IRON
METABOLISM
A
Review
with
Special
Consideration
of
Iron
Requirements
During
Normal
Infancy
By Phillip Sturgeon, M.D.
Department of Hematology Research, Los Angeles Children’s Hospital
This study was supported in part by a grant from the Leukemia Research Foundation of California
and grant A-182 C2 from the National Institute of Arthritis and Metabolic Diseases of the National
In-stitutes of Health, Public Health Service.
Dr. Sturgeon is a Markle Scholar in Medical Science.
ADDRESS: 4614 Sunset Boulevard, Los Angeles 27, California.
267
PLAN OF PRESENTATION
INmoDucrIoN
IRON ExciuTIoN
IRON ABSORPTION
Ferrous versus Ferric
Ferritin and the Mucosal Block Per Cent Absorbed
Food Iron versus Medicinal Iron Effect of Vitamin C
IRON TRANSPORT
Iron Binding Capacity
Serum Iron
IRON STORAGE
Hemosiderin in Bone Marrow Sideroblasts
Direct Measurement
COPPER AND Fm ERYTHROCYTE
PROTO-PORPHYRIN
IRON KINETICS
Turnover of Serum Iron Suxixsui AND CoNci.usloNs APPENDICES
A. Pathogenesis of the Physiologic Anemia of Late Infancy
1. Iron stores, blood volume and con-centration of hemoglobin
2. Myoglobin
3. Parenchymal Iron
4. Total Body Iron at Birth 5. Rate of Weight Increase
6. Nutritional Requirement
7. Placental Transfusion
8. Summary of Endogenous and
Exo-genous Factors in Total Iron Nu-trition
B. Supplementation with Intramuscular Iron
SUMMARY AND CoNusIoNs TO APPENDICES
A AND
B
INTRODUCTION
I
RON METABOLISM has been studiedcx-tensively in the normal adult male;
marked variations have been observed in a
variety of pathologic and physiologic
condi-tions. Many of these conditions are
spe-cifically of interest to the pediatrician.
Physiologic alterations of iron metabolism
are observed in pregnancy,4 the neonatal period,” both early and late infancy7 and even possibly to some degree in most of childhood.71#{176} Among the pathologic conditions associated with profound ab-normalities in iron metabolism are those resulting from depletion of body iron. These include “nutritional” iron deficiency anemia in children,7’8,’1 and iron deficiency13 and
chronic anemia of blood donors22 in adults.
Excessive accumulation of iron within one of the several metabolic pathways, at tile
expense of others, is found in inflammatory
states, especially in severe chronic infec-tions.21 Acute toxicity results from ingestion of large amounts of iron when the capacity of serum to transport iron is exceeded, leading to “iron poisoning.”1720 Profound alterations in the internal metabolism of
iron are encountered in most blood
tissues, in some cases leading progressively to loss of function and death, is encountered in transfusion hemosiderosis, pulmonary
1’ 16 and hemochroma-tosis.’5’ 23, 26 The latter condition, although rare, has stimulated numerous investiga-tions which have provided a large propor-tion of our knowledge of normal and ab-normal metabolism of iron.
The pediatrician concerned especially
with academic or an institutional type of
practice will observe many if not all of the above conditions. Their clinical manage-ment depends on knowledge of normal iron metabolism and its alteration in disease. This knowledge, however, is of equal im-portance to physicians concerned with gen-eral nutrition, prophylaxis and treatment of iron deficiency anemia in infants and supportive therapy of non-hematoiogic dis-eases, such as chronic infections associated with mild degrees of anemia. Moreover, the recent addition to the list of hematinics of
a nontoxic intramuscular iron that appears
to be practical and effective makes it even more important that the principles of iron metabolism be fully appreciated by all engaged in the prophylaxis and therapy of iron deficient states. The inability of the
body to excrete iron and the consequent
danger of overloading with iron (exogen-ous hemochromatosis) should be of special concern to those who administer blood transfusions and other forms of parenteral iron.
The aims of this report, therefore, are to
focus attention on currently accepted con-cepts of iron metabolism and recent contri-butions to the problem, especially as they relate to the “physiologic anemia of late infancy”; #{176}to point out areas in the field of
a The term “physiologic anemia of late infancy”
is employed to denote the condition of microcyto-sis, hypochromasia and reduced mean cell hemo-globin, in the absence of frank anemia that is ob-served in a large proportion of normal infants from
6 to 18 iionths of age. The interested reader is
referred to the detailed extensive studies of Guest,
Brown and \Ving,’ which characterize the
erythro-cyte morphology of this age period. Their study
also emphasizes that reduction in size and
hemo-pediatrics where additional studies of iron metabolism are needed; and to briefly sum-marize literature dealing with use of intra-muscular iron and to make a preliminary report of our experiences with this material in the prophylaxis of the physiologic anemia of late infancy. A critical evaluation of the innumerable facets of iron metabolism will not be attempted; for such an evaluation,
interested readers are referred to the
ex-tensive reviews published by 2124 Drabkin26 and Josephs.27
IRON EXCRETION
In general the plan in this review is to consider, as one would with most other metabolites, first, digestion and absorption, then, intermediary metabolism and,
ulti-mately, excretion. However, the absence of
an excretory route for iron is a unique feature of its metabolism and appreciation
of this is of such importance to the proper
clinical use of iron that it will be dealt with first.
Prior to 1937 it was commonly stated that iron was absorbed in the first part of the
small bowel and that any taken up in
ex-cess was excreted into the colon.28 How-ever, McCance and Widdowson, in a series of balance studies on normal people in iron equilibrium, showed that overloading with
parenterally administered iron was not
compensated for by increased fecal or urinary excretion.29 They established con-elusively that iron found in the feces repre-sents only unabsorbed iron, and that the intestine has no power of regulating the amount of iron in the body by excretion. They confirmed that urinary excretion of iron is negligible, and also showed that increased ingestion of iron was not ac-companied by elevated urinary
excre-930 There remains no doubt today that
globin content of the erythocyte is an early mani-festation of iron deficiency prior to the appearance
of frank anemia. The gradual decrease in
con-centration of hemoglobin (luring the first 3 months
of life is not under consideration in this review.
This may be termed the “physiologic anemia of
iron, once it gains access to the body, has no organ of excretion, hence the capacity to regulate the amount of iron in the body resides exclusively in the absorption mechanism.
Estimates of loss of iron from the normal adult male vary from 1 to 1.5 527
Iron is lost in the urine up to 0.5 mg./
2931 bile, 0.2 mg./day (in the dog)’2
and sweat, negligible.” As iron is toxic when present in the body in increased amounts, it is apparent15 that there must be a rigid control of the amount absorbed. It is not surprising either, in view of the above, that the ability of the body to absorb iron, even under conditions of great de-mand, is v,ry limited.
