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Community and International Nutrition

Exclusive Breast-Feeding for 6 Months, with Iron Supplementation, Maintains Adequate Micronutrient Status among Term, Low-Birthweight, Breast-Fed Infants in Honduras¹

Department of Nutrition and Program in International Nutrition, University of California, Davis, CA

²To whom correspondence should be addressed. E-mail: kgdewey{at}ucdavis.edu.

	ABSTRACT

TOP ABSTRACT SUBJECTS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
There is little information on the risk of micronutrient deficiencies during the period of exclusive breast-feeding. We evaluated this among term, low-birthweight (LBW; 1500–2500 g) infants in Honduras. Mother-infant pairs were recruited in the hospital and assisted with exclusive breast-feeding during the first 4 mo. At 4 mo, infants were randomly assigned to either continue exclusive breast-feeding to 6 mo (EBF; n = 59) or be given iron-fortified complementary foods (rice, chicken, fruits, and vegetables) from 4 to 6 mo while continuing to breast-feed (SF, n = 60). Blood samples were collected at 2, 4, and 6 mo and analyzed for hemoglobin (Hb), hematocrit, plasma ferritin, % transferrin saturation, vitamin A, vitamin B-12, folate, zinc, and erythrocyte folate. Infants with Hb < 100 g/L at 2 or 4 mo were given medicinal iron supplements for 2 mo; the proportion administered iron drops did not differ significantly between groups. There was no significant effect of complementary foods on indices of vitamin A, B-12, folate, or zinc status. Among infants not given medicinal iron at 4–6 mo, iron status was higher in the SF group than the EBF group. In those given medicinal iron at 4–6 mo, iron status was higher in the EBF group, suggesting that complementary foods interfered with iron utilization. About half of the infants were anemic by 2 mo, before the age when complementary foods would be recommended. This supports the recommendation that LBW infants should receive iron supplementation in early infancy. Given that infants given iron supplements did not benefit from complementary foods at 4–6 mo, we conclude that exclusive breast-feeding for 6 mo (with iron supplementation) can be recommended for term, LBW infants.

KEY WORDS: • breast-feeding • low birth weight • micronutrient • complementary feeding • anemia

In response to accumulating evidence regarding the benefits of exclusive breast-feeding for both mothers and infants, the World Health Organization issued a statement in May, 2001 recommending exclusive breast-feeding for 6 mo (1). The background document (2) cautioned that further research was required on the risk of micronutrient deficiencies within the first 6 mo, especially in susceptible infants. Low-birthweight³ (LBW; <2500 g) infants are susceptible to micronutrient deficiencies because they are more likely than normal-birthweight infants to be born to malnourished mothers and to have lower reserves of nutrients such as iron (3), zinc (4), and vitamin A (4,5). LBW infants represent up to 30% of births in some developing countries (6). Therefore, it is important to evaluate the risk of micronutrient deficiencies in LBW infants during the period of exclusive breast-feeding.

We previously reported the results of a randomized trial comparing breast milk intake, total energy intake, and growth of LBW infants in Honduras who were given complementary foods beginning at 4 mo of age or exclusively breast-fed until 6 mo of age (7). We selected only infants who were born at term, and thus were considered small for gestational age. The results indicated no growth advantage of complementary feeding before 6 mo, even with hygienically prepared foods of high nutritional quality. In this paper, we present results on the micronutrient status of the same infants, focusing on the following two questions: 1) What is the proportion of exclusively breast-fed LBW infants with indices of iron, zinc, folate, vitamin B-12, and vitamin A status suggestive of nutrient deficiencies during the first 6 mo? and 2) Does consumption of high-quality complementary foods from 4 to 6 mo have any effect on indices of micronutrient status?

	SUBJECTS AND METHODS

TOP ABSTRACT SUBJECTS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
    Study design. The study was a prospective, observational study from birth to 4 mo of age, followed by a randomized intervention trial from 4 to 6 mo. At 4 mo, infants who remained eligible were randomly assigned (by week of birth) to either the exclusive breast-feeding (EBF) group, which continued exclusive breast-feeding to 6 mo, or the solid foods (SF) group, which was exclusively breast-fed for 4 mo and continued breast-feeding while receiving complementary foods from 4 to 6 mo. Mothers in the SF group were encouraged to maintain their baseline (4 mo) nursing frequency. Lactation guidance was provided to all subjects in the hospital and every week thereafter in the home. Subjects were not informed of their group assignment until they had completed the first 4 mo of the study. The protocol was approved by the Human Subjects Review Committee of the University of California, Davis.

