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OCR for page 380
19
Protein and Amino Acids
Protein is a macronutrient of major importance in human nutrition.
Plant and animal proteins are composed of more than 20 individual amino
acids. Within the body, amino acids are used for a wide variety of structural
proteins and enzymes; and they serve as a source of energy, carbon, and
nitrogen.
Protein has an energy value of approximately 5.5 kcaVg. Of this, ap-
proximately 4 kcaUg is used during metabolism; the unmetabolized portion
is excreted as urea and other compounds. For meeting metabolic needs
and promoting satisfactory rates of protein synthesis, the diet must provide
amino acids of adequate quality and quantity.
Amino acids and nitrogen are available to mammals through degra-
dation of proteins and other nitrogenous compounds. Mammals can syn-
thesize nonessential amino acids de novo, if energy and suitable forms of
carbon and nitrogen are available. Thus, net requirements for nonessential
amino acids can be met both by dietary protein and by endogenous synthe-
sis of amino acids. Ordinarily, the following amino acids are considered to
be essential amino acids, because they cannot be synthesized by mammals:
histidine, isoleucine, leucine, lysine, methionine + cystine, phenylalanine
+ tyrosine, threonine, tryptophan, and valine (N RC, 1989~. Thus, these
must be provided in adequate amounts by the diet. Other amino acids,
such as arginine and taurine, may functionally appear to be essential during
fetal and infant development in some species (Gaull, 1983; Sturman, 1986;
Visek, 1986), because the metabolic pathways have not yet fully developed
to adult levels and because the amount needed to cover growth and net new
380
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PROTEIN AND AMINO ACIDS
TABLE 19-1 Factorial Estimate of Protein
Components of Weight Gain in a Normal Full-Term
Pregnancya
Component
Weight, g Protein, g
Fetus3,400440
Placenta650100
Amniotic fluid8003
Uterus970166
Blood1,25081
Extracellular fluid1,680135
Total8,750925
a Modified from Calloway (1974), after Hytten and Leitch (1971),
with permission.
381
protein accretion is high. Developmental immaturity of biochemical path-
ways may also limit conversion of pairs of metabolically related essential
amino acids, such as conversion of phenylalanine to tyrosine.
In postnatal life, ingested protein is hydrolyzed to amino acids, which
are absorbed and carried via the portal system to the liver. The amino
acids then enter the systemic circulation and are distributed throughout the
body. The liver is an especially active site for synthesis of protein from i
amino acids. Since considerable reutilization of amino acids occurs, there
is synthesis and degradation of more protein daily than has been ingested.
IMPORTANCE
Pregnancy complicates the already complex metabolism of amino acids.
Expansion of blood volume and growth of the maternal tissues require
substantial amounts of protein (Table 19-1~. Growth of the fetus and
placenta also places protein demands on the pregnant woman. Thus,
additional protein is essential for the maintenance of a successful pregnancy.
However, a review of the processes controlling these changes in maternal
protein metabolism is beyond the scope of this chapter.
Maternal protein restriction, alone and in combination with energy
restriction, results in consistently decreased fetal growth in many species
(Fattet et al., 1984; Hill, 1984; Lederman and Rosso, 1980; Pond et
al, 1988; Rosso, 1977a,b, 1980; Rosso and Streeter, 1979~. These models
demonstrate not only decreased body weight and growth but also decreased
numbers of cells and a variety of biochemical changes. A particular concern
is that the developing fetus may or may not adequately compensate for some
of the effects of maternal protein deprivation, and effects may even span
generations.
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382
DIETARY INTAKE AND NUTRIENT SUPPLEMENTS
AMINO ACID UTILIZATION
The fetus receives a continuous stream of amino acids from the mother
via the placenta (Battaglia, 1986~; the amino acids cross the placenta by
a complex series of transport systems, probably including both active and
facilitated transport systems. Transport systems may differ on the maternal
and fetal sides of the placenta, and different classes of amino acids are
transported by different placental systems (Battaglia, 1986; Eaton and
Yudilevitch, 1981; Lemons and Schreiner, 1983; Schneider et al., 1979;
Smith, 1986; Yudilevitch and Sweiry, 1985~. Amino acid concentrations
are typically somewhat higher in the fetus than in the mother (Cetin et
al., 1988; Soltesz et al., 1985; Yudilevitch and Sweiry, 1985~. Moreover,
the placenta is very active metabolically, and in laboratory animals, it plays
an important role in nitrogen metabolism (Meschia et al., 1980~. Because
of the complexity of the transport processes and placental metabolism, it
is difficult to predict the effect of altered maternal protein intake on fetal
amino acid metabolism, both in terms of the total quantitative amino acid
flux and in terms of relative changes in the fluxes of individual amino acids.
The fetus must handle rapid entry of both exogenous and endogenous
amino acids, and it must provide for the rapid accretion of new protein
(Battaglia, 1986~. Studies in the unstressed fetal lamb have shown rapid
turnovers of leucine and lysine in amounts severalfold higher than umbilical
uptakes of the amino acids from the placenta (Battaglia, 1986~. More
recently, turnover measurements of the nonessential amino acid glycine
have suggested the interconversion of glycine and serine in the fetal liver
(Marconi et al., 1989~. The sheep fetus also appears to catabolize amino
acids to urea at a rapid rate (Lemons et al., 1976~.