IRON ABSORPTION
The absorption of iron salts has been studied extensively and in most aspects there is general concurrence. Figure 1 represents diagramatically the essential fea-tures of iron digestion and its preparation for passage through the intestinal epi-thelium. Studies with radioisotopes indicate that in the vicinity of 10 per cent of a large dose (1 mg./kg.) of ferrous chloride is ab-sorbed by normal adult subjects. Similar
experiments in individuals with iron
de-ficiency anemia revealed assimilation of from 24 to 80 per cent of iron, given in tile
same dosage.34 There is agreement that at least in the human, and possibly the dog, iron is only absorbed in the ferrous
state;’4’ thus it is not surprising tilat
ferrous salts are more effective for
hemo-globin regeneration than ferric salts. The
quantity of elemental iron required to achieve an optimal response in the therapy of iron deficiency anemias has been shown to be less with the ferrous form both in adultssc and children.” It has also been
demonstrated that the ferrous form is more
effective in the prophylaxis of the physio-logic anemia of late ‘“ Moreover,
studies on adult males with the use of radioactive iron have provided confirmation that the ferrous form is used more efficiently in both normal and anemic subjects.3’
In addition, these studies demonstrated that
responses to the ferrous salt are more
con-sistent, and that it is well tolerated and
637 Thus, today there appears
to be no valid reason for the use of ferric iron salts in the oral therapy of iron de-ficiency states.
It is widely accepted tilat iron absorp-tion occurs principally in the
duo-2427 in the guinea pig and in the
horse this has been demonstrated by the
accumulation of ferritin within the mucosal
cells following feeding of Studies
in tile dog, the guinea pig and the rat using isotopic technics have also indicated an
absorption gradient, being highest in the
region of the pylorus and diminishing
toward the Although direct
evidence for absorption of iron through such a restricted segment of the bowel in
infants is not available, and seems
techni-cally impossible to obtain, it may be
in-ferred that such a gradient does exist. Absorption is enhanced by reducing
sub-stances in the stomach and duodenum,
es-pecially vitamin
C.44
These substances assistIRON
DIGESTION
DIETARY STOMACH DUODENUM, MUCOSAL CELLS
I Organic Iron Fi”’ . Digsstion and Lib.rotion-.F.”4]!..R.duCtiOfl ,Assimilotion,IOS J
Medicinal Iron F. -,F. F. SlEmtcr.tion, 90%-.
L I I JL I
FIG. 1. Organic or food iron occurs in the ferric state and, after liberation by digestive processes in the
stomach, must be reduced to and held in the ferrous state to be absorbed. The same is true for medicinal
ferric iron saits. Ordinarily 10 per cent or less is absorbed by the normal individual, the remainder is
INTESTINAL
Fe
LUMEN
BLOOD Iron Binding Protein
(Siderophilin) in the reduction of ferric iron, as it occurs
naturally in foods, to ferrous iron. They also hinder the oxidation of ferrous iron salts to the ferric state.44’ Absorption is
hindered by ions which form relatively
stable iron complexes, such as phosphates and other substances which form insoluble salts with iron.4’
The absorption of iron is not a matter of simple diffusion across a membrane; in-stead, there is a metabolic transportation through the intestinal mucosa (Fig. 2). This may be mediated at least in part by a pro-tein elaborated by the mucosal cells, fern-tin, which has a great affinity for iron. The chemical nature of fernitin, its discovery and physiologic functions have been re-viewed recently by Granick.8 Although the data are far from conclusive it appears that
the intestinal mucosa is impermeable to
fen-rous iron in concentrations encountered physiologically. However, when iron is present the mucosal cells elaborate apofer-ritin which reacts with ferrous iron, con-verting it to a fernic protein complex termed
fernitin. Thence, transport of iron across the cells and adjacent capillary membrane and release into the portal system requires a reduction process and reconversion to the ferrous state. It appears that at this point iron absorption is regulated primarily by the degree of existing chronic anemia. If profound anemia does exist, the process proceeds at a relatively rapid rate; this occurs irrespective of the state of iron ston-age.’2’ 34, 49 For instance, the anemic
pa-tient with a high concentration of iron in the serum and who is greatly overloaded with storage iron (as from repeated blood transfusions) continues to assimilate iron at an increased rate. This iron will only add to the overloaded state. Therefore, the common practice of routinely prescribing iron for infants and children, who are in all probability in a state of iron overload
(familial jaundice, congenital hypoplastic anemia, Cooley’s anemia, aplastic anemias, chronic inflammatory states, erythroblasto-tic infants who have had repeated small transfusions), will not correct the anemia
IRON
ASSIMILATION
(Mucosol Block’)
Fe4” #{149}
\ Oxidation
\ Reduction ,/
\ /
\ I
\ I
4 I
F?
,,Apofsrritin ±F.rritin
r
Synthesis“S.. Dsgradatiosm1l
FIG. 2. Passage of iron through the intestinal epithelial cell is not a matter of simple diffusion. \Vithi:i physiologic limits and in most therapeutic conditions, ferrous iron enters the cell and, after oxidation to
the ferric state, reacts with the protein apoferritin to form ferritin in which state the iron is retained
within the mucosal cell. Then it is released from ferritin, reduced and in the ferrous state enters the
circulation. Initially some iron may pass directly into the circulation but very quickly a “block” develops
and additional iron does not gain immediate access to the circulation but may be retained within the
but will contribute to the state of iron over-loading, and in some instances hasten the development of hemochromatosis.5#{176}
Con-versely, published reports” and studies in
progress from this laboratory on normal in-fants indicate that, despite biochemical evi-dence of meager or totally depleted iron stores, the infant who has approximated an optimum concentration of hemoglobin will assimilate little iron, even when given in large therapeutic doses. Haskins et al.52 observed a comparable situation in adults
who have regained normal concentrations
of hemoglobin following depletion of the iron stores by repeated phlebotomies.
The precise mechanism for regulating
passage of iron through intestinal epithe-hum is not understood. Original observa-tions on dogs by Hahn and associates’3 that within 1 hour following a relatively large oral dose of iron salt, a second dose was often poorly absorbed, led to the con-cept of “mucosal block.” It was also ob-served by Granick#{176} that feeding of iron was immediately associated with an in-crease in ferritin in the gastrointestinal mu-cosa, and it was suggested, therefore, that
the formation of ferritin was related to the
“mucosal block” phenomenon. Josephs,27 however, emphasizes that as ferritin func-tions mostly as an iron storage compound, a complete reversal of this idea might be proposed: He suggests that ferritin in the intestinal mucosa enables the organism to retain an extra quantity of iron within the mucosal epithelium that would be other-wise only briefly present in the duodenum. Thus the assimilation process would pro-ceed over a longer time despite the periodic presence of iron in the intestinal tract. He also suggests that the mechanism for limit-ing absorption resides elsewhere than in the ferritin reaction. Granick in one of his later papers is also more reserved in his attitude toward the possible central im-portance of ferritin in the genesis of the “mucosal block.” He states, “Whether changes in concentrations of ferritin are
directly connected with the mucosal block
or whether the presence of much ferritin merely indicates that the mucosal cell is saturated with respect to some iron com-pounds more closely allied with the mucosal block has not been determined.”8 Figure
2 summarizes some of the aspects of iron
assimilation considered above.
The percentage absorption and utiliza-tion of iron from naturally occurring foods, rather than medicinal salts as considered above, has been studied only recently. It has been found that there are significant differences which must be considered, if a physiologic concept of iron assimilation rather than a therapeutic concept is to be had. Moore and Dubach44 incorporated radioactive iron metabolically into several foods, and fed them to normal and anemic subjects. They found that both the anemic and normal individuals absorbed only 10
per cent or less of iron in food. This is in
sharp contrast to the results with simple iron salts; iron deficient subjects regularly absorb several times more iron than normal persons.34’ 54 If the above data are
ap-proximately applicable to infants and one considers the amount of iron in an infant’s
diet, the percentage absorbed, the require-ments for growth, plus, in the case of iron deficiency anemia, that needed to make up
the deficiency, one may realize the futility
of trying to correct such a deficiency within
a 3 months period by dietary means alone.
