Nitrogen Balance

The oldest (and most widely used) method of following changes in body N is the N balance method. Because of its simplicity, the N balance technique is the standard of reference for defining minimum levels of dietary protein and essential amino acid intakes in humans of all ages ( 44, 45). Subjects are placed for several days on a specific level of amino acid and or protein intake and their urine and feces are collected over a 24-h period to measure their N excretion. A week or more may be...

Chapter Lipids Sterols and Their Metabolites

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Dietary Considerations

Fat intake of the average North American diet represents 38 of total calories consumed (8, 9). Over 95 of the total fat intake is composed of TG the remainder is in the form of PL, free Fa, CH, and plant sterols. Total dietary TG in the North American diet is about 100 to 150 g per day. In addition to dietary intake, lipids enter the gastrointestinal tract by release from mucosal cells, biliary expulsion into the lumen, and bacterial action. In almost no other instance can food choice influence...

Turnover Of Proteins In The Body

As indicated above, proteins in the body are not static. Just as every protein is synthesized, it is also degraded. Schoenheimer and Rittenberg first applied isotopically labeled tracers to the study of amino acid metabolism and protein turnover in the 1930s and first suggested that proteins are continually made and degraded in the body at different rates. We now know that the rate of turnover of proteins varies widely and that the rate of turnover of individual proteins tends to follow their...

The Concept of Conditional Essentiality

Snyderman (17) found that premature infants, in whom many enzymes of amino acid metabolism develop late during gestation, required cystine and tyrosine (which are dispensable for most full-term infants) to ensure nitrogen retention and maintain their normal plasma levels. Cystine and tyrosine were thus essential for premature infants. Rudman and associates (18, 19) subsequently proposed the term conditionally essential for nutrients not ordinarily required in the diet but which must be supplied...

Formation and Breakdown of Glycogen

Glucose is first enzymically phosphorylated, then reacted with uridine triphosphate to give uridine diphosphate glucose (UDP-glucose). The enzyme glycogen synthetase fits this onto a preexisting glycogen chain (primer and or protein backbone), splitting off the UDP. The glucose residue added by 1 4 linkages is attached at the outer end of the molecule so that the branches of the glycogen tree become elongated. After 11 glucose residues, another enzyme called the branching enzyme (amylol (1 4)...

Dietary Carbohydrates

Carbohydrates (Fig, 3,1) thus exist as a vast family of naturally occurring compounds and derivatives of these compounds. Fortunately, only a small number of them are commercially significant and used in the food industry, while a similar number are of metabolic importance. Dietary carbohydrate is a major nutrient for both man and omnivorous animals. Human adults in the Western world obtain approximately half their daily caloric requirements from dietary carbohydrates in the developing...

Carbohydrate Intolerance

There is a range of clinical disorders in which sugar digestion or absorption is disturbed and gives rise to sugar intolerance, creating symptoms by the undigested or unabsorbed sugar and causing water to enter the intestine, which activates peristalsis and induces passage of frequent fluid stools. The undigested carbohydrate can also enter the colon and become fermented into diarrheic agents. The disorders are usually classified as (a) congenital or (b) secondary to some other disease, to...

Historical Perspective

The understanding and assessment of energy requirements in humans have been enhanced by the advent of indirect calorimetry. In indirect calorimetry, the type and rate of substrate oxidation and energy are measured in vivo from gas exchange measurements. This method in combination with other measurement techniques permits investigation of numerous aspects of metabolism, heat production, energy requirements of physical activity, and altered energy metabolism in injury and disease. The development...

R VCOz V n

Because 1 g of urinary nitrogen arises from approximately 6.25 g protein, the protein oxidation rate p (in g min) is given by the equation Thus, indirect calorimetry allows calculation of net rates of nutrient oxidation. It is important to appreciate that indirect calorimetry measures the net appearance by oxidation of a substrate. Moreover, it is important to understand that there is a slight difference in the heat produced per liter of O 2 consumed when one compares carbohydrate, lipid, and...

Creatine and Creatinine

Most of the creatine in the body is found in muscle, where it exists primarily as creatine phosphate. When muscular work is performed, creatine phosphate provides the energy through hydrolysis of its high-energy phosphate bond, forming creatine with transferal of the phosphate to form an ATP. The reaction is reversible and catalyzed by the enzyme ATP-creatine transphosphorylase also known as creatine phosphokinase . The original pathways of creatine synthesis from amino acid precursors were...

