Recent Advances in Nutritional Sciences
Maternal Nutrition and Fetal
Development1,2
the past decade, compelling epidemiological studies have
linked IUGR with the etiology of many chronic diseases in
adult humans and animals (Table 1) (3). These intriguing
findings have prompted extensive animal studies to identify
the biochemical basis for nutritional programming of fetal
development and its long-term health consequences [e.g., (4 –
8)]. This article reviews the recent advances in this emerging
area of research.
Guoyao Wu,†3 Fuller W. Bazer, Timothy A. Cudd,*
Cynthia J. Meininger,† and Thomas E. Spencer
Departments of Animal Science and of *Veterinary Physiology and
Pharmacology, Texas A&M University; and †Cardiovascular
Research Institute, The Texas A&M University System Health
Science Center; College Station, TX 77843
The Intrauterine Environment as a Major Factor
Contributing to IUGR. Multiple genetic and environmen-
KEY WORDS:
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epigenetics
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fetus
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growth
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Undernutrition and IUGR. Maternal undernutrition during gestation reduces placental and fetal growth of both domestic animals and humans (1,3). Available evidence suggests
that fetal growth is most vulnerable to maternal dietary deficiencies of nutrients (e.g., protein and micronutrients) during
the peri-implantation period and the period of rapid placental
development (4 – 6). In animal agriculture, fetal undernutrition frequently occurs worldwide. For example, the nutrient
uptake of grazing ewes in the western United States is often
⬍50% of the National Research Council (NRC) requirement
(11). Unsupplemented grazing ewes lose a significant amount
of body weight during pregnancy, and their health, fetal
growth, and lactation performance are seriously compromised
(11). In pigs, a disproportionate supply of nutrients along the
uterine horn results in 15–20% low-birth-weight piglets (⬍1.1
kg), whose postnatal survival and growth performance are
severely reduced. Therefore, the poor performance of certain
livestock during the postnatal growth and finishing phases may
be a consequence of growth restriction in utero.
Undernutrition in pregnant women may result from low
intake of dietary nutrients owing to either a limited supply of
food or severe nausea and vomiting known as hyperemesis
gravidarum (12). This life-threatening disorder occurs in
1–2% of pregnancies and generally extends beyond the 16th
week of gestation (12). Pregnant women may also be at increased risk of undernutrition because of early or closelyspaced pregnancies (13). Since pregnant teenage mothers are
themselves growing, they compete with their own fetuses for
nutrients, whereas short interpregnancy intervals result in
maternal nutritional depletion at the outset of pregnancy. Low
pregnancy
Maternal nutrition plays a critical role in fetal growth and
development. Although considerable effort has been directed
towards defining nutrient requirements of animals over the
past 30 y, suboptimal nutrition during gestation remains a
significant problem for many animal species (e.g., cattle, pigs,
and sheep) worldwide (1). Despite advanced prenatal care for
mothers and fetuses, ⬃5% of human infants born in the U.S.
suffer from intrauterine growth retardation (IUGR)4 (2). Over
1
Manuscript received 14 May 2004.
Supported in part by grants from USDA (2000 –2290, 2001– 02259 & 2001–
02166) and NIEHS P30-ES09106.
3
To whom correspondence should be addressed. E-mail: g-wu@tamu.edu.
4
Abbreviations used: BH4, tetrahydrobiopterin; IUGR, intrauterine growth
retardation; NO, nitric oxide; NOS, nitric oxide synthase; NRC, National Research
Council; ODC, ornithine decarboxylase; SAM, S-adenosylmethionine.
2
0022-3166/04 $8.00 © 2004 American Society for Nutritional Sciences.
2169
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tal factors contribute to IUGR (1). Although the fetal genome
plays an important role in growth potential in utero, increasing evidence suggests that the intrauterine environment is a
major determinant of fetal growth. For example, embryotransfer studies show that it is the recipient mother rather than
the donor mother that more strongly influences fetal growth
(9). There is also evidence that the intrauterine environment
of the individual fetus may be of greater importance in the
etiology of chronic diseases in adults than the genetics of the
fetus. For instance, in twin pregnancies, a baby with fetal
growth retardation is more likely to develop noninsulin dependent (type-II) diabetes mellitus than a sibling with normal
fetal growth (10). Among intrauterine environmental factors,
nutrition plays the most critical role in influencing placental
and fetal growth (3).