An approximate calculation reveals that in the vicinity of 20 to 25 mg. of dietary iron per day would be required. #{176}This could be
0 An infant who consumes 1 egg yolk (0.5 to
1.0 mg. Fe), 7 to 17 gm. of a cereal mixture
“un-usually rich in iron” (2.25 to 4.5 mg. Fe), 1 quart of milk (.14 to 2.4 mg. Fe) and some
pur#{233}edspin-ach (1.22 rng./100 gm.) per day will ingest from
3.4 to 6.8 mg. of iron per day (in the vicinity of
0.5 mg./kg. for a 1-year-old infant).” If a maximum
of 10 per cent (egg yolk, 1.1-8.4, median 3.1 per
cent; meat, 2.7-33.1, median 7.6 per cent) is
re-tamed as indicated by the studies of Moore and
Dubach,” then approximately 0.5 mg. will be
available for hemoglobin synthesis each (lay.
Be-ginning at 8 months, an 18-pound normal infant would gain in 3 months approximately 3 pounds”
approxi-obtained from 20 to 40 eggs or % pound of meat per day for 3 months or twice this amount of food if the deficiency had to be corrected in 6 weeks. Because studies such as those by Moore and Dubach have not been performed on infants, the applicability of such data to the consideration of iron nu-trition in infants is purely speculative.
In-direct appraisal of iron assimilation in in-fants has been made through the use of balance studies,” influence on concen-tration of hemoglobin8’ 51 and oral iron tolerance studies.61 Few reliable studies on
the influence of various dietary regimens on
concentration of hemoglobin of infants are
available. Brokaw et al.60 could demonstrate no difference in concentrations of hemo-globin in normal infants fed solids early, compared to those carried exclusively on
iiately 40 mi/pound” or a total of 120 ml.
Dur-ing that time, if a concentration of hemoglobin of
12 gm./100 ml. is to be maintained, 12 x 1.2 or
14 gm. of hemoglobin will have to be formed.
There are 3.4 mg. of iron per gram of Hb.” “ thus,
approximately 45 mg. of iron (14 gm. Hb. x 3.34
mg. Fe/gm. Hb. = 47.6 mg. Fe) will be required
or 0.5 mg/day for 90 days. It is apparent
there-fore that an iron enriched diet of the above type,
assuming 10 per cent absorption, would provide
only enough iron for maintenance. No allowance
has been made, however, for increase due to
growth of the myoglobin mass which has the same
iron content as hemoglobin.” A rough allowance
of an additional 10 mg. could be made and if one
assumes it would be physiologically advantageous
for an infant to build up or maintain a storage
reserve comparable to that in the adult male, then
an additional allowance of approximately 20 per
cent would have to be included.” The inference
from this type of calculation is that the growing
infant’s demands for iron can barely be met from
dietary sources. These calculations are consistent
with the observation of Brokaw at al. who found
that the early introduction of cereals, vegetables
and eggs had no effect on the hemoglobin levels of
infants through the first year of life. Following
the same line of reasoning, if the 8-month.infant
had an anemia of 5 gm. of Hb./100 ml. there
would be an additional iron deficit of (18 pounds x
40 mI/pound x 7 gm. Hb./100 ml. x 3.4 mg. Fe./
gin. Hb.) 170 mg. This would require an additional
assimilation of approximately 2 mg. (170 mg. Fe/ 90 days) of iron per day or the oral ingestion of
food containing 20 mg. of iron each day (twice the
iron requirement of the adult male.
milk through the first 12 months of life. In general, balance studies have not been performed in great numbers, especially in
early infancy, and the results from various
authors are not always in agreement and are therefore difficult to interpret. The ex-tensive studies by Josephs’7 cover a wide age range. His data indicate that infants on diets providing 0.3 mg. of iron/kg/day assimilated approximately 30 per cent of the iron. With intakes of less than 0.1 mg./
kg./day, he found no retention. In an
in-fant under 3 months of age, on intakes of less than 0.2 mg./kg., he found a negative balance. This finding is difficult to reconcile with the present concept that the bowel does not excrete iron. However, Feuilien’s recent iron balance studies6264 on infants did not show a negative balance but other-wise they confirmed to a great extent those of Josephs. He found, moreover, even a
greater mean retention, 53 per cent. He also
showed that cow’s milk as consumed by the infant has 3 to 4 times as much iron as human milk, and that the percentage re-tention is equal to, or greater than, that from human milk. He concluded that anemia in infants fed artificially is not due to a lack of iron in the diet : “In fact these infants often ingest 3 to 4 times more iron than infants of the same age breast fed”62 and “the fact anemia is more frequent among artificially fed infants than among breast fed is due neither to lack of iron in the diet nor to a deficient absorption.”63
that in healthy adults 80 per cent of ab-sorbed iron appears in hemoglobin within
2 weeks34’ 65 and that recovery of
unab-sorbed isotope in the feces is accurate to within 10 per cent, thus giving a reasonable estimate of the amount of iron assimilated but not utiilized for hemoglobin formation. He is also of the opinion that the balance experiments are laborious and even though meticulously done, are subject to the not inconsiderable errors involved in the
analy-sis of foods and of feces for iron.44 Similar
opinions have been voiced by others; Bal-four and associates’4 state : “Difficulties in iron analyses are considerable, particularly in feces, where phosphorous-iron com-pounds introduce serious errors and make figures for iron recovery too low, thus giving erroneous impressions of positive iron bal-ance.” There is obviously need for addi-tional studies of the percentage assimilation and utilization of food iron for hemoglobin formation, especially in infants.
With regard to medicinal iron, Josephs states that intakes of as high as 1.0 to 2.0 mg./kg./day are required before retention occurs, and mentions that the intestinal epithelium behaves differently toward in-organic iron and food iron. Again, these concepts are contrary to the data of Moore
et al. who found, using the isotopic technic, better utilization of medicinal than food iron, at least in anemic individuals.44
The effect of vitamin C in enhancing the assimilation of iron occurring naturally in food is not as clear as is its effect on iron salts. Feuillen64 supplemented infants diets with 100 mg. of vitamin C daily but could not demonstrate, with balance studies, increased assimilation of food iron or small supplements of inorganic iron.
J
osephs” maintains that no relationshipbe-tween vitamin
C deficiency
or excess
and
iron absorption has ever actually been
dem-onstrated. Moore et however, found
ascorbic acid or foods which contained as-corbic acid greatly enhanced the absorption of food iron. If such findings are applicable to infants, it becomes more difficult to see how or why there is a tendency for infants
whose diets are adequate and supplemented with vitamin C to develop a physiologic
anemia of late infancy, not to mention frank
iron deficiency anemia. If it is a fact that they do develop such an anemia, it would indicate in those who show the changes, that the above figures are not applicable or that there is a serious breakdown in the normal absorption process. Should Josephs and Feuillen’s findings of such high per-centage retention of food iron by infants prove generally correct, and if vitamin C would further enhance the absorption of iron in infants, then it would not be out of the realm of possibility to treat iron de-ficiency anemia by dietary methods 0 In general it appears that there is need for clarification between those who have
per-0 For instance, using the example given in the
preceding footnote, the 8-month-old 18-pound
infant had a hemoglobin iron deficit of 170 mug.
