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Part III. Glossary of terms used in protein quality evaluation
Many terms and ratios have been used in relation to protein quality. This glossary is an attempt to define, and hence standardize, the terminology used in protein quality evaluation. The presence of terms does not necessarily imply approval by the working group but merely recognition that the term has been proposed and is in use. New terms that are equivalent to others already in use have been so identified. It would often be more accurate for the words "claimed to indicate" to precede some of the definitions because the working group could not always agree that the indices, in fact, measure the parameters they are purported to measure. Indeed, criticism of many of the terms defined in this glossary will have been found throughout the text. The relationship of several of the biologically determined indices to each other can be seen by examination of figure 2.
The committee has not yet considered adopting the International System of Units (Sl) as has been recommended by some; however, some of the definitions of the terms in current use have been modified where necessary to use the new units, and a short final paragraph has been added in which the IUNS terminology is indicated. The use of capital letter acronyms, e.g., "NPU" rather than "npu" for net protein utilization, has, however, been retained for this edition.
The word "protein" has a common usage, and the reader is referred to the more precise meanings of crude protein, reference protein, protein calories, and protein-energy ratio. In most of the methods described for the determination of nutritive value, nitrogen is used as an index of protein, and thus the definitions relate strictly to the nutritive value of the nitrogen of foods. Where possible, each term has been defined in a general sense, and the responsibility of more precise meaning is left to the user. Thus, in general, "net protein utilization" is defined as the fraction of food nitrogen intake that is retained, but the conditions under which a paticular measurement has been made should be stated, e.g., the age or weight, sex, and species of the animal used, its energy intake, the composition of the diet fed (especially the protein and energy contents), details of the experiment, including the period of measurement, the environmental temperature, and the previous nutrition of the animals. Other factors may relate to cage size and whether the animals are housed singly or together.
References follow each definition.
The following abbreviations have been used:
A = absorbed nitrogen = I - (F - FK)
B = body nitrogen BK = body nitrogen at zero nitrogen intake Bo = body nitrogen at zero time F = faecal nitrogen FK = metabolic nitrogen (endogenous faecal) I = intake nitrogen S = integumental and miscellaneous nitrogen SK = obligatory integumental and miscellaneous nitrogen U = urinary nitrogen UK = endogenous urinary nitrogen
B and BK are measured at the end of the test period in animals fed the test diet and non-protein diet, respectively. Bo is measured on a control group of animals at the beginning of the experimental period.
A/E ratio
The relationship between the content of an individual essential amino acid in food protein (A) and the total essential amino acid content (E). It may be expressed either as milligrams of the individual amino acid per gram of total essential amino acids, or as grams of nitrogen from the individual amino acid per gram of nitrogen from all essential amino acids used. Rarely used.
Protein Bequirements, FAO Nutrition Meetings Report Series, no. 37 (FAO, Rome,1965).
Amino acid score
( mg of amino acid in 1 g of test protein ) / ( mg of amino acid in 1 g of reference protein)
In practice, equivalent to "chemical score" and "protein score" (qq.v.), although "chemical score" as originally defined was relative to the amino acid composition of egg protein. Expressed either as a ratio to unity (recommended), or on a percentage scale. May sometimes be used for other than the limiting amino acid. Energy and Protein Requirements, FAO Nutrition Meetings Report Series, no. 52 (FAO, Rome, 1973).
Aromatic amino acids (arom AA)
The total of tyrosine and phenylalanine used for scoring purposes. Protein Requirements, FAO Nutrition Studies, no. 16 (FAO, Rome, 1957).
Available amino acids
Amino acids in the food of an animal that are available for protein anabolism. These may be measured directly by bioassay, or indirectly by special chemical or microbio logical methods.
K.J. Carpenter, Biochem. J., 77: 604 (1960).
J.E. Ford, Brit J. Nutr., 16:409 (1962).
Available Iysine value (ALV)
A chemical determination of Iysine in a form that will react with fluorodinitrobenzene
(FDNB). More recently termed "FDNB-available" or "FDNB-reactive Iysine" to avoid confusion with bioassays for available amino acids (q.v.).
K.J. Carpenter, Nutr. Abst Revs., 43: 424 (1973).
R. Hurrell and K.J. Carpenter,Proc. Nutr. Soc., 32: 55A (1973).
The term "chemically available Iysine" has also been used. This procedure involves a chemical determination of Iysine with and without deamination by sodium nitrite by standard ion-exchange procedures. The difference following deamination is an indica tion of available Iysine.
