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Statement Concerning Application of the Recommended Method D. M. Hegsted The subcommittee is in agreement that under idea, conditions the probability approach which is recommended appears to offer the best available approach to esti- mating the proportion of the population at risk of nut ri tonal deficiency from dietary survey data. As the report notes, dietary surveys are used as a basis for determining the magnitude of inadequate nutrition, serve as a basis for food assistance programs, etc., and a general instrument for formulation of nutrition policy. I believe that the recommendation that the probability approach be adopted is premature. The method has not been adequately tested. It appears likely that this approach will grossly overestimate the extent of inade- quate nutrition in the country and, thus, lead to inap- propriate policy decisions. - The two major requirements for the application of the probability approach are a) that there be available reasonably accurate estimates of the mean requirement for each nutrient and an estimate of the range of require- ments within the population group, and b) that the survey methodology yield satisfactory estimates of the usual nutri- ent intakes of the group. ~ -Dissenting statements prepared by individual members of a committee are not subject to the normal National Research Council review processes, nor are they subject to committee or staff editing or review. They appear exactly as the dis- senting committee members prepare them. The Research Coun- cil neither endorses nor takes responsibility for the con- tent of the statements. 104
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105 NUTRIENT REQUIREMENT INFORMATION Currently available estimates of the mean require- ment for any nutrient are confined to few age-sex groups, usually young adult men or women. Presumably the dietary survey data will be analyzed according to the format of the Recommended Dietary Allowances which provide values for 17 or more age-sex groups. If so, "requirement curves for each nutrient for each of these age-sex groups must be inferred from the limited data available. Just how this can be done to yield reliable values is unclear and one must assume that such derived values will be less accurate than values based upon actual data. There are even less data on the range of require- ments within any age-sex group. The report places con- siderable reliance on the assumption that protein require- ments determined by nitrogen balance have a standard devia- tion of about 15% of the mean value. It must be empha- sized that this value is the total variance of require- ments determined by this technique which includes the variation due to error as well as biologic variation. It is the latter which is of importance. Since the estimated mean requirement for nitrogen balance may vary by as much as 50% depending upon the experimental protocol, particu- larly the prior diet of the individual subjects, it is apparent that the error term must be large and one must assume that the true biologic variation is relatively small. In all estimates of the variability in nutrient requirements derived from compilation of values in the literature the error term must be similarly large since the experimental protocol, background of the subjects, etc., differ. The other example of differences in requirements is based upon blood losses in women of the childbearing age which can be translated into differences in iron require- ments. The utility of this example, however, is compro- mised by many factors which influence iron absorption, such as the adaptive response of the individual, the form of iron in the diet, and the amounts of various factors which promote or inhibit iron absorption. Indeed, it appears that such factors are more important in deter- mining the availability of iron than the amount of iron actually consumed. One may make general assumptions about average levels of iron and of these f actors in the diet in establishing recommended levels of iron intake, but these
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106 have little applicability for the individual. We have no methodology for including these factors in the probability approach, and estimates of the proportion of the population at risk of iron deficiency inevitably suffer. The recent analysis of the extent of iron deficiency in the American population for the NHANES data indicates that the proba- bility approach overestimates the extent of the problem. Currently available information on the nutritional status of Americans and recent developments in nutrition inevitably shift the interest in dietary surveys and how they should be interpreted. Nutritional deficiency disease is clearly not a major public health problem. Ate major chronic diseases are the major public health problem, and these are nutrition related. Interest in dietary surveys in the future should focus on these problems rather than nutritional deficiency. Thus, to varying degrees, the report focuses attention on the wrong problem and fails to indicate how the survey data should be examined relative to these more important issues. Nutritional standards related to excessive consump- tion of fat, cholesterol, sugar, or salt can probably not be derived by attempting to develop curves related to requirements or toxicity. Rather, these are devel- oped by attempting to accommodate desirability, feasibil- ity, acceptability, etc. Whether approaches similar to those outlined for the probability approach can be adapted to these issues is unclear at this time. Recent evidence implicates vitamin A, carotene, and vitamin C in the etiology of cancer. If intake of these materials is relevant to cancer, it will have a profound effect upon the interpretation of dietary surveys since even modest effects upon cancer prevalence are immeasur- ably more important than the defined nutritional defi- ciencies. Yet the likelihood that one can develop reli- able estimates of requirements to prevent cancer seems relatively remote at this time, and the probability approach can probably not be applied. The subcommittee notes that for a considerable nether of nutrients the information on requirements and/or range of requirements is virtually nonexistent, the probability approach cannot be applied, and estimates of the proportion of the population at risk cannot be made. No doubt this