The diet provided 3.4 to 6.8 mg. of iron per day
(or approximately 0.7 mg/kg.). If retentions as
great as 91 per cent (see Feuillen’s Table I”) were
achieved, then 5 mg. of iron would be available
each day. The anemia could be corrected in ap-proximately 34 days. The requirements for growth
as calculated in the previous example would not
be greater than 0.7 mg/day. Under the conditions
presently postulated a diet providing 1 mg. of iron
per day would make that amount of iron
avail-able for the daily hemoglobin increment. Even this
much iron could not be obtained, however, from
a quart of milk per day according to the
concen-tration of iron found in milk by Feuillen and
Plumier,” (0.43 mg/i.) and Steams,’ (0.38 to
0.7 mg./1.). Niccum et al. cite other authorities,
however, who list substantially higher values.” It
is apparent from the previous considerations and
calculations that the percentage assumed for the
amount of iron absorbed has by far the widest
range (1 to 100 per cent) compared to any of the
other factors (blood volume, total iron at birth,
iron content of food, and rate of growth). Thus, if
only 1 to 10 per cent of food iron is absorbed, then
even an iron rich diet (6.8 mg. in an 18-pound
infant or up to 1.5 mg/kg.) may not suffice, even for growth. Therefore, the difference between those
infants who develop the physiologic anemia of
late infancy or even frank iron deficiency anemia
and those who do not, may have little or no
rela-tion to the iron content of the diets, the rate of
growth or the total body iron at birth. It is
formed balance studies and those who have IISe(l other methods to assess iron assimila-tion. Very few points have been settled to
the satisfaction of all workers. There
cer-tainly is need for additional studies of
those aspects of the problem which are
unique to the rapidly growing infant.
IRON TRANSPORT
Once iron has passed the intestinal
mu-cosa, it is picked up in the blood stream.
Within the blood it is transported entirely
in the plasma, bound by a specific protein, transferrin (siderophilin).6 Because it has been conclusively proven that there is no exchange between iron of hemoglobin within erythrocytes and serum iron,67 neither the erythrocytes nor hemoglobin iron enter into a consideration of iron trans-port. Siderophilin is a globulin having a molecular weight of approximately 90,000, the electrophoretic mobility of
beta-glob-ulin, and is present in a concentration of
approximately 0.24 gm./100 ml. of serum;
it forms 3.3 per cent of the total plasma
proteins. In the normal adult, only one-third of it is in combination with iron, resulting in a concentration of serum iron of approxi-mately 130 g./100 ml. and leaving a residual binding capacity of 220 &g./100 ml. of serum. Rath et al.6’ devised a relatively simple test for the measurement of iron binding capacity which has been applied to the study of normal and anemic adults and infants.’#{176} The central role of
sidero-philin in the transport of iron in plasma
is depicted in Figure 3.
Iron deficiency anemias in adults are as-sociated with increased iron-binding ca-ty66’ while in states of iron excess
(aplastic anemias, hemochromatosis,
trans-fusion hemosiderosis) the iron-binding
ca-pacity is nearly or completely saturated.’8
In normal infants and children studies of
iron-binding capacity have been reported
from Sweden by Hagberg8 and in the United States by Smith et al. and from the author’s laboratory.’ Although
differ-ences in degree exist, and the number of
observations reported by Smith et al. are
PLASMA
TRANSPORT
Cl1 Bon#{149}Morrow
::osot ____________________ /
+.iOtiOn + Sderophitsn 4SideroIn(V,UTZAT ON 7ac
Spleen -- STORAGE -ave’
Fr r tin
H.mo,7erin
[7,
$20 Doys
FIG. 3. Ferrous iron from within the intestinal mucosal epithelium is picked up by the circulation,
oxidized to the ferric state in combination with siderophilin (the specific plasma iron-binding beta
globulin) and transported to sites of storage and utilization. Siderophilin also picks up daily a much
larger (uant3ty of iron released from the normal destruction of 1I 120 of the total erythrocyte mass. There
is sufficient excess of siderophilin to react with twice as much iron as is normally present; the excess
siderophilin represents the serum iron-binding capacity. This latter plus the serum iron represents the
REVIEW ARTICLE
few during the critical age period from 1 to 6 months, all workers find, compared to normal adults, an increased total iron-binding capacity (range 300 to 500, mean
400 tg./100 ml.) in late infancy, and con-dude that the data indicate a state of iron depletion. In instances of frank iron de-ficiency anemia, Smith et al. and Hagberg report even a greater increase in iron-bind-ing capacity, in the vicinity of 400 to 650 g./100 ml. of serum.
In adults with infection the iron-binding capacity is reduced.” Few reports on the influence of infection on the iron-binding capacity in children are available. Hag-berg found with severe infections, lasting four days or more, slight lowering of the iron-binding capacity. After cessation of fever, the values had returned to normal within 7 days.
The concentration of serum iron [5.1.] in the adult male is approximately 130 p.g./100 ml. of serum.7” 69, 70 However, the
concentration is quite variable; the above value is found in the morning and it may be reduced to half that amount by late evening.7’ Studies of diurnal variation in in-fants and children have been reported by Maurer72 who found only a relatively small fluctuation of from 10 to 40 p.g./100 ml. from 5 AM. to 10 P.M. In view of the fact that much of the data from normal infants are consistent with an iron deficient state, it is interesting to note that Paterson et al.’3 find in adults a reduced diurnal variation in iron deficiency anemia and many other blood
dyscrasias. Rapid changes in concentration
of serum iron occur following a variety of
stimuli. With infections in adults#{176} the
con-centration of iron decreases to extremely low levels (24 to 64 p.g.) within 24 hours;7t similar changes are noted in the dog on pro-duction of sterile 6975 In the dog
and the rat, cortisone and the adrenal cortex have been shown to play a role in the main-tenance of tile normal concentration of
Se-* The subject of anemia of infection has been
thoroughly reviewed by Cartwright and Win-trobe.2’
rum iron and the production of hypofer-remia associated with infection or other
stress.” The influence of infection on the
level of serum iron in infants has been studied by Hagberg.8 He found a lowering (of 37 to 70 &g.) within 2 days of the onset of infection, returning to normal 2 to 4 days after the fever subsided. Therefore, it would appear that, in infants, serum iron and iron-binding capacity are no more labile to stressful stimuli than in adults or, in propor-tion to the initial levels, even less so.
Values for normal concentrations of serum iron in infants and children have been reported by Hagberg,’ Smith et al.7 and Sturgeon.’ Hagberg’s and Sturgeon’s methods were essentially the same, using orthophenanthroline for final development
of a color;68 both find practically identical
values for normal adult males (137 and 134
tg./100 ml., respectively). Smith et al.7
em-ployed Bipyridine for development of a
color;’7 with this method they report 163
ig./100 ml. as the normal mean value for adult males.’4 There are some differences in the findings of these investigators in chil-dren but all reported, compared to adults, hypoferremia (and elevated iron-binding
capacity as mentioned previously)
essen-tially after the first half year of life. Smith
et al. report a mean value of 132 &g. for the
ages of 1 to 6 months, with a range from
52 to 201 in 6 normal infants. Hagberg’s
mean value of 99 .g. for 52 determinations
on infants 2 to 6 months of age is dis-tinctly lower than the mean value found by Smith. Sturgeon’ found a mean value of 53 .g./100 ml. in 22 normal infants, averaging 4 months of age. This investigator found
the values rose rapidly to concentrations
ill the vicinity of 80 to 100 g./100 ml. by 16 to 24 months of age. These latter values
are in close agreement with those of Hag-berg and Smith for the older age range. All of these investigators have indicated that the low values are suggestive of an iron deficiency state, despite the absence of frank anemia in the children studied.