Models for Whole Body Amino Acid and Protein Metabolism

The limitations to using tracers to define amino acid and protein metabolism are largely driven by how the tracer is administered and where it is sampled. The simplest method of tracer administration is orally, but intravenous administration is preferred to deliver the tracer systemically to the whole body into the free pool of amino acids. The simplest site of sampling of the tracer dilution is also from the free pool of amino acids via blood. Therefore, most approaches to measuring amino acid...

Amino Acid Pools and Distribution

The distribution of amino acids is complex. Not only are there 20 different amino acids incorporated into a variety of different proteins in a variety of different organs in the body, but amino acids are consumed in the diet from a variety of protein sources. In addition, each amino acid is maintained in part as a free amino acid in solution in blood and inside cells. Overall, a wide range of concentrations of amino acids exists across the various protein and free pools. Dietary protein is...

Criteria of Essentiality

Criteria for establishing whether or not a dietary constituent is an essential nutrient were implicit in the types of investigations that had provided the basis for the concept of nutritional essentiality. Later they were elaborated in more detail as follows 1. The substance is required in the diet for growth, health, and survival 2. Its absence from the diet or inadequate intake results in characteristic signs of a deficiency disease and, ultimately, death 3. Growth failure and characteristic...

Measurement of the Kinetics of Individual Amino Acids

Kic Leucine Model Isotope

As an alternative to measuring the turnover of the whole amino-N pool per se, the kinetics of an individual amino acid can be followed from the dilution of an infused tracer of that amino acid. The simplest models consider only essential amino acids that have no de novo synthesis. The kinetics of essential amino acids mimic the kinetics of protein turnover as shown in Figure 2 10. The same type of model can be constructed but cast specifically in terms of a single essential amino acid, and the...

Biosynthesis And Function Of Eicosanoids

Arachidonic Acid Bound Pghs

Some of the most potent effects of PUFA are related to their enzymatic conversion into a series of oxygenated metabolites called eicosanoids, so-named because their precursors are PUFA with chain lengths of 20 carbon units. Eicosanoids include PG, thromboxane TXA , leukotrienes LT , hydroxy fatty acids, and lipoxins. PG and TXA are generated via cyclooxygenase CO enzymes, whereas LT, hydroxy acids, and lipoxins are produced from lipoxygenase LO metabolism. Under stimulation, rapid and transient...

Fructose

Fructose, a monosaccharide ketohexose, is present either as the free hexose honey, soft drinks, sweets, biscuits, apples, pears or is produced from hydrolysis of the dietary disaccharide sucrose yielding glucose and fructose . In humans, it is absorbed largely intact into the portal blood and is almost totally cleared in a single passage through the liver. Thus, there is essentially no appreciable fructose in the blood. After a large oral dose of 1 g free fructose kg...

Pathways Of Amino Acid Synthesis And Degradation

Urea Degrade

Several amino acids have their metabolic pathways linked to the metabolism of other amino acids. These codependencies become important when nutrient intake is limited or when metabolic requirements are increased. Two aspects of metabolism are reviewed here the synthesis only of nonessential amino acids and the degradation of all amino acids. Degradation serves two useful purposes a production of energy from the oxidation of individual amino acids 4 kcal g protein, almost the same energy...

Methods Of Measurement

Many methods of measuring energy expenditure have become available over the years, and they vary in complexity, cost, and accuracy 12 . It is important to gain an appreciation of the differences in the methods and of their applications in laboratory and other settings. The techniques used to measure total daily energy expenditure and its components are briefly described below. A more detailed explanation of the laboratory methods of measuring energy expenditure has been published 13 . The most...

Chapter References

Nutritional essentiality a changing paradigm. Report of the 12th Ross Conference on Medical Research. Columbus, OH Ross Products Division, Abbott Laboratories, 1993. 2. Sauberlich HE, Machlin LJ, eds. Ann NY Acad Sci 1992 669 1-404. 3. McCormick DB. The meaning of nutritional essentiality in today's context of health and disease. In Roche AF, ed. Nutritional essentiality a changing paradigm. Report of the 12th Ross Conference on Medical Research. Columbus, OH Ross Products...

