20 amino acids pdf

Because of methodologic problems, it is difficult to attain exact measurement of intake and output of nitrogen Hegsted, , to determine the time required for adjustment at altered levels of protein intake Rand et al. Most studies of protein requirements have been short. Because of the methodologic problems cited above, longer studies should provide a better basis for determining protein requirements c ; they would permit the measurement of variables such as alterations in lean body mass or in growth rate of children, which respond more slowly to dietary inadequacy.

In the few long-term studies that have been reported, investigators have explored the usefulness of various biochemical indices e. Because the human body can adapt to low and high intakes of nitrogen, there is a substantial difference between intakes barely sufficient to compensate for losses or to permit growth and intakes that may be associated with harmful effects.

Since there are few criteria by which to evaluate the significance of the rate of protein turnover and pool size, value judgments must be made as to what is desirable in adults. For children, the protein required for growth is relatively small compared to that needed for maintenance.

Nevertheless, satisfactory growth is a sensitive indicator of protein nutritional status. The requirement for protein is reasonably well established for the very young child and the young male adult. For other age groups, much less information is available, and protein needs are estimated in part by interpolation or extrapolation based on reasonable biological principles. In determining the requirement for protein, the subcommittee first considered requirements for the essential amino acids.

The essential amino acid requirements of infants, children, men, and women were studied extensively from to Estimates of amino acid requirements for various age groups are listed in Table In a novel approach to examining these requirements, the need for four amino acids was examined in children whose diets were strictly controlled because of inborn errors of metabolism and who were developing normally Kindt and Halvorsen, Requirements determined in this way during the first 3 years of life are in good agreement with the values for isoleucine, leucine, phenylalanine plus tyrosine, and valine given in Table for infants and 2-year-old children.

The requirement for histidine has not been quantified beyond infancy. Requirement values are difficult to establish because deficiency symptoms occur only after long periods of low intake.

The relatively low requirements estimated for adults have been confirmed by Inoue et al. Studies of whole body lysine, leucine, valine, and threonine oxidation rates suggest that adult requirements for these essential amino acids have been underestimated.

These new estimates have been challenged on methodologic and theoretical grounds Millward and Rivers, and require further confirmation.

Studies on requirements for individual essential amino acids in the elderly are inconsistent. Some suggest that requirements are increased in the elderly; others indicate that they are decreased Munro, In the one study in which the same methodology and design were applied to the elderly as in a study of young men, no differences in requirements between age groups were found Watts et al.

The pattern of requirement for essential amino acids in the elderly is accepted to be the same as for younger adults. The data demonstrate the unsatisfactory state of knowledge concerning amino acid requirements. The values in Table are the best available and serve as the basis for calculation of amino acid requirement patterns at various ages and for procedures for the amino acid scoring of diets see below.

In establishing an RDA for protein, three steps were followed: 1 The subcommittee first estimated the average requirement for reference proteins i. These were based on requirements of various age groups for essential amino acids and for total protein. These patterns of requirement were reviewed in relation to U. Adults To determine the protein requirements of young male adults, WHO reviewed evidence from both short- and longterm nitrogen balance studies. On the basis of recalculated data from the short-term studies, the international group proposed a mean requirement of 0.

Several relatively long-term studies 58 to 89 days involving single levels of protein intake yielded similar estimates of requirement for subjects consuming egg-protein diets. By averaging the two sets of balance data i.

This is accepted to be the average daily requirement for reference proteins. No data were available on the coefficient of variation for long-term studies, but for short-term studies, the mean coefficient of variation was estimated to be Thus, 0.

The international group examined the data from several short-term studies in which men were fed habitual mixed diets of ordinary food. The requirements were predicted to be 0. The adult requirement for absorbed protein appears not to differ between reference and practical diets.

There are fewer data for young adult women, but there is evidence Calloway and Kurzer, that requirement values, when adjusted for body weight, are not substantially different from those for young adult men.

Accordingly, the recommended allowance for reference protein is 0. The Elderly The protein content of the adult body diminishes with age.

More specifically, nonmuscle mass is little affected by age, whereas muscle diminishes extensively and is compensated for by an increase in body fat. These changes in muscle mass are related to whole-body protein turnover and changes in the rate of protein synthesis Uauy et al. Serum albumin levels and daily albumin synthesis also decrease in elderly people who consume diets with adequate protein Gersovitz et al.