R.M. Allison, W.M. Laird, and R.L.M. Synge, Brit J. Nutr., 29: 51 (1973).
Biological value (BV)
The proportion of absorbed nitrogen that is retained for maintenance and/or growth, i.e., DB/A, or
[I - (F - FK) - (U - UK) ] / [ I - (F - FK)]
It may also include sweat nitrogen losses and would then be defined as
[I - (F - FK) - (U - UK) - (S - SK) ] / [ I - (F - FK) ]
If the correction of metabolic and endogenous losses is not made, the value is termed apparent biological value, i.e.,
[ I - F - U ] / [ I - F]
May also be defined in terms of carcass nitrogen, in which case the definition for BV and apparent BV become
BV = DB / A = ( B - BK ) / I - ( F - FK )
Apparent BV = B - B0 / I - F
Expressed either as a ratio to unity (recommended) or on a percentage scale. K. Thomas, Arch. Anat Physiol., Lpz., Physio l. Abstr., 219 (1909). H.H. Mitchell,J. Biol. Chem., 58: 873 (1923).
Calculated protein efficiency ratio (C-PER)
An in vitro assay for the prediction of the protein efficiency ratio (PER) (q.v.) of food proteins.
L.D. Satterlee, H.F. Marshall, and J.M. Tennyson, J. Am. Oil. Chem. Soc., 56: 103 109 (1979).
Chemically available Iysine See "Available Iysine value."
Chemical methods The most theoretically sound chemical method is that of chemical score (q.v.), especially when based on available amino acids (q.v.). Other methods that are technically simpler, but empirical by nature, include the measurement of nitrogen solubility (C.M.Lyman, W.Y. Chang, and J.R. Couch, J. Nutr., 49: 678 [1953] ), total sulphur (D.S. Miller and D.J. Naismith, Nature [Londonl, 182: 176 [1958] ), protein quality index (PQI) (q.v.), dye-binding capacity (q.v.), and dye-binding Iysine (q.v.).
Chemical score
The content of each essential amino acid in a food protein is expressed as a percentage of the content of the same amino acid in the same quantity of a protein (real or hypothetical) selected as a standard. The original standard used was egg protein. The amino acid showing the lowest percentage is called the "limiting amino acid" (q.v.), and this percentage is the chemical score. The concept is applicable to both available amino acid (q.v.) and total amino acid data. Score is thus dependent on the standard chosen. It is frequently used interchangeably with "amino acid score" and "protein score." May now also be expressed as a fraction.
R.J. Block and H.H. Mitchell, Nutr. Abstr. Revs., 16: 249 (1946).
Crude protein
Nitrogen content multiplied by the conventional factor 6.25: crude protein = N x 6.25.
Digestibility
The proportion of food nitrogen that is absorbed:
A /I = [ I - (F - FK) ] / I = true digestibility
This value is often termed "true digestibility of nitrogen." If the correction for metabolic losses in faeces is not made, the value is termed "apparent digestibility":
I = apparent digestibility
See also "enzyme methods."
Dye-binding capacity (DBC)
Procedures that involve binding of protein with certain dyes that may correlate both with protein quality and protein quantity. The usual reactions involve the basic amino acids. Dyes used include Cl Acid Orange (Orange G) and Cl Acid Black I (Amido Black 10 B).
H. Frankel-Conrati and M. Cooper, J. Biol. Chem., 154: 239 (1944).
A.L. Lakin, "Evaluation of Protein Quality by Dye-Binding Procedures," in J.W.G.
Porter and B.A. Rolls, eds., Proteins in Human Nutrition (Academic Press, New York and London, 1973), p. 179.
Dye-binding Iysine (DBL)
The reactive Iysine content of a food as measured by the difference between its dye binding capacity before and after treatment with a reagent (usually propionic anhydride) to block specifically the basic epsilon-amino group of Iysine units in peptide chains.
R.F. Hurrell, P. Lerman, and K.J. Carpenter, J. Food Sci., 44: 1221 (1979).
A.F. Walker, Brit J. Nutr., 42: 455 (1979).
Enzyme methods Liberation of amino acids from proteins for the determination of availability and the testing of protein damage, Pepsin, trypsin, and pancreatin have been proposed. Also used to measure digestibility. See "Pepsin digest residue (PDR)." E. Menden and H.D. Cremer, Nutr. Dieta, 8: 188 (1966).
