Metabolism Distribution and Regulation
Once absorbed, carbohydrates are carried to the liver A . There, fructose and galactose are converted into glucose. Some of the absorbed glucose reaches the peripheral bloodstream, where it is recognized by pancreatic receptors. This triggers increased insulin secretion by the p-cells and reduces glucagon secretion. These hormonal changes provide a signal which affects the entire metabolism absorption of glucose into the liver, muscle cells, and fatty tissues is increased, and its conversion...
Lipoprotein Lipase
The activity of endothelial lipoprotein lipase LPL determines the various uptake levels of fatty acids in different tissues. LPL hydrolyzes triglycerides in high-triglyceride lipoproteins like chylomicrons and VLDL to 2-monoglycer-ides and fatty acids. It also effects the transfer of phospholipids and apopro-teins to HDL. Recent research shows that LPL is further responsible for specific binding of lipoproteins to cell surfaces and receptors. These functions point to a central role of the...
The RDA and DRI
Early recommendations for nutrient intakes date back to the mid-1800s when, in the Lancashire district in England, nutrient intake recommendations were established because of a famine. The purpose, however, was solely to ensure adequate minimal nutrient intakes for the population and the army. In 1941, the U.S. National Research Council first issued recommendations which had the goal of achieving perfect health in the population. These Recommended Dietary Allowances RDA were updated in...
Digestion and Absorption 1
Protein digestion begins in the stomach, initiated by pepsins. Pepsins split peptide bonds involving Phe or Tyr R-groups. These endopeptidases split polypeptides inside the molecule are secreted by the gastric lining as inactive precursors, pepsinogens, which are activated at acidic pH. Acids further facilitate the enzymes' catalytic attack by denaturing the proteins. The activity of the gastric enzymes normally produces large fragments poly-, oligopep-tides . After stomach resections or in...
During the postabsorptive phase A
muscles release mainly Gln and Ala. The gastrointestinal tract, too, absorbs Gln, releasing Ala and cirtulline in turn. All AA are absorbed by the liver, the N released as ammonia, and the carbon skeleton processed for further use. Acidosis causes inhibition of hepatic Gln uptake. The kidneys then pick up Gln and convert it to ammonia, which combines with H ions that need to be excreted, forming NH4 . All H ions that need to be excreted due to acidosis are removed in this form. The AA Arg and...
Postprandial Lipid Distribution
Lipids absorbed into the mucosa cells 1 after intraluminal hydrolysis A are re-esterified inside the cells and reach the circulatory system 2 in the form of chylomicrons via the lymphatic system. Chylomicrons CM can be found in the blood within 1-2 hours after a meal. With a half-life of 4-5 minutes, they are extremely short-lived however, after a high-fat meal, their influx continues for hours. After modification by lipoprotein lipase LPL, see p. 100 , the remnants are absorbed into the liver...
Fructose and Galactose
Fructose, which has no influence on the release of insulin, is metabolized in the liver A . Which metabolic pathway is used depends on the presence or absence of other monosaccharides. In case of simultaneous supply of glucose and fructose, fructose is transformed into intermediate products of the glycolytic pathway. Since only one phosphorylated product results from the splitting of fructose-1-phosphate, the remaining glyceraldehyde has to be processed via a different pathway first. Like...
Structure and Properties
Carbohydrates are hydroxylized aldehydes or ketones and are the most abundant of organic compounds. They serve as fuel, energy storage, basic building blocks in DNA and RNA, and as structural elements of bacterial and plant cell walls. Carbohydrates are bonded to other macronutrients in glycoproteins and glycolipids, which play important roles in cell membranes, as for instance in cell-cell recognition. Food carbohydrates are built almost exclusively from the monosaccharides, glucose, fructose,...
Metabolism Glucose Storage
The glucose stored in the liver as glyco-gen can be considered a reservoir for buffering blood glucose levels A . Since glucose is the essential energy source of the CNS, blood glucose levels are regulated within a tight margin through the interplay of glucagon and insulin. Muscle glycogen is not directly integrated into the blood glucose regulation system. Since muscles lack glucose-6-phosphatase, their cells are unable to convert glucose-6-phosphate formed during glycogen breakdown into...