Amino acids dietary requirements

In the fed state, when there is abundant protein and carbohydrate, dietary protein is hydrolyzed to amino acids. Those amino acids not required for protein synthesis are converted to 2-oxoacids by the aminotransferases. The 2-oxoacids are then converted into lipids and carbohydrate for storage. Glutamate dehydrogenase catalyzes the formation of ammonia from the excess amino groups derived from the amino acids this ammonia is excreted as urea. 

The amino acid molecules that are immediately available for use in metabolic processes are referred to as the amino acid pool. In animals, amino acids in the pool are derived from the breakdown of both dietary and tissue proteins. Excreted nitrogenous products such as urea and uric acid are output from the pool. Amino acid metabolism is a complex series of reactions in which the amino acid molecules required for the syntheses of proteins and metabolites are continuously being synthesized and degraded. Depending on current metabolic... 

The synthesized or synthetic proteins previously discussed in terms of the genetic code are not necessarily the same as those that start out as nutritional or dietary proteins. However, digestive enzymes break down dietary proteins into the amino acid pool required for subsequent cell protein synthesis, as has been described. The proteins so synthesized variously become cellular structural components, enzymes and hormones, and blood plasma proteins. The latter are involved in cellular osmotic balances, that is, in ensuring a stable pressure difference across the cell walls. These blood proteins also transport substances through the vascular system, and assist in promoting inununity (Chaney, in Devlin, 1986, p. 1179ff). 

Organisms vary widely in their ability to synthesize amino acids. Dietary amino acid requirements for organisms vary from all to none. Mammals require about half of the amino acids in their diet for growth and maintenance of normal nitrogen balance (see here). Amino acids that must be provided in the diet to meet an animal s metabolic needs are called essential amino acids (Table 20.1). Those that need not be provided because they can be biosynthesized in adequate amounts are called nonessential amino acids. 

Carnivores rely on a protein-rich diet and produce new biomass primarily from dietary amino acids, although the enzymes required for de novo amino acid synthesis are present (Garmes et al., 1998). Bone collagen, muscle (meat) and apatite were analyzed for a set of modern southern African herbivores and carnivores (Lee-Thorp et al., 1989). The isotopic analyses showed i C enrichment in bone collagen, apatite and muscle, and depletion in lipids. Difference in values between herbivores and carnivores indicates a trophic effect, which for carbon in bone collagen is 2.5-3%o (Fig. 2). 

The amino acid tyrosine is the starting point in the synthesis of the catecholamines and of the thyroid hormones tetraiodothyronine (thyroxine T4) and triiodothyronine (T3) (Figure 42-2). T3 and T4 are unique in that they require the addition of iodine (as T) for bioactivity. Because dietary iodine is very scarce in many parts of the world, an intricate mechanism for accumulating and retaining T has evolved. 

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-... 

While the dog is a carnivore, it is able to adapt to an omnivorous diet. Requirements for dietary sources of energy, amino acids, glucose precursors, fatty acids, minerals, vitamins, and water have been established based on recommendations by the National Research Council (NRC, 1985). Adult beagles maintained in a laboratory environment function well with one feeding of standard laboratory chow per day. In safety assessment testing, however, some compounds may induce serious dietary deficiencies through induced loss of appetite, malabsorption, or vomiting, and, in these cases, it may be advisable to provide a dietary supplement. 

Table III. Dietary Required Amino Acids (mg/100 g Fresh Fruit) in Some Fruits...

Humans have a limited capacity to synthesize amino acids de novo, but extensive interconversions can occur. Those amino acids which cannot be formed within the body and must be supplied by the diet are called essential . Members of this group, which includes the branched chain amino acids leucine and valine, and also methionine and phenylalanine, are all dietary requirements. Such essential amino acids may be chemically converted, mainly in the liver, into the non-essential amino acids. The term non-essential does not equate with not biochemically important but simply means they are not strict dietary components. 

Ah 20 types of amino adds are required for protein synthesis. These amino adds can be derived from digesting dietary protein and absorbing their constituent amino acids or, alternatively, by synthesizing them de novo. 

The problem to be solved with respect to the chemical reactions that constitute metabolism and sustain life is that, without the action of catalysts, they are far too slow. Let s consider the digestion of the proteins themselves, an important constituent of our diet. In an enviromnent similar to that of our digestive system, several tens of thousand years would be required to digest half of the protein content of a typical meal in the absence of a catalyst. Clearly, this will not do. In reality, the stomach secretes one protein catalyst, the enzyme pepsin, and the pancreas secretes several enzymes that catalyze the digestion of proteins. In the presence of these enzymes, dietary proteins are fully digested and reduced to their basic constituents, the amino acids, in a matter of hours. Obviously, these enzymes are enormously potent catalysts." ... 

The presence of proteins in the diet is essential for health. An important question, therefore, is what is the minimal amount of protein that must be provided to maintain health It is not an easy question to answer. Even when no protein or amino acid is consumed, in an otherwise adequate diet, urea is lost from the body due to body protein break down. The daily loss of protein is about 0.34 g per kg or about 24 g protein each day for a 70 kg person (i.e. when no protein is consumed). However, this amount does not represent the minimal intake required, since other factors, (such as the amount of energy consumed, other components in the diet, and trauma physical activity can affect this amount.) The recommended dietary allowance (RDA) for a young adult is 0.8 g per kg per day (Table 8.6). 

C. The main role of dietary proteins is provision of the amino acid building blocks for synthesis of cellular proteins, many of which require daily renewal to maintain physiologic functions and respond to the needs of the body. 

Dietary supplements are products taken by mouth that contain an ingredient intended to supplement the diet, such as vitamins, minerals, herbs or other botanicals, amino acids, and substances such as enzymes, organ tissues, and metabolites. Dietary supplements come in many forms, including extracts, concentrates, tablets, capsules, gel caps, liquids, and powders. They have special requirements for labeling. In the U.S., the Dietary Supplement Health and Education Act of 1994 states that dietary supplements are considered foods, not drugs. (Note that some dietary supplements are used in conventional medicine for example, folic acid... 

Excessive heat can cause destruction of amino acid residues. The amino acid most susceptible to direct heat destruction is cystine. Although not an essential amino acid, cystine does have a sparing effect on the dietary requirement for methionine. As a result, cystine destruction can be nutritionally important. In addition, many vegetable proteins are limiting in the sulfur amino acids. Cystine destruction would be particularly harmful for these proteins. 

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