Definition

What are carbohydrates The formal definition is a class of substances having the formula C n H2O n, i.e., the molar ratio of C H O is 1 2 1. This definition, however, fails for oligosaccharides, polysaccharides, and sugar alcohols viz., sorbitol, maltitol, mannitol, galactitol, and lactitol . Of the complex carbohydrate macromolecules known, the main member is plant starch and the animal polymer glycogen , but the group includes pectins, cellulose, and gums. Simple carbohydrates include the...

The Concept Of Nutritional Essentiality

The concept of nutritional essentiality was firmly established less than 100 years ago. It arose from observations that certain diseases observed in human populations consuming poor diets could be prevented by including other foods in the diet and that failure of animals fed on diets composed of purified components or restricted to one or a few foodstuffs to grow and survive could similarly be corrected by including another food or an extract of the food in the diet. The food constituents that...

End Product Approach

The earliest model of whole-body protein metabolism in humans was applied by San Pietro and Rittenberg in 1953 using 15N glycine 65 . Glycine was used as the first tracer because glycine is the only amino acid without an optically active a-carbon center and therefore is easy to synthesize with a 15N label. At that time, measurement of the tracer in plasma glycine was very difficult. Thus, San Pietro and Rittenberg proposed a model based upon something that could be readily measured, urinary...

Basic Definitions

The amino acids that we are familiar with and all of those incorporated into mammalian protein are alpha-amino acids. By definition, they have a carboxyl-carbon group and an amino nitrogen group attached to a central a-carbon Fig 2.1 . Amino acids differ in structure by substitution of one of the two hydrogens on the a-carbon with another functional group. Amino acids can be characterized by their functional groups, which are often classified at neutral pH as a nonpolar, b uncharged but polar,...

Digestion And Absorption

Mixed Micelles

Digestion of dietary lipids and their metabolites evokes a series of specific processes that enable absorption through the water-soluble environment of the gut Table. 4.2 . Digestion begins in the oral cavity with salivation and mastication. Lingual lipase, released from the serous glands of the tongue with saliva, starts the hydrolysis of free FA from TG. Mechanical dispersion by chewing enlarges the surface area upon which lingual lipase can act. Lingual lipase cleaves at the sn-3 position,...

Historical Highlights

The initial history of carbohydrates is the story of sugar cane and the human passion for sweetness. Although there is some dissension, sugar cane's origin is thought to be Papua New Guinea. It was probably cultivated from wild plants still in existence about 10,000 years ago at the time of the global Neolithic agricultural revolution. The slow diffusion of migrants carried it to India, Southeast Asia, and China. Sugar was mentioned by an Indian author in 325 BC. After the Arabs defeated the...

Factorial Method

When a person is placed on a protein-free diet, rates of amino acid oxidation and urea production decrease over several days as the body tries to conserve its resources, but amino acid oxidation and urea production do not drop to zero Fig. .Z . There is always some obligatory oxidation of amino acids and urea formation and miscellaneous losses of N Tabj 2.,10 . The factorial method assesses all routes of loss possible for adult humans on an N-free diet. The minimum daily requirement of protein...

Acknowledgments

The authors extend special thanks to Catherine Vanstone for her invaluable contribution to research-ing and writing this chapter. The graphic art work of Helen Rimmer is also gratefully acknowledged. Finally, appreciation is extended to Fady Ntanios and Andrea Papamandjaris, who offered suggestions on improving the quality and composition of the chapter. Abbreviations BS bile salt CH cholesterol CE cholesterol ester DG diglyceride EFA essential fatty acid EFAD essential fatty acid deficiency FA...

Intracellular Movement And Biosynthesis Of Lipids

SAFA are biosynthesized in the extramitochondrial compartment by a group of enzymes known as FA synthetases. Compared with many animal species, human FA synthesis occurs predominantly in the liver and is much less active in adipose tissue. The FA biosynthetic pathway is almost identical in all organisms examined to date. The starting point is acetyl-CoA. Acetyl-CoA and oxaloacetate are cleaved from citrate, which is transported from the mitochondria. The first reaction in the FA biosynthetic...