Dietary Intake of Protein

Ingested protein is digested in a stepwise fashion in the stomach, small intestinal lumen, and small intestinal mucosal cells (Chapter 12). Peptides formed in the intestinal lumen are absorbed into the mucosal cells and degraded to free amino acids. The outflow of amino acids to the portal vein does not reflect the amino acid composition of the ingested protein. Thus, alanine levels increase two-to fourfold, and glutamine, glutamate, and aspartate are absent. These changes arise from amino acid interconversions within the intestinal cell. 

With the exception of the branched-chain amino acids (valine, leucine, and isoleucine), most amino acids 

Formation of other molecules from amino acid precursors, e.g., nonessential amino acids, purines, pyrimidines, porphyrins, and plasma proteins (such as albumin, VLDL, and transferrin), for its needs and those of other tissues  

Utilization of the amino acid carbon skeletons in gluconeogenesis or glycogen or lipid synthesis and 

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Positive Nitrogen Balance when dietary intake of proteins is greater than the requirement for endogenous protein synthesis. 

The average daily dietary intake of protein in North America is about 100 g compared with an estimated requirement for adults of 50 to 70 g. In addition to dietary protein, another 50 to 60 g of protein enters the intestinal contents daily in Gl secretions and from desquamated mucosal cells. Normal daily fecal loss of protein is about 10 g. 

As in the other disorders of urea synthesis, the plasma level of the blocked amino acid, in this case citrulline, varied with protein intake, the level dropping to half with restriction of protein (M6, M8, M12). Excretion of citrulline in the urine was high, in one case ranging from 0.5 g to 2.5 g per day, and in the other from 0.15 g to 1.39 g per day, depending on dietary intake of protein. This compares with a normal excretion of less than 1 mg per day. 

An adult has a requirement for a dietary intake of protein because there is continual oxidation of amino acids as a source of metabolic fuel and for gluconeogenesis in the fasting state. In the fed state, amino acids in excess of immediate requirements for protein synthesis are oxidized. Overall, for an adult in nitrogen balance, the total amount of amino acids being metabolized will be equal to the total intake of amino acids in dietary proteins. 

Collectively, the possible effects of high dietary intakes of protein and phosphate on urinary calcium excretion and enhanced bone resorption, respectively, along with the possibility of reduced calcium absorption with advancing age, argue for recommending an ample intake of calcium. 

Two amino acids—cysteine and tyrosine—can be synthesized in the body, but only from essential amino acid ptecutsots (cysteine from methionine and tyrosine from phenylalanine). The dietary intakes of cysteine and tytosine thus affect the requirements for methionine and phenylalanine. The remaining 11 amino acids in proteins are considered to be nonessential or dispensable, since they can be synthesized as long as there is enough total protein in the diet—ie, if one of these amino acids is omitted from the diet, nitrogen balance can stiU be maintained. Howevet, only three amino acids—alanine, aspartate, and glutamate—can be considered to be truly dispensable they ate synthesized from common metabolic intetmediates (pyruvate, ox-... 

The amino acid L-tryptophan is the precursor for the synthesis of 5-HT. The synthesis and primary metabolic pathways of 5-HT are shown in Figure 13-5. The initial step in the synthesis of serotonin is the facilitated transport of the amino acid L-tryptophan from blood into brain. The primary source of tryptophan is dietary protein. Other neutral amino acids, such as phenylalanine, leucine and methionine, are transported by the same carrier into the brain. Therefore, the entry of tryptophan into brain is not only related to its concentration in blood but is also a function of its concentration in relation to the concentrations of other neutral amino acids. Consequently, lowering the dietary intake of tryptophan while raising the intake of the amino acids with which it competes for transport into brain lowers the content of 5-HT in brain and changes certain behaviors associated with 5-HT function. This strategy for lowering the brain content of 5-HT has been used clinically to evaluate the importance of brain 5-HT in the mechanism of action of psychotherapeutic drugs. 

Small-molecule antioxidants include glutathione, ascorbic acid (vitamin C), vitamin E and a number of dietary flavonoids. Because humans, in contrast to most other animals, are unable to synthesize vitamin C, this important antioxidant must be supplied entirely from dietary intake. Other proteins, such as thioredoxin and metallothionein, may also contribute to some extent to the cellular antioxidant pool. 

L-tryptophane is the precursor of serotonin and other biological substances like tryptamine, kynure-nine and quinolinic acid. Furthermore, it is an essential substrate in the protein synthesis. The dietary intake of L-tryptophane might increase the production of serotonin. For this reason the aminoacid is used for the therapy of light sleeping disorders. 

