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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
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
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
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
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
250 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
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
252 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
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
254 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
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. REFERENCES Abramson, J.H., C. Slome, and C. Kosovsky. 1963. Food frequency interview as an epidemiological tool. Am. J. Public Health 53:1093-1101. Allen, L.H., E.A. Oddoye, and S. Margen. 1979. Protein-induced hypercalciuria: a longer term study. Am. J. Clin. Nutr. 32:741-749. Beaton, G.H., J. Milner, P. Corey, V. McGuire, M. Cousins, E. Stewart, M. de Ramos, D. Hewitt, P.V. Grambsch, N. Kassim, and J.N Little. 1979. Sources of variance in 24-hour dietary recall data: implications for nutrition study design and interpretation. Am. J. Clin. Nutr. 32:2546-2559. Beaton, G.H., J. Milner, V. McGuire, T.E. Feather, and J.N Little. 1983. Source of variance in 24-hour dietary recall data: implications for nutrition study design and interpretation. Carbohydrate sources, vitamins, and minerals. Am. J. Clin. Nutr. 37:986-995.
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