Mothers and infants were recruited at the two main maternity hospitals in San Pedro Sula, Honduras. Initial selection criteria were as follows: 1) birthweight 1500–2500 g; 2) gestational age 37 wk [based on a standardized assessment (8)]; 3) maternal age 15 y; 4) mother did not plan to work outside the home before 6 mo postpartum; and 5) mother willing to exclusively breast-feed for 6 mo. Twins and infants with severe medical conditions were excluded.

Subjects were visited weekly from birth to 6 mo to assess infant growth and morbidity. In addition, observers stayed in the homes for 12 h at 14, 19, and 24 wk postpartum to record infant feeding patterns and measure solid food intake in the SF group. Infant blood samples were collected at 2, 4, and 6 mo. In a 50% subsample (subjects who were willing to stay overnight at our central facility), we assessed breast milk intake by test-weighing for 24 h at 4 and 6 mo (7).

    Blood sampling and analysis. Infant blood samples were collected at 2, 4, and 6 mo. Blood was collected in the morning by venipuncture (using a butterfly needle) into 4-mL vacutainer tubes containing lithium heparin. Care was taken during the collection of blood samples to minimize light exposure, which can affect vitamin A levels. Measurements of hemoglobin (Hb), hematocrit (Hct), and mean corpuscular volume (MCV), and assessment of a blood smear were performed using standard methods at a local laboratory with well-documented quality control procedures. An aliquot of whole blood was transferred to a separate vial and frozen immediately for later analysis of RBC folate levels. The remaining blood was centrifuged at 3400 x g for 5 min and 0.5 mL of plasma was transferred to each of two amber vials (for analysis of vitamin A, folate, vitamin B-12_, ferritin, and C-reactive protein) and one clear vial (for analysis of iron and zinc). Plasma samples were frozen at –20°C for later analysis at the Clinical Nutrition Research Unit, UC Davis.

Plasma ferritin concentration was determined by double antibody RIA (Diagnostic Products). Transferrin and C-reactive protein were analyzed by rate nephelometry (Beckman Immunochemistry Systems). Plasma iron and zinc concentrations were analyzed by flame atomic absorption spectrophotometry (9). Plasma vitamin A was measured by HPLC. Plasma and RBC folate and plasma vitamin B-12 were analyzed by radioassay (Bio-Rad, Quantaphase II). All analyses were done in duplicate with the exception of folate and vitamin B-12, which were done only once due to lack of sufficient sample. When the CV between duplicates was not within an acceptable range (e.g., <10%) the sample was reanalyzed.

Infants identified as anemic (Hb < 100 g/L) at any of the blood sampling time points (2, 4, and 6 mo) were given iron supplements [5 mg/(kg · d)] for 2 mo and then retested. Infants given iron were considered separately in data analyses in which iron status was the outcome.

    Infant dietary intake. The complementary foods for the SF group were provided in jars (Beech-Nut Nutrition) and included rice cereal with applesauce (fortified with iron sulfate, ascorbic acid, thiamin, niacin, and riboflavin), chicken, fruit (banana and pear with pineapple, both fortified with ascorbic acid), and vegetables (carrots, squash, and mixed vegetables). Rice cereal was fed 2 times a day; chicken, a fruit and a vegetable were each fed once each day. In the subsample of infants with data on breast milk intake, total nutrient intakes at 6 mo were calculated from both breast milk and solid foods, using assumed values for breast milk micronutrient concentrations (10). Nutrient intake from solid foods was calculated on the basis of data collected at 24 wk, using nutrient composition data provided by the manufacturer and values for folate and vitamin B-12 from The Food Processor Plus (ESHA Research).

    Maternal vitamin-mineral supplement use. Information on maternal consumption of vitamin or mineral supplements during pregnancy was obtained by asking the mother which products she had consumed and by examining the product labels. For each of the 5 nutrients examined in this study (iron, vitamin A, vitamin B-12, folate, and zinc), prenatal supplement use was coded simply as "any" or "none" because no quantitative information was obtained on the doses consumed.

