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APPENDIX B
Derivation of Criteria for Interpreting
Iron Intake in Women
As discussed in Chapter 5, when nutrient require-
ments are symmetrically distributed around the mean, the
probability assessment approach is relatively insensi-
tive to the shape of the requirement distribution. This
is not true when the distribution is markedly asymmet-
rical, as for iron requirements of menstruating women.
For this reason, it is important to estimate the charac-
teristics of the distribution of iron requirements for
this group.
In agreement with the FAD/WHO Expert Group (FAD/WHO,
1970), the iron losses are divided into two components:
basal losses via the skin, urine, and feces (excreted
iron rather than unabsorbed iron) and the losses in the
menses. The need for absorbed iron to balance these
losses is then estimated using the upper limit of
absorption of dietary iron that can be expected in
persons ingesting a mixed diet, who are in need of iron,
but maintaining body iron stores. The development of
these components of the final estimate is described
below.
An isotopic technique has been used to measure basal
iron losses for adult men under various conditions
(Green et al., 1968). For the purpose of this appendix,
the data obtained with this technique have been extrapo-
lated to women on the basis of relative metabolic size,
as reflected by basal metabolic rate (BAR). The mean
basal iron loss derived in this manner is approximately
0.67 mg/day. mere are few data on the variability of
these losses, other than those in the original studies
of men. A coefficient of variation (CV) of approxi-
mately 15% used for this exercise results in a range
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116
from about 0.47 to 0.87 mg/day. For simplicity' a basal
loss of 0.87 mg/day was accepted for all wa~en--a small
overestimation of actual need.
Iron content of the menses is the major factor affect-
ing the distribution of iron needs among menstruating women.
Several studies have established that there is considerable
variation among women but a similarity from cycle to cycle
for individual women. Thus, losses for a population of
women should be fairly similar to the distribution of iron
requirements used in the probability approach. Suitable
data on iron losses have been provided in the reports of two
large population studies (Cole et al., 1971; Hallberg et al.,
1966), which are supported by the findings from a number of
smaller studies (see Beaton, 1974). A simple examination of
the distribution of observed iron losses would lead to an
underestimate of both loss and requirements of women replete
with iron because women with high blood losses tend to have
low hemoglobin levels (i.e., a tendency toward anemia). To
circumvent this, the distribution of blood losses was con-
verted to iron losses by using a standard hemoglobin concen-
tration rather than the hemoglobin level of the study sub-
ject. The resultant distributions for the two studies were
then merged and found to be in good agreement. A log-normal
distribution model that fit the data reasonably well (Beaton,
1974) was used for modeling. Expressed in terms of natural
logarithms, the menstrual iron loss distribution may be de-
scribed as having a mean of -0.81 and a standard deviation
of 0.84.
Iron absorption is a regulated process, and within the
limits of bioavailability of dietary iron, the body will
absorb sufficient iron to meet one's needs and will reject
(i.e., absorb with lower efficiency) iron above these needs.
Since the objective is to estimate the lowest intake of
dietary iron that will maintain iron balance in relation to
known losses, there is a need to estimate the upper limit of
iron absorption. As iron depletion increases, the effi-
ciency of iron absorption also increases.
After reviewing various kinds of information, the FAG/
WHO (1970) committee suggested that the upper limit of
absorption was approximately 20% among subjects consuming
diets relatively rich in meat and other animal proteins.
Since the nature of different diets affects iron bioavaila-
bility (Monsen et al., 1978), the upper limit suggested by
the FAD/WHO committee was much lower for subjects consuming
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predominantly cereal diets. The 20% upper limit absorption
figure is appreciably higher than the commonly quoted aver-
age iron absorption of adult men. Nonetheless, it hen been
used in the models presented in this report. To examine
the effect of defining the requirement in terms of some
iron-depleted state (e.g., mild anemia), one need only alter
the estimate of the upper limit of iron absorption by
increasing it.
To apply this model in the assessment of intake, the
following algorithms were adopted:
Available iron = 0I x UL, where 0I = observed intake
(mean intake for the frequency interval) and UL = upper
limit of absorption, i.e., 20% for the iron replete state.
