Implementing food-based dietary guidelines for - United Nations ...

S52
A new paradigm for calculating the
probability of nutrient adequacy
Because the ANR and its standard deviation describe a
distribution of requirements, it is possible to calculate
the probability that usual long-term intake at a given
level is adequate. Such probability estimates require
knowledge of the distribution of requirements and
are not possible if the requirement is stated as a single
number. The calculation is based on the assumption
that intake is independent of requirement. That is, a
person with a higher requirement for a nutrient does
not necessarily select a diet that is higher in the nutrient.
This assumption is clearly violated for energy
intake, but there is no persuasive evidence that intakes
are related to requirements for vitamins and minerals.
On the assumption of independence of intake and
requirement, the probability that a person’s usual intake
is adequate can be statistically determined from the
distribution of requirements that is described by the
ANR and its variation (fig. 1). For example, if a person’s
usual intake is equal to the ANR, the probability
of adequacy (using the chosen criterion of adequacy)
is, by definition, 50%, because it meets the needs of
half of all the people who were studied. If a person’s
usual intake is equal to the ANR plus 2 SD of the
requirement, the probability of adequacy is about 98%,
because only 2% of individuals in the group would have
a higher requirement. If an individual’s usual intake
is equal to the ANR minus 2 SD, then the probability
of inadequacy is only 2%, because almost everyone
would have a higher requirement. A simple statistical
algorithm can calculate the probability of adequacy for
any given intake based on the ANR and its distribution
(by calculating the area under the requirement curve,
such as the one in figure 1, which is to the left of the
intake value) [5]. Note that it is possible to calculate
either the probability of adequacy (the area to the left of
the intake value) or the probability of inadequacy (the
area to the right of the intake value). The probability of
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FIG. 1. Graph of a hypothetical distribution of nutrient
requirements and adverse effects as the amount of a nutrient
consumed increases
inadequacy is 100 minus the probability of adequacy.
For a group of individuals, the prevalence of adequacy
(or inadequacy) can be estimated as the average probability
of adequacy (or inadequacy) within the group.
Although it is necessary to know (or estimate) the
distribution of requirements to calculate the probability
of adequacy (for individuals) or the prevalence
of adequacy (for groups), requirements do not have
to be normally distributed. For example, iron requirements
for menstruating women are skewed, but a
requirement distribution can be described in a tabular
format, as was done for the dietary reference intakes
(DRIs) for the United States and Canada [6]. For such
nutrients, an individual’s probability of adequacy can
be determined simply by locating the correct table (for
the person’s age and sex) and selecting the probability
of adequacy that corresponds to the person’s usual
intake. However, as discussed in more detail below,
a shortcut to the probability approach for estimating
the prevalence of adequacy for groups (the “cutpoint”
method) cannot be used if the requirement distribution
is not symmetrical.
Using NIVs for assessment and planning
for individuals
Assessing nutrient intakes of individuals
Goal
The goal of assessing the intake of an individual is to
determine the probability that the person’s usual diet
is meeting his or her nutrient needs and whether the
person is potentially at risk for adverse effects from
excessive intake [2, 7, 8]. However, it is difficult to
know if an individual’s nutrient intake is adequate,
because the person’s actual nutrient requirements are
usually unknown, and an accurate measure of the
person’s usual, long-term nutrient intake is almost
never available. Although the probability of adequacy
can be calculated, as described above, the result is only
meaningful if the usual nutrient intake for a person
is known. Because of day-to-day variation in intakes,
it is usually necessary to observe a person’s diet over
many days when an accurate estimate of usual intake
is needed. This is seldom feasible, so calculation of the
probability of inadequacy for an individual may not
be meaningful. Indeed, it may be more appropriate to
monitor physiological measures (such as blood markers
of anemia) rather than rely on intakes that were
observed for a small number of days.
Calculating the confidence of adequacy
S. P. Murphy and H. H. Vorster
Another approach to assessing an individual’s intake
is to calculate the confidence that the usual intake is
adequate, which considers the number of days on
which the intake was observed, as well as how far the