Questions thus arise concerning the extent to which changes in protein metabolism affect protein requirements. Dietary protein needs might be expected to change during the aging process, i. There is surprisingly little information on which to base recommendations for protein intake in the elderly.

The early literature on this subject was reviewed by Irwin and Hegsted Only a few studies have been conducted in the more recent past, and their results are inconsistent. Zanni et al. According to Gersovitz et al. In both of these studies, body weight was maintained; however, energy intake was lower in the study by Gersovitz and colleagues , suggesting that activity patterns may have been different in the two groups or that nitrogen balance was improved by the higher food energy intake in the study by Cheng et al.

Variations in activity level, disease prevalence, and use of therapeutic drugs are all potentially confounding variables. The recommended allowance for reference protein 0. Because of the difference in body composition, this allowance is higher per unit of lean body mass and should allow for some decrement in utilization efficiency.

Pregnancy Additional protein is required during pregnancy for the mother and the fetus Hytten and Leitch, Maternal protein synthesis increases in order to support expansion of the blood volume, uterus, and breasts, and fetal and placental proteins are synthesized from amino acids supplied by the mother.

The magnitude of the required increase in dietary intake remains uncertain, since different methods of estimation yield different figures. The factorial method of estimating requirements for pregnancy is based on the amount of protein present in the fetus, placenta, and maternal tissues, including blood Hytten and Leitch, It is calculated that g of protein is deposited during a pregnancy involving The rate of nitrogen retention is not constant; for the first through third trimester, nitrogen deposition is estimated to be 0.

Evidence from animal studies suggests, however, that relatively more protein may be stored during early gestation and mobilized at later stages of pregnancy Naismith, ; Naismith and Morgan, Protein turnover is increased by the twelfth week of pregnancy de Benoist et al.

Consequently, increased protein needs during pregnancy may be more uniform across time than the figures of Hytten and Leitch indicate.

Nitrogen balance data can also be used to estimate protein requirements during pregnancy. Almost all older balance studies have indicated that more nitrogen is retained during pregnancy than is predicted on the basis of fetal and placental growth and maternal tissue hypertrophy Calloway, In more recent studies Appel and King, ; Johnstone et al.

Nevertheless, data on changes in body weight and body potassium indicate that nitrogen retention under laboratory conditions is somewhat greater than can be accounted for only by the fetus and maternal supportive tissue Appel and King, ; King et al. This suggests that protein might be retained at sites other than those now recognized, for example, in skeletal muscle.

This possibility is supported in part by analyses of animal carcasses King, Dietary surveys in developed countries indicate that pregnant women eating self-selected diets generally consume somewhat larger amounts of protein than theoretical requirements. Moreover, satisfactory levels of protein intake tend to be associated with improved reproductive outcome Higgins et al.

However, such epidemiological data are confounded by the strong dietary correlation between protein and energy, as well as their metabolic interrelationships.

Despite these discrepancies between factorial and nitrogen balance estimates of nitrogen gain during pregnancy, the subcommittee concurs with WHO that the estimate of requirement should be based on the factorial method. The additional allowance of reference protein needed to support the deposition of new tissue is calculated to be 1. There is also a maintenance requirement associated with the added lean tissue. To allow for this and because of the uncertainty about the rate of tissue deposition, the subcommittee recommends an additional allowance of 10 g of reference protein per day throughout pregnancy.

Analyses of human milk composition in the United States, based on a study of 40 mothers in the first 4 months of lactation, show a fall in protein content from 1. The coefficient of variation is taken to be The allowance is calculated as follows:. No recent data from the United States have been published. If the international figure is accepted, then the additional allowance in this period is The average increase in body protein is about 3.

As the growth rate drops rapidly after the first year of life, the maintenance requirement represents a gradually increasing proportion of the total protein requirement. For the first months of life, requirements are based on intake data because of the difficulty in accurately estimating allowances for growth and maturation of body composition. Infants breastfed by healthy, well-nourished mothers or fed human milk by bottle can grow at a satisfactory rate for about 4 months Butte et al.

Protein d intakes by breastfed infants range from 2. Breastfed infants in the United States grow satisfactorily at a mean protein intake of 1.

Probability assessment suggests that the true requirement appears to be less than mean intake; the figure proposed is 1. The subcommittee has, however, accepted an average intake of 1. The protein needs of an infant up to 4 months of age will be met if the energy needs are met, provided the food is human milk or a formula that contains protein of a quality and quantity equivalent to that of human milk.

