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Article

Less Than 80% of Absorbed Iron Is Promptly Incorporated into Erythrocytes of Infants¹

Department of Pediatrics, University of Iowa, Iowa City, IA and ^* Department of Preventive Medicine and Community Health, The University of Texas Medical Branch, Galveston, TX

²To whom correspondence should be addressed.

	ABSTRACT

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS Procedures Statistical analysis RESULTS DISCUSSION REFERENCES

 
Erythrocyte incorporation of an administered iron isotope has been used as a surrogate for iron retention on the assumption (validated in normal and iron-deficient adults) that 80–100% of the retained isotope is promptly incorporated into circulating erythrocytes. This assumption has not been validated in infants or children. The purpose of our study was to determine concurrently in normal infants absorption and erythrocyte incorporation of the stable isotope, ⁵⁸Fe. In a preliminary study (Study 1), we demonstrated that fecal excretion of ingested isotope occurs predominantly during the first 4 d after administration but continues beyond 7 d after ingestion, that is, beyond the point at which isotope in feces can be explained either by excretion of isotope that failed to enter enterocytes or by exfoliation of isotope-enriched enterocytes. In Study 2, we administered ⁵⁸Fe to nine younger (age 20–69 d) and nine older (age 165–215 d) term infants and collected feces for 11 d. Geometric mean retention of ⁵⁸Fe by the younger infants was 31.2% of intake at 4 d and 26.9% at 11 d, and by the older infants, 35.0% at 4 d and 32.5% at 11 d. Erythrocyte incorporation of ⁵⁸Fe 14 d after ingestion was 5.2% of the dose by the younger infants and 12.5% by the older infants. Utilization of retained (11 d) isotope thus was 19.8% by the younger infants and 38.3% by the older infants. We conclude that far less than 80% of retained isotope is promptly incorporated into erythrocytes (utilized) by infants.

KEY WORDS: • iron absorption • erythrocyte incorporation of iron • infants • iron balance studies

	INTRODUCTION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS Procedures Statistical analysis RESULTS DISCUSSION REFERENCES

 
In both industrialized and less industrialized countries, iron deficiency is the most common nutritional deficiency disorder of infants and young children. Infants are particularly at risk because the amount of iron needed for growth is so great that the abundant iron stores present at birth are commonly exhausted by 4–6 mo of age (Fomon 1993a ). Iron deficiency in infancy and early childhood is of concern because it is a step in the development of iron deficiency anemia, and iron deficiency anemia is associated with evidence of delayed cognitive development (Lozoff 1998 , Pollitt 1999 ). There is, therefore, an urgency to find effective means of preventing iron deficiency anemia and its forerunner, iron deficiency.

Strategies for preventing iron deficiency require precise knowledge of the availability of iron from the diet and from dietary supplements, and this knowledge must come from studies of iron absorption under defined conditions. (We use the term "absorption" here in the nonspecific fashion in which it is most commonly used in the literature; we shall later offer a specific definition applicable to this study.) The abundant data from studies of iron absorption by adults may not be predictive of absorption by infants and young children, and it is therefore necessary to obtain the information in the specific population at risk.

Absorption of iron can be determined by studies in which iron intake and fecal excretion of iron are determined concurrently over several days. However, such metabolic balance studies are cumbersome and labor-intensive; when iron absorption is a small percentage of intake, as it often is, the results are subject to large errors. Use of iron isotopes in metabolic balance studies greatly decreases the errors associated with determination of iron intake but does not decrease the errors related to accurate determination of fecal excretion.

Whole-body counting at an appropriate time after administration of the radioisotope, ⁵⁹Fe, permits determination of isotope retention without the necessity of fecal collection. With meticulous attention to calibration of the counter, adjustment for background radiation and the inclusion of preliminary whole-body counting within 24 h after administration of ⁵⁹Fe, the results with whole-body counting are accurate, and such results can be used as a standard against which to judge other methods. However, whole-body counting and other methods that involve use of radioisotopes are no longer considered acceptable for studies of children.