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107 is true, but it seems rather unlikely that these nutrients can be simply ignored in evaluating dietary survey data. Finally, the subcommittee recommends that research to improve knowledge of nutrient requirements be expanded in order to make the application of the probability approach more feasible. No doubt this is desirable, but given the fact that nutritional deficiency does not appear to be a major public health issue, it is quite unlikely that such research can be a national research priority except for a few nutrients. Thus it seems unlikely that the necessary data required for more accurate application of the proba- bility approach will be available any time in the near future. ESTIMATES OF NUTRIENT INTAKE The report recognizes that the reporting of food intake is subject to gross errors but, in my opinion, falls to adequately consider the impact of such errors on the relia- b~lity of the estimated extent of malnutrition. The application of the probability approach rests upon the assumption that the errors in reported food intake and calculated nutrient intake are randomly distributed, i.e., that it is equally likely that under- and overreporting occur with equal frequency and extent. No doubt the reliability of the data collected in any survey will depend upon the methodology used, but most of the data available do not support the supposition and, even if it is true that mean intakes are correctly reported (over- and underreporting are equal), the method will still apparently overestimate the extent of undernutrition. Chapter 6 refers to many of the reports which have attempted to validate the reliability of dietary recall or food records. While the conclusions vary, a large number of investigators find that such methods often yield grossly low estimates of energy consumption, in some cases to the surprising extent of 30 to 40 percent underestimate. It should be noted that most such estimates refer to energy intake only. It can be assigned that total energy intake will be similar whether the subjects are free-living or under controlled conditions if weight is maintained and physical activity is similar. It is much more difficult, or perhaps impossible, to obtain reliable estimates of intake of other nutrients in free-living subjects. It is
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108 reasonable to assume that most subjects will be more likely to forget to recall or record food eaten than they will list foods not eaten. Furthermore, most of the literature reports have obviously been obtained with relatively well- motivated subjects. This cannot be assumed in large scale surveys where motivation may be minimal, and instructions to the subjects will generally be less extensive than in well-designed studies. However, even if the mean intake of a group is cor- rectly recorded, it still seems likely that the proba- bility approach will yield excessive estimates of the extent of malnutrition. It is certain that a substantial part of the survey population will underestimate their individual average intake either because of error or because the two or three days derring which intake is measured were atypically low. When nutrient intakes vary greatly from one day to the next as they do in the U.S., often 100%, it is inevitable that a substantial number of atypical intakes will be recorded. The probability approach (or any other approach that has been suggested) is concerned only with the nether of low intakes, the "tail end" of the distribution of intakes. These remain to be counted as in risk of deficiency even though over- estimates might yield a correct mean value for the group. The calculation of "usual intakes" by subtracting the intraindividual variance from the total variance no doubt diminishes the extent of erroneously recorded low intakes, but when data are available for only two or three days, it is not eliminated. The consumption, for example, of a potent source of carotenoids once a week might provide an adequate intake of vitamin A. Another criticism of the probability approach is that it provides no estimate of the severity of deficiency. Subjects whose consumption is only modestly below standard are grouped with those well below standard. The report recommends that multiple standards be applied in order to provide an estimate of severity but, at this time, only the RDAs are available as standards. We can anticipate obvious problems if agencies establish their own "require- ment curves to meet their own needs or expectations.
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109 CONCLUSION It appears to me that the probability approach rests upon a weak foundation both with respect to the data on nutrient requirements and, especially, the survey meth- odology. Most of the errors, biases, and variability in the data collected are likely to result in substantial overestimates of the extent of undernutrition. The cal- culated examples in the body of the report do not appear to be reassuring since, except for protein, they indicate large numbers at risk of apparently all other nutrients examined. his seems inconsistent with the reasonably well-based conclusion that nutritional deficiencies are not a public health problem of any magnitude in this country. The probability approach gives the impression that the estimates of undernutrition are rather precise since the utility of the extensive statistical calculations is not apparent otherwise. The identification of such "problems" will presumably call for public health solutions. Unless the "problems" are real, which I doubt, this will result in large expenditures of money and effort with little or no benefit. Indeed, no solution appears to be in sight nor are there other data indicating that the problems exist. Dietary surveys are useful in indicating food pat- terns and provide data for rough comparisons of differ- ences between groups, time trends in food consumption, etc. Until some data are available which indicate that the estimates of undernutrition obtained by application of the probability approach are available, however, I do not believe we should recommend its application. The most appropriate reck foundations of the subcommittee should be that the probability approach deserves further study but, at this tome, the extent of under- or malnutrition can not be determined from dietary survey data.
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