A modified factorial procedure for calculating the protein needs of infants and children was examined by the WHO group. These estimates were compared with data from the few studies of nitrogen balance in the age group 6 months to 9 years. Neither the factorial nor balance estimates were consistently higher or lower across age groups.

Because there are large gaps in the experimental data, the modified factorial approach was used by the WHO group to estimate needs for all children.

The international values are accepted by this subcommittee in establishing average requirements for reference protein beyond age 3 months. Table provides examples of the steps involved in the calculations of daily allowances by the modified factorial approach.

The coefficient of variation in requirements calculated in this manner is assumed to be The tabulated values for reference protein allowances for the various age and sex groups are listed in Table To convert these values to daily allowances of average U.

Since the requirements of both amino acids and protein differ among age and sex groups, the quality of protein required to meet these needs will also vary. Recommended Allowances of Reference Protein and U. Dietary Protein. A pattern of requirements for amino acids in the total dietary protein is calculated by dividing each essential amino acid requirement by the recommended allowance of reference protein for the given age group see Table The requirement pattern for infants is based on quantitative amino acid requirements Table divided by the reference protein allowance of infants 3 to 4 months of age 1.

WHO accepted for this age group a pattern based on the average composition of human milk protein. Both patterns are shown in Table The variation in reported composition of human milk proteins is large, and a substantial portion of breast milk nitrogen is nonprotein. Given the difficulties in estimating the factors that affect the patterns, the values are in surprisingly good agreement.

Only for tryptophan is the difference substantial. This subcommittee concludes that the composition of human milk should be used as a reference pattern for formulation of human milk substitutes for infants and as a guide to supplementary feeding of infants through 6 months of age.

The amino acid pattern shown for 2-year-old children should be applied to children ages 2 to 6, and that shown for children 10 to 12 years should be used for ages 6 to 13 years. The adult pattern is applicable to children above age 13 and adults. For children above 6 months and less than 2 years of age, a combination of the infant and preschool child figures should be used to evaluate a total diet.

These values and recommendations are in general agreement with those of WHO There are no established amino acid requirements and, hence, no amino acid patterns for pregnancy and lactation. The international group suggested that a pattern for the lactating woman's total diet could be developed by summing the adult protein and additional lactation allowances, calculating the proportion of each in the total, and applying the adult pattern to its fractional part 0.

A similar calculation is theoretically possible for pregnancy, but more difficult because of the variety of tissues and proteins deposited.

Amino Acid Scoring of Dietary Protein Quality To adjust for amino acid composition, a score is calculated according to the most limiting amino acid, i.

The amino acid score should be based on the appropriate pattern for age. Only four essential amino acids are likely to affect the protein quality of mixed human diets: lysine, the sulfur-containing amino acids methionine plus cystine , threonine, and tryptophan. Scoring patterns for all the essential amino acids presented in Table are important for the formulation of special purpose diets in clinical practice.

Food consumption data from the U. Despite wide variations in food energy intake, this proportion remains similar for both sexes and all age groups except infants. There is also little change as a function of household income, urbanization, or race. Food items likely to be underreported in surveys e. The importance of grain products as suppliers of protein is not always realized, particularly in populations ingesting diets rich in animal products.

The amino acid pattern in the diet consumed by children ages 1 to 3 years and all persons surveyed is given in Table The pattern is uniform between the age groups and meets the requirement pattern levels for all age groups except infants.

The U. Therefore, no adjustment to the recommended allowance for reference protein is required for people consuming a typical U. Digestibility The amino acid score alone may lead to an overestimation of the capacity of some proteins to meet physiological requirements unless digestibility is taken into account. When the amino acid score is multiplied by digestibility, it becomes analogous to the biologically determined net protein utilization NPU.

The NPU is the product of biological value comparable to amino acid score and true protein digestibility. Differences in digestibility result from intrinsic differences in the nature of food protein and the nature of the cell wall, from the presence of other dietary factors that modify digestion e.

There are few data on the digestibility of specific amino acids in food proteins, and any differences are not captured in measurements of overall protein digestibility.

Although it is known that there are differences between the pattern of amino acids in food protein, fecal matter, and portal blood, it is not now possible to provide finer adjustment than overall digestibility. Representative data on the digestibility of some selected proteins are shown in Table A more comprehensive listing of protein digestibility can be found in reports by Hopkins and FAO The true digestibility of reference proteins is assigned a value of for translating requirements for reference proteins to recommended levels of intake for ordinary mixtures of dietary proteins.