Essential amino acid (EAA)
An amino acid that cannot be synthesized from materials normally present in the diet at a rate commensurate with normal bodily needs. Also called "indispensable amino acid" by some investigators.
W.C. Rose,Physiot. Rev., 18: 109 (1938).
Essential amino acid index (EAA index)
The geometric mean of the ratios of the essential amino acids in a protein to those of a standard (usually egg protein).
B. L. Oser, J. Am. Dietet Assoc., 27: 396 (1951) .
Essential to non-essential amino acid ratio (E/N ratio)
1. The molar ratio of essential to non-essential amino acids in samples of body fluids.
M.E. Swendseid, J. Villabos, and B. Friedrich, J. Nutr., 80: 99 (1963). 2. The ratio of essential amino acids to non-essential amino acids in a food protein; comparison here is preferable in terms of nitrogen. See "Indispensable/dispensable amino acid nitrogen ratio ( I/D ratio)." Protein Requirements, FAO Nutrition Meetings Report Series, no.37 (FAO, Rome, 1965).
Essential amino acid to total amino acid ratio (E/TN ratio) The proportion of the total nitrogen (T) derived from essential amino acids (E). This may be expressed in various ways, e.g., as milligrams of essential amino acids per gram of total nitrogen content or per gram of crude protein. Protein Requirements, FAO Nutrition Meetings Report Series, no. 37 (FAO, Rome, 1965).
Food efficiency
An undefined term, also called "food conversion efficiency," showing the relationship between gain by the animal and the food consumed. Thus: weight gain food eaten
These may also be expressed in units of energy. S.Brody, Bioenergetics and Growth ( Reinhold, New York, 1945).
This ratio is increasingly used in toxicology as an overall assessment of food whole" someness and in agricultural practice. The present position where food efficiency can be either food/growth or growth/food is confusing. Unless the term is used under conditions of isonitrogenous intake, it bears no relationship to protein quality. The group deplores the confusion resulting from the use of this term in protein quality evaluation.
FDNB-available Iysine value (ALV) See "Available Iysine value (ALV)."
Gross protein value (GPV) A method for testing the supplementary values of protein to basal cereal diets. Obtained by measuring the difference in body weights of a group of animals fed a basal cereal diet and the test proteins, and a control group fed the basal diet alone. Values are calculated per gram of test protein eaten and referred to the value obtained, using casein as a standard. Usually measured using chicks fed diets containing 8 per cent and 11 per cent protein (basal and test diets, respectively). Several modifications of the procedure are in the literature and these depend on whether or not allowance is made for feed consumption of the basal groups, and whether comparison is made to the 3 per cent protein supplement or 11 per cent total protein intake. N. J. Daghir, B. Ayyash, and P.L. Pellett,J. Sci. FoodAgric., 20: 349,1969.
V. Heiman, J.S. Carver, and J.W. Cook, "Gross Value of Protein Supplements for Poultry," Bulletin no. 338 (Agriculture Experiment Station, Pullman, Wash., USA, 1 940).
Indispensable amino acid See "Essential amino acid."
Indispensable/dispensable amino acid nitrogen ratio (I/D ratio)
The ratio of indispensable (essential) amino acid nitrogen to dispensable (non essential) amino acid nitrogen. See "Essential to non-essential amino acid ratio (E/N ratio)."
W.P. Stucki and A.E. Harper,J. Nutr., 78: 278 (1962).
Limiting amino acid (LAA) The essential amino acid of a protein present in the lowest proportion as compared to the same quantity of another protein (real or hypothetical) selected as a standard. The apparent limiting amino acid in a protein is thus dependent on the standard chosen. The true limiting amino acid in a protein is, however, the amino acid limiting growth in a biological experiment. See "Chemical score," "Amino acid score," and "Protein score."
Linear regression analysis See "Regression analysis, linear."
Microbiological methods These methods have been used to measure individual amino acids after hydrolysis; protein quality is then estimated by chemical score (q.v.). They have also been used to determine the overall capacity of a protein to support growth. See "Relative nutritive value. 1." Also used to assess amino acid availability.
Net dietary protein
Obtained by multiplying crude protein content (q.v.) by NPUop (as fraction). To be distinguished from reference protein (q.v.) and utilizable protein (q.v.). See also "Net protein value (NPV)."
