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Nutrition Department, The Pennsylvania State University, University Park, PA 16802
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONREFERENCES |
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KEY WORDS: • iron deficiency • adolescence • pregnancy • anemia
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONREFERENCES |
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). His estimates are that 46% of the world’s 5- to
14-y-old children are anemic, with the overwhelming majority of this
anemia occurring in individuals from the developing world. In addition,
48% of the world’s pregnant women are anemic; 56% of pregnant women
from the Third World are anemic. It is unclear how many of these are
adolescents, nor is it certain how much of the anemia is due to iron
deficiency and how much to parasitic infections, vitamin A deficiency,
folate and B-12 deficiencies, and generalized undernutrition.
In the United States, recent national surveys document the amount of
anemia and iron deficiency in the adolescent portion of the population.
An examination of the distribution of hemoglobin (Hb) concentrations
and iron status indices in the U.S. population from either the National
Health and Nutrition Examination Survey (NHANES) II or III data sets
reveals the clear effect of the adolescent growth spurt on iron
metabolism and iron requirements. The prevalence of iron deficiency
averaged between 8 and 10% for girls aged 12–19 in the NHANES III
survey of the U.S. population (Dallman et al. 1996
).
This is a higher percentage than had been determined in the 1976–1980
national survey. In boys, the estimated prevalence was <1% in this
same age group. This prevalence in adolescent boys is diminished
greatly from the nearly 11% estimated prevalence derived from the
NHANES II survey (Expert Scientific Working Group 1985
).
Iron balance is the difference between iron retention and iron
requirements and has been well described over the past 50 years
(Beard et al. 1996
). The retention of iron, frequently
called the absorbed iron, is the product of iron intake and the
bioavailability of that dietary, supplemental or contaminant iron. The
excess iron that accumulates beyond that necessary for the daily
requirement is stored within the core of the ferritin molecule. This
stored ferritin iron is then available for cellular iron needs should
dietary intake fall below the organ needs. When this negative iron
balance persists for a period of time, the iron stores are depleted and
the iron supply to the essential iron pools of the body is diminished.
Functional consequences then result from insufficient
iron-dependent functioning for oxygen transport, oxidative
metabolism, nuclear metabolism and gene transcription. Clinical
sequelae to this poor iron status include anemia, poor immune function
and decreased work performance. Poor fetal outcomes may occur if iron
deficiency occurs in the first trimester of pregnancy (see review by
Beard et al. 1996
).
The dynamics of iron movement in humans is well described and is
displayed in Figure 1
(Bothwell et al. 1979
). Iron lost from red cell mass
turnover averages 0.38 mg Fe/d in adults, bile losses between 0.22 and
0.28 mg Fe/d, desquamated gastrointestinal cells ~0.24 mg Fe/d and
urinary losses of ~ 0.5–1.0 mg Fe/d. In adolescents, the amount
of iron moving from one compartment to another is likely to be modified
slightly on the basis of body size and the onset of menses in the
female portion of the adolescent population (Rossander-Hulthen and Hallberg 1996
). There are no clear data to indicate that
these numbers are appreciably different in adolescent boys and girls
once body size is considered (Hallberg 1996
)
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,
Rossander-Hulthen and Hallberg 1996
). Total iron
requirements for adolescent boys (Fig. 2
) are computed from the increased iron requirements for the expansion of
the total blood volume (0.18 mg/d in boys and 0.14 mg/d in girls on
average) and the increase in the total body essential iron pool with
the increase in the lean body mass (0.55 mg/d in boys and 0.33mg/d in
girls median additional requirements). The increase in iron
requirements for the red cell mass includes both the increase in total
blood volume as well as the increase in mean Hb concentration from the
preadolescent years through the adolescent growth spurt. The mean Hb
concentration in the U.S. increases from 130 to 133 g/L in adolescent
girls and to 141g/L in adolescent boys during this time period. The
increase in mean total iron requirements may exceed 1.8 mg/d or more
than double those requirements of preadolescent males (Fig. 2)
.
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). Hallberg concludes
that menstrual losses in adolescents do not differ from those of
reproductive age women although actual data on volume of menstrual
losses were reported in his most recent treatment of those data. The
mean menstrual blood loss of 84 mL/period (Hallberg 1996
), assuming a mean Hb of 133 g/L, provides an estimate of
0.56 mg of additional iron per day. The 10th percentile for this is
0.17 mg/d and the 90th percentile is 1.08 mg of additional iron per
day. These additional requirements for iron to balance the menstrual
blood losses may increase the daily iron requirements to as much as 2.1
mg/d in girls at the 75th percentile of blood loss. Fairweather-Tait (1966) estimates the range of iron requirements for adolescent boys to be between 1.45 and 2.03 mg/d based on data derived from United Kingdom and European surveys. Similarly, she estimates the iron requirements for adolescent girls before menses to be between 1.22 and 1.46 mg/d, and after menses to be between 1.39 and 2.54 mg/d. Thus, although there are some small quantitative differences between her estimations and those of Hallberg, the fundamental conclusion that iron requirements nearly double during adolescence remains intact.