In normal and iron-deficient adults, it has been demonstrated that 14 d after ingestion of a radioisotope, 80–100% of the retained isotope (as determined by whole-body counting) is generally present in erythrocytes (Heinrich and Fischer 1982 , Larsen and Milman 1975 ). Determination of erythrocyte incorporation of iron has therefore been used widely in adults as a surrogate for iron absorption (Bothwell et al. 1979 , Lynch 1984 , Skikne and Baynes 1994 ). Because conditions in which an iron isotope can be used as a tag or label for dietary iron have been well defined, much of our current knowledge about factors affecting iron absorption in adults has been obtained by using erythrocyte incorporation of radioiron (Cook et al. 1972 , Hallberg 1981 , Lynch 1984 ). Many investigators (Abrams et al. 1997 , Davidsson et al. 1994 , Engelmann et al. 1998 , Hertrampf et al. 1986 , Hurrell et al. 1998 , Kastenmayer et al. 1994 , Rios et al. 1975 , Stekel et al. 1986 ) have assumed that in infants and children, as in adults, 80–100% of absorbed iron is promptly incorporated into erythrocytes, and iron absorption has been estimated on this basis. Our reluctance to accept this assumption without validation led to the study reported here.

Current methodology permits the determination of erythrocyte incorporation of iron with the use of stable rather than radioisotopes of iron (Janghorbani et al. 1986 ) and studies of erythrocyte incorporation of a stable iron isotope have been conducted with infants and children (Abrams et al. 1997 , Davidsson et al. 1994 , Engelmann et al. 1998 , Fomon et al. 1988 , 1993c , 1995 and 1997 , Hurrell et al. 1998 , Kastenmayer et al. 1994 ). For determining the relative bioavailability of an iron isotope from different foods or with different modes of administration (e.g., fasting or included with a feeding), the method is quite satisfactory. However, interpretation of results in absolute terms (the quantity absorbed) is hampered by the lack of data concerning the proportion of retained iron that appears promptly in circulating erythrocytes of infants and children.

This study was undertaken to obtain information in normal infants on the relation between absorption and erythrocyte incorporation of an iron isotope.

	SUBJECTS AND METHODS

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS Procedures Statistical analysis RESULTS DISCUSSION REFERENCES

 
Study design.

Study 1 was designed to determine the pattern of fecal excretion of ⁵⁸Fe after administration of a dose of the isotope. An attempt was made to collect one fecal specimen each day for 12–14 d after isotope administration, and these specimens were analyzed to determine the extent of ⁵⁸Fe enrichment of the stools. We were surprised to find that the feces were still appreciably enriched with the isotope 2 wk after its ingestion, and this observation influenced the design of Study 2. In addition to the stools collected daily for 12–14 d, a few fecal specimens obtained 26–112 d after ⁵⁸Fe dosing were obtained from three of the infants participating in Study 1 and from one infant participating in Study 2; these stools were also examined for ⁵⁸Fe enrichment.

Study 2 was designed to determine concurrently absorption and erythrocyte incorporation of ⁵⁸Fe. The isotope was given in two doses on the same day. Fecal collections were performed for 11 d in three successive pools (Pool 1, 96 h; Pool 2, 72 h; Pool 3, 96 h), and blood was obtained before and 14 d after dosing. Because of the possibility that the extent of absorption and of erythrocyte incorporation of iron would be age related, we studied one group of infants < 90 d of age and another group > 150 d of age. We also determined plasma ferritin concentration, which is inversely correlated with absorption and erythrocyte incorporation of iron (Bezwoda et al. 1979 , Charlton et al. 1977 , Cook et al. 1974 , Disler et al. 1975 , Heinrich et al. 1977 , Walters et al. 1975 ).

The study protocols were reviewed and approved by the University of Iowa Committee on Research Involving Human Subjects. The study procedures were explained to one or both parents and written consent was obtained.

Definition of terms.

In work with stable isotopes of iron, it is necessary to correct for the background presence of the isotope. Unless specified otherwise, we shall use the term, "⁵⁸Fe," to refer to isotopic label (i.e., after correction for background).