    Data analysis. Data were analyzed using EpiInfo and PC-SAS. Analysis of covariance was used to determine the effect of the feeding intervention on micronutrient status at 6 mo, controlling for initial micronutrient status at 4 mo and iron supplementation at 4–6 mo. When sample sizes permitted, the interaction between intervention group and iron supplementation was also examined. Other potential confounders (infant sex, birthweight, gestational age, and weight gain from birth to 4 mo; maternal height, BMI, parity, education, prenatal vitamin/mineral supplements, number of visits for prenatal care, and socioeconomic status) were included in these analyses but were not retained in the final models because they did not alter the results.

There is uncertainty regarding appropriate cutoff values for defining abnormal micronutrient status during infancy. For the purposes of this analysis, we used the following cutoffs: Hb < 100 g/L; Hct < 0.30; MCV < 70 fL; plasma ferritin < 12 or < 20 µg/L (11); plasma transferrin saturation < 12% (12); plasma vitamin A < 0.35 (13) or < 0.7 µmol/L (12); plasma folate < 11.3 nmol/L (1 SD below the mean for breast-fed infants in Norway; 14); RBC folate < 215 (the 5th percentile among breast-fed infants in Norway; 14) or < 317 nmol/L (12); plasma vitamin B-12 < 96 or < 136 pmol/L (15); and plasma zinc < 9.2 µmol/L (16,17).

	RESULTS

TOP ABSTRACT SUBJECTS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
    Sample size and characteristics. Figure 1 shows the study design, the number of subjects given iron supplements, and Hb status. In total, 222 infants were enrolled and 163, 128, and 119 were assessed at 2, 4 and 6 mo, respectively. At 2 and 4 mo, there were no significant differences between participants and nonparticipants with respect to infant sex, gestational age, or birthweight, length, ponderal index, or head circumference, nor in maternal height, BMI, age, education, income, or prenatal care. Characteristics of subjects who completed the intervention phase (n = 119) and those who dropped out between 4 and 6 mo (n = 9) were discussed previously (7). Dropouts had significantly lower birthweight, head circumference, Apgar score at 5 min, and maternal age. Of those who remained in the study through 6 mo, 44% were male, and mean values were 2364 ± 137 g for birthweight, 23.3 ± 3.3 kg/m² for maternal BMI, and 5.7 ± 2.7 y for maternal education. Characteristics of those who completed the study did not differ significantly between the EBF and SF groups (7).

View larger version (32K): [in this window] [in a new window]   FIGURE 1 Flow chart of study design, attrition, iron supplementation, and hemoglobin status. Reasons for exclusions and dropouts are presented elsewhere (7). The number of infants given iron supplements at 2 and 4 mo is larger than the number who were anemic because some nonanemic infants were given iron supplements on doctor’s orders.

    Nutrient intake. There was no significant difference in total energy intake, but the SF group consumed significantly more protein, iron, zinc, vitamin A, and vitamin C than the EBF group at 6 mo (Table 1). The difference in iron intake was very large because the cereal product was iron-fortified. The difference in vitamin A intake was large because of the fruits and vegetables consumed by the SF group. Vitamin B-12 intake was significantly lower in the SF group due to the decrease in breast milk intake (although chicken was one of the foods provided to the SF group, mean consumption of chicken was only 22 g/d).

View larger version (11K): [in this window] [in a new window]   FIGURE 2 Cumulative probability of Hb remaining above 100 g/L at 2 (n = 163), 4 (n = 128), and 6 mo (n = 119) among infants in the EBF and SF groups. The EBF group was exclusively breast-fed from 4 to 6 mo; the SF group received solid foods as well as breast milk from 4 to 6 mo.

Plasma ferritin concentrations were very high at 2 mo, and declined over time even in infants given iron supplements. Despite the fact that about half of the infants at 2 mo had Hb < 100 g/L, none of them had a ferritin concentration < 12 µg/L, and only 2 had a ferritin value < 20 µg/L. At 4 mo, the percentage with ferritin < 12 µg/L was 2.6% (excluding subjects with elevated C-reactive protein values). At 6 mo, infants who had never received iron supplements were more likely to have ferritin < 12 µg/L than those who had received iron at 2 and/or 4 mo (23.1 vs. 2.6%, P = 0.01, Fisher’s exact test). Within the subgroup that did not receive iron supplements at 4–6 mo, the decrease in ferritin between 4 and 6 mo was significantly greater in the EBF than in the SF group. Due to the robbery of the blood samples and resulting very small sample sizes in the subgroup given iron supplements at 4–6 mo, there was insufficient statistical power to compare the change in ferritin from 4 to 6 mo between the EBF and SF groups in this subgroup.