Blood pressure effect. Fruit juice, administered intravenously hy infusion to dogs at a dose of 3 mL/minute for 100 minutes, was active. Initial effect was a decrease in hlood pressure . Oil, administered to male weanling rats at a dose of 10% of diet for 5 weeks, produced significantly higher hlood pressure than other groups. Systolic hlood pressure was found related to the dietary intakes of saturated and unsaturated fatty acids. Prenatal exposure of the rats to a maternal low-protein diet abolished the hypertensive effect of the coconut oil dieH . Butyryl cholinesterase activity. Oil was administered to rats at different doses with or without clofibrate for 15 days. The hypolipidemic action of clofibrate was not... 

Adequate dietary intake of calories and protein essential for successful freaf ment... 

Sagara M, Kanda T, Njelekera M et al. Effects of dietary intake of soy protein and isoflavones on cardiovascular disease risk factors in high risk, middle-aged men in Scotland. J. Am. Coll Nutr. 23, 85-91, 2004. 

Carnitine, L-3-hydroxy-4-(trimethylammonium)butyrate, is a water-soluble, tri-methylammonium derivative of y-amino-jS-hydroxybutyric acid, which is formed from trimethyllysine via y-butyrobetaine [40]. About 75% of carnitine is obtained from dietary intake of meat, fish, and dairy products containing proteins with trimethyllysine residues. Under normal conditions, endogenous synthesis from lysine and methionine plays a minor role, but can be stimulated by a diet low in carnitine. Carnitine is not further metabolized and is excreted in urine and bile as free carnitine or as conjugated carnitine esters [1, 41, 42]. Adequate intracellular levels of carnitine are therefore maintained by mechanisms that modulate dietary intake, endogenous synthesis, reabsorption, and cellular uptake. 

Experiments with rats have shown that the branched-chain a-keto acid dehydrogenase complex is regulated by covalent modification in response to the content of branched-chain amino acids in the diet. With little or no excess dietary intake of branched-chain amino acids, the enzyme complex is phosphorylated and thereby inactivated by a protein kinase. Addition of excess branched-chain amino acids to the diet results in dephosphoiylation and consequent activation of the enzyme. Recall that the pyruvate dehydrogenase complex is subject to similar regulation by phosphorylation and dephosphorylation (p. 621). 

Dietary requirements for AAs and protein usually are stated as proportions of the diet. However, the level of feed consumption has to be taken into account to ensure that the total intake of protein and AAs is appropriate. The protein and AA requirements derived by the NRC (1994) relate to poultry kept in moderate temperatures (18-24°C). Ambient temperatures outside of this range cause an inverse response in feed consumption i.e. the lower the temperature, the greater is the feed intake and vice versa (NRC, 1994). Consequently, the dietary levels of protein and AAs to meet the requirements should be increased in warmer environments and decreased in cooler environments, in accordance with expected differences in feed intake. These adjustments are designed to help ensure the required daily intake of AAs. 

Chromium(III) is an essential nutrient required for normal energy metabolism. The National Research Council (NRC) recommends a dietary intake of 50-200 ig/day (NRC 1989). The biologically active form of an organic chromium(ni) complex, often referred to as GTF, is believed to function by facilitating the interaction of insulin with its cellular receptor sites. The exact mechanism of this interaction is not known (Anderson 1981 Evans 1989). Studies have shown that chromium supplementation in deficient and marginally deficient subjects can result in improved glucose, protein, and lipid metabolism. 

Calcium is absorbed in the intestine by two distinct mechanisms, an active process that is vitamin D dependent and another that is vitamin D independent. When the dietary intake of calcium is low, the intestinal uptake of calcium occurs by an active transport process that is vitamin D dependent. Active transport is most efficient in the duodenum and proximal jejunum, areas of the intestine that have a pH close to 6.0 and where calbindin, a calcium transport protein, is present. However, the amount of calcium absorbed in the ileum may be greater than that absorbed in the duodenum and jejunum... 

The human requirement of niacin is related to the intake of tryptophan. Animal proteins contain approximately 1.4 percent of tryptophan, vegetable proteins about 1 percent. A dietary intake of 60 mg of tryptophan is considered equivalent to 1 mg of niacin. When this is taken into account, average diets in the United States supply 500 to 1,000 mg tryptophan per day and 8 to 17 mg niacin for a total niacin equivalent of 16 to 33 mg. The RDA for adults, expressed as niacin, is 6.6 mg per 1,000 kcal, and not less than 13 mg when caloric intake is less than 2,000 kcal. 

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