Several investigators have studied the effect of direct amino acid infu-
sion in experimentally induced growth retardation in fetal animals (Charl-
ton and Johengen, 1985; Fatter et al., 1984; Mulvihill et al., 1985~. These
studies have demonstrated at least partial restitution of birth weight with di-
rect nutritional supplementation. However, there is no evidence that amino
acid supplementation of normally grown fetuses significantly increases birth
weights above those achieved by controls.
ESTIMATED REQUIREMENTS
Information regarding total protein requirements during pregnancy has
been provided through the factorial approach, balance studies, turnover
studies, and epidemiologic surveys (see Chapter 12~. As noted above,
there are theoretical and experimental differences of opinion regarding
requirements for protein and amino acids.
The results of body composition studies in human and nonhuman
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PROTEIN AND AMINO ACIDS
383
species have formed the basis for estimation of protein accretion in the
fems. Hytten and Leitch (1971) reviewed classic studies of human body
composition (Kelly et al., 1951; Widdowson and Dickerson, 1964) and esti-
mated fetal protein requirements to be approximately 440 g over the course
of pregnancy and the placental protein requirement to be an additional 100
g (Bible 19-1~. Other reviewers, using much of the same published data on
humans, estimated a nitrogen accumulation of 50 to 60 g for a full-term
3,300-g fetus (Sparks, 1984; Ziegler et al., 1976~. The data on which such
factorial estimates are based are limited, however, and lacking in important
details such as accurate gestational age. The results differ because of differ-
ences in mathematical modeling and data bases (Hytten and Leitch, 1971;
Sparks, 1984; Ziegler et al., 1976~. At the standard estimate of 6.25 g of
protein per gram of nitrogen, this would amount to 310 to 375 g of protein
per human fetus at full term somewhat lower than previous estimates.
Both approaches estimating fetal amino acid and nitrogen requirements
demonstrate that the fetus and placenta present a substantial demand for
amino acids from the mother.
Nitrogen is found in many compounds other than protein. Nucleic
acids and polyamines are two such compounds that may be of particular
relevance to the growing fetus. In detailed studies of the chemical compo-
sition of the guinea pig fetus, approximately 20% of the nitrogen content
was found in compounds other than protein (Sparks et al., 1985~. If this is
also true of the human fetus, its protein content and requirements may be
lower than current estimates.
Using the factorial approach and assuming a 40-week gestation and
a 3,300-g newborn, Hytten and Leitch (1971) estimated that 925 g is the
total increment in body protein during pregnancy (Table 19-1~. More
recent nitrogen balance studies (Appel and King, 1979; Johnstone et al.,
1981) suggest that nitrogen retention approaches the factorial estimate, if
adjustment is made for unmeasured losses.
Turnover studies have indicated that protein turnover increases early
and remains elevated throughout pregnancy (de Benoist et al., 1985; Fitch
and King, 1987; Jackson, 1987~. Some investigators have expressed tech-
nical concerns about using turnover measurements to estimate protein
requirements during pregnancy (Fitch and King, 1987~. All human stud-
ies to date have used nonessential amino acids to measure the turnover
of protein in pregnant women, further complicating the interpretation of
these data.
The deposition of protein is not necessarily linear throughout preg-
nancy. Early during pregnancy, the fetal component is minimal, whereas
the requirement for maternal volume expansion and tissue growth may be
substantial. Late in pregnancy, the fetus may account for the major increase
in protein needs. The additional requirement averaged over gestation ap
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384
DIETARY INTAKE AND NUTRIENT SUPPLEMENTS
pears to be roughly 3 to 4 g of protein per day. If it is assumed that there
is a 15% variation in birth weight and that dietary protein is converted at
70% efficiency, the requirement for protein would be an additional 6.0 g/day
averaged over pregnancy, but the demand is highest (10.7 g/day) in the last
trimester (NRC, 1989~. On the basis of these and other considerations,
a maternal protein intake of 10 gJday over the Recommended Dietary
Allowance (RDA) for protein (i.e., a total of 60 g/day) is recommended
throughout pregnancy. This subcommittee notes that most foods that are
good sources of protein (e.g., grains, flesh foods, milk, cheese, and dried
peas and beans) are also good sources of many other nutrients and thus
their use should be encouraged as part of a balanced diet during pregnancy.
USUAL INTAKES
As discussed in Chapter 13, usual protein intakes by pregnant women
in the United States range from 75 to 110 g/day. The estimated average
intakes of protein by low-income women enrolled in the Supplemental
Food Program for Women, Infants, and Children (WIC) were higher than
the 1980 RDA of 74 g/day, even before participation in the program (Rush
et al., 1988~. However, inadequate energy intake may contribute to protein
deficiency if there is compensatory catabolism of protein and amino acids
to meet energy needs. Thus, the adequacy of dietary protein must be
considered in the context of total nutrient intake.