We define absorption as entry of iron from the intestinal lumen into the enterocyte with subsequent transfer of the iron from the enterocyte to the circulation. According to this definition, unabsorbed iron includes iron that fails to enter the enterocyte and iron that enters the enterocyte but is returned to the intestinal lumen with exfoliation of the enterocyte. The life span of villous cells in the adult human duodenal and jejunal mucosa is 5–7 d (Klein and McKenzie 1983 ), and the life span of cells in similar locations in the infant’s intestines is presumably no longer than that of the adult. Therefore, it is likely that fecal excretion of the isotope during the first 4 d after isotope administration (our fecal Pool 1) consists of isotope that never entered the enterocytes, of isotope that entered the enterocytes but was returned to the lumen when the enterocytes were sloughed, and a small amount of absorbed isotope that was reexcreted into the lumen. We suspect that fecal excretion of isotope from d 5 to 7 (Pool 2) consists mainly of isotope sloughed with enterocytes and reexcreted isotope. Fecal enrichment beyond 7 d (Pool 3) presumably consists almost entirely of reexcreted isotope. Our operational definition of absorption is ingested dose of ⁵⁸Fe minus the ⁵⁸Fe excreted in the feces during the first 4 d; our definition of retention is dose minus ⁵⁸Fe excreted in feces, and is applicable to any duration of excretion, although in relation to this study, we shall consider primarily 11 d.

Subjects.

The seven subjects in Study 1 (1 boy, 6 girls) ranged in age from 54 to 165 d at the time of isotope administration. They included four normal term Caucasian infants with birth weights > 2500 g, a set of twins born at 37 wk gestation with birth weights of 2765 and 2355 g, respectively, and one infant born at 39 wk gestation with birth weight of 2295 g.

The subjects in Study 2 were 17 normal infants with birth weights > 2500 g. With the exception of one American-Asian infant, they were Caucasian. Infants were enrolled in the following two age groups: "younger infants," ranging from 20 to 69 d of age at the time of isotope administration and "older infants," ranging from 165 to 215 d of age (Table 1 ). One infant (Subject 6459) was studied at both ages and one infant (Subject 6455) in the older age group had participated in Study 1.

All infants in Study 1 and all but three of the infants in Study 2 had been fed commercially available iron-fortified infant formulas from birth to the time of enrollment. Three of the older infants in Study 2 (Subjects 7353, 7302 and 6455) had been breast-fed initially and then fed iron-fortified formulas. After enrollment, infants were fed a low-iron formula for 1 wk before administration of ⁵⁸Fe and throughout the 14 d (Study 1) or 11 d (Study 2) of fecal collections. The formula (Similac, Ross Products Division, Abbott Laboratories, Columbus, OH) provided 2.0 mg Fe/L or 0.714 mg/MJ (0.3 mg/100 kcal). Infants >140 d of age were also permitted to receive other foods low in iron.

Administration of ⁵⁸Fe.

Isotopically enriched ⁵⁸Fe in elemental form was obtained from Isotec S.A. (St. Quentin, France, ⁵⁸Fe abundance 91.90 atom%) and from Cambridge Isotope Laboratory (Cambridge, MA, ⁵⁸Fe abundance 93.2 atom%). A precisely weighed amount of isotopically enriched ⁵⁸Fe was converted to ferrous sulfate and made up to volume as previously described (Janghorbani et al. 1986 ). For administration, a precisely weighed amount of solution containing ~6.8 µmol (0.4 mg) or 13.6 µmol (0.8 mg) ⁵⁸Fe was made up to 5 mL in a 50 g/L glucose solution containing 10 mg ascorbic acid. In Study 1, a single oral dose of 13.6 µmol (0.8 mg) of ⁵⁸Fe was given to each infant 3 h after a morning feeding, except Subject 7252, who was given two doses of 6.8 µmol (0.4 mg) each with an interval of 4 h between doses. No feedings were given for 1 h after dosing. In Study 2, each infant was admitted to the Lora N. Thomas Metabolism Ward after a morning feeding at home. Three hours after that feeding, a dose of 6.8 µmol (0.4) mg of ⁵⁸Fe was given. One hour later, a feeding was given, and 3 h after this feeding, a second dose of 6.8 µmol (0.4 mg) of ⁵⁸Fe was given. The isotope was delivered directly into the back of the oral cavity by syringe in a small volume to decrease the likelihood of regurgitation. For the next hour, the infant was not fed but remained in the ward and was observed closely for the possibility of regurgitation before being discharged. We had planned to eliminate from the study any infant for whom there appeared to be a possibility of loss of isotope, but regurgitation did not occur. Where reference to "dose of ⁵⁸Fe" is made, it concerns the total 13.6 µmol (0.8 mg) of ⁵⁸Fe.