Transferrin saturation declined from a mean of 54.7% at 2 mo to 35–37% at 4–6 mo. There were no cases of low transferrin saturation (<12%) at 2 or 4 mo and only 1 case at 6 mo. Among those not given iron supplements at 4–6 mo, transferrin saturation values at 6 mo did not differ between the EBF and SF groups. Despite the very small sample size of the subgroup given iron supplements at 4–6 mo, there was a significant difference in transferrin saturation at 6 mo between the EBF and SF groups in this subgroup (higher in the EBF group). As a result, the interaction between intervention group and iron supplementation was significant (P = 0.04).

    Vitamin A, folate, vitamin B-12, and zinc. Table 4 shows data for vitamin A, folate, vitamin B-12, and zinc status for all infants at 2 and 4 mo, and Table 5 shows data on these indices by intervention group at 4 and 6 mo.

Plasma folate concentrations were 24, 41, and 35 nmol/L at 2, 4, and 6 mo, respectively. There were 4 infants with low values (<11.3 nmol/L) at 2 mo, and none at 4 or 6 mo. There was no significant difference in plasma folate at 6 mo between the EBF and SF groups, nor any significant interaction between intervention group and iron supplementation.

RBC folate concentrations increased from 367 to 453 nmol/L between 2 mo and 4–6 mo. A substantial proportion of the infants had values below the usual cutoff for adults (317 nmol/L), i.e., 43, 14, and 25% at 2, 4, and 6 mo, respectively. The proportion with values < 215 nmol/L was17, 5, and 6% at 2, 4, and 6 mo, respectively. Controlling for the baseline value at 4 mo, there was no significant difference in mean RBC folate at 6 mo between intervention groups, nor any significant interaction between intervention group and iron supplementation. At 6 mo, 40% of the EBF infants had a value below the usual cutoff for adults (317 nmol/L), compared with only 9% of the SF group (P < 0.01). However, the EBF and SF groups differed in this respect before the intervention at 4 mo (23 vs. 5% with RBC folate < 317 nmol/L, respectively; P = 0.01); when this was taken into account, the difference at 6 mo was not significantly associated with consumption of solid foods.

Plasma vitamin B-12 concentrations were 215, 249, and 210 pmol/L at 2, 4, and 6 mo, respectively; 16–23% had a value < 136 pmol/L (considered marginal) and 6–10% had a value < 96 pmol/L (considered deficient). There was no significant difference between intervention groups in plasma vitamin B-12 at 6 mo, nor any interaction between intervention group and iron supplementation.

Plasma zinc concentration ranged from 11.1 µmol/L at 2 mo to 11.8 µmol/L at 6 mo. The percentage with values < 9.2 µmol/L was 18.9% at 2 mo, but only 6–7% at 4 and 6 mo. Plasma zinc at 6 mo did not differ significantly between intervention groups. The percentage with values < 9.2 µmol/L at 6 mo was 2% in the EBF group and 9% in the SF group.

    Relation of infant micronutrient status to maternal prenatal vitamin-mineral supplement use. There was no significant association between infant iron, vitamin A, vitamin B-12, or zinc status and maternal prenatal consumption of supplements containing these nutrients. Infant plasma folate concentrations were unrelated to whether the mother took prenatal folate supplements, but RBC folate concentrations were significantly higher among infants of mothers who consumed prenatal folate supplements than in those whose mothers did not: 387 ± 161 (n = 102) vs. 322 ± 136 (n = 42) nmol/L at 2 mo, respectively (P = 0.02), and 474 ± 184 (n = 55) vs. 381 ± 127 (n = 23) nmol/L at 6 mo (P = 0.009); the difference was not significant at 4 mo. The difference at 6 mo remained significant when controlling for intervention group.

	DISCUSSION

TOP ABSTRACT SUBJECTS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Of the micronutrients examined in this study (iron, zinc, vitamin A, folate, and vitamin B-12), the results indicate that these term, low-birthweight, exclusively breast-fed infants were at risk for iron deficiency, whereas the risk of other micronutrient deficiencies was either low or difficult to ascertain. A major challenge in making these assessments is the lack of well-substantiated cutoff values for diagnosing micronutrient deficiencies during infancy. We will address this issue below for each of the micronutrients assessed.