ABSTRACT Nutrition is the major intrauterine environmental factor that alters expression of the fetal genome
and may have lifelong consequences. This phenomenon,
termed “fetal programming,” has led to the recent theory of
“fetal origins of adult disease.” Namely, alterations in fetal
nutrition and endocrine status may result in developmental
adaptations that permanently change the structure, physiology, and metabolism of the offspring, thereby predisposing individuals to metabolic, endocrine, and cardiovascular
diseases in adult life. Animal studies show that both maternal undernutrition and overnutrition reduce placentalfetal blood flows and stunt fetal growth. Impaired placental
syntheses of nitric oxide (a major vasodilator and angiogenesis factor) and polyamines (key regulators of DNA and
protein synthesis) may provide a unified explanation for
intrauterine growth retardation in response to the 2 extremes of nutritional problems with the same pregnancy
outcome. There is growing evidence that maternal nutritional status can alter the epigenetic state (stable alterations of gene expression through DNA methylation and
histone modifications) of the fetal genome. This may provide a molecular mechanism for the impact of maternal
nutrition on both fetal programming and genomic imprinting. Promoting optimal nutrition will not only ensure optimal fetal development, but will also reduce the risk of
chronic diseases in adults. J. Nutr. 134: 2169 –2172, 2004.
WU ET AL.
2170
TABLE 1
Hormonal imbalance, metabolic disorders, and diseases in
adult animals and humans with prior experience
of intrauterine growth restriction
Hormonal imbalance
Increased plasma levels of glucocorticoids and renin; decreased
plasma levels of insulin, growth hormone, insulin-like growth
factor-I, and thyroid hormones
Metabolic disorders
Insulin resistance, -cell dysfunction, dyslipidemia, glucose
intolerance, impaired energy homeostasis, obesity, type-II
diabetes, oxidative stress, mitochondrial dysfunction, and aging
Organ dysfunction and abnormal development
Testes, ovaries, brain, heart, skeletal muscle, liver, thymus, small
intestine, wool follicles, and mammary gland
Cardiovascular disorders
Coronary heart disease, hypertension, stroke, atherosclerosis
Overnutrition and IUGR. Significant health problems for
animals (particularly companion animals) and women of reproductive age also result from being overweight or obese due
to overeating. Overnutrition can result from increased intake
of energy and/or protein. Extensive studies have shown that
maternal overnutrition retards placental and fetal growth, and
increases fetal and neonatal mortality in rats, pigs, and sheep
(14). Results of recent epidemiological studies indicate that
almost 65% of the adult population in the U.S. is overweight
[defined as a body mass index (BMI) ⬎ 25 kg/m2], while 31%
of the adult population is obese (defined as BMI ⬎ 30 kg/m2)
(15). Many overweight and obese women unknowingly enter
pregnancy and continue overeating during gestation (16).
These women usually gain more weight during the first pregnancy and accumulate more fat during subsequent pregnancies. Maternal obesity or overnutrition before or during pregnancy may result in fetal growth restriction and increased risk
of neonatal mortality and morbidity in humans (16).
Health Problems Associated with IUGR. IUGR causes
both perinatal and neonatal medical complications. For example, IUGR is responsible for about 50% of nonmalformed
stillbirths in humans (2). Infants who weigh ⬍2.5 kg at birth
have perinatal mortality rates that are 5 to 30 times greater
than those of infants who have average birth weights, while
those ⬍1.5 kg have rates 70 to 100 times greater (2). Surviving
infants with IUGR are often at increased risk for neurological,
respiratory, intestinal, and circulatory disorders during the
neonatal period. Both epidemiological and experimental evidence suggest that IUGR contributes to a wide array of metabolic disorders and chronic diseases in adults (Table 1). For
example, individuals exposed to the Dutch winter famine of
1944 –1945 in utero had higher rates of insulin resistance,
vascular disease, morbidity, and mortality in adulthood (17).
A cohort study of 15,000 Swedish men and women born
Biochemical Mechanisms of IUGR. The lack of knowledge about the mechanisms of IUGR has prevented the development of effective therapeutic options, such that the
current management of growth-restricted infants is empirical
and is primarily aimed at selecting a safe time for delivery (2).
Because nutritional and developmental research often involves invasive tissue collections and surgical procedures, it is
neither ethical nor practical to conduct these experiments
with the human placenta and fetus. Thus, animal models (e.g.,
mice, rats, pigs, and sheep) are instrumental for defining the
mechanisms of IUGR and developing therapeutic means.
Available evidence, which is discussed in the following sections, suggests that arginine [a nutritionally essential amino
acid for the fetus (19)] plays a key role in development of the
conceptus (embryo/fetus, associated placental membranes, and
fetal fluids).
Crucial Roles of NO and Polyamines in Placental
and Fetal Growth. Arginine is a common substrate for nitric
oxide (NO) and polyamine syntheses via NO synthase (NOS)
and ornithine decarboxylase (ODC) (19). NO is a major
endothelium-derived relaxing factor, and plays an important
role in regulating placental-fetal blood flows and, thus, the
transfer of nutrients and O2 from mother to fetus (20). Likewise, polyamines regulate DNA and protein synthesis, and
therefore, cell proliferation and differentiation (19,21). Thus,
NO and polyamines are key regulators of angiogenesis (the
formation of new blood vessels from preexisting vessels) and
embryogenesis (22), as well as placental and fetal growth (Fig.
