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

Establishing nutrient intake values
Annex
Example of deriving an average nutrient
requirement (ANR): vitamin A
Vitamin A functions in many biological systems:
11-cis-retinal, the active aldehyde form of vitamin A,
is part of rhodopsin, required for vision in the brain,
for normal growth and maintenance of the cornea
and conjunctival membranes of the eye (to prevent
xerophthalmia), and as a regulator of genes that form
structural proteins, enzymes, and retinol-binding proteins
in the blood, including those involved in normal
embryonic development and immune function related
to cytokines and T and B lymphocyte production [7].
It is not surprising that some of these functions continue
normally at comparatively low levels of intake,
while others are depressed or inactive, resulting in
detectable changes in indicators that denote vitamin
A inadequacy.
In selecting the indicators or functional outcomes
to use to establish an ANR and an NIV x for vitamin A,
a number of choices are available (see Annex table 1).
In reviewing research and studies available in the year
2000, the DRI process selected as the main criterion
to establish adequacy for vitamin A in Canada and
the United States adequate liver stores, with adequacy
defined as at least 20 µg retinol/g liver tissue. This
resulted in an EAR (estimated average requirement,
TABLE 1. Possible indicators for evaluating vitamin A adequacy
S49
equivalent to an ANR) of 625 µg/day for men and 500
µg/day for women 19 years of age and older [7].
However, it is recognized that significant liver retinol
stores are not required for normal function. Thus, dark
adaptation was identified as the most sensitive criterion
for which adequate data were available to establish an
average requirement. There were data supporting the
ability of dark adaptation to identify individuals with
inadequate intakes of vitamin A, and there were data
from individuals inadequate in vitamin A whose dark
adaptation was measured and who then were given various
levels of vitamin A and underwent additional dark
adaptation tests to ascertain at what level of intake the
function returned to normal. The level at which dark
adaptation returned to normal (defined as at least –4.6
± 0.3 log candela/m 2 ) in half of those studied (while
dark adaptation in the other half remained abnormal)
was 300 µg/day. This could be considered the ANR
for vitamin A based on dark adaptation. One of the
primary limitations of selecting this criterion was that
there were only 13 individuals studied in this way.
According to this example, when the selected criterion
or indicator of adequacy is dark adaptation, the
ANR is 300 µg; this is less than half the amount identified
if liver stores at the level of 20 µg/g are chosen (650
µg) [7]. This demonstrates that the important question
that must be asked in establishing nutrient requirements,
and a potential barrier to harmonizing such
Function Test Definition of adequacy
Dark adaptation Dark adaptometry: final dark-adapted
threshold after 35–40 min
Circulating levels of vitamin
A
≥ – 4.6 ± 0.3 log candela/m 2
Pupillary response test A high threshold in response to incremental
pulses of light = low retinal sensitivity due to
vitamin A deficiency
Serum or plasma retinol concentration; retinol-binding
protein concentration
Total liver reserves (TLR) Isotope dilution using TLR = F × dose × [(H:
D) – 1]; F = efficiency of storage of early
administered dose; dose = dose of labeled
retinol; H:D = ratio of H to 2 H-retinol in
plasma after equilibration
Relative dose–response
(RDR)
Modified relative dose–
response (MRDR)
Conjunctival impression
cytology (CIC)
Change in plasma retinol concentration in
response to a test dose after 5 h
Amount of test dose vitamin A 2 (dehydroretinol)
appearing in plasma after 5 h
Microscopic examination of periodic acid
Schiff–hematoxylin-stained epithelial cells
Immune function Humoral antibody responses and cell-mediated
immunity
Source: Institute of Medicine [7].
Considered insensitive indicators and thus
not used
TLR ≥ 20 µg/g retinol
RDR < 20%
MRDR < 20%
Lack of abnormal cells
Normal number of natural killer cells and
normal cytotoxic activity