500
BC
Sl
TIBC
Ftc.. 4. The relation of the concentration of serutui iron (SI.) to the serum iron-binding capacity (I.B.C.) and the total serum iron-l)inding capacity (T.I.B.C.) in normal adult males and various “physiologic”
and pathologic conditions. Note the lack of consistent relation of the serum iron or iron-binding
capacity to the relative rate or efficiency of iron assimilation. Note also the similarity of the pattern of the 1-year-old infant, tile frankly iron deficient sul)ject and pregnant women at term. There is evidence for enhanced iron assimilation in inflamnlatorv states; this is based on the appearance of increased iron
deposits in tissues. However, experiments reported by Gubler et a!. (J. Biol. Chem., 184:563, 1950)
demonstrated reducc(1 assinlilation of iron in the experimentally infected rat.
Iron
Adult Deficiency
infant or
Adult
IRON
ASSIMILATION Normal (10%) increased
and the state of iroll storage. Ill 1OIl
de-ficiency and post-hemorrhagic states the
concentration of serum iron is reduced’8’ 69
and there is depletion of storage iron.7#{176} In hemolytic and refractory anemias there
is hyperferremia’8 and increased iron
stores.72 Ill pregnailcy there is an increased
demand for iron, and the concentration of serum iron is reduced, and the iron
bind-ing capacity greatly increased.’ Figure 4
summarizes the relation between the con-centration of serum iron and iron-binding
capacity under a variety of circumstances.
The mechanism for tile release of
traiis-Port iron from its protein-bound state in the serum to sites of utilization or storage
has not been studied extensively. Laurell6ui
has reviewed the current theories and from
consideration of tile known facts proposes
that siderophilin-iron complex does not leave the serum at sites of utilization but
3Weeks Infection Hemolytic
I year old Ante portum Infont or Anemia
infant Adult Aplastic Anemia
lit Day of Lift
7 2 Increased increased
tilat the iron is liberated intravascularl>’
from siderophilin, the latter remaining in
the serum.
The anabolic pathways concerned in the incorporation of iron into protoporphyrin to form heme, and the ultimate formation of hemoglobin, have not been studied. That such does occur very rapidly, within a few
hours, will be brought out later. The
re-verse process, i.e., the release of hemoglobin from “worn out” 120-day-old erythrocytes and its catabolism, likewise has received
little attention. It has been shown that iron
released from broken down erythrocytes
is rapidly swept hack into the circulation, SOOll to appear in newly formed
hemo-78 However, if there is excess
stor-age iron, a greater fraction of the iron
re-cently liberated from erythrocytes or
as-sinlilated is deposited as storage iron.
/
-i; Fe (.-Enzyme?)
I1
FERRITIN (non stainable
water soluble
‘1’
I
HEMOSIDERIN (stainable
partIculate
\ transportation of iron in the serum from sites of absorption and hemoglobin break-down to those of utilization and storage.
IRON STORAGE
Storage iron in the normal adult com-prises 20 per cent of the total body iron or a quantity equal to approximately 25 per cent of the hemoglobin iron.2” 27 It exists
as an intracellular iron-protein complex in
1 of 2 forms. Normally the largest portion of the iron is in ferritin (60 to 70 per cent); it has a molecular weight of approximately 460,000, is water soluble and contains 17
to 23 per cent iron; it cannot be demon-strated by methods for staining iron.7’ Hemosiderin comprises the rest of storage iron; its chemical composition is similar to that of ferritin, but it contains as much as 35 per cent iron by dry weight. It con-sists of large clusters of iron hydroxide, is water insoluble and is thought to be a less readily available form of iron than ferritin. The hemosiderin granules give a positive
iron stain and are large enough to be
visible microscopically. As mentioned
ear-her, some ferritin is found in the mucosa of
the first part of the intestine following
ingestion of iron but the bulk of it is
lo-cated in the reticuloendothelial and
par-enchymal cells of the liver and spleen.
Under conditions of iron overload other
parenchymal cells (pancreas, adrenals)
con-tam significant quantities of iron.” 21, 27,
40, 79, 80 Figure 5 provides a diagram of the
metabolic pathways of iron through the
various storage compounds found in the
liver, spleen and bone marrow.
Direct measurement of the quantity and
type of iron stored in various organs
neces-sitates weighing the organs involved,
moval of hemoglobin iron, homogenizing
an aliquot, water extraction to fractionate
ferritin from hemosiderin and determination
of iron in the 2 fractions.8’ The amount of tissue require(l precludes the performance
of such measurements on normal
indivi-duals. For estimation of the state of iron
IRON
STORAGE
LIVER PARENCHYMAL CELL
or
Siderophilin
4-Fe4 Sideroph
Macrophage, Bone Marrow, Spleen, Liver
FIG. 5. The iron siderophilin nuolecules reach sites of storage or utilization whereupon the iron is
re-leased and enters the cell in the reduced state. If it is to be stored, rather than synthesized into
hemo-globin, etc., it reacts with apoferritin to form a ferric hydroxide complex, ferritin. The latter, under
con-ditions of ample iron storage, condenses further to hernosiderin which is more stable and contains
rela-tively more iron in proportion to the amount of protein. When iron reserves are needed, the process is
stores under abnormal and physiologic conditions, less accurate indirect ap-proaches are employed.
The concentration of serum iron is in-fluenced by the state of the iron stores; thus, if all other factors which influence the level of serum iron are considered (time of
day, freedom from infection, adrenal
dys-function) a reduced concentration of serum iron is indicative of depleted iron stores,’8 while increased values for serum iron are found in states associated with excessive iron deposition, such as hemochromato-sis.’5 Observations of the former type have been made on normal infants by Hagberg,8 Smith7 and Sturgeon;’ all have concluded that the hypoferremia observed in so many normal infants is likely attributable to de-pletion of iron stores.
