required to meet energy expenditure, the body will spend more time in the fed state than the fasting state there will be more accumulation of nutrient reserves than utilization. The result of this is an increase in body size, and especially an increase in adipose tissue stores. If continued for long enough, this will result in overweight or obesity, with potentially serious health consequences — see Chapter 6. 

In the fasting state (sometimes known as the post-absorptive state, as it begins about 4-5 hours after a meal, when the products of digestion have been absorbed) metabolic fuels enter the circulation from the reserves of glycogen, triacylglycerol and protein... 

In the fasting state, which is the normal state between meals, these reserves are mobilized and used. Glycogen is a source of glucose, while adipose tissue provides both fatty acids and glycerol from triacylglycerol. Some of the relatively labile protein laid down in response to meals is also mobilized in fasting, and the amino acids are used both as a metabolic fuel and, more importantly, a source of citric acid cycle intermediates for gluconeogenesis. 

The cause of obesity is an intake of metabolic fuels greater than is required for energy expenditure, so that excess is stored, largely as fat in adipose tissue reserves. The simple answer to the problem of obesity is therefore to reverse the balance reduce food intake and increase physical activity and hence energy expenditure. 

If the intake of metabolic fuels is lower than is required for energy expenditure, the body s reserves of fat, carbohydrate (glycogen) and protein are used to meet energy needs. Especially in lean people, who have relatively small reserves of body fat, there is a relatively large loss of tissue protein when food intake is inadequate. As the deficiency continues, so there is an increasingly serious loss of tissue, until eventually essential tissue proteins are catabolized as metabolic fuels — a process that obviously cannot continue for long. 

Not only have the body s fat reserves been exhausted, but there is wastage of muscle as well, and as the condition progresses there is loss of protein from the heart, liver and kidneys, although as far as possible essential tissue proteins are protected. As discussed in section 9.2.3.3, protein synthesis is energy expensive, and in marasmus there is a considerable reduction in the rate of protein synthesis, although catabolism continues at the normal rate (section 9.1.1). The amino acids released by the catalysis of tissue proteins are used as a source of metabolic fuel and as substrates for gluconeogenesis to maintain a supply of glucose for the brain and red blood cells (section 5.7). 

During moderate- to high-intensity exercise, glycogen (a fuel reserve that helps maintain normal body processes) can be depleted within 60 to 90 minutes. Blood sugar levels drop as the glycogen reserves are used up, and lactic add (a by-product of glucose metabolism) builds up in muscle... 

The demand for the consumption of fuel reserves can be regarded as a form of metabolic stress. This is characterized by a low activity of insulin relative to stress hormones catecholamines, corticotropin, glucocorticoids and glucagon. Such an hormonal balance can occur in starvation, diabetes, trauma and under the influence of certain toxins. As we have seen earlier, it can also be a response to the ingestion of large amounts of ethanol, fructose or fat. 

Fatty acids stored in adipose tissue, in the form of neutral TAG, serve as the body s major fuel storage reserve. TAGs provide concentrated stores of metabolic energy because they are highly reduced and largely anhydrous. The yield from complete oxidation of fatty acids to CO2 and H2O is nine kcal/g of fat (as compared to four kcal/g of protein or carbohydrate, see Figure 27.5, p. 357). 

Fats and oils serve as energy reserves for the organism. Because they are in a lower oxidation state than carbohydrates, they provide more energy per gram when they are metabolized (see the Focus On box Energy Content of Fuels in Chapter 5). 

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