Almost 50% of these LBW infants had Hb < 100 g/L at 2 mo. The appropriate cutoff value to use for defining anemia at 2 mo is unclear, given that this age represents the physiologic nadir in Hb concentration during infancy. However, in a study of 91 term, normal birthweight, predominantly breast-fed Danish infants, mean Hb at 2 mo was 115 g/L and the 5th percentile was 101 g/L (18). Results from randomized trials with LBW infants (19) suggest that iron deficiency is the usual cause of low Hb, which is not surprising given the relation between birthweight and liver iron reserves. In our previous study of Honduran infants (20), Hb values at 6 mo were strongly associated with birthweight, i.e., among unsupplemented, exclusively breast-fed infants who weighed <3000 g at birth, 50% had Hb levels < 103 g/L, compared with only 5% of infants weighing >3000 g at birth. The present results confirm previous observations that LBW breast-fed infants are at high risk for iron deficiency within the first 6 mo of life. Although the prevalence of low Hb and Hct values was high, very few infants had low values for plasma ferritin at 2 or 4 mo or for percentage transferrin saturation at any age. Ferritin concentration is normally very high at birth and declines steeply during the first 6 mo. In this cohort, ferritin concentration was not correlated with Hb or Hct values at any age (data not shown). Because of the rapid hematological changes that occur during early infancy, it is unclear what cutoff values to use for ferritin and transferrin saturation.

The effects of providing iron-fortified complementary foods on iron status in these infants depended on whether they were receiving supplemental iron drops. In those who did not receive iron drops at 4–6 mo, Hb, Hct, MCV, and ferritin levels at 6 mo were higher in the SF group than the EBF group. However, in those who received iron drops at 4–6 mo, Hb, Hct, and transferrin saturation levels were higher in the EBF group than in the SF group. This suggests that iron utilization from iron drops was superior in those who were still exclusively breast-fed, perhaps due to lower iron absorption when complementary foods were part of the diet. Over the entire 6-mo study period, the cumulative probability of remaining nonanemic at 6 mo without the use of iron drops was virtually identical in the two intervention groups (Fig. 2). Given that 48% of the cohort became anemic within the first 2 mo, supplemental iron is advisable long before complementary foods are recommended. Our results suggest that maintenance of exclusive breast-feeding while providing supplemental iron drops will maximize the response to iron treatment among LBW infants.

Plasma zinc concentrations in our LBW cohort were higher than those of breast-fed infants in Finland (21) and Denmark (22). In comparison with values for small-for-gestational-age infants in Chile who were randomly assigned to Zn-supplemented or placebo groups (23), mean values in this study were higher than those for the placebo group and lower than those for the Zn-supplemented group at 2 and 4 mo, but higher than those of both Chilean groups at 6 mo. The percentage with plasma zinc < 9.2 µmol/L was 19% at 2 mo, but only 5–7% at 4–6 mo. It is uncertain whether this cutoff value is appropriate, given that the mean values in Denmark and Finland were close to this level and the 10th percentiles in those two populations were 6–8 µmol/L at 2 mo and 6–7 µmol/L at 4–6 mo. There are many limitations to using plasma zinc as an index of zinc status, particularly for breast-fed infants, in whom it is difficult to control for postprandial variation. Nonetheless, these results provide no evidence that zinc deficiency is more common among term, LBW, exclusively breast-fed infants in Honduras than it is among healthy, exclusively breast-fed infants in affluent populations. Even though mean zinc intakes of infants in the SF group were twice those of the EBF group, there was no significant effect of intervention group on plasma zinc or growth (7), which supports the conclusion that zinc status was adequate. In the zinc supplementation trial among small-for-gestational-age infants in Chile (23), there was a positive effect of zinc on linear growth from birth to 6 mo in infants who received some infant formula, but not in those who were exclusively breast-fed for at least 4 mo. This implies that exclusive breast-feeding in early life may be at least partially protective against zinc deficiency, perhaps due to the high bioavailability of zinc from human milk and the reduced risk of diarrhea. Whether exclusively breast-fed, term, LBW infants will benefit from supplemental zinc may depend on prenatal zinc nutriture. In Bangladesh, LBW infants of mothers who received zinc supplements during pregnancy had a lower risk of diarrhea, dysentery, and impetigo during the first 6 mo than LBW infants whose mothers received placebo, although there was no effect on birthweight, postnatal growth from birth to 6 mo, or serum zinc concentrations (24). In a subsequent zinc supplementation trial of Bangladeshi infants from 4 to 24 wk of age, there were positive effects of zinc on weight gain and lower respiratory infection among infants with plasma zinc < 9.2 µmol/L at 4 wk, but not among infants with higher baseline plasma zinc concentrations (25).

