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1 ~
Assessment of
Nutrient Needs
In considering nutrient supplementation for pregnant women, the sub-
committee reviewed nutrient requirements and evidence regarding whether
those requirements can be and are ordinarily met by dietary means. Among
the lines of evidence reviewed were overt (clinical) signs of deficiency, re-
sults of laboratory and functional tests, and dietary intake data. Special
attention was given to physiologic changes during pregnancy changes that
make it particularly difficult to assess the nutrient requirements and nutri-
tional status of pregnant women.
NUTRIENT HOMEOSTASIS
Unless the usual dietary intake of a nutrient is severely deficient, the
body is able to maintain relatively constant tissue levels of most nutrients,
even in the face of the wide variations in intake that occur from meal to
meal and day to day. The same homeostatic mechanisms also afford some
protection against longer-term, chronic marginal deficits or excesses in the
intakes of specific nutrients.
Four major types of homeostatic responses can help to maintain tissue
levels when dietary intakes are low: use of body stores (applicable to
most vitamins and minerals), an increased absorption of the nutrient (e.g.,
calcium, iron, zinc, magnesium, copper, and carotene), reduced excretion
in urine (e.g., of sodium and calcium), and a slowing down of nutrient
utilization or turnover (e.g., of protein). These responses are described in
more detail for individual nutrients in Chapters 14 to 19.
245
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246
DIETARY INTAKE AND NUTRIENT SUPPr FMENTS
Physiologic changes that occur in pregnancy stimulate some of these
homeostatic responses, regardless of the nutritional status of the mother,
thereby increasing the supply of nutrients to help meet increased demands.
Most notably, there is more efficient absorption of several minerals, such as
calcium (Heaney and Skillman, 1971) and iron (Svanberg et al., 1976a,b),
and urinary excretion of some nutrients, e.g., riboflavin, is reduced (Sauber-
lich, 1978~. However, the utilization and turnover rate of most nutrients are
probably increased during pregnancy, although there are few quantitative
data concerning this issue. To some extent, it is considered normal for
tissue stores of nutrients~specially vitamins and minerals to be drawn
upon during pregnancy. Thus, a drop in tissue levels may be of concern
only if such levels were initially low and become depleted during pregnancy,
or if there is evidence that stores are not repleted at a later date.
ASSESSMENT OF NUTRITIONAL STATUS
Nutrient deficiency or excess in an individual progresses in a continuum
from the initial signs to severe tissue pathology. One of the difficulties
in assessing nutrient status, especially in the early stages of deficiency
or excess, is the lack of precision and sensitivity in most methods. In
addition, most changes in nutritional status can be observed only if the
homeostatic mechanisms described above are unable to modulate body
nutrient levels. The likelihood that the homeostatic mechanisms will be
inadequate increases in proportion to the severity and duration of the
specific nutrient deficiency.
A variety of methods have been developed for use in the assess-
ment of nutrient status. Conceptually, they fall into two categories: static
measurements, which include determination of nutrient levels in blood or
tissues, and functional measurements, which include a wide range of tests
to determine the adequacy of nutritional status to support the functions of
subcellular constituents, cells, tissues, organs, biologic systems, or the whole
body. The advantages and limitations of static as compared with functional
measurements have been reviewed by Solomons and Allen (1983~.
In the assessment of nutritional status during pregnancy, static mea-
surements include determination of overt signs of clinical deficiency, which
are extremely rare in pregnant women in the United States and Canada;
estimates of tissue stores; and determination of levels of nutrients or
metabolites in maternal and infant blood and in other body tissues and flu-
ids. Functional measurements include determination of activity of nutrient-
dependent enzymes or amounts of hormones, nutrient-dependent metabolic
or structural changes, rates at which maternal and infant anthropometric
measurements change, and indicators of the course and outcome of preg
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ASSESSMENT OF NUTRIENT NEEDS
247
nancy, including maternal and infant health, gestational duration, birth
weight, and infant neurobehavioral development.