1). These crucial roles of NO and polyamines are graphically
illustrated by the following findings. First, inhibition of NO
synthesis by NOS inhibitors in rats or the absence of NO
synthesis in eNOS-knockout mice results in IUGR (23). Second, inhibition of polyamine synthesis prevents mouse embryogenesis, and inhibition of placental polyamine synthesis reduces placental size and impairs fetal growth (21). Third,
IUGR in humans is associated with impaired whole body NO
synthesis (24) and with decreases in arginine transport, eNOS
activity, and NO synthesis in umbilical vein endothelial cells
(25). Finally, maternal arginine deficiency causes IUGR, increases fetal resorption and death, and increases perinatal
mortality in rats, whereas dietary arginine supplementation
reverses fetal growth restriction in rat models of IUGR induced by hypoxia or inhibitors of NOS (26).
Unusual Abundance of the Arginine-Family Amino
Acids in the Conceptus. We recently discovered that arginine is particularly abundant in porcine allantoic fluid (4 –5
mmol/L) at d 40 of gestation (term ⫽ 114 d), when compared
with its maternal plasma level (0.13– 0.14 mmol/L) (27). Remarkably, concentrations of arginine and its precursor ornithine in porcine allantoic fluid increase by 23- and 18-fold,
respectively, between Days 30 and 40 of gestation, with their
nitrogen accounting for ⬃50% of the total free ␣-amino acid
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birth weights and preterm deliveries in adolescent pregnancies
are more than twice as common as in adult pregnancies, and
neonatal mortality in adolescent pregnancies is almost three
times higher than for adult pregnancies (13). Further, placental insufficiency results in reduced transfer of nutrients from
mother to fetus, thereby leading to fetal undernutrition and
IUGR (1). Finally, due to competition for nutrients, multiple
fetuses resulting from assisted reproductive technologies are
often at risk of undernutrition and therefore fetal growth
restriction (2). Thus, various nutritional and pathological conditions can result in IUGR.
between 1915 and 1929 provides by far the most convincing
evidence for the close association between reduced fetal
growth rate and increased risk of death from ischemic heart
disease (18). Thus, the intrauterine environment of the conceptus may alter expression of the fetal genome and have
lifelong consequences. This phenomenon is termed “fetal programming,” which has led to the recent theory of “fetal origins
of adult disease” (3). Namely, alterations in fetal nutrition and
endocrine status may result in developmental adaptations that
permanently change the structure, physiology and metabolism
of the offspring, thereby predisposing individuals to metabolic,
endocrine, and cardiovascular diseases in adult life.
NUTRITION AND FETAL GROWTH
nitrogen in allantoic fluid (27). Most recently, we found that
citrulline (an immediate precursor of arginine) is very rich (10
mmol/L) in ovine allantoic fluid at Day 60 of gestation (term
⫽ 147 d) (28). Concentrations of citrulline and its precursor
glutamine in ovine allantoic fluid increase by 34- and 18-fold,
respectively, between Days 30 and 60 of gestation, with their
nitrogen representing ⬃60% of total ␣-amino acid nitrogen in
ovine allantoic fluid (28). The unusual abundance of the
arginine-family amino acids in fetal fluids is associated with
the highest rates of NO and polyamine syntheses in ovine
placentae in the first half of pregnancy (29,30), when their
growth is most rapid (1). These novel findings support the
proposed crucial roles of the arginine-dependent metabolic
pathways in conceptus development (Fig. 1).
IUGR and Impaired Syntheses of NO and Polyamines in the Conceptus. Maternal undernutrition and
hypercholesterolemia during pregnancy (frequently occurring
in obese subjects) have profound effects on the synthesis of
NO and polyamines. For example, feeding a low-protein diet
to pregnant pigs decreases arginine concentration by 21–25%
in fetal plasma, allantoic fluid, and placenta, at Day 60 of
gestation (31). In addition, allantoic fluid concentrations of
arginine and ornithine decrease by ⬃45% in hypercholesterolemic pigs, compared with normocholesterolemic pigs, at Day
40 of gestation (31). Further, placental NOS and ODC activities are 40 – 45% lower in protein-deficient pigs than in protein-adequate pigs (4). Similarly, placental NOS activity is
reduced by 26% in hypercholesterolemic pigs compared to
normocholesterolemic pigs (4). The decreases in substrate
availability and enzyme activity contribute to impaired placental syntheses of NO and polyamines in both protein-deficient and hypercholesterolemic pigs (4,31).