Since the mixed protein of a typical U. Adjustment of Allowances for Dietary Quality Adjustment for exceptional dietary patterns can be made by deriving a weighted digestibility factor based on the digestibilities of the principal protein sources consumed and an amino acid score based on their contribution of essential amino acids. Such adjustment would rarely be warranted for the U.

Shown in Table is an example of calculations required to make an adjustment for an unusual diet—one in which the usual consumption pattern is reversed, i.

A comparison of the amino acid pattern with the requirement patterns in Table shows that lysine is low for the preschool age group and tryptophan is borderline. The amino acid pattern meets the requirement patterns of older children and adults, i. Thus, the protein allowance for a 3-year-old child is 1.

For older children and adults, an adjustment of the allowance would be made only for digestibility. There is little evidence that muscular activity increases the need for protein, except for the small amount required for the development of muscles during physical conditioning Torun et al.

Vigorous activity that leads to profuse sweating, such as in heavy work and sports, and exposure to heat increases nitrogen loss from the skin, but with acclimatization to a warm environment, the excessive skin loss is reduced and may be partially compensated by decreased renal excretion WHO, In view of the margin of safety in the RDA, no increment is added for work or training. No added allowance is made here for the usual stresses encountered in daily living, which can give rise to transient increases in urinary nitrogen output Scrimshaw et al.

It is assumed that the subjects of experiments forming the basis for the requirement estimates are usually exposed to the same stresses as the population generally. Extreme environmental or physiological stress increases nitrogen loss Cuthbertson, Infections, fevers, and surgical trauma can result in substantial nitrogen loss through the urine and greatly increased energy expenditure.

Therefore, severe infections and surgery should be treated as clinical conditions that require special dietary treatment. During convalescence from an illness that has led to protein depletion, requirements for both protein and energy are elevated because of the need to replace wasted tissues, just as they are during periods of rapid growth.

Premature infants also require special consideration with regard to amino acid composition of the formulas and level of protein intake. The RDAs for protein are summarized in Table The RDA for infants was 2. The present RDA for the 6- to month age group is somewhat lower, having been based on newer data on observed intakes of healthy children and theoretical growth requirements.

Human milk or an equivalent substitute i. RDAs for other age and sex groups are essentially the same as the figures, but the derivation and justification are different, reflecting both new data and reexamination of older data.

The revised allowance for pregnancy is more heavily influenced by theory than by new evidence; the allowance for lactation is in accord with new information on breast milk production by women in the United States. Because the system for disposal of excess nitrogen is efficient, protein intakes moderately above requirement are believed to be safe.

Brenner et al. There is supportive evidence from studies in animals, but not in humans on this point. Urinary calcium excretion increases with increased protein intake if phosphorus intake is constant. If phosphorus intake increases with protein intake, as it does in U. It has been suggested, but not demonstrated, that a habitual high intake of protein might contribute to osteoporosis. This seems unlikely based on present evidence, at least for the range of intake by most people in the United States.

Habitual intakes of protein in the United States are substantially above the requirement, and although there is no firm evidence that these intake levels are harmful, it has been deemed prudent to maintain an upper bound of no more than twice the RDA for protein NRC, Specific food proteins have greater cereals or lesser milk percentages of nitrogen.

The WHO report contains extensive references to the original investigations. The reader is referred to that report for full documentation.

The term protein requirement conventionally encompasses specific and nonspecific amino acid and amino nitrogen requirements. Turn recording back on. National Center for Biotechnology Information , U. Search term. Nitrogen Balance Nitrogen balance is the difference between nitrogen intake and the amount excreted in urine, feces, and sweat, together with minor losses occurring by other routes.

The Requirement for Reference Protein and Its Variability Adults To determine the protein requirements of young male adults, WHO reviewed evidence from both short- and longterm nitrogen balance studies. Amino Acid Requirement Pattern According to Age A pattern of requirements for amino acids in the total dietary protein is calculated by dividing each essential amino acid requirement by the recommended allowance of reference protein for the given age group see Table Protein utilization in pregnant and nonpregnant women.

