Metabolic fuels

Although there is a requirement for energy sources in the diet, it does not matter unduly how that requirement is met. There is no requirement for a dietary source of carbohydrate — as discussed in section 5.7, the body can make as much carbohydrate as is required from proteins. Similarly, there is no requirement for a dietary source of fat, apart from the essential fatty acids (section 4.3.1.1), and there is certainly no requirement for a dietary source of alcohol. However, as discussed in section 7.3.2, diets that provide more than about 35 0% of energy from fat are associated with increased risk of heart disease and some cancers, and there is some evidence that diets that provide more than about 20% of energy from protein are also associated with health problems. Therefore, as discussed in section 7.3, the general consensus is that diets should provide about 55% of energy from carbohydrates, 30% from fat and 15% from protein. 

Although there is no requirement for fat in the diet, fats are nutritionally important and, as discussed in section 1.3.3.1, there is a specific mechanism for detecting the taste of fats in foods. 

Unlike fats and carbohydrates, there is a requirement for protein in the diet. In a growing child this need is obvious. As the child grows, and the size of its body increases, so there is an increase in the total amount of protein in the body. 

Adults also require protein in the diet. There is a continuous small loss of protein from the body, for example in hair, shed skin cells, enzymes and other proteins secreted into the gut and not completely digested. More importantly, there is turnover of tissue proteins, which are continually being broken down and replaced. Although there is no change in the total amount of protein in the body, an adult with an inadequate intake of protein will be unable to replace this loss, and will lose tissue protein. Protein turnover and requirements are discussed in Chapter 9. 

In addition to metabolic fuels and protein, the body has a requirement for a variety of mineral salts, in small amounts. Obviously, if a metal or ion has a function in the body, it must be provided by the diet, as the different elements cannot be interconverted. Again, the need is obvious for a growing child as the body grows in size, so the total amounts of minerals in the body will increase. In adults, there is a turnover of minerals in the body, and losses must be replaced from the diet. 

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Many metabolic fuels are oxidized in the mitochondrial matrix. Pyruvate is oxidatively decarboxylated to acetyl-CoA by the pyruvate dehydrogenase complex (PDH)... 

In the liver, its major function is to ptovide glucose for extrahepatic tissues. In muscle, it setves mainly as a ready source of metabolic fuel fot use in muscle. 

A SUPPLY OF METABOLIC FUELS IS PROVIDED IN BOTH THE FED STARVING STATES (Figure 27-1)... 

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

For several hours after a meal, while the products of digestion are being absorbed, there is an abundant supply of metabolic fuels. Under these conditions, glucose is the major fuel for oxidation in most tissues this is observed as an increase in the respiratory quotient (the ratio of carbon dioxide produced to oxygen consumed) from about 0.8 in the starved state to near 1 (Table 27-1). 

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

Figure 27-1. Metabolic interrelationships between adipose tissue, the liver, and extrahepatic tissues. In extrahepatic tissues such as heart, metabolic fuels are oxidized in the following order of preference (1) ketone bodies, (2) fatty acids, (3) glucose. (LPL, lipoprotein lipase FFA, free fatty acids VLDL, very low density lipoproteins.)...

Table 27-1. Energy yields, oxygen consumption, and carbon dioxide production in the oxidation of metabolic fuels.

In adipose tissue, the effect of the decrease in insulin and increase in glucagon results in inhibition of lipo-genesis, inactivation of lipoprotein lipase, and activation of hormone-sensitive lipase (Chapter 25). This leads to release of increased amounts of glycerol (a substrate for gluconeogenesis in the liver) and free fatty acids, which are used by skeletal muscle and liver as their preferred metabolic fuels, so sparing glucose. 

Table 27-2. Plasma concentrations of metabolic fuels (mmol/L) in the fed and starving states.

Besides water, the diet must provide metabolic fuels (mainly carbohydrates and lipids), protein (for growth and turnover of tissue proteins), fiber (for roughage), minerals (elements with specific metabolic functions), and vitamins and essential fatty acids (organic compounds needed in small amounts for essential metabolic and physiologic functions). The polysaccharides, tri-acylglycerols, and proteins that make up the bulk of the diet must be hydrolyzed to their constituent monosaccharides, fatty acids, and amino acids, respectively, before absorption and utilization. Minerals and vitamins must be released from the complex matrix of food before they can be absorbed and utifized. 

Cortisol is an important component of the body s response to physical and psychological stress. Nervous signals regarding stress are transmitted to the hypothalamus and the release of CRH is stimulated. The resulting increase in cortisol increases levels of glucose, free fatty acids, and amino acids in the blood, providing the metabolic fuels that enable the individual to cope with the stress. A potent inhibitor of this system is cortisol itself. This hormone exerts a negative-feedback effect on the hypothalamus and the adenohypophysis and inhibits the secretion of CRH and ACTH, respectively. 

Tabernero, A., Vicario, C. and Medina, J. M. Lactate spares glucose as a metabolic fuel in neurons and astrocytes from primary culture. Neurosci. Res. 26 369-376,1996. 

In the second stage, the building blocks are degraded by various pathways in tissues to a common metabolic intermediate, acetyl CoA. Most of the energy contained in metabolic fuels is conserved in the chemical bonds (electrons) of acetyl CoA. A smaller portion is conserved in reducing nicotinamide adenine dinucleotide (NAD) to NADH or flavin adenine dinucleotide (FAD) to FADH. Reduction indicates the addition of electrons that may be free, part of a hydrogen atom (H), or a hydride ion (H ). 

Release rates of exudates from phytoplankton range from 0 to 80% of carbon fixed. These rates are dependent on species composition, physiological state, nutrient deficiency, temperature, and Ught limitation. Some evidence suggests that exudation is a mechanism for release of excess organic matter from cells when nutrient availability is too low to enable their usage as metabolic fuel. 

Lactate versus Alanine as Metabolic Fuel The Cost of Nitrogen Removal The three carbons in lactate and alanine have identical oxidation states, and animals can use either carbon source as a metabolic fuel. Compare the net ATP yield (moles of ATP per mole of substrate) for the complete oxidation (to C02 and H20) of lactate versus alanine when the cost of nitrogen excretion as urea is included. 

When a cell or organism has more than enough metabolic fuel to meet its energy needs, the excess is generally converted to fatty acids and stored as lipids such as triacylglycerols. The reaction catalyzed by acetyl-CoA... 

TABLE 23-5 Available Metabolic Fuels in a Normal-Weight 70 kg Man and in an Obese 140 kg Man at the Beginning of a Fast... 

Randle, P.J. (1995) Metabolic fuel selection general integration at the whole-body level. Proc. Nutr. Soc. 54, 317-327. 

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