CRITERIA FOR DEFICIENCY
Deficiency of protein is difficult to assess, both because of protein's
dynamic and complex metabolism and because protein deficiency is gen-
erally associated with deficiencies of other nutrients and energr. Classic
signs of protein deficiency include poor growth, muscular weakness, poor
hair growth, and low serum albumin, which may result in edema. Classic
protein deficiency is rare in the general U.S. population, occurring primar-
ily in people with serious illness or injury rather than as a result of poor
dietary intake. However, protein-energy malnutrition is relatively common
in other areas of the world, especially among children, and it is associated
with decreased birth weight. It is difficult, however, to isolate the effect of
protein malnutrition from that of energy intake.
The results of most common laboratory tests used to assess protein
deficiency show changes during pregnancy. With the increase in plasma
volume, there is a decreased concentration of albumin and certain other
blood constituents. However, some blood proteins, especially those whose
levels are influenced by estrogen, increase during pregnancy. Urea nitrogen
and alpha amino nitrogen levels decrease.
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PROTEIN AND AMINO ACIDS
385
SUPPLEMENTATION STUDIES
A large body of literature documents the results of protein supplemen-
tation programs during pregnancy in regions where malnutrition is found.
Many of these studies are examined in Chapter 7. In Guatemala, studies
were conducted to determine the effects of a protein-energy supplement
and a low energy supplement on maternal and newborn outcomes among
chronically malnourished rural women (Delgado et al., 1982; Lechtig et
al., 1975, 1978) (see Chapter 7, Table 7-2B). In these widely cited stud-
ies, investigators found minimal change in birth weight and no effect on
gestational duration among women receiving either supplement. Post hoc
analysis demonstrated a significant positive effect of spontaneous energy
intake on birth weight and maternal weight gain regardless of the protein
content of the supplement.
In Colombia, investigators examined the effect of a supplement con-
taining 20 g of protein and 150 kcal of energy given to poor urban women
in Bogota. They found an approximately 50-g increase in birth weight in
the supplemented group and no effect on gestational duration (More et al.,
1979~. In Taiwan, supplementation with both protein and calories failed to
statistically increase the birth weights of infants born to poor rural women
(Adair and Pollitt, 1985; Adair et al., 1983; McDonald et al., 1981; Wohlleb
et al., 1983~.
Studies in the developed world have also demonstrated minimal
changes in birth weight as a result of protein supplementation. In the
United Kingdom, protein-energy supplementation of pregnant Asian wom-
en in Birmingham led to significantly higher maternal weight gains than did
energy supplements alone; however, only the supplement that contained
vitamins in addition to protein and energy was associated with a significant
increase in birth weight (Viegas et al, 1982a,b). Rush et al. (1980) found sig-
nificant decreases in both gestational length and birth weight and marginally
significant increases in mortality and preterm birth rate with high-densi~
protein supplementation of poor women in Harlem, New York. Adams
and colleagues (1978) reported that a high-protein supplement given to
women in San Francisco resulted in a 45-g decrease in birth weight, com-
pared with controls, and a 140-g decrease in birth weight, compared with
infants of mothers who were provided with energy supplements. Reviews of
WIC have demonstrated minimal eRects of program participation on birth
weight, gestational duration, or the incidence of low birth weight (Kennedy
and Kotelchuck, 1984; Metcoff et al., 1985; Rush et al., 1988~.*
In a comprehensive review of the literature on supplementation, Rush
and colleagues (1984) reported an inverse relationship between birth weight
*An average WIC package for pregnant women provides 90 to 1,000 kcal of energy and 40 to 50
g of protein daily.
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386
DIETARY INTAKE AND NUTRIENT SUPPLEMENTS
and protein density in supplements. An increase in prematurity has not
been generally associated with supplements that provide protein-to-energy
ratios comparable to those found in usual diets.
~ summarize, in many studies, protein-energy supplements have been
given to pregnant women in an effort to determine the effect on maternal
and fetal outcomes. In many of them, no significant changes were found
in either birth weight or gestational duration; in others, small changes
from a 30- to 100-g increase in birth weight were observed. The biologic
importance of changes of this magnitude (1 to 3% of the full-term birth
weight) is not certain. It is difficult to interpret these studies because of
variations in baseline nutritional status, composition of the supplements,
and other characteristics; it is particularly problematic to separate the effect
of the protein from that of the energy in the supplements.
RECOMMENDATIONS REGARDING SUPPLEMENTATION
On the basis of the estimated additional needs for protein and energy
during pregnancy and the usual intake of these nutrients from the U.S. diet,
the subcommittee concludes that the additional requirement for protein
during pregnancy can be met from dietary sources. Because evidence
suggests possible harm from specially formulated high-protein supplements,
the use of special protein powders or specially formulated high-prote~n
beverages should be discouraged.
CLINICAL IMPLICATIONS
· A moderate increase In the use of food sources of protein, such as
whole grains, milk, and legumes, as part of a balanced diet, is encouraged
during pregnancy since these foods are valuable sources of other nutrients.
· Assessment of adequacy of protein status is most important in
women whose energy intake Is low.
· Use of specially formulated protein supplements (e.g., protein pow-
ders) is not recommended during pregnancy.
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Representative terms from entire chapter:
birth weight