In nonpregnant, nonlactating women, a number of static measurements
and the first two types of functional measurements have been found to give
useful information about nutrient status' especially with regard to vitamins
and iron. However, blood levels of some nutrients are maintained within
a narrow range, e.g., the calcium level is controlled by hormones and the
magnesium level by renal reabsorption. This prevents them from being
useful in assessment of nutrient status, even in the nonpregnant state.
Assessment of the nutritional status of pregnant women is complicated
by altered nutrient needs in conjunction with shifts in plasma volume,
hormone-induced changes in metabolism, and changes in renal function and
resulting urinary excretion, as described in the Food and Nutrition Board's
report Laboratory Indices of Nutniional Status in Pregnancy(NRC, 1978~.
Nutrient concentrations in blood or plasma during pregnancy are decreased
by hemodilution, but they may be increased, decreased, or unaffected by
the concentration of carrier proteins. Placental transfer of nutrients varies
from nutrient to nutrient. The impact of these factors changes over the
course of pregnancy, so that a laboratory value that is normal during week
12 of gestation may be abnormal at week 34. Furthermore, the pattern of
change differs for each nutrient. Nutrient status parameters usually revert
to normal after delivery, suggesting that these changes reflect physiologic
adjustments to pregnancy rather than a state of deficiency (NRC, 1978~.
These complexities in the assessment and interpretation of the nutri-
tional status of pregnant women explain in part the relative sparseness of
such data and the difficulties faced by the subcommittee in evaluating the
need for nutrient supplements. Changes in static measurements, such as
blood levels of nutnents, very likely result from the physiologic changes
of pregnancy rather than from a nutrient deficiency. For this reason, the
subcommittee gave more weight to evidence showing that usual nutrient
intakes were inadequate to support optimal function than to evidence of
decreases in static levels of nutrients. For example, low serum zinc levels
might be regarded as normal during pregnancy, but if linked to low dietary
zinc intake and adverse pregnancy outcomes, this would provide a strong
argument for recommending supplementation with this nutrient.
ASSESSMENT OF NUTRIENT NEEDS
Nutrient needs can be defined as the amount of each nutrient required
in the diet to support optimal metabolism; functions of cells, tissues, and
organs; and the maintenance of adequate tissue stores. Four principal
methods have been used to estimate nutrient requirements: evidence from
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248
DIETARY INTAKE AND NUTRIENT SUPPLEMENTS
epidemiologic studies relating intake to outcome, the factorial approach,
balance studies, and nutrient turnover.
Epidemiologic approaches include observational and experimental stud-
ies, each of which is useful in investigating possible determinants of health
outcomes. Observational studies, which comprise cohort, case-control, and
cross-sectional investigations, have been used to assess associations be-
tween estimates of usual nutrient intake and measures of biologic outcome
such as anemia or infant birth weight. The cross-sectional approach has
proven to be an important method for evaluating nutrients (such as iron)
for which deficiency states are identifiable and relatively common. For
example, population-based data comparing iron intake with iron status in
nonpregnant women in the United States were used as supporting evidence
that the Recommended Dietary Allowance (RDA) for iron could safely
be lowered (NRC, 1989~. Cohort investigations have been conducted to
assess pregnancy outcomes for women with low, intermediate, and high
hemoglobin concentrations during early and midgestation (Murphy et al.,
1986~. Case-control studies, in which a sample of affected individuals
(cases) is compared with a group of individuals who are free of the dis-
order (controls), present a particularly useful approach for studying rare
pregnancy outcomes, such as neural tube defects. Finally, experimental
studies, specifically, randomized controlled clinical trials, provide a power-
ful method for determining the safety and efficacy of nutrient supplements
in preventing pregnancy complications and improving outcomes.
In the factorial approach,the total nutrient requirement is calculated
by summing the estimated amounts required by category of need. Dur-
ing pregnancy, these needs include increased requirements for maternal
tissue expansion or metabolism; placental growth; and growth, nutrient
storage, and metabolism in the fetus. An additional amount is added to
the estimated sum of nutrient requirements for these purposes to allow for
endogenous secretion and incomplete maternal intestinal absorption. This
approach has been used to estimate the maternal requirements for protein;
some minerals such as iron, calcium, and zinc; and vitamins A and C.
