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Nutrient Requirements as ~ Basis
for Dietary Evaluation
When using nutrient requirements as a basis for dietary
evaluation, a number of factors must be considered. For
example, there is great variability among similar people
as well as different interpretations of adequacy and
deficiency. All these factors must be considered in
order to use nutrient requirements most effectively.
VARLa8ILITY OF NUTRIENT REQUIREMENTS
Like all other biological features, nutrient requ~re-
ments vary among se ~ ngly similar persons. Although
variability generally applies to all nutrients, its nature
is known to be specific to certain nutrients.
Dietary standards are usually described as recommended
dietary allowances, safe levels of intake, or other simi-
lar terms. A distinction must be made between these and
the term requirements, which is used in this report. Over
the past few decades, groups charged with the development
of standards for dietary intake recognized the variability
of nutrient requirements; nevertheless, they designated a
single point in the distribution as the recommended dietary
allowance (RDA) (FAO/WHO/UNU, in press; Health and Welfare,
Canada, 1983; NRC, 1980). For protein, for example, a sin-
gle point was chosen to estimate the dietary intake level
adequate to meet the needs of almost all healthy persons in
a specified age or sex group. The point was established
after examining the data base for each nutrient and then
making scientific judgments about the position and nature
of the requirement distribution. By definition, the point
chosen to meet the needs of almost all persons lies near
the upper tail of the requirement distribution (NRC, 1974).
10
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When there is information on which to base inferences about
the actual distribution, as for protein, the mean plus 2 SD
has been identif fed as the recommended intake ( FAO/WHO/UNU,
-in press; Health and Welfare, Canada, 1983; NRC, 1980).
For most nutrients, the distribution has not been explic-
itly described, and the relationship between recommended
intake and requirement distribution has not been explored
in detail. me principle still holds, however, that the
recommended intake level generally exceeds the estimated
requirements and, hence, the needs of almost all persons.
Dietary standards for energy intake are different from
those for specific nutrients because the level published
for energy is usually the estimated mean requirement, i.e.,
one-half of the persons are expected to have higher needs
and one-half, lower needs (FAO/WHO/UNU, in press; Health
and Welfare, Canada, 1983). Some reports provide a range
around the median energy requirement (NRC, 1980); others
present an estimate of the variance of energy requirements
(FAO/WHO/UNU, in press; Health and Welfare, Canada, 1983).
Nutrient requirements of specific persons can only be
expressed by referring to the probability (FAO/WHO/UNU, in
press) or likelihood that each level of observed intake is
inadequate. In a probability approach, therefore, the
underlying distribution of requirements among similar
persons must be recognized. This is in contrast to using
a fixed cutoff point delineating inadequate from adequate
nutrient intake, which fails to recognize those persons
whose intake and requirement may both be below the cutoff
point. If the cutoff point is set below the RDA, both
intake and requirement of some persons may be higher than
the cutoff point.
The period during which a specified requirement must
be met is seldom defined (FAO/WHO/UNU, in press; Health
and Welfare, Canada, 1983). Requirement estimates are
usually related more generally to levels of usual or
habitual intake (FAO/WHO/UNU, in press; Health and
Welfare, Canada, 1983; ARC, 1980). They do not refer to
intake on a particular day, unless that is a reliable
measure of the usual intake. Nutrient requirements as
used for the analysis in this report consist of a aistri-
bution of usual dietary intakes required to maintain an
adequate or acceptable physiological or nutritional state.
For a few nutrients, requirement information simply is
not available, and meaningful analysis of the adequacy
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of dietary intake must await the development of further
knowledge of requirement distributions. For others, there
is some information, but its precision may be low.
Nevertheless, coupling available information about
requirement with other information about factors expected
to affect requirement can permit the development of
informed judgments about requirements for various age and
sex groups. As a result, it should be feasible to use a
probability approach to improve present inferences about
the adequa Q of dietary intake. It is important that
priority be given to the nutrients that are most likely to
present public pow cy problems in the United States.
For a few nutrients and age groups, better information
about requirements appears to be available, and one can
be more confident in their application. On the whole,
however, there is a clear need for research on nutrient
requirements. It is important that priority be given to
shone nutrients which are most likely to present public
policy problems in the United States. Such refinement
should permit the development of improved statistical
approaches to survey interpretation through the use of
information about nutrient requirements. Until exact
information about requirements is available, the resultant
inferences about prevalence must be considered imprecise,
although the probability approach is superior to other
possible methods.
Information about mean requirements and characteristics
of their distributions for some but not all nutrients may
be found in the reports of committees charged with devel-
oping recommended intakes. However, the development and
presentation of information needed for all nutrients have
not been included in the mandate to such committees. This
does not mean that the info` - ation is unavailable but,
rather, that it may be necessary to undertake a special
effort to examine the literature and develop the required
data bare. m e present subcommittee die not attempt such
a search and notes that skills and experience not repre-
sented in its membership would be needed to perform the
requisite task.
LEVELS OF REQUIREMENT
Multiple definitions of adequacy are possible ( ee ge r
the prevention of clinical deficiency symptoms, the main-
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tenance of specified levels of the nutrients or their
metabolites in tissues, the maintenance of enzyme activity
at specified levels), and each of these could be associ-
ated with a different dietary requirement. Thus, it is
possible to establish a family of requirement curves
marking different definitions of adequacy.
Estimates of average requirement based on these dif-
ferent criteria can be derived from the early nutrition
literature. The earliest marker for nutritional adequacy
was the prevention of clinically detectable signs of mal-
function. Estimates based on deficiency avoidance were
provided in some early dietary standards, along with
recommended intakes for improved nutrition, e.g., in the
1963 Dietary Standard for Canada (Committee on Revision of
the Canadian Dietary Standard, 1964). Recent RDA reports
(NRC, 1980) provide some of the information needed for the
proposed approach. Such information can also be found in
reports issued by FAO and WHO (e.g., FAD/WHO, 1961, 1970).