	Procedures

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Study 1.

Using disposable diapers (Huggies, Kimberly-Clark, Neenah, WI), an attempt was made to collect one fecal specimen of sufficient size each day for 14 d after isotope administration. Stool was scraped into plastic containers by the parents and kept refrigerated until shipped to the laboratory.

Study 2.

A stool specimen for a baseline measurement was obtained from each infant before ⁵⁸Fe was administered, and quantitative fecal collections were started at the time of administration of the first dose of ⁵⁸Fe. For most infants, feces were collected entirely in the home with disposable diapers. However, in five subjects, collection of Pools 1 and 3 was made partly in the metabolism ward and partly in the home; Pool 2 was collected always in the home. Collections in the ward were made during the daytime using methods previously described (Fomon 1993b ), and continued in the home during the evening and night hours using disposable diapers. Stool wipes (acid-washed cloth) were used in the ward and in the home. In the home, soiled diapers and wipes were placed individually in sealable plastic bags labeled with name, date and time, and were kept frozen until transported to the laboratory, where they were stored at -20°C until processed.

Using a disposable spring-loaded device (Tenderfoot, International Technidyne, Edison, NJ), blood samples were obtained by heel stick before administration of ⁵⁸Fe (baseline) and 14 d after administration.

Laboratory methods.

Diapers were processed by cutting out the portion of each diaper that contained stool. The outer plastic sheet was removed, and stool plus diaper plus wipes, if any, were placed in a 250-mL porcelain crucible. The soiled portion of the diaper was in some cases small enough to be accommodated by one crucible; in other cases, two or more crucibles were required. The crucible contents were dried in an oven at ~85°C and then covered with porcelain lids and placed in a muffle furnace, where the temperature was gradually increased from 150 to 250°C, kept at 250°C until smoke was no longer detectable (usually 24 h), and then raised incrementally to 525°C and kept there for 24 h. After cooling, ash was taken up in 2.2 mol/L HNO₃, and dissolved ashes belonging to the same fecal pool were combined and stored in polypropylene bottles. Feces collected in the metabolism ward (without the use of diapers) were ashed in identical fashion. Iron concentration of dissolved ashes was determined by atomic absorption spectrophotometry (model 560 Perkin Elmer, Norwalk, CT). The ⁵⁸Fe/⁵⁷Fe isotope ratio (IR⁵⁸Fe/⁵⁷Fe) was determined by inductively coupled plasma mass spectrometry as previously described for blood (Fomon et al. 1995 ).

Blood was analyzed for hemoglobin concentration by Coulter Counter model M430 (Coulter Electronics, Hialeah, FL) and for IR⁵⁸Fe/⁵⁷Fe as previously described (Fomon et al. 1995 ). Plasma was analyzed for ferritin concentration by RIA, using the Quantimune kit (catalog number 190-2001, Bio-Rad Laboratories,Hercules,CA).

Data analysis.

The dose of ⁵⁸Fe was calculated from the measured weight of the administered ⁵⁸Fe-enriched ferrous sulfate solution and the ⁵⁸Fe concentration of the solution. Fecal excretion of ⁵⁸Fe derived from the dose (⁵⁸Fe*_excr) was calculated from the total iron content (Fe_f) of the relevant fecal pool (e.g., Pool 1) and the IR⁵⁸Fe/⁵⁷Fe of that fecal pool (IR_f) as follows:

where ⁵⁸A* is the abundance of ⁵⁸Fe in the dose, ⁵⁷A is the natural isotopic abundance of ⁵⁷Fe, IR₀ is the fecal IR⁵⁸Fe/⁵⁷Fe at baseline (before ⁵⁸Fe administration), ⁵⁷A* is the isotopic abundance of ⁵⁷Fe in the dose, and IR* is the IR⁵⁸Fe/⁵⁷Fe in the dose. Absorption was calculated as dose minus excretion period 1, and 11-d retention as dose minus excretion periods 1 + 2 + 3.