B.S. Platt and D.S. Miller,Proc. Nutr. Soc., 18: vii (1959).
Net dietary protein calories per cent (NDpCal%) An estimate of the utilizable protein content of a diet in terms of calories expressed as a percentage of the total metabolizable energy:
NDpCal% = PCal% x NPUop (as fraction)
Metabolizable energy of protein taken as 4 kcal/g. See "Protein calories per cent (PCal%)." B.S. Platt, D.S. Miller, and P.R. Payne, "Protein Values of Human Food," chap. 28 in
J.F. Brock, ea., Recent Advances in Clinical Nutrition (Churchill, London, 1961).
Net dietary protein energy ratio (NDpE) Replaces NDpCal% when using the International System of Units (Sl). NDpE = protein energy ratio x NPUop as fraction. Numerically equivalent to NDpCal%/100.
Net protein ratio (NPR)
The weight gain of a test animal plus weight loss of a control animal per gram of protein consumed. Thus: weight gain of average weight loss of animals
NPR = test animal + fed basal (non-protein) diet protein (N x 6.25) consumed by test animal
Both 10- and 14 day growth periods have been recommended. An improvement on protein efficiency ratio (q.v.) in that an allowance for maintenance is made by use of a non-protein control group. Similar in concept to net protein utilization (q.v.) but calculated from body weight rather than body nitrogen. As defined above, the ratio is not on a percentage or unity scale. See also "Nitrogen growth index (NGI)," "Protein retention efficiency (PRE)," and "Relative net protein ratio (RNPR)." A.E. Bender and B.H. Doell, Brit. J. Nutr., 11:140 (1957).
Net protein utilization (NPU)
The proportion of nitrogen intake that is retained, i.e., the product of biological value
(q.v.) and digestibility (q.v.).
NPU = BV x D = (DB / A) x (A / I) = DB / I = [ I - (F - FK) - (U - UK) ] / I
May also be defined in terms of carcass nitrogen when DB / I = B - BK / I
In this case, digestibility is included in the index and cannot be expressed separately unless faecal analysis is performed.
If the measurement of NPU is made under standard conditions, with the protein intake at 10 per cent (100 9 per kg diet) or below, the value is termed standardized. If a food or diet is fed as it is consumed without dilution or addition, it is termed NPU operative (NPUop). Similar to net protein ratio (q.v.) if body weight is used to calculate body N.
If the correction for endogenous losses is not made, the value is termed apparent NPU, i.e.,
Apparent NPU = ( I - F - U) / I or ( B - B0 ) /I
This function was used by H.V. McCollum and N. Simmonds, The Newer Know/edge of Nutrition (Macmillan, New York, 1929), and is identical with "productive protein value." Expressed in relation to unity (recommended) or on a percentage scale. D.S. Miller and A.E. Bender, Brit J. Nutr., 9: 382 (1955). D.S. Miller and P.R. Payne, Brit J. Nutr., 15: 11 (1961).
Net protein value (NPV)
A term used to compare protein concentrates. The product of the percentage of crude protein and NPU measured with diets containing 10 per cent protein, i.e., crude protein % x NPU10. Similar, but not equivalent, to utilizable protein (q.v.).
H.H. Mitchell, Am. Soc. Anim. Prod., p. 55,1922.
Nitrogen balance
Apparent nitrogen retention (see "Nitrogen retention"), i.e., l - F - U.
Nitrogen-balance index (NBI) The slope of the line relating nitrogen balance to absorbed nitrogen. In many circumstances this is equivalent to biological value (q.v.). If nitrogen intake is used in place of absorbed nitrogen, the values are equivalent to net protein utilization (q.v.). J.B. Allison, Physiol. Rev., 35: 664 (1955).
Nitrogen conversion factors
Various factors have been proposed for the conversion of the nitrogen content of different foods to protein content; they may range from 5.18 for almonds to 6.38 for milk.
D.B. Jones, US Department of Agriculture Circular 183 (USDA, Washington, D.C.,1931).
O. L. Merril and B.K. Watt, Agriculture Handbook no. 174 ARS (USDA, Washington, D.C., 1955). See table 3.
Nitrogen efficiency ratio (NER)
Weight gain per weight of nitrogen eaten, i.e., protein efficiency ratio (q.v.) x 6.25.
H.R. Rosenberg, in A.A. Albanese, ea., Protein and Amino Acid Nutrition (Academic
Press, New York 1959).
