GLUT 4 Insulin Responsive Glucose Transporter

Glucose is transported across the cell membranes of adipocytes (fat cells), and its rate of transport can be speeded up 20- to 30-fold within 2 or 3 minutes by addition of insulin, without evidence of protein synthesis. Studies showed that this stimulation of glucose transport was due in part to translocation of GLUT 1 from an intracellular pool into the membrane. Careful quantitative measurements showed, however, that this could account for only a 12- to 15-fold increase in glucose transport....

Digestion and Absorption of Protein

Two organs have particular, potentially important, regulatory roles during feeding the gut and liver. All dietary intake passes first through the gut and then through the liver via portal blood flow. Digestion of protein begins with pepsin secretion in gastric juice and with proteolytic enzymes secreted from the pancreas and the mucosa of the small intestine (133). These enzymes are secreted in their pro (or zymogen) form and become activated by cleavage of a small peptide portion. Pancreatic...

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Table 2.9 Composition of the Major Nitrogen-Containing Species in Urine CSucKmpWtKr noftfiitr tam Pi Cmcul IflMrtwc m Table 2.9 Composition of the Major Nitrogen-Containing Species in Urine Nitrogen appears in the feces because the gut does not completely absorb all dietary protein and reabsorb all N secreted into the gastrointestinal tract ( Fig 2.6). In addition, N is lost from skin via sweat as well as via shedding of dead skin cells. There are also additional losses through hair, menstrual...

Criteria of Essentiality

Criteria 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...

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...

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...

Energy Needs Of Specific Populations

Below, we examine recent applications of doubly labeled water methodology in healthy and diseased older individuals to understand better daily energy requirements and the regulation of energy balance. We consider several diseases that are associated with negative energy balance and generalized wasting. Heart failure is an increasing important and frequent clinical problem, with the highest prevalence observed in the elderly ( 24). The incidence of heart failure increases 50-fold between the...

Key Aspects Of Energy Expenditure

Changes in body energy content occur through changes in the balance between daily intake and energy expenditure. Energy intake is episodic, derived primarily from the carbohydrates, proteins, and fats in foods consumed. Total daily energy expenditure for theoretical and analytic purposes can be divided into several components Figure 5.1. The components of daily energy expenditure. Figure 5.1. The components of daily energy expenditure. The resting metabolic rate (RMR) represents the largest...

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...

Dietary Fiber

Dietary fiber was originally defined as the remnants of plant cell walls not hydrolyzed by the alimentary enzymes of man but the definition was subsequently modified to include all plant polysaccharides and lignin which are resistant to hydrolysis by the digestive enzymes of man ( 7). The fiber, both soluble and insoluble, is fermented by the luminal bacteria of the colon. High-fiber diets maintained for the long term reduce the incidence of colon cancer, but the mechanisms(s) involved rest on...

Protein and Amino Acid Needs in Disease

Most of the discussion to this point has centered around amino acid and protein metabolism in normal individuals. Although the effect of disease on amino acid and protein requirements is beyond the limits of this introductory chapter, a few important general points need to be made. The first is that energy and protein needs are tied together, as illustrated in Eigure2J.6. When metabolic rate rises, body protein is mobilized for use as a fuel (amino acid oxidation) and for supply of carbon for...

Study of Glucose Transporters by Use of Transgenic and Knock Out Mice

Although a host of metabolic inhibitors are available for use in examining metabolic pathways, their specificities are often questionable. With molecular biologic techniques, however, metabolic pathways can be changed in quite specific ways, even in the intact animal. A protein (e.g., enzyme carrier) can be overexpressed, expressed in a tissue that normally does not contain it, or eliminated in any particular cell type. Site-directed mutations allow a molecule to be dissected and particular...

Nutrient Interactions

Many substances that are physiologically, but not nutritionally, essential are synthesized from specific essential nutrients. If the products of the synthetic reactions are present in the diet, they may exert sparing effects that reduce the need for the precursor nutrients. Less phenylalanine and methionine are required, particularly by adults, when the diet includes tyrosine and cystine, for which they are, respectively, specific precursors. Birds, which do not...

Digestion And Absorption

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,...

Measurement of the Kinetics of Individual Amino Acids

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 2,1l0. The same type of model can be constructed but cast specifically in terms of a single essential amino acid, and the...

Degradation of Specific Proteins

Measurement of protein degradation is much more limited in terms of the methods available. To measure protein degradation, the protein must be prelabeled. Three methods have been used (a) removal of the protein from the body, followed by iodination with radioactive iodine, and reinjection into the body to follow the disappearance of the labeled protein (b) administration of a labeled amino acid to label proteins via incorporation of the tracer during protein synthesis, followed by measurement...

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...

Biosynthesis And Function Of Eicosanoids

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 (IXA), leukotrienes (LI), hydroxy fatty acids, and lipoxins. PG and IXA are generated via cyclooxygenase (CO) enzymes, whereas LI, hydroxy acids, and lipoxins are produced from lipoxygenase (LO) metabolism. Under stimulation, rapid and...

Turnover Of Proteins In The Body

Proteins 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...

Essential Fatty Acid Requirements

In studies on EFA, C18 2n-6 and C20 4n-6 have been emphasized because mammals have an absolute requirement for the n-6 family of FA. EFA are required for stimulation of growth, maintenance of skin and hair growth, regulation of CH metabolism, lipotropic activity, and maintenance of reproductive performance, among other physiologic effects. On a molecular level, EFA are components of specific lipids and maintain the integrity and optimal levels of unsaturation of tissue membranes. Because EFA...