Examination of aspirated bone marrow for hemosiderinS2 furnishes a reliable index of iron stores in man; this technique has been employed frequently in adults as a routine clinical method.8385 Marrow par-tides are mounted unstained or stained with Berlin blue and examined microscop-ically for hemosiderin granules. The sam-pies are graded into 4 categories, according to the number of hemosiderin granules present (none to trace, slight, moderate and heavy). This method has revealed more storage iron in normal males than females, increased iron stores in patients with infec-tion, pernicious anemia, cirrhosis of the liver or hemolytic anemia. Reduction in hemosiderin granules is found in iron
de-ficiency states. Stevens et al.8’ stress that a
normal erythrocyte count, concentration of
hemoglobin, and value for serum iron may be associated with no marrow hemosiderin, and that such a situation represents a sub-clinical state of iron deficiency; i.e., the iron stores are depleted without hemato-logic or blood chemical manifestations of iron lack. Of 31 normal subjects studied by Stevens, Coleman and Finch,8’ hemosiderin
in bone marrow was graded as moderate in 6, in 21 as slight and in 4 as none to trace. Of the latter 4, 2 were young women in tile menstrual age range and 2 were children. Smith et al.86 have recently reported such
observations on patients ranging from a few months to 7 years of age, some being essen-tially free of disease, while most suffered from a wide variety of disorders. Only in
young infants was it possible to demonstrate
significant amounts of iron-containing ma-terial in the specimens from normal subjects. Here again is evidence of depleted iron stores in normal infants past the first few months of life. Beutler and Drennan84 dem-onstrated that stainable iron appeared in the marrow rapidly after treatment of iron de-ficiency anemia with parenteral iron, but more slowly or not at all after oral adminis-tration of iron. They stressed, therefore, the importance of continuing oral iron therapy long after the blood picture has returned to normal.#{176}
Indirect estimates of iron stores in adults
have been made by weekly 500 ml. phle-botomies continued to the point that all of the tissue iron had been mobilized.70 It was assumed that the first signs of depletion of iron stores became manifest when the cir-culating erythrocyte mass and the rate of hematopoiesis were reduced. Finch et al. found that under these circumstances a normal adult could lose from 1,200 to 1,500 mg. of iron (equivalent to S I. of blood or approximately half the total blood volume) over a period of 3 to 4 months before signs of depletion appeared. He also concluded that prolonged feeding of iron prior to phlebotomy had little effect on the size of the stores, and that once depleted the stores were rebuilt slowly, requiring one year or more either on a normal diet or a diet supplemented with oral iron. When one notes that the normal infant, in associa-tion with rapid growth during the first year, “phlebotomizes” himself to an equivalent of
2 to 3 times the blood volume (as much as half the blood volume every 2 months) because of the rapidly expanding circula-tory capacity, and if one relates this to the above observation of Finch et al., it is not
0 The opposite viewpoint may be taken logically,
namely, once the hemoglobin has reached a
nor-mal concentration, iron absorption is so poor that
it is futile to attempt to replenish iron stores via
REVIEW ARTICLE
surprising that the measurements which have been made during normal infancy indicate depletion of iron stores.
The number of sideroblasts (normoblasts containing iron granules) have recently been shown by Kaplan and Zuelzer87 to
vary with conditions associated with
in-creased or decreased iron stores. Sidero-blasts are demonstrated by staining bone marrow films, obtained in the routine man-ner, for iron with prussian blue. The iron ap-pears as small, light to dark blue, intracyto-plasmic granules up to 0.5 p.m. in diameter. The percentage of sideroblasts in the marrow of normal infants (20 to 90 per cent) did not
vary much from those in infants with a
variety of nonhematologic conditions or
with hemolytic anemia. However, in iron deficiency anemia the percentage of sidero-blasts was greatly reduced (0.5 to 15 per cent). In general a rough direct correlation
was found between the concentration of
serum iron and the percentage of
sidero-blasts. It would be interesting to know whether in normal adult males the
percent-age of sideroblasts is higher than in normal
infants, inasmuch as all other studies mdi-cate that the latter are relatively depleted
of iron.
Direct measurements of tissue iron in
newborn infants were performed earlier in the century with rather discrepant results. Gladstone’s88 results have been recently
confirmed in the report of Widdowson and
Spray8’ and McCance and Widdowson.Du
The finding by the latter of an average of
273 mg. of iron (201 to 372 mg.) in 6 infants
weighing 3,000 gm. or more is probably representative. They showed that relatively little iron is “stored” in the liver at birth (10 to 25 mg).#{176}Gladstone also concluded that there is no evidence, microscopically or chemically, of large or progressive dep-osition of iron in the liver during the last 4 months of intra-uterine life. Therefore, there is unanimity of opinion today that the full-term infant is born with an
insig-0 In a 1-year-old child weighing 10 kg., this
would be approximately equivalent to 0.3 to 0.9
gm. Hb./100 ml. of blood.
nificant reserve or store of iron in the liver.7 27
Smith8’ determined the hemosiderin iron in livers obtained post mortem from infants who died of a variety of conditions, but eliminated those who were thought to have iron deficiency and those who had received iron by mouth or by blood transfusion.
Al-tilough it is not possible to evaluate the
precise influence of the terminal illnesses on these infants, it is reasonable to suppose that there would usually he a tendency for the illnesses to increase the amount of iron stored in the liver; it has been shown that inflammatory illnesses favor an increase in storage iron by diversion of plasma iron and assimilated iron to the liver.’4’ ‘
Smith’s results show distinctly lower levels of hemosiderin iron during the second year of life.
Although there is need for additional and
confirmatory studies of iron storage in
in-fants, results obtained from the studies
discussed above indicate that iron storage,
in terms of the adult male standard, is
greatly reduced during normal infancy
(after 4 months of age), and that this
prob-ably persists through the second year of life. There is also much evidence that iron in food is poorly absorbed and available in such small quantities (Stearns” and Brockaw’#{176}) that scarcely enough is provided to meet the demand for hemoglobin forma-tion, to say nothing of iron for storage. This concept is contrary to the statement by Josephs : “likewise during periods of growth the iron may be retained beyond the needs for hemoglobin formation” (p. 153, ref. 27). However, it is not clear from Josephs’ state-nlent that it pertains to human infancy; moreover, supporting experimental data are not cited.
COPPER AND FREE ERYTHROCYTE
PROTOPORPHYRIN IN RELATION
TO IRON METABOLISM
Myoqlobin
a
cyf oc h r 0me Iron of copper and iroll. The subject has been
reviewed b’v Cartwright.’2 In more recent
publications, it was brought out that
swine on a low copper diet had impaired
ability to absorb iron from the
gastrointes-tinal tract, an incomplete mobilization of iron from tissues and an inability to utilize
parenterally administered iron for
hemo-globin synthesis. They also demonstrated that copper deficient swine developed an anemia resembling iron deficiency anemia
ill most respects, except that the
concen-tration of serum copper was reduced.’4 However, in iron deficiency anemia there is
characteristically an increase in
concentra-tion of serum copper.’3 The normal con-centration of serum copper in adult males is 105 .g./100 ml. with a standard deviation
of ± 16.” Serunl copper increases in
pregnancy,’ infection and iron deficieny
states,’3 with values ranging from 150 to
250 tg./100 ml. Studies of serum copper in normal infants reported from the author’s
laboratory’ reveal a mean value beyond
8 months of age of 140 and values ranging
as high as 200 .g./100 ml. This high concen-tration has been interpreted as further evi-dence of iron depletion during infancy.’
The concentration of free erythrocyte protoporphyrmn varies in relation to serum iron in a manner similar to that of serum copper”3 in infection, pregnancy and iron
deficieny states; it is increased in all of
these conditions. Studies of its occurrence in normal infancy originally reported from this laboratory’ have been recently con-firmed by Yi-Yung Hsia et al.” Greatly increased values are observed in numerous otherwise apparently normal infants, thus providing additional evidence for iron de-pletion during this age. period.”