Nitrogen growth index (NGI)
The slope of the line, using linear regression analysis, relating growth to nitrogen intake. Cf. "Nitrogen-balance index." In some circumstances, equivalent to net protein ratio (q.v.). See also "Relative nutritive value. 2," and "Protein value."
J.B. Allison, R.W. Wannemacher, M.T. Spoerlein, and E. Middleton, Fed. Proc., 18: 516 (1958).
Nitrogen-incorporation efficiency (NIE) Equivalent to apparent NPU.
W.P. Stucki and A.E. Harper, J. Nutr., 78: 278 (1962).
Nitrogen retention
Dietary nitrogen retained in the body, i.e., I - (F - FK) - (U - UK). If the correction for endogenous losses is not made, the quantity is termed "apparent nitrogen reten tion," i.e., l - F - U. In common usage, equivalent to nitrogen balance only when positive.
Non-specific nitrogen
Nitrogen that is metabolically available but that leads to minimal toxicity at the levels used.
N.S. Scrimshaw, V.R. Young, R. Schwartz, M.L. Piche, and J.B. Das,J. Nutr., 89: 9 (1966).
C. Kies, Fed. Proc., 31: 1172 (1972).
Pepsin digest residue (PDR)
Ratio of essential amino acids liberated from a protein by pepsin hydrolysis to the total essential amino acids present. See "Enzyme methods."
A.L. Sheffner, G.A. Eckfeldt, and H. Spector, J. Nutr., 60: 105 (1956).
Plasma amino acid ratio (PAA)
[(B - A)/ R ] x 100
A = concentration of specific amino acid in plasma after an 18-hour fast (mg/100 ml).
B= postprandial plasma concentration (average level in five consecutive hourly blood samples drawn after a meal) (mg/100 m1).
R = requirement for specific amino acid in question (9 amino acid per 16 9 N). J.B. Longenecker and N.L. Hause, Am. J. Clin. Nutr., 9: 356 (1961).
Precision
The ability of an estimate to discriminate quality among proteins is a function of both how different the estimates are and also the random error or coefficient of variation of the estimate.
Productive protein value (produktiver Proteinwerk - PPVv) Apparent NPU. See "Net protein utilization (NPU)." D. Hotze l, Z. Tierernaehr. Fu ttermittelkd., 13: 193 (1958) .
Proportionality
An estimate should be proportional. A material with half the potency of another should yield estimates that are half the value.
Protein
See the introductory paragraphs to this Glossary. See also "Nitrogen-conversion factors," "Crude protein," "Protein calories," and "Protein-energy ratio."
Protein calories
The metabolizable energy (kcal) of crude protein (q.v.), i.e., N x 6.25 x 4, or N x 25.
Protein calories per cent (PCal%)
Protein calories (q.v.) expressed as percentage of total metabolizable energy (kcal).
Protein efficiency ratio (PER)
Weight gain per weight of protein eaten. Values are usually measured using rats.
Originally measured at different levels of protein and the maximum value quoted; later conventionally fed at 10 per cent protein; standardized procedure uses diets containing 9.09 per cent protein.
T.B. Osborne and L.B. Mendel, J. Biol. Chem., 32: 369 (1917).
AOAC, Official Methods of Analysis of the Association of Official Analytical
Chemists, 11 tn ed. (Washington, D.C., 1970) .
Protein-energy ratio
The metabolizable energy of crude protein (N x 6.25) expressed as a fraction of the total metabolizable energy. Numerically equivalent to PCal%/100.
Protein quality index (PQI)
An empirical index, now obsolete, which assigns a value to a protein on the basis of its solubility in various solvents including enzyme preparations.
H.J. Almquist, E.L. Stokstad, and E.R. Halbrook, J. Nutr., 10: 193 (1935).
Protein rating
The product of the protein efficiency ratio (q.v.) of the protein of a food, multiplied by the grams of
protein in a "reasonable daily intake."
J.A. Campbell and D.G. Chapman,J. Canad. Dietet Assoc., 21: 51 (1959).
Protein requirement
Used loosely to describe the overall protein needs of population groups.