Beaton, G. Protein requirements of infants: a reexamination of concepts and approaches. Block, G. Dresser, A. Hartman, and M. Macronutrients and fats. Brenner, B. Meyer, and T. Dietary protein intake and the progressive nature of kidney disease: the role of hemodynamically mediated glomerular injury in the pathogenesis of progressive glomerular sclerosis in aging, renal ablation, and intrinsic renal disease.

Butte, N. Garza, E. Smith, and B. Human milk intake and growth in exclusively breast-fed infants. Calloway, D. Nitrogen balance during pregnancy. Winick, editor. Nutrition and Fetal Development. Menstrual cycle and protein requirements of women. Cheng, A. Gomez, J. Bergan, T. Lee, F. Monckeberg, and C. Comparative nitrogen balance study between young and aged adults using three levels of protein intake from a combination wheat-soy-milk mixture.

Cho, E. Anderson, R. Wixom, K. Hanson, and G. Long-term effects of low histidine intake on men. Cuthbertson, D. Physical injury and its effects on protein metabolism. Munro, editor; and J.

Allison, editor. Mammalian protein Metabolism, Vol II. Academic Press, New York. Jackson, J. Hall, and C. Whole-body protein turnover in Jamaican women during normal pregnancy. Human Nutr. Food and Agriculture Organization, Rome. Energy and Protein Requirements. Technical Report Series No. World Health Organization, Rome. Ferris, A. Vilhjalmsdottir, V. Beal, and P. Diets in the first six months of infants in western Massachusetts. Energy-yielding nutrients. Fitch, W. Protein turnover and 3-methylhistidine excretion in nonpregnant, pregnant and gestational diabetic women.

Fomon, S. Infant Nutrition , 2nd ed. Saunders, Philadelphia. Protein requirements of term infants. Fomon, editor; and W. Heiod, editor. Energy and Protein Needs During Infancy. Filer, Jr. These deficiencies are usually present in poorer parts of the world or elderly adults with inadequate care. Kwashiorkor and marasmus are examples of more severe clinical disorders caused by malnutrition and inadequate intake of essential amino acids. Kwashiorkor is a form of malnutrition characterized by peripheral edema, dry peeling skin with hyperkeratosis and hyperpigmentation, ascites, liver malfunction, immune deficits, anemia, and relatively unchanged muscle protein composition.

It results from a diet with insufficient protein but adequate carbohydrates. Marasmus is a form of malnutrition characterized by wasting caused by inadequate protein and overall inadequate caloric intake. Amino acid generic structure. Contributed and created by Michael Lopez, B. This book is distributed under the terms of the Creative Commons Attribution 4.

Turn recording back on. National Center for Biotechnology Information , U. StatPearls [Internet]. Search term. Author Information Authors Michael J. Affiliations 1 Augusta Un. Introduction Essential amino acids, also known as indispensable amino acids, are amino acids that humans and other vertebrates cannot synthesize from metabolic intermediates. Fundamentals Amino acids are the basic building blocks of proteins, and they serve as the nitrogenous backbones for compounds like neurotransmitters and hormones.

Mechanism Although there are twenty amino acids required for human protein synthesis, humans can only synthesize about half of these required building blocks. Clinical Significance The classification of essential and nonessential amino acids was first reported in nutritional studies done in the early s.

Review Questions Access free multiple choice questions on this topic. Comment on this article. Figure Amino acid generic structure. References 1. Dietary essentiality of "nutritionally non-essential amino acids" for animals and humans. Exp Biol Med Maywood. Hou Y, Wu G. Nutritionally Essential Amino Acids. Adv Nutr. Reeds PJ. Dispensable and indispensable amino acids for humans. J Nutr. Curr Genomics. Protein - Which is Best?

J Sports Sci Med. Amino acid composition and chemical evaluation of protein quality of cereals as affected by insect infestation. Plant Foods Hum Nutr. LaPelusa A, Kaushik R. Physiology, Proteins. Amino acids: metabolism, functions, and nutrition. Amino Acids. Amino acid synthesis deficiencies. Handb Clin Neurol. Amino acids biosynthesis and nitrogen assimilation pathways: a great genomic deletion during eukaryotes evolution.

BMC Genomics. Less is more: selective advantages can explain the prevalent loss of biosynthetic genes in bacteria. Genome sequence of the endocellular bacterial symbiont of aphids Buchnera sp. The amino acid requirements of adult man. Nutr Abstr Rev. Benjamin O, Lappin SL. Biochemistry, Essential Amino Acids. In: StatPearls [Internet]. In this Page. Related information.

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