Maternal undernutrition in sheep (50% of NRC requirements) between Days 28 and 78 of gestation decreases (P
⬍ 0.05) concentrations of arginine, citrulline, and polyamines
in maternal plasma, fetal plasma, and allantoic fluid by 23–
30% at Day 78 of gestation (32). Notably, concentrations of
biopterin [an indicator of de novo synthesis of BH4 (an essential cofactor for NOS)] in fetal plasma, amniotic and allantoic
fluids are reduced by 32–36% in underfed ewes, compared with
control ewes (G. Wu, Texas A&M University, College Station, TX, unpublished results), indicating reduced availability
of BH4 for NO production in the conceptus. These changes
would impair placental and fetal NO synthesis, thereby resulting in reduced placental-fetal blood flows in underfed ewes (1).
Consistent with these findings, maternal undernutrition impairs NO-dependent vasodilation and increases arterial blood
pressure in the ovine fetus (33). Similarly, uterine and umbilical blood flows are reduced in overnourished adolescent sheep
(14), suggesting a reduction in NO generation by vascular
endothelial cells of the uterus and placentae. In obese subjects,
high levels of low-density lipoprotein and/or hypercholesterolemia are expected to impair endothelial NO synthesis
through mechanisms involving: 1) reduced availability of BH4
likely due to oxidative stress; 2) reduced expression of NOS;
and 3) inactivation of NOS due to its close association with
caveolin-1 (34). These results have led to our hypothesis that
impaired placental syntheses of NO and polyamines may provide a unified explanation for IUGR in response to the two
extremes of nutritional problems with the same pregnancy
outcome (Fig. 1).
Molecular Mechanisms of Fetal Programming. Nutritional insult during a critical period of gestation may leave a
permanent “memory” throughout life, and some of the effects
(e.g., insulin secretion and action) may be gender-specific (5).
There is growing evidence that maternal nutritional status can
alter the epigenetic state of the fetal genome and imprint gene
expression. Epigenetic alterations (stable alterations of gene
expression through covalent modifications of DNA and core
histones) in early embryos may be carried forward to subsequent developmental stages (6). Two mechanisms mediating
epigenetic effects are DNA methylation (occurring in 5⬘positions of cytosine residues within CpG dinucleotides
throughout the mammalian genome) and histone modification
(acetylation and methylation) (35). CpG methylation can
regulate gene expression by modulating the binding of methylsensitive DNA-binding proteins, thereby affecting regional
chromatin conformation. Histone acetylation or methylation
can alter the positioning of histone-DNA interactions and the
affinity of histone binding to DNA, thereby affecting gene
expression (35).
DNA methylation is catalyzed by DNA methyltransferases,
with S-adenosylmethionine (SAM) as a methyl donor (35).
SAM is synthesized from methionine and ATP by methionine
adenosyltransferase. One-carbon unit metabolism, which depends on serine, glycine, and B vitamins (including folate,
vitamin B-12, and vitamin B-6), plays an important role in
regulating the availability of SAM (6). Thus, DNA methylation and histone modifications may be altered by the overall
availability of amino acids and micronutrients. This notion is
supported by several lines of evidence. First, a deficiency of
amino acids results in marked reduction in genomic DNA
methylation and aberrant expression of the normally silent
paternal H19 allele (an imprinted gene) in cultured mouse
embryos (36). Second, uteroplacental insufficiency causes hypomethylation of p53 gene in postnatal rat kidney (7), as well as
global DNA hypomethylation and increased histone acetyla-
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➡
FIGURE 1 Proposed mechanisms for fetal growth restriction in
underfed and overfed dams. Both maternal undernutrition and overnutrition may impair placental syntheses of NO and polyamines, and
therefore placental development and utero-placental blood flows. This
may result in reduced transfer of nutrients and O2 from mother to fetus,
and thus fetal growth restriction. mTOR, mammalian target of rapamycin. The symbol “ ” denotes reduction.
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WU ET AL.
2172
tion in postnatal rat liver (8). Third, maternal supplementation of methyl donors and cofactors (folic acid, vitamin B-12,
choline, and betaine) increases CpG methylation at the Avy
locus of agouti mice, and the methylation patterns are retained
into adulthood (6). It remains to be determined whether
maternal nutrition affects CpG methylation of the genes for
NOS, GTP cyclohydrolase I (the rate-limiting enzyme for BH4
synthesis) and ODC, or alters histone modifications, in the
uterus, placenta, as well as fetal and postnatal tissues (e.g., the
vascular bed, adipose tissue, liver, kidney, skeletal muscle, or
pancreas). Nevertheless, epigenetics may provide a molecular
mechanism for the impact of maternal nutrition on fetal
programming of postnatal disease susceptibility and on
genomic imprinting (the parent-of-origin-dependent expression of a single allele of a gene) (6 – 8).
Concluding Remarks and Perspectives. Placental and
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