More precisely defined, for each subgroup of the population, as the sum of obligatory nitrogen losses together with the special nitrogen needs (where applicable) of growth, pregnancy, and lactation. These requirement values (mg N/kg/day) are then adjusted upwards by factors to allow first for the inefficiency of nitrogen utilization and then for individual variability. After multiplication by body weight and conversion to protein (N x 6.25), followed by further upward adjustment to allow for protein quality, these values become the safe practical allowance (SPA) or recommended dietary allowance (RDA) for protein (g/day) for specific population groups.
May be defined by other criteria for the young child. Energy and Protein Requirements, FAO Nutrition Meetings Report Series, no. 52 (FAO, Rome, 1973). Food and Nutrition Board, National Research Council, Recommended Dietary Al/owances, 9th ed. ( National Academy of Sciences, Washington, D.C., 1980).
Protein-retention efficiency (PRE)
Net protein ratio (q.v.) x 16
A.E. Bender and B.H. Doell, Brit J. Nutr., 11: 140 (1957).
Protein score
Measures the extent to which a food or food combination supplies the limiting amino acid as compared to the provisional pattern. See "Chemical score" and "Amino acid score."
Protein Requirements, FAO Nutrition Meetings Report Series, no. 37 (FAO, Rome,1965).
Protein value (PV) The slope of the line relating growth response to nitrogen or protein intake. Similar to nitrogen growth index (q.v.). When expressed in relation to a standard protein (e.g., lactalbumin), becomes RNV (see "Relative nutritive value. 2") or PRV (see "Relative protein value"), depending on whether the zero nitrogen intake data are included or excluded.
Protein value cost index (PVCI)
Suggested for comparison of relative costs of supplementary protein foods. Multiple of grams of protein per US cent and PER (see "Protein efficiency ratio").
H.A.B. Parpia, M. Swaminathan, and V.A. Daniel, Nutr. Rep. /nt, 5: 233 (1972).
Reference pattern
The pattern of amino acids in reference protein (q.v.). Also used in a less rigorous manner to mean a pattern of amino acids used for reference.
Reference protein A hypothetical protein of high biological value containing a specified pattern of amino acids. (Hypothetical because it is assumed to have the same quality at any dietary level; this is an invalid assumption for food proteins.) Used by FAO (FAO Nutrition Studies, no. 16 [1957] ) for stating protein requirements. Similar, but not equivalent, to net protein value and utilizable protein (qq.v.).
Regression analysis, linear Standard statistical procedure to determine the slope (m) and intercept (C)of the best straight-line relationship between two variables X and Y in the equation Y = mX + C. Used to estimate the slope of the line for slope-ratio assay procedures (q.v.), such as
NGI and RPV. Also used to calculate the intercept with zero balance for the relative nitrogen retention (RNR) procedure. See chapter 10.
Relative
The term is used preceding a defined index when that index is expressed in relation to the value obtained at the same time, under the same conditions, with a standard protein taken as unity. May also be expressed in terms of percentage.
Relative growth index Later named "relative nutritive value" (q.v.). D.M. Hegsted and Y. Chang,J. Nutr., 85: 159 (1965).
Relative net protein ratio (RNPR)
NPR of a test protein expressed as a fraction of that obtained with a standard high quality protein, taken as unity (recommended) or as a percentage.
Relative nitrogen retention (RNR)
The ratio of the intercepts of the regression lines relating nitrogen balance and nitrogen intake for a test protein and a standard protein.
V.R. Young, W.M. Rand, and N.S. Scrimshaw, Cereal Chem., 54: 929 (1977).
Relative nutritional value (RNV) The growth response of Streptococcus zymogenes to proteins (following partial digestion with papain) as a percentage of the response to casein. J.E. Ford, Brit J. Nutr., 14: 485, and 16: 409 (1960).
Relative nutritive value (RNV)
Originally named "relative growth index."
D.M. Hegsted and Y. Chang,J. Nutr., 87: 19 (1965).
D.M. Hegsted, R. Neff, and J. Worcester, J. Agric. Food Chem., 16: 192 (1968).
Relative protein value (RPV) The slope of the straight portion of the line relating growth response to nitrogen intake, i.e., protein value (PV) (q.v.) expressed on a scale relative to 1.00 for a standard high-quality protein. This was originally lactalbumin. The slope should not include the zero (non-protein) data, and is thus to be distinquished from RNV (see "Relative nutritive value. 2"). Growth response may be expressed as live weight, body water, or body nitrogen. See chapter 4.