Erythrocyte incorporation of ⁵⁸Fe was calculated from the IR⁵⁸Fe/⁵⁷Fe in blood, hemoglobin concentration and body weight, assuming a blood volume of 65 mL/(kg·d) and 3.47 mg Fe/g hemoglobin, using a formula (Fomon et al. 1995 ) analogous to the formula given above for fecal isotope excretion. Incorporation was expressed as a percentage of the ingested dose. Incorporation expressed as a percentage of absorbed or of 11-d retained isotope is referred to as "utilization."

	Statistical analysis

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Values (%) for absorption, retention, erythrocyte incorporation and utilization of ⁵⁸Fe, and values for plasma ferritin concentration were transformed to natural logarithms before statistical analysis to compensate for nonnormal distributions (Cook et al. 1969 ). Thus statistical comparisons pertain to geometric mean values throughout, but for purposes of comparison with the literature, arithmetic means are also provided. Descriptive, associative and comparative statistics were performed with SAS version 6.12 (SAS Institute, Cary NC) using Pearson correlations, regression and general linear models procedures with and without covariate adjustment for ferritin. Significance was set at the = 0.05 error rate, and all tests are stated at the per comparison error rate, not adjusted for experiment-wise error rates involving multiple variables.

	RESULTS

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS Procedures Statistical analysis RESULTS DISCUSSION REFERENCES

 
Study 1

Figure 1 presents the log IR⁵⁸Fe/⁵⁷Fe of each individual stool sample against time for the seven infants during the 14 d after ⁵⁸Fe administration. ⁵⁸Fe-enrichment of the stools was high during the first 96 h after dosing and was relatively low and gradually declining thereafter. The mean IR for stools obtained 12–14 d after dosing was 0.1563, which was significantly greater (P < 0.001) than the baseline value of 0.1330. The slope of the fecal IR on time from 7 to 12 or to 14 d postdose was significantly negative for each of the infants, ranging from -0.018 to -0.003. The combined regression of IR was significantly negative with a slope of -0.011 from 4 to 14 d (P < 0.01) and a slope of -0.010 (P = 0.024) from 7 to 14 d. The IR of two specimens obtained 26 d after dosing were 0.1359 and 0.1345, and the IR of one specimen obtained 42 d after dosing was 0.1343. These values were significantly (P < 0.05) greater than the baseline value of 0.1330. The IR values of five fecal specimens collected from three infants 77 to 112 d after dosing ranged from 0.1327 to 0.1333 and did not differ significantly from baseline values.

View larger version (20K): [in this window] [in a new window]   Figure 1. Fecal isotope excretion by infants during the 14 d after isotope administration. The figure shows fecal isotopic enrichment expressed as the 58Fe/57Fe isotope ratio (IR) on a natural logarithmic scale vs. time after 58Fe administration. Each dot indicates the IR for a single fecal specimen, and the dots for each of the seven infants are connected by a line. The geometric mean IR for stools obtained 12–14 d after dosing was 0.1563, which was significantly greater (P < 0.001) than the mean baseline value of 0.1330.

Age, gender, body weight, and concentrations of hemoglobin and plasma ferritin at the time of ⁵⁸Fe administration are presented for each infant in Table 1 . Also included are concentrations of hemoglobin and plasma ferritin 14 d after ⁵⁸Fe dosing. In the younger infants, hemoglobin concentrations ranged from 95 to 169 g/L at the time of dosing (the highest value was that of the youngest infant), and in the older infants, from 108 to 143 g/L at the time of dosing. Geometric mean plasma ferritin concentration of the younger infants was 156 µg/L at the time of dosing and 116 µg/L 14 d later; corresponding values for the older infants were 36 and 30 µg/L.