Iron available to meet menstrual loss = (OI x UL) -
0.87, where 0.87 mg/day is the assumed basal loss of iron
(see comments above), and the position in the normal dis-
tribution (Z score) is calculated as:
Z = Ln [(OI x UL) - 0.87] - (-0.81),
0.84
where -0.81 is the mean of the distribution of logarithms of
menstrual iron losses, 0.84 is the standard deviation of
that distribution, and the probability that the observed
intake would be inadequate to meet iron losses is computed
by an algorithm describing the cumulative area under the
normal distribution curve to the right of Z. This phase of
the calculation is identical with that used for nonloga-
rithmic distribution models.
Beaton (1974) attempted to validate this model by com-
paring predicted prevalences of inadequate intake with pre-
dicted response to iron administration. He based the latter
on the probability of response associated with observed
hematocrit, using data from a population study by Garby et
_. (1969a,b). There was reasonable agreement when hema-
tologic data from Nutrition Canada and from the Ten-State
Nutrition Survey were examined by a probability approach and
then compared with assessments based on dietary data from
1-week studies. The model described above has been used to
estimate dietary iron requirements in the recent revision of
Recommended Nutrient Intakes for Canadians, which contains
further discussion on this topic (Health and Welfare, Canada,
1983). With this model, the current Canadian recommended
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118
intake of iron (14 mg/day) would be adequate to meet the
needs of all but approximately 5% of menstruating women,
whereas the U.S. recommended intake (18 mg/day) would meet
the predicted needs of all but about 2% to 3% of women.
REFERENCES
Beaton, G. H. 1974. Epidemiology of iron deficiency.
Pp. 477-528 in A. Jacobs and M. Worwood, eds. Iron in
Biochemistry and Medicine. Academic Press, New York.
Cole, S. R., W. Z. Billewicz, and A. M. Thomson. 1971.
Sources of variation in menstrual blood loss. J. Obstet.
Gynaecol. Br. Commonw. 78:933-939.
FAD/WHO (Food and Agriculture Organization/World Health
Organization). 1970. Requirements of Ascorbic Acid,
Vitamin D, Vitamin B12, Folate, and Iron. Report of a
Joint FAD/WHO Expert Group. WHO Technical Report Series
No. 452. FAO Nutrition Meetings Report Series No. 47.
World Health Organization, Geneva.
Garby, L., L. Irnell, and I. Werner. 1969a. Iron defi-
ciency in women of fertile age in a Swedish community.
II. Efficiency of several laboratory tests to predict
the response to iron supplementation. Acta Med. Scand.
185:107-111.
Garby, L., L. Irnell, and I. Werner. 1969b. Iron de~i-
ciency in women of fertile age in a Swedish community.
III. Estimation of prevalence based on response to iron
supplementation. Acta Med. Scand. 185:113-117.
Green, R., R. Charlton, H. Seftel, T. Bothwell, F. Mayet,
B. Adams, C. Finch, and M. Layrisse. 1968. Body iron
excretion in man: A collaborative study. Am. J. Med.
45:336-353.
Hallberg, L., A.-M. Hogdahl, L. Nilsson, and G. Rybo. 1966.
Menstrual blood loss--a population study: Variation at
different ages and attempts to define normality. Acta
Obstet. Gynaecol. Scand. 45:320-351.
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119
Health and Welfare, Canada. 1983. Recommended Nutrient
Intakes for Canadians. Compiled by the Committee for
the Revision of the Dietary Standard for Canada. Bureau
of Nutritional Sciences, Food Directorate, Health Pro-
tection Branch, Department of National Health and
Welfare. Canadian Government Publishing Centre, Ottawa.
Monsen, E. R., L. Hallberg, M. Layrisse, D. M. Hegsted,
J. D. Cook, W. Hertz, and C. A. Finch. 1978. Esti-
mation of available dietary iron. Am. J. Clin. Nutr.
31:134-141.
Representative terms from entire chapter:
iron losses