Repletion methods The ability of a protein to support repletion of a previously depleted animal has been used to measure protein values. Techniques include the measurement of the regeneration of body weight (P.R. Cannon, E.M. Humphreys, R.W. Wissler, and L.E. Frazer, J. Clin. Invest., 23: 681 11944] ), plasma proteins (W.T. Pommerenke, H.B. Slevin, D.H. Kariker, and G.H. Whipple, J. Exp. Med., 61: 261 [1935] ), serum albumin and globulin (R.D. Seeley, Am. J. Physiol., 144: 369 [1945] ), haemoglobin, (M.N. Moorjani and V. Subrahmanyan, Indian J. Med. Res., 38: 145 [1950] ), liver protein, (R.M. Campbell and H.W. Kosterlitz, J. Physiol., 107: 383 [1948]; S. Mokady, S. Viola, and G. Zimmerman, Brit J. Nutr., 23: 491 [1969] ), liver enzymes (J.N. Williams and C.A. Elvehjem, J. Biol. Chem., 181: 559 [1949] ), and liver RNA (C.B. Mendes and J.C. Waterlow, Brit J. Nutr., 12: 74 11958] ).
Reproducibility
The ability of an adequate method to yield similar results when applied in different laboratories and/or in repeated analyses within a laboratory.
Slope ratio assays A general term to describe assays of the dose-response type where the slope of the response is measured. For most assays, dose {X) is protein or nitrogen intake, while response (Y) would be body weight, body nitrogen, body water, or nitrogen balance. The straight-line portion of the relationship is used for the calculation of slope and intercept by linear regression analysis (see "Regression analysis, linear"). Examples of such assays in this glossary are nitrogen growth index, relative protein value, relative nitrogen retention, and nitrogen balance index. Relative nutritive value (sense 2) is also a slope-ratio assay, but requires the more complex multiple regression analysis for its calculation.
Standard protein
A high-quality protein used in a biological assay procedure as a reference. Should be determined at the same time and under the same conditions as the assay procedure being used. Not identical to reference protein.
Sulphur amino acids (SAA) The total of methionine and cystine used for scoring purposes. Units are as for other amino acid data, i.e., mg/g N or mg/16 9 N. Cystine is not an essential amino acid but can be synthesized from methionine. Cystine in a diet can thus "spare" methionine, and the total of the two has been found more satisfactory for scoring purposes than methionine alone. Sometimes called "total sulphur amino acids." Protein Requirements, FAO Nutrition Studies, no.16 (FAO, Rome, 1957).
Tetrahymena protein efficiency ratio (T-PER) An in vitro prediction of protein efficiency ratio (q.v.) using the growth of Tetrahymena thermophila together with an enzymic determination of digestibility. H.W. Hsu, N.E. Sutton, M.O. Banjo, L.D. Satterlee, and J.G. Kendrick, Fd. Technol., 32 ( 12): 69 - 73 ( 1978).
Total sulphur amino acids (total SAA) See "Sulphur amino acids."
Utilizable protein The potential maximum amount of protein present that can be utilized. Similar to reference protein (q.v.), but to be distinguished from net dietary protein (q.v.). A multiple of protein content (g/kg) and a quality index (as a fraction).
Note on the International System of Units (Sl) - Conversion Factors and
Recommended Terminology 1. Expression of concentration
Because the unit of mass has been defined as a kilogram unit: a. Concentrations expressed as mass per unit weight (w/w) are to be expressed per kilogram. Values should thus be expressed as nanograms, micrograms, milligrams, or grams per kilogram. b. Concentrations expressed as mass per unit volume (w/v) should be expressed per litre. c. Composition of concentration should not be expressed on a percentage basis.
As a further extension of this principle, the common ratios used in protein nutri tional studies such as digestibility, biological value (BV), net protein utilization (NPU), protein-energy ratio, etc. should be expressed as decimal fractions of unity rather than percentages.
The amounts of available nutrients can thus be obtained from analytical data by direct multiplication.
2. Use of the joule
The joule (J) can be used as a unit of energy instead of the calorie, with a conversion factor of 4.184 J = 1 calorie. 1,000 joules = 1 kilojoule (kJ), and 1,000 kilojoules = 1 megajoule (MJ). Protein-energy concentration as a fraction then replaces PCal%. The maintenance of the calorie as a unit of nutritional energy has recently been recommended by Moore (T. Moore, "The Calorie as a Unit of Nutritional Energy," World Rev. Nutr. Dietet., 26: 1-25 [1977] ).