    Excretion of ⁵⁸Fe. Table 2 presents data on excretion of ⁵⁸Fe during the three fecal collection periods. It is evident that isotope excretion occurred mainly during Period 1. Excretion was 67.5% of the dose in the younger infants and 58.9% in the older infants (P = 0.19). As expected from Study 1, excretion of ⁵⁸Fe continued in Periods 2 and 3, albeit at a low and declining rate. There was no apparent difference in excretion rate between younger and older infants.

    Erythrocyte incorporation of ⁵⁸Fe. Erythrocyte incorporation of ⁵⁸Fe averaged 5.2% of the dose in the younger infants and 12.5% in the older infants (P = 0.06). Incorporation of ⁵⁸Fe was not significantly correlated with plasma ferritin in either age group (Table 3 ). However, incorporation of ⁵⁸Fe was significantly correlated with absorption of ⁵⁸Fe in the younger infants (r = 0.74, P = 0.023) but not in the older infants.

	DISCUSSION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS Procedures Statistical analysis RESULTS DISCUSSION REFERENCES

 
Study 1 provided unequivocal evidence that fecal excretion of ingested isotope continues beyond 7 d after ingestion, that is, beyond the point at which the presence of the isotope in feces can be explained either by excretion of isotope that failed to enter enterocytes or by exfoliation of isotope-enriched enterocytes. Thus, the isotope present in feces beyond 7 d after ingestion must represent reexcretion. Our data suggest that the feces continue to be detectably enriched with the isotope for 6–11 wk after isotope administration.

In Study 2, we quantitated isotope excretion for 11 d after isotope administration. As was to be expected on the basis of the results of Study 1, the quantity excreted over 11 d was larger than the quantity excreted over 4 d. Consequently, the quantity absorbed (4-d retention) was greater than the quantity retained over 11 d. A problem in comparing results of earlier studies that use different approaches to determining iron "absorption" relates to the different times at which the endpoint is reached. Studies conducted by the metabolic balance method are commonly of 3 or 4 d duration. Balance studies without the aid of an iron isotope have been performed in term infants (Feuillen 1954 , Josephs 1939 , Maurer et al. 1934 , Schulz-Lell et al. 1987 and 1991 , Stearns and Stinger 1937 , Wallgren 1933 ) and in preterm infants (Dauncey et al. 1978 ), and metabolic balance studies with an iron isotope have been carried out in term infants (Fairweather-Tait et al. 1987 , Garby and Sjölin 1959 , Martinez et al. 1998 , Schulz and Smith 1958 ) and preterm infants (Ehrenkranz et al. 1992 , Gorten et al. 1963 , Widness et al. 1997 ). The endpoint has been 14 d in ⁵⁹Fe whole-body counting studies of term infants (Götze et al. 1970 , Heinrich et al. 1969 , Saarinen and Siimes 1977 , Saarinen et al. 1977 ), and in double isotope studies of preterm infants (McDonald et al. 1998 , Zlotkin et al. 1995 ).

It may be calculated from the data in Table 2 that excretion of ⁵⁸Fe from 4 to 11 d after isotope administration accounted for 6% of total 11-d isotope excretion for the younger infants and 3.6% for the older infants. Although our fecal collections were limited to 11 d, we estimate that for the total interval from 4 to 14 d after ⁵⁸Fe administration, isotope excretion by the younger infants might have reached 7 or 8% of the total 14-d excretion, and isotope retention might therefore have been as much as 8% less if determined at 14 rather than at 4 d after isotope administration. Nevertheless, it is by no means certain that the difference in elapsed time between isotope administration and the point at which retention is determined introduces more uncertainty than errors in the methods themselves.

All of the approaches are subject to procedural and methodologic errors. In balance studies that do not include administration of an isotope, the major difficulties are accurate measurement of intake and fecal excretion of iron. As already noted, these errors can be somewhat decreased by inclusion of an isotope and by designing the study so that iron retention will be relatively high.