Balance studies are conducted to estimate nutrient requirements based
on measurements of all dietary intake and physiologic loss of the nutrient
and its metabolites. During pregnancy, a nutrient requirement is considered
to be that amount necessary to replace obligatory maternal nutrient losses
and to allow for normal growth of fetal and maternal tissues and for
accretion of nutrient stores. This approach has been used to estimate
requirements for protein and some minerals during pregnancy.
Measurements of nutrient turnover (utilization and replacement) can
provide additional information about the dynamics of nutrient metabolism.
This relatively new approach has been used to quantify the rate at which
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ASSESSMENT OF NUTRIENT NEEDS
249
some nutrients are transferred from the mother to the fetus and the rate
of nutrient turnover in both the mother and fetus.
Because each of these approaches has its limitations, there is sometimes
a lack of agreement, on both theoretical and experimental grounds, regard-
ing nutrient requirements. Observational epidemiologic studies are useful
for establishing the relationship between nutrient intake and adequacy in
population groups not in individuals. Unless other potentially important
factors are controlled effectively, the results of epidemiologic studies can
be misleading. The factorial approach requires assumptions that are not
always testable, and it neglects the dynamic nature of metabolism. Bal-
ance studies are technically tedious and difficult for many reasons; their
usefulness is limited by the body's ability to maintain balance over a wide
range of intakes (Mertz, 1987~. Turnover measurements have provided
valuable insights into metabolism, but for most nutrients, the models for
estimating requirements from such measurements have yet to be developed.
Nonetheless, these multiple approaches seine to bracket general estimates
of nutrient requirements.
ASSESSMENT OF DIETARY ADEQUACY
Dietary assessment selves as the foundation for appropriate nutrition
counseling and intervention. It enables investigators and clinicians to iden-
tify both poor and desirable food habits and dietary patterns, and thus
is fundamental in determining the risk of inadequate intakes of specific
nutrients, possibilities for dietary improvement, and the potential need for
supplementation of individual pregnant women. Food intake data can be
collected by a variety of methods, both quantitative (where the goal is
to collect accurate information on the usual total daily intake of specific
nutrients) and qualitative (such as an assessment of food groups or food
patterns). Useful reviews of dietary assessment methods include those by
Block (1982) and 1bdd et al. (1983~. However, few studies have been
conducted to validate methods for use with pregnant women (Abramson
et al., 1963; Hunt et al., 1983; Krebs-Smith and Clark 1989; Suitor et al.,
1989), and there is a need for research to develop and validate practical
methods of assessing dietary intake for use in routine prenatal care.
Quantitative intake data are useful in several areas, e.g., in nutrition
research, in the evaluation of effects of interventions, and in surveys;
however, even the most accurate methods for obtaining information on
nutrient intakes are imprecise (Medlin and Skinner, 1988; Quandt, 1986),
or they may alter food intake.
Methods for collecting quantitative data include 24-hour recalls and
food records. In the recall method, the individual is asked to recall the
types and amounts of the foods and beverages consumed during the previous
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DIETARY INTAKE AND NUTRIENT SUPP~F;MENTS
day. This is a relatively inexpensive and practical quantitative method for
collecting data about 1 day's intake, but its accuracy is limited by such
factors as memory, lack of knowledge about the ingredients used in foods,
and an inability to describe portion sizes correctly (Beaton et al., 1979~.
In the food record method, the subjects record the type and amount of
items consumed, preferably immediately after eating. One of the problems
with this method is that individuals must be cooperative and able to record
detailed information, resulting in a bias toward better-quality data on
women who are concerned about their diet and who are well educated
(Sampson, 1985~.
Estimation of intake over 1 day is very unlikely to represent a person's
usual nutrient consumption (Garn et al., 1976) because of the wide variation
in day-to-day food and nutrient intake. A single day's intake can be used
to estimate the nutrient intake of a group of individuals, but this is not the
usual intent in clinical practice. For such nutrients as energy and protein,
which are consumed relatively consistently from day to day (Beaton et al.,
1979), assessment of average intake over 3 days is sufficient to estimate
usual intake by an individual. Unfortunately, many days would have to be
sampled to obtain an accurate estimate of the usual intake of nutrients
such as vitamins A and C, for which intake is highly variable (Beaton et
al., 1983~.