Of our LBW cohort, <5% had plasma vitamin A concentrations < 0.35 µmol/L at 2, 4, or 6 mo. The cutoff value of 0.35 µmol/L for infants 1 mo old (rather than 0.7 µmol/L) was suggested by Lindblad et al. (13) based on their observation that infants normally have lower serum vitamin A levels than adults. In our cohort, there was no significant effect of consuming complementary foods rich in provitamin A carotenoids on plasma vitamin A concentrations at 6 mo. This supports the conclusion that vitamin A deficiency was uncommon.

Of the LBW infants, <3% had a plasma folate concentration < 11.3 nmol/L, but 5–43% (depending on age and the cutoff value used) had low RBC folate. The appropriate cutoff values for these indices during infancy are unclear. In exclusively breast-fed Finnish infants, plasma folate concentrations were 34 nmol/L at 2 mo and 50 nmol/L at 4–6 mo, and were about 2–3 times higher than their mothers’ plasma values (who were taking folate supplements during lactation) (26). The lowest value observed was 11.3 nmol/L. Similar plasma folate values were found for breast-fed infants in Norway (14) and the United States (27). In the Norwegian infants, mean RBC folate concentration was 619 nmol/L, whereas in the U.S. group, it was 900 nmol/L from 2 to 6 mo. Compared with these values, our cohort of LBW infants had somewhat lower mean plasma concentrations (24 nmol/L at 2 mo and 41 nmol/L at 4–6 mo) and substantially lower RBC concentrations (368–440 nmol/L). There was no significant difference in plasma or RBC folate between the EBF and SF groups; this is not surprising, however, given that estimated total folate intake did not differ significantly between groups (although one limitation of our study is that we did not measure the vitamin content of the mothers’ breast milk). In the absence of well-documented cutoff values for infants, it is difficult to draw conclusions about the prevalence of folate deficiency in this cohort. However, the very low prevalence of low plasma folate levels and the decline in prevalence of low RBC folate levels between 2 and 4–6 mo (from 16.7 to 5%) both suggest that prenatal folate status may have been more of a limiting factor than intake of folate from breast milk. This is consistent with our finding that RBC folate levels at 2 and 6 mo were significantly higher among infants whose mothers had taken prenatal folate supplements than in those whose mothers had not.

The appropriate cutoff value for plasma vitamin B-12 in infants is also not well documented. Few (10%) of the LBW infants had a value < 96 pmol/L, but 16–23% of the infants had a value < 136 pmol/L. Mean values were 210–250 pmol/L. Median plasma vitamin B-12 concentration of 47 macrobiotic children (10–20 mo of age) in the Netherlands was 149 pmol/L, compared with 404 pmol/L in the omnivorous controls (15). Of the macrobiotic children, 19% were considered to be deficient (<96 pmol/L) and 45% had levels < 136 pmol/L. In our LBW cohort, plasma vitamin B-12 concentration was not significantly correlated with MCV (a marker of macrocytic anemia) at any age, nor was there any evidence of hypersegmented neutrophils; thus the clinical implications for those with low plasma concentrations are unclear. There was no significant difference in plasma vitamin B-12 between the EBF and SF groups, even though estimated total vitamin B-12 intake was significantly lower in the SF group. However, we assumed that breast milk vitamin B-12 concentration was normal, which may not have been the case given that 31% of periurban lactating women in nearby Guatemala had low milk vitamin B-12 concentrations (28).

One unforeseeable limitation of our study was the loss of a substantial proportion of blood samples during a robbery at our study facility. As a result, the comparisons of changes in micronutrient status between 4 and 6 mo between the EBF and SF groups are based on a smaller sample size than intended. However, we found no evidence that the loss of samples introduced a bias that would have affected our conclusions.

We conclude that iron deficiency is a concern among LBW breast-fed infants, but that there is little evidence of zinc or vitamin A deficiency in this population. No conclusions can be drawn about the prevalence of folate or vitamin B-12 deficiency until there is a better understanding of appropriate cutoff values for infants. The results support the recommendation that LBW infants should receive medicinal iron drops beginning at 1–2 mo of age (29). Among the study infants who were not given iron drops at 4–6 mo, iron-fortified complementary foods had a positive effect on iron status, but in those who were receiving iron drops at 4–6 mo, complementary foods appeared to interfere with iron utilization. Consumption of complementary foods from 4 to 