This factorial method of estimation of iron requirements has many
implicit assumptions that are based on relatively sparse data. These
assumptions are as follows: 1) basal iron losses can be
scaled to body size to convert the adult basal loss data to younger and
smaller individuals; 2) menstrual blood flow volume
distributions in adolescent girls are similar to those of adult women
of reproductive age; 3) iron content of lean body mass in
growing organs is similar to that in the fully formed adult organ. Some
of these assumptions may carry considerable risk (#2),
whereas other assumptions are likely reasonable (#1,
#3). Some faith in the factorial method, however, is derived
from an examination of the prevalence data and dietary intake data.
That is, the estimated prevalence, based on requirements and dietary
intakes, matches the measured prevalence in many cases (Hallberg and Rossander 1991
).
Pooled estimates of iron intakes for adolescents have been reported by
Fairweather-Tait (1996)
and are derived from survey data
in both the United Kingdom and continental Europe. Female teens average
10 mg Fe/d up until age 15 and then seem to increase intake to 13–14
mg/d. In contrast, teen boys showed a gradual but steady increase in
intake from 10 mg/d at age 11 y to 15 mg/d at age 16 y and
then a large increase to >20 mg/d at 17 y and beyond. This
suggests that iron intakes may be adequate to meet requirements to
prevent depletion of iron stores in many young females, but are
insufficient to actually increase iron stores substantially. Because
efficiency of iron absorption declines as iron status increases, a
large increase in intake is necessary to increase significantly the
mean plasma ferritin of the adolescent female population
(Hallberg and Rossander 1991
).
Within the context of this symposium, it is worthwhile reminding
ourselves of the iron costs of pregnancy (Allen 1997
,
Viteri 1997
). These calculations are again based on a
factorial method of estimating iron needs rather than true empirical
determinations of iron costs (Fig. 3
). The sum of the costs for expansion of the red cell mass in the second
and third trimester, the growth of the fetus and placenta in the second
and third trimester, and then blood losses at delivery can reach 1290
mg of iron. There is a considerable variation in this, however, and
much uncertainty regarding the blood loss at delivery. The amenorrhea
of pregnancy must be considered in this iron balance equation and may
constitute a saving of as much as 290 mg of iron over the 9 mo of
pregnancy. When the lactational period is considered, this iron savings
may rise as high as 400 mg or more (Fig. 3)
. The average iron
requirement over this period of time then can be computed to be ~4 mg
Fe/d. The efficiency of iron absorption will increase dramatically in
the second and third trimesters in response to the normal decline in
iron status and will compensate in part for the increased iron
requirements (Barrett et al. 1994
). In many women, it is
uncertain whether diet alone can provide the additional iron needs of
pregnancy (Allen 1997
), and the need for iron
supplementation is actively debated (Beard 1998
,
Hallberg 1998
, Viteri 1997
).
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONREFERENCES |
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1. ACC/SCN Second Report on the World Nutrition Situation 1992;Vol. 1 Global And Regional Results. ACC/SCN WHO, Geneva, Switzerland.
2. Allen L. Pregnancy and iron deficiency: unresolved issues. Nutr Rev 1997;55:91-101[Medline]
3. Barrett J.F.R., Whittaker P. G., Williams J. G., Lind T. Absorption of non-heme iron from food during normal pregnancy. Br. Med. J. 1994;309:45-63
4. Beard J. L. Weekly iron intervention: the case for intermittent iron supplementation. Am. J. Clin. Nutr. 1998;68:209-212[Medline]
5. Beard J. L., Dawson H., Pinero D. Iron metabolism: a comprehensive review. Nutr Rev 1996;54:295-317[Medline]
6. Bothwell T., Charlton R., Cook J., Finch C. E. Iron Metabolism in Man 1979 Blackwell Scientific Oxford, England.
7. Dallman P., Looker A. C., Johnson S. L., Carroll M. Influence of age on laboratory criteria for the diagnosis of iron deficiency anemia and iron deficiency in infants and children. Hallberg L. Asp N.-G. eds. Iron Nutrition in Health and Disease 1996:65-74 John Libbey & Co.
8.
Expert Scientific Working Group Summary of a report on assessment of the iron nutritional status of the United States population. Am. J. Clin. Nutr. 1985;42:1318-1330
9. Fairweather-Tait S. Iron requirements and prevalence of iron deficiency in adolescents. An overview. Hallberg L. Asp N.-G. eds. Iron Nutrition in Health and Disease 1996:137-148 John Libbey & Co.
10. Hallberg L. Iron requirements, iron balance and iron deficiency in menstruating and pregnant women. Hallberg L. Asp N.-G. eds. Iron Nutrition in Health and Disease 1996:165-182 John Libbey & Co.
11. Hallberg L. Daily iron supplementation: why it is necessary Am. J. Clin. Nutr. 1998;68:213-217
12.
Hallberg L., Rossander-Hulthen L. Iron requirements in menstruating women. Am J. Clin. Nutr. 1991;54:1047-1058
13. Rossander-Hulthen L., Hallberg L. Prevalence of iron deficiency in adolescents. Hallberg L. Asp N.-G. eds. Iron Nutrition in Health and Disease 1996:149-156 John Libbey & Co London, UK.
14. Viteri F. E. Iron supplementation for the control of iron deficiency in populations at risk. Nutr. Rev. 1997;55:195-209[Medline]
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