For these reasons, a pregnant woman's risk of dietary inadequacy may
be assessed more efficiently and practically by a food frequency or diet
history questionnaire. In these approaches, the woman is asked (either
orally or in written form) for the usual frequency with which specific foods
are consumed over time. Both methods are useful for detecting poor
dietary patterns and low intake of specific food groups; food frequency
questionnaires have also provided a practical means of collecting data
on relationships between dietary patterns and either nutrition or health
outcomes (see the review by Sampson, 1985~. Examples of interview forms
used for this purpose have been published (Sampson, 1985; Williams,
1989~. The accuracy of the nutrient estimates may improve somewhat if
the questionnaire includes portion size (Block and Hartman, 1989), but the
addition of such questions would increase the time required to complete
the form and may make the approach less practical in the clinical setting.
If food intake information is collected carefully by the diet recall or diet
record method, it can be used to calculate the intake of specific nutrients.
This is done most efficiently by using one of the many diet analysis programs
available for either personal or mainframe computers (Frank and Pelican,
1986~. Responses to food frequency questionnaires and diet histories are
often evaluated by comparing the usual number of servings of foods in
specific food groups to a recommended number of servings (see Williams,
1989, for examples), but there are also computer programs for estimating
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ASSESSMENT OF NUTRIENT NEEDS
251
usual daily nutrient intake (NCI, 1988; Willett et al., 1985~. The best
method to use in a particular clinic depends on the purposes of the data
collection and practical considerations within the clinic. Practical methods
for collecting dietary data from pregnant women, often tailored to specific
ethnic groups, are available from the state departments of health, from
the Supplemental Food Program for Women, Infants, and Children, the
American Dietetic Association, and the American College of Obstetricians
and Gynecologists.
In general, the dietary assessment methods outlined above serve to
identify nutritionally unsound dietary practices. However, additional ques-
tions can be asked, as appropriate, to determine whether the pregnant
woman has special problems that may affect her dietary adequacy. The
following are examples of problems that may be identified by questioning
and reviewing the woman's medical/obstetric chart.
· Low income and inadequate access to food.
· Avoidance of certain types of food because of intolerance or aver-
sion, e.g., avoidance of milk because of lactose intolerance or because of
fad diets or cultural practices involving food taboos (Cassidy, 1982~.
· Adherence to completely vegetarian diets. (The diets of lacto-
ovovegetarians are more likely to be nutritionally adequate.)
· Consumption of substantial amounts of alcohol or use of tobacco
or illicit drugs (see Chapter 20~.
· A life-style that is unlikely to support adequate acquisition, prepa-
ration, or consumption of food, e.g., that of a busy professional person as
well as that of a poor woman living without a partner.
· Diet restriction in an attempt to control weight.
· The practice of pica (consumption of nonfood substances such as
laundry starch).
· Unhappiness with being pregnant.
Clinical information that may signify a risk of nutritional problems
includes:
· Substantial under- or overweight
Early teenage pregnancy
Multiple gestation (e.g., twins or triplets)
Anemia
1b the extent possible, the subcommittee recommends that poor dietary
practices be improved by appropriate interventions. These may include
general nutrition education, individualized diet counseling, and referral to
food assistance programs or to programs (e.g., the Expanded Food and
Nutrition Education Program) that promote improved food acquisition or
preparation practices. If, in the judgment of the clinician, such interventions
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DIETARY INTAKE AND NUTRIENT SUPPLEMENTS
are likely to be or have been unsuccessful, then recommendation of a
multivitamin-mineral supplement may be the only practical strategy to
improve nutrient intake. Later chapters distinguish between nutrients that
warrant careful attention and those for which intake is likely to be adequate
to meet the increased demands of pregnancy.