To this point, the various steps in the
nletabohsm of iron have been segregated
into distinct compartments. Progression through tllese compartments has been
viewed stepwise with relatively long pauses
at each step. However, studies of tile rates of movement of iron show that there is rapid permeation of iron more or less si-multaneously through some, and into several
IRON
KINETICS
FIG. 6. The central circular wide black line represents the relative amount of iron that enters
and leaves the circulation daily (the iron turnover), and the arrows to and from tile various
surround-ing compartments represent the relative amount
withdrawn and contributed daily by each. Tile
hemoglobin circuit accounts for the great majority because of its relatively huge ..size, despite the 120-day life span of each molecule. Although there is no organ for iron excretion the amount assimilated
per day in the normal adult male (1 mg.) viil equal
the amount lost (not shown in diagram); likewise
at equilibrium each conupartnient will contribute
to the daily turnover the amount withdrawn by
that compartment. UIBC represents the
unsatu-rated iron-binding capacity; SI, the serum iron;
the sum of the 2 values equals the serum total
iron-binding capacity (TIBC).
if not all, of the metabolic compartments. In normal subjects the main flow of iron is into bone marrow for the formation of hemoglobin, while under pathologic condi-tions there is diversion of a considerable amount of iron into storage depots.2 ‘ Figure
6 represents schematically the turnover of iron Witilill tile body.
IRON KINETICS
assimilation of iron salts is known. Iron “tolerance” curves in adults74 and children” show a distinct rise within 2 hours of
in-gesting large amounts (1 gm. or 4 mg./kg.)
of ferrous sulfate. Radioactive iron has been detected in the plasma of anemic dogs within 2 hours of administration and in circulating erythrocytes within 4 hours.’7 The appearance of radioactive iron in
fer-ritin in dog liver has been detected within
1 hour of intravenous injection.’8
Consideration of the dynamics of iron metabolism in terms of ferro-kinetics does not end with formation of the metabolically active compounds of iron (hemoglobin,
myoglobin, cytochromes and catalase). It
is well established that the hemoglobin molecule has a short life span (120 days within the erythrocyte) compared to that of the individual. The life span of the other iron compounds is not known” ‘#{176}#{176}but it
may be assumed that there is continuous anabolism and catabolism and that the rates are different from that of hemoglobin.101 The iron released from the catabolism of these compounds, as pointed out originally in the opening paragraph of this paper, is not excreted, hence, must be fed back into anabolic iron pathways. Thus, once an atom of iron is assimilated it may enter any one of several metabolic circuits and is con-tinuously routed through these circuits the remainder of the individual’s life.
It appears highly probable that all iron released from the catabolism of iron com-pounds finds its way back into the serum’0’
before being resynthesized into storage iron or into new metabolically active iron com-pounds. At equilibrium the concentration of serum iron and rate of turnover represent the net effect of all of these anabolic and catabolic processes going on simultaneously plus assimilation (Fig. 6). Knowledge of the magnitude of the contribution of each circuit to the serum iron equilibrium is necessary to appreciate the relative im-portance of each.
In adult males, studies with radioactive iron show that on an average of every 90 minutes, half of the total serum iron has
been removed;b01103 conversely, if the con-centration of serum iron remains constant,
an equal quantity of iron will have to be returned into the circulation. The total quantities involved may be derived from knowledge of the total serum or plasma
volume (2200 ml.) and concentration of
se-rum iron (130 ,.g./100 ml.). Thus, in adult males the total iron in circulation amounts to approximately 3.0 mg. and, in the course
of 24 hours, 27 mg. of iron are turned
over.’0” ‘#{176}The average daily assimilation
and loss of iron in adult males is 1 mg.;15’ 27
thus dietary sources contribute negligible amounts to the total iron processed daily through the serum. Calculation of the rate of hemoglobin metabolism shows this to be the main source of iron withdrawal and addition to the serum iron circuit.
If the survival period of the hemoglobin molecule is taken as the same length of time
as that of the erythrocyte, the total hemo-globmn mass is synthesized and destroyed
every 120 days, or 1/120 (0.83 per cent)
every day. Based on an estimated volume
of erythrocytes of 30 ml./kg., the
erythro-cyte volume in a 77 kg. man amounts to
2310 ml. of which one-third is hemoglobin,
or 770 gm. For every gram of hemoglobin there are 3.4 mg. of iron, or a total of
2310 mg. of iron (this calculation shows that there is close to 1 mg. of iron per ml. of packed erythrocytes). Thus, daily hemo-globmn metabolism utilizes and releases
2310 mg.
or 21 mg. of iron per day. It is
120
obvious that the previous calculation of the total iron turnover in the plasma ex-ceeds that derived from hemoglobin catab-olism and from assimilation by approxi-mately 5 mg. (25 per cent).101’ ‘#{176}This
cx-cess represents the iron contributed to, and removed from, the circulation by the myo-globin-, cytochrome-, and storage iron cir-cuits.
growing infant will make an additional con-sideration necessary in such calculations. Growth of the organism will be paralleled
by increase in the size of each of the circuits
or compartments considered above. Hence, during periods of rapid growth the total quantities of iron going into the various circuits will not equal that returned
be-cause a certain proportion will be retained,
as a result of the increasing capacity of the
circuit. Therefore, assimilation from the diet will have to make up the deficit. How-ever, it is well established that the slightly hypochromic microcytic erythrocytes of older infants (physiologic anemia of late
infancy) are associated with: 1) anemia
(re-duction in size of the hemoglobin circuit);25
2) reduced concentration of serum
and 3) no evidence of bone marrow iron stores and reduced liver iron stores.8’
The above facts and previous considera-tion pertaining to normal adult males, mdi-cate that in those infants exhibiting the
physiologic anemia of late infancy (and
the associated evidence of reduced iron
stores) the quantity of dietary iron and the
efficiency of gastrointestinal absorption
can-Ilot keep pace with the deficit in tile meta-bolic iron pool created by rapid growth. It is apparent tilat loss of hemoglobin iron to other circuits having a greater demand for iron, and the reduction in concentration of hemoglobin resulting from rapid growth of the blood volume during tile first year of life, constitute a strain on iron nutrition far in excess of that seen during any other period of life.
SUMMARY AND CONCLUSIONS
The infant has no route or organ for the
physiologic excretion of iron. Clinically
in-significant quantities are lost through the skin and gastrointestinal tract.
Assimilation of iron by the adult does
not exceed 10 per cent of that ingested in
tile food; natural foods have a relatively
low content of iron. There is evidence from
i)alance studies tilat at least some infants
may have a greater capacity to absorb iron than adults; confirmatory studies using
iso-topic labeled iron in naturally occurring food, especially milk, would be most help-ful in considering and re-evaluating this
phase of iron nutrition in the infant.
The intestinal mucosa tends to act as a barrier to the rapid entrance of iron into the circulation (mucosal block).
Iron in serum is transported bound to a serum globulin (siderophilin); the latter is present in a two- to threefold excess of the concentration of iron in the serum (this represents the serum iron-binding capacity). Relative to adults, many normal infants
have a reduced concentration of serum
iron and increased iron-binding capacity. Similar changes are seen in iron deficiency states.
Storage iron is found primarily in the
liver and spleen in 2 chemical forms. One,
ferritin, is detectable only by chemical means, and the other, hemosiderin, is
vis-ible microscopically and takes iron stains.
The content of the latter in bone marrow
is reduced in iron deficiency states. Also,
hemosiderin is not found in the bone mar-row of normal infants.
The time required for the assimilation of iron, including metabolic transport across the intestinal mucosa, through the serum iron pool, into the bone marrow and out into the circulating erythrocytes, is very short; as little as 4 hours.