Throughout the world, dietary standards and recommended
intakes are based on basic philosophies that differ in
detail ( IONS, 1983a,b) but have ~ similar goal of estab-
lishing levels of intake that will maintain a state of
nutriture beyond the mere prevention of clinical deficiency
disease. For example, adequate iron intakes are regarded
as those that maintain reasonable iron stores rather than
those that merely stabilize mild anemia or maintain hemo-
globin at physiologically normal levels (FAD/WHO, 1970;
Health and Welfare Canada, 1983; NRC, 1980). Similarly,
vitamin C requirements have been set at a level that is
sufficient to establish and maintain metabolic pools
(Health and Welfare Canada, 1983; NRC, 1980) or to main-
tain tissue levels (FAD/WHO, 1970) rather than just to
prevent scurvy. Requirements for other nutrients are
determined in analogous ways. The criteria selected for
the same nutrient often vary between reports, even within
the some country (e.g., ascorbic acid in Canada).
Levels of intake and requirement estimates lower than
those given in many recently published dietary standards
appear to be consistent with the absence of clinical signs
of ill health. Thus, some definitions of requirement may
be more desirable that others, depending on the purposes
of the dietary assessment. Interpretations of dietary
intake data in relation to estimated requirements require
consideration of the particular biochemical, physical,
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clinical, or functional criteria that were used to
establish the requirement.
A multilevel assessment procedure can be used by devel-
oping a series of requirement distribution estimates, each
referring to a defined criterion of adequacy. By using
the several requirements, one can calculate a series of
estimates of the prevalence of inadequate intakes. A1-
though the RDA reports may not provide the appropriate
information for determining such a family of requirement
estimates, this absence does not mean that the information
is not available. Such information may not have been pre-
sented because those reports are intended primarily for
use in developing a single requirement to meet the needs
of all healthy individuals.
FIXED CUTOFF POrNTS
It is a common practice to use fixed cutoff points to
estimate the adequacy of nutrient intake. In this method,
estimates of the prevalence of inadequate intake have been
based on a fixed proportion of the RDA. The rationale has
been based on the recognition that the RDAs, designed to
include virtually all healthy individuals, have included
margins of safety--often generous margins. Hence, if
applied as a criterion, an RDA would clearly lead to an
overestimate of the prevalence of inadequacy. Therefore,
the proportions selected as cutoffs have varied--somet~mes
two-thirds, sometimes three-quarters, and sometimes 70~.
There has not been a clear rationale for the selection of
the level.
The use of fixed cutoff points is conceptually similar
to population-based screening for unrecognized disease
(e.g., Rogan and Gladen, 1978). The well-known terms used
to describe problems of medical screening are similar to
those encountered with the use of cutoff points (Habicht,
1980). Thus, both the teems and the screening approach
have been used to examine the fixed cutoff point method.
Regardless of the cutoff point selected, some persons
who meet their nutrient requirement will be identified as
having inadequate intake. Conversely, some who do not
meet their requirement will be identified as adequately
nourished. The term sensitivity is applied to the ability
of a test to detect truly affected individuals; specifi-
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city refers to the ability of a test to identify truly
unaffected individuals. Misclassification occurs when
people are designated not at risk when they are truly
affected by the condition (false negatives) or at risk
when they are actually unaffected (false positives).
statistical decision~making, a similar concept is used,
with sensitivity and specificity corresponding to Type I
(a ) and Type II (D ) errors in hypothesis testing. In
Figure 3-l, the distribution of those who truly fail to
meet their requirement and the distribution of those who
truly meet their requirement are plotted. For the purpose
of illustration, it is assumed that persons can be cla~si-
fied with an absolutely accurate test.
AREA OF
OVER LAP
I NAD EQUATE
ADEQUATE
INTAKE OF NUTR I ENT X
FIGURE 3-1. The distributions of people who truly fail
to meet their requirement (inadequate) and
those who truly meet it (adequate) for a
hypothetical nutrient X.
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This figure is useful in gaining an understanding of
the implications of the fixed cutoff point approach.
Presumably, a cutoff point would be selected somewhere
in the area of.overlap between the distribution of truly.
adequate and truly inadequate intake. For this tube done,
the sensitivity and specificity of the cutoff point would
have to be detenmlned (Habicht et al., 1982). Brownie and
Habicht (1984) have developed a strategy for selecting the
optimal cutoff point under certain conditions.
To most important conclusion from these considerations
{Brownie and Habicht, 1984) is that the choice of the
optimal cutoff point to estimate prevalence or changer in
prevalence depends upon a rather exact estimate of the
prevalence being sought--an impossibility. Estimates of
prevalence using less than optimal cutoff points can be
corrected (Rogan and Gladen, 1978) by taking the sensitiv-
ity and specificity of the cutoff point into account .
Although this approach is theoretically possible, no such
data are presently available and, more importantly, it is
probably impracticable to acquire such data with the pre-
cision required. It i. not rational to select the cutoff
as a proportion of the highest requirement in the popula-
tion.
When fixed cutoff points are used without these correc-
tions, estimation of the prevalence of inadequate intake
is in error and the magnitude, the extent, and even the
direction of the error cannot be estimated. Recogniz-
ing the drawbacks in the use of fixed cutoff points, the
subcommittee concluded that a different approach was
required to analyze the adequacy of dietary intake. The
probability approach proposed in this report avoids the
limitations of the fixed cutoff points.
Representative terms from entire chapter:
fixed cutoff