As already mentioned, whole-body ⁵⁹Fe counting is the most precise method of determining iron retention but requires meticulous attention to procedural aspects of the counting; unfortunately, several of the reports of whole-body ⁵⁹Fe counting in infants fail to give details of the methods and validation procedures. We consider the whole-body counting studies of Heinrich and co-workers (Götze et al. 1970 , Heinrich et al. 1969 ) to be most relevant to consideration of our data. Normal infants similar in age to those in our study were included; the dose of isotope was given between meals with ascorbic acid, and the iron content of the dose was similar to that used in our study. Fortunately, the studies of Heinrich and co-workers were done with great care and whole-body counting was done before ⁵⁹Fe administration (permitting correction for background) as well as 8 to 12 h and 14 d after ⁵⁹Fe administration (Heinrich et al. 1966 ).

Heinrich et al. (1969) determined iron retention in this manner with a large number of term and preterm infants. Arithmetic mean iron retention by 39 iron-sufficient term infants < 3 mo of age was 20.7% of intake. In a subsequent publication (Götze et al. 1970 ), which probably included some of the same infants as the report just mentioned, ⁵⁹Fe retention by iron-sufficient term infants with birth weights >2500 g was reported in three age groups. The second age group included 42 infants 1–3 mo of age, roughly corresponding to the age range of our younger infants. Arithmetic mean 14-d retention was 29% of intake, a value quite similar to our 11-d (arithmetic) mean retention of 27.8% of intake. The third age group studied by Götze et al. (1970) included 25 infants 4–6 mo of age with arithmetic mean retention of 37% of intake. This value is nearly identical to our 11-d value of 36.8% for infants from 165 to 215 d of age. Because of the similarity of our findings to the whole-body counting findings of Götze and co-workers, we conclude that the 11-d retention values we obtained by metabolic balance studies are valid, and we have no reason to suspect that the 4-d absorption values are less valid.

There are no previous reports concerning term infants in which retention of an iron isotope has been combined with determination of erythrocyte incorporation of the isotope, but four reports have been published concerning preterm infants. Two of these reports (Ehrenkranz et al. 1992 , Widness et al. 1997 ) were based on metabolic balance studies with ⁵⁸Fe, and two (McDonald et al. 1998 , Zlotkin et al. 1995 ) were based on variations of the double isotope method. The double isotope method is based on the assumption that the availability of iron for erythropoiesis is identical when administered orally or intravenously. This assumption has been validated in adults under specified conditions (Lunn et al. 1967 , Pitcher et al. 1965 ), but has not been validated in infants. Moreover, the provision of substantial amounts of an iron isotope intravenously over 12–24 h (Zlotkin et al. 1995 ) and the nonconcurrent administration of oral and intravenous doses of isotope (McDonald et al. 1998 ) have not been validated even in adults. We have therefore elected to omit a review of the double isotope studies and to consider only the ⁵⁸Fe metabolic balance studies.

For purposes of comparing our results with those from the two studies of preterm infants, we have based our utilization data on the basis of 11-d retentions and have used arithmetic means (Table 4 ). Ehrenkranz et al. (1992) administered ⁵⁸Fe between feedings in a total iron dose of ~0.27 mg without ascorbic acid to preterm infants (mean body weight 1.43 kg) and conducted metabolic balance studies for 7 d. Mean retention of iron was 41.6% of intake and mean utilization was 28.7%. Widness et al. (1997) determined ⁵⁸Fe retention in erythropoietin-treated and control (untreated) infants (mean body weight 1.43 kg) by 10-d metabolic balance studies after administering a dose of 6.0 mg of ⁵⁸Fe-enriched ferrous sulfate with ascorbic acid. The difference in ⁵⁸Fe retention between treated and control groups was not significant; the arithmetic mean retention of ⁵⁸Fe for all subjects was 34.0% of the dose, and utilization was 12.1%. Thus, the data on utilization of ⁵⁸Fe provided by Ehrenkranz et al. (1992) and Widness et al. (1997) demonstrated for preterm infants, as we have demonstrated for term infants, that utilization of retained iron by infants is substantially less than the 80–100% observed in adult subjects (Heinrich and Fischer 1982 , Larsen and Milman 1975 ).

	FOOTNOTES

 
¹ Supported by U.S. Public Health Service grant HD 32938.

Manuscript received June 7, 1999. Initial review completed August 24, 1999. Revision accepted October 11, 1999.

	REFERENCES

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS Procedures Statistical analysis RESULTS DISCUSSION REFERENCES

 

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