The vast majority of iron metabolized
internally comes from the daily breakdown
of hemoglobin and, to a lesser extent, from other iron compounds such as myoglobin,
cytochrome and ferritin. The dietary iron
assimilated constitutes only a small per-centage of the daily total iron turnover. Studies of iron turnover rates
(ferro-kinet-ics) have not been performed in infants; the magnitude and significance of the addi-tional factor created by rapid growth in such considerations is not known at pres-ent.
The phyisologic anemia of late infancy
is associated with evidence of depletion of
APPENDIX A
Quantitative Considerations on the
Pathogene-sis of the Physiologic Anemia and
“Nutri-tional” Iron Deficiency Anemia in
Late Infancy
From the material considered in the previous sections, it may be concluded that the newborn infant does not have signifi-cant iron stores at birth (average about
1/12 of the total body iron.’#{176} Relative to
the 100 per cent increase in iron that will be required by 1 year of age, were the same concentration of iron to be main-tamed in the several iron compartments (storage, transport, hemoglobin, myoglobin,
cytochromes), this value becomes
approxi-mately 1/24. Available data indicate that the demands of parenchymal tissues (myo-globin and cytochromes) for iron, though relatively small, are met at the expense of all others.’#{176}4’HO It is apparent, therefore, that either assimilation from the diet must provide approximately a threefold increase in iron for these remaining compartments or they must be “depleted” and “diluted”
by growth, hence the concentration of iron
reduced.
J
osephs has presented calculations, based on consideration of blood volume, concen-tration of hemoglobin, estimated needs for tissues and rate of growth, for an “average baby” born at term and followed up to 2 years of age. He compares the total iron present at birth to that required by this hypothetical infant at various ages, and the difference represents the amount which must be assimilated from the diet to pre-vent depletion of stores, hemoglobin iron or both. The conclusion drawn from his calculations is that stores supposed to be present at birth play an extremely small part in supplying the needs of the growing infant for iron, and that the blood of the newborn infant is the major source of iron to be used later in maintaining levels of hemoglobin. In this opinion there is general concurrence.” 8, 88, However, based onrecently available studies of blood volume in the newborn and on concentrations of
hemoglobin in umbilical cord blood, there appears to be ample reason to reduce the estimates of tile contribution of the new-born infant’s hemoglobin as a source of iron for future use. It is also apparent, from
recently available data, that in estimates of
this sort one must consider not only the mean values for concentration of hemoglo-bin, blood volume, and rate of growth, but
also consider the problem utilizing values
toward the outside limits of the normal
range. Tables I, II and III list values
im-portant to such calculations as assumed for
3 hypothetical normal newborn infants, “A,”
“B” and “C.”
Considerations of this type in the past
have tended to overestimate the mean value and range for the concentration of hemo-globin at birth. Josephs assumes that a mean value of 20 gm./100 ml. is conserva-tive.27 Yet concentration of hemoglobin in cord blood recently determined in the au-thor’s laboratory yielded substantially lower values; 20 gm./100 ml. was not observed in a single normal newborn infant. The method employed, the precision and tech-nique of standardization, have been de-scribed elsewhere.’ When applied to 33 normal adult males in the Los Angeles area, a mean value of 15.7 gm./100 ml. was ob-tamed; a widely accepted normal value for such a population.10’ Our data on 77
speci-mens of blood from the umbilical cord show
a range of 12.8 to 19.2 gm./100 ml., a mean
of 15.73 and a standard deviation of 1.61 gm. Mollison et al.b06 recently published normal values derived from a study of 133
normal infants. They found a 95 per cent
range of 13.6-19.6, a mean of 16.55 and a
of close to 16 gm. and a range of 13 to 19
gm./100 ml. These are the values employed
in the calculations that follow.
Blood volume at birth has been estimated
by Mollison et al. by an isotope dilution
method.’#{176}T In 34 normal infants they found
a mean value of 84.7 ml./kg. and a range
of from 70 to 100 ml./kg., approximately. In connection with the present calculations, it is of considerable interest that Mollison
et a!. expected on theoretical grounds that the blood volume would increase with the venous hematocrit. The value for blood volume of 90 ml./kg. for the largest hypo-thetical infant, also having the highest con-centration of hemoglobin (19 gm./100 ml.), and a value of 80 ml./kg. for the small in-fant, with the lowest concentration of hemo-giobin, are consistent with Mollison’s oh-servations and theoretical considerations.
The total blood hemoglobin iron at birth is derived from multiplying (3.4 mg. of iron/gm. Hb.) times the total hemoglobin (gm. Hb./100 ml. X blood volume -i-- 100).
The amount of iron in the liver and spleen at birth is taken from the data of McCance and Widdowson.#{176}#{176}It is assumed that all of
tllis is available for future use in
hemoglo-bin or other tissues having greater demand for iron, such as myoglobin. The iron con-tent of myoglobin is the same as that of hemoglobin, 3.4 mg/gm.,58 but there is a discrepancy of 500 to 700 per cent in esti-mates of the myoglobin content of muscle.
Drabkin26 lists the 70 kg. man, assuming 43 per cent muscle mass, as having 40 gm. of myoglobin, whereas Bi#{246}rk” estimates,
assuming the same muscle mass, 200 gm. as
the myoglobin content. According to Drab-kin’s estimates myoglobin constituted only a relatively insignificant fraction of the total hemoglobin iron (5 per cent). Bi#{246}rk,how-ever, considers it equivalent to 25 per cent of hemoglobin iron. Should the values
given by BiOrk be applicable to infants, the
iron requirement for this compartment dur-ing the first year is magnified by growth another two- to threefold. Thus the total iron in myoglobin in an infant at 1 year,
as-suming a threefold weight increase, would
equal 50 per cent of the hemoglobin iron at birth. The growing infant, therefore, would not only be diluting the concentration of hemoglobin by a rapidly increasing blood volume but would be decreasing it by the withdrawal of two-thirds of this iron to function in the more vital compound, myo-globin.
The role of skeletal muscle myoglobin and other iron compounds having a more
vital requirement for iron than hemoglobin
(parenchymal iron) is probably
overesti-mated in the above considerations. First,
although practically nothing has been done
to evaluate the iron requirements of these
substances in infants, in the few studies by
Bi#{246}rkin this age range certain trends are apparent. He found a low content of myo-globin at the latter part of fetal life, in-creasing to almost normal values at the
beginning of the second year (3 per cent of
the dry weight of muscle is myoglobin).
The value for the mean myoglobin content of abdominal musculature in 3 infants under 1 month of age was 0.62 gm./100 gm. as
against 1.67 in 4 children 1 month to 1
year (mean 7 months), and 1.43 in muscu-lature from extremities (average age 9 months). He brought out, however, that the efficiency of myoglobin extraction is not 100 per cent and that this inefficiency is more pronounced the younger the age. In general it appears from Bi#{246}rk’swork that a reasonable estimate of the myoglobin
content of skeletal muscle at 1 year of age
is 2 per cent of the dry weight (Fig. 66, p. 107, ref. 99). It is also reasonable to al-low an additional 0.5 per cent for unex-tracted myoglobin and other iron com-pounds or a total of 2.5 per cent. At birth, as the myoglobin content is reduced, an allowance of 1.5 per cent appears reasona-ble. That the values forming the basis of
these latter estimates are in need of
re-examination is emphasized by Bi#{246}rkwho
states, “obviously great uncertainty
char-acterizes these values. Thus the majority of
these particular cases can hardly be defined