RECOMMENDED DIETARY ALLOWANCES
There is considerable variation in nutrient requirements among in-
dividuals within a population. In recognition of this problem, and after
considering data on both nutrient requirements and their variability by us-
ing the four approaches described above, the Food and Nutrition Board set
RDAs at a level high enough to meet the requirements of nearly all healthy
individuals in the U.S. population (NRC, 1989~. Energy recommendations
are the only exception, since intake of energy above requirements causes
obesity. The extent of the data base on which the RDAs are based varies
from nutrient to nutrient, but the needs of most pregnant women will be
met, by definition, even when their nutrient intakes fall somewhat below
the RDAs.
The subcommittee concluded that a nutrient intake that is lower than
the RDA for pregnancy, by itself, is an insufficient basis for recommending
supplementation. Given the difficulties of assessing usual nutrient intake
and nutritional status during pregnancy, the subcommittee concluded that
~ . .
~ decision lo recommend routine supplementation should be based on
evidence that usual intakes are inadequate to support optimal maternal
and fetal health and function.
Although the subcommittee did not rely on the RDAs as the sole basis
on which to judge the adequacy of usual dietary intakes of nutrients, it did
use the RDAs for several other purposes. In Chapter 13, for example, the
average nutrient intakes of pregnant women are compared with the RDAs.
If the average intake of a nutrient exceeds the RDAs, the subcommittee
is less concerned about the need for supplements. A similar approach
was used in the report Nutrition Monitonng in the United States (LSRO,
1989~. When the subcommittee recommended supplements, the RDAs
were considered in suggesting the amount of supplement to be provided.
FACTORS INFLUENCING NUTRIENT REQUIREMENTS
AND THE NEED FOR SUPPLEMENTATION
Factors that may increase nutrient needs above the ordinary demands
of pregnancy are poor nutritional status, young maternal age, a multiple
pregnancy, closely spaced births, a continued high level of physical activity,
certain disease states (e.g., malabsorption), and use of such substances
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ASSESSMENT OF NUTRIENT NEEDS
253
as cigarettes, alcohol, some legal drugs (e.g., antibiotics and phenytoin),
and illegal drugs. For women whose nutrient needs are increased by one
or more of these factors, an increased supply of selected nutrients may
be indicated. Ordinarily, this is most effectively accomplished with food,
because an increased need for specific vitamins and minerals is often
accompanied by an increased need for energy and for other nutrients not
provided in multivitamin-mineral supplements.
Attempts have been made to categorize pregnant women as being at
low, moderate, or high risk of nutrient inadequacy. Methods for making
such assignments have included comparison of food or dietary patterns
and nutrient intake with various standards or using such criteria as young
maternal age, multiple gestation, closely spaced births, and substance abuse
to identify high-risk women. Results of laboratory tests have also been used
for this purpose, but this is not a practical approach for most nutrients.
Unfortunately, there are no validated guidelines on how to use infor-
mation based on such criteria to assign women to a specific risk category,
because there has been no determination of the relative risk that the mother
or fetus will have one or more deficiencies if the mother meets such cri-
teria. Nor has it been established which individual women in a category
are likely to benefit from extra nutrients through food or supplements or
which women are likely to be harmed if there is no intervention. Whether
or not a level of dietary or supplementary intake of a nutrient reduces the
risk of pregnancy complications and adverse pregnancy outcomes can be
determined most convincingly when there are data on many women from
carefully controlled, randomized studies. Such studies are rare because of
their high cost, difficulty in recruiting subjects, and ethical considerations.
Thus, the decision to provide special dietary intervention or nutrient
supplementation must be made on an individual basis, using the best
judgment of the health professional. The information provided in later
chapters is intended to assist in such judgments.
NUTRIENI-NUTRIEN7r INTERACTIONS
Supplements pose a much greater potential problem of nutrient-
nutrient interactions than does the typical U.S. diet. An excess of one
nutrient in a supplement may interact with another nutrient in the supple-
ment or in food thereby affecting absorption or utilization adversely or,
less often, beneficially. Numerous such interactions have been identified.
The effects result from competitive biologic interactions (Hill and Matrone,
1970) and a variety of other mechanisms.
