Dietary requirements essential amino acids

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-... 

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. 

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. 

Humans have no dietary requirement for protein, per se, but, the protein in food does provide essential amino acids (see Figure 20.2, p. 260). Ten of the twenty amino acids needed for the synthesis of body proteins are essential—that is, they cannot be synthesized in humans at an adequate rate. Of these ten, eight are essential at all times, whereas two (arginine and histidine) are required only during periods of rapid tissue growth characteristic of childhood or recovery from illness. 

Dietary protein provides essential amino acids. The quality of a protein is a measure of its ability to provide the essential amino acids required for tissue maintenance. Proteins from animal sources, in general, have a higher quality protein than that derived from plants. However, proteins from different plant sources may be combined in such a way that the result is equivalent in nutritional value to animal protein. 

In addition to their importance as essential amino acids for humans, the quantitative determination of cysteine and methionine seems to be growing in importance in the animal feed industry. The dietary requirements for the sulfur amino acids tend to be very high in many animals. This is presumably due to the magnitude of hair/feather growth and the fact that the structural proteins that comprise hair/feathers often have high cyst(e)ine content. 

The liver plays a central role in the synthesis of nearly all circulating proteins. Plasma contains 60-80 g/L of protein and this is turned over at a rate of approximately 250 g/day. A variety of proteins are constructed in the liver using amino acids (Aa) as their basic building blocks. Amino acids are categorised as essential and non-essential , the former being a requirement of dietary intake as they cannot be constructed in vivo, whereas the latter can be synthesised hepatically. The essential amino acids are further categorised as branched-chain amino acids (BCAA leucine, valine, isoleucine) or aromatic amino acids (AAA phenylalanine, tyrosine, methionine) according to their structure. Table... 

It is not strictly correct to regard niacin as a vitamin. Its metabolic role is as the precursor of the nicotinamide moiety of the nicotinamide nucleotide coenzymes, nicotinamide adenine dinucleotide (NAD) and NADP, and this can also be synthesized in vivo from the essential amino acid tryptophan. At least in developed countries, average intakes of protein provide more than enough tryptophan to meet requirements for NAD synthesis without any need for preformed niacin. It is only when tryptophan metabolism is disturbed, or intake of the amino acid is inadequate, that niacin becomes a dietary essential. 

There is probably little or no requirement for any preformed niacin in the diet, because it is likely that average intakes of protein (at least in developed countries) wUl provide enough tryptophan to meet requirements (Section 8.3). Assuming that the average diet provides some 15% of energy from protein, and this protein meets the reference pattern for essential amino acids and provides 14 g of tryptophan per kg of dietary protein, this implies an intake of... 

Methionine, USP. An adequate diet should provide the methionine ncccs.sary for normal metabolism in the human.. Viethionine is considered an essential amino acid in humans. It is the precursor in the biosynthesis of -adcnosylnie(hio-niiic, which is an important methylating coenzyme involved In a variety of methylations (e.g.. N-me(hyla(ion of norepinephrine to form epinephrine and O-methylation of catecholamines catalyzed by ca(cchul-CI-mcthyl(iansfcra.ses). Adenosylmcthionine also participates in the methylation of pho.sphatidylcthanolaininc to form phosphatidylcholine, but this pathway is not efficient enough to provide all of the choline required hy higher animals hence, adequate dietary availability of choline is ncces.sary. ... 

Protein is an essential nutrient for human growth, development, and homeostasis. The nutritive value of dietary proteins depends on its amino acid composition and digestibility. Dietary proteins supply essential amino acids, which are not synthesized in the body. Nonessential amino acids can be synthesized from appropriate precursor substances (Chapter 17). In human adults, essential amino acids are valine, leucine, isoleucine, lysine, methionine, phenylalanine, tryptophan, and threonine. Histidine (and possibly arginine) appears to also be required for support of normal growth in children. In the absence from the diet of an essential amino acid, cellular protein synthesis does not occur. The diet must contain these amino acids in the proper proportions. Thus, quality and quantity of dietary protein consumption and adequate intake of energy (carbohydrates and lipids) are essential. Protein constitutes about 10-15% of the average total energy intake. 

Dietary protein provides organic nitrogen and the essential amino acids. The quantitative estimation of protein requirement must take into account the quality of protein, as determined by its essential amino acid composition. Dietary protein should provide all of the essential amino acids in the appropriate amounts. If the concentration of one amino acid is significantly greater or less than that of the others (in a protein or amino acid mixture), utilization of the others may be depressed and will be reflected in growth failure. 

Of the 20 amino acids in proteins, the body can readily synthesize eight if an appropriate nitrogen source is available. Two others can be synthesized from other amino acids of the diet tyrosine from phenylalanine and cysteine from methionine. The rest must be provided in the diet (Chapter 17), since the body can synthesize none or an insufficient amount. The dietary requirement depends on several factors. Beside essential amino acids, the diet should provide the nitrogen required for synthesis of the nonessential amino acids. 

A young child is in an automobile accident that requires surgical intervention and substantial recovery time in the hospital. A consultation with a nutritionist results in a specific dietary plan. The plan included the supplementation of an amino acid that is typically considered a nonessential amino acid. Which of the following amino acids is an essential amino acid under conditions of enhanced growth or surgical recovery ... 

Chlorella contains protein, chlorophyll, dietary fiber, and large amounts of minerals and vitamins. The protein of Chlorella contains all the essential amino acids required for the nutrition of animals and humans. 

Copper is recognized as an essential metalloelement like sodium, potassium, magnesium, calcium, iron, zinc, chromium, vanadium and manganese [1]. Like essential amino acids, essential fatty acids and essential cofactors (vitamins), essential metalloelements are required for normal metabolic processes but cannot be synthesized de novo and daily dietary intake and absorption are required. The adult body contains between 1.4 mg (22 pmol) and... 

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. 

Other Dietary Requirements In addition to providing energy, the diet provides precursors for the biosynthesis of compounds necessary for cellular and tissue structure, function, and survival. Among these precursors are the essential fatty acids and essential amino acids (those that the body needs but cannot synthesize). The diet must also supply vitamins, minerals, and water. 

What are essential amino acids Some species, including humans, cannot synthesize all the amino acids required for protein synthesis and must therefore obtain these essential amino acids from dietary sources. About half of the standard 20 amino acids are essential in humans including arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. 

Seed storage proteins of grain crops meet the major dietary protein requirement of over half of the world population. However, seed proteins in general are deficient in some essential amino acids and are of poor nutritional quality (Table 13.1). 

A diet of only glucose would be adequate to fulfill all the energy and mass requirements for carbohydrates. However, the requirements for amino acids and fats are more stringent, because there are amino acids and fats which are considered essential from a dietary point of view. 

The seeds of legumes and cereal grains provide humans with an estimated 70 percent of their dietary protein requirements, but the proteins in these seeds are also deficient in certain essential amino acids. Researchers have examined the possibility of genetically engineering the genes that code for these proteins to alter their amino acid composition, but they are complex, multigene families. 

---