Interactions may occur at any stage in the digestion, absorption, trans-
nort. metabolism. utilization. or excretion of nutrients. Among the micronu
r -- -A -a --, ~ __ _,
tnents, there are interactions among trace elements, between vitamins and
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DIETARY INTAKE AND NUTRIENT SUPPLEMENTS
trace elements, and between vitamins. For example, iron inhibits the ab-
sorption of zinc (Hambidge et al., 1987), and zinc inhibits the absorption
of copper (Festa et al., 1985~. Major minerals such as calcium, phosphorus,
and magnesium also participate In Important Interactions among themselves
and with other nutrients. For example, calcium interferes with the absorp-
tion of both iron and zinc (Seligman et al., 1983), and protein increases
urinary calcium losses (Allen et al., 1979) and vitamin B6 requirements
(NRC, 1989~.
Subsequent chapters include further discussions of nutrient-nutrient
interactions and highlight reasons why the routine use of supplements
should be viewed with caution. It is important to avoid a situation in which
supplementation with one nutrient increases the need for another, so that
yet one more nutrient must be added to the supplement.
NUTRIENT TOXICITIES
The use of high levels (e.g., >10 times the RDA) of supplements, which
are often self-prescribed, by a substantial portion of the general public has
led to concern about nutrient toxicities. Women in the childbearing years
are among the frequent users of vitamin-mineral supplements. The vulner-
ability of the fetus to nutrient toxicity is an additional concern. An extensive
review of this topic is found in an earlier Food and Nutrition Board report
(Pitkin, 1982~. Unique features of the maternal-fetal relationship make it
difficult to predict the extent to which an excess of a nutrient will cross the
placenta, accumulate, and harm the fetus. Nutrients that potentially can
exert toxic effects include iron; zinc; selenium; and vitamins A, B6, C, and
D. Even when there is no conclusive evidence that teratogenicity or signs
of toxicity result from excessive intake of a specific nutrient, Pitkin (1982)
argued that potential for harm, in the absence of any clear benefits, is a
sound reason for caution in the use of supplements.
According to the subcommittee, pharmacologic (high) doses of nu-
trients should be prescribed during pregnancy only when there is solid
evidence of a beneficial effect, as in the treatment of vitamin dependency
states.
SUMMARY
In the laboratory and in routine clinical practice, there are serious
limitations in diagnosing states of nutritional deficiency and in determining
who may benefit from supplementation. Given that biochemical changes in
pregnancy are often poorly understood, correction and prevention of func-
tional impairments caused by nutrient deficiency are the most important
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ASSESSMENT OF NUTRIENT NEEDS
255
criteria on which to judge the need for, and benefits of, nutrient supple-
mentation. Subsequent chapters of this report provide information relative
to making decisions regarding the desirability of supplementation.
CLINICAL IMPLICATIONS
· The usefulness of laboratory tests for assessment of nutritional
status in pregnant women is limited by changes in blood levels of nutrients
or in nutrient-dependent enzymes or reactions, which most likely resect
normal physiologic changes that occur in pregnancy rather than a state of
nutrient deficiency.
· Dietary assessment Is recommended for all pregnant women in the
United States. It is important to have information on the usual dietary
intake of individual women before recommendations can be made about
their need for specific nutrient supplements.
· The most practical method of evaluating the adequacy of usual
food intake patterns is to use some type of food frequency questionnaire.
· In addition to questions about food intake, it is important to ask
pregnant women whether they have special problems that may affect their
dietary adequacy and to consider clinical information that may signify
additional nutritional risk
1b the extent possible, the approach should be to remedy poor
dietary practices by appropriate interventions. Supplements may fail to
supply all the nutrients needed, and they raise concerns about the adverse
effects of nutrient-nutrient interactions and toxicities. If recommendations
to change dietary intake are judged likely to be unsuccessful or insufficient,
then recommendation of a nutrient supplement may be indicated.
· If nutrient supplements are recommended to pregnant women for
any reason, the subcommittee urges clinicians to provide counseling at the
same time in the proper use of supplements to prevent overdosing.
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Representative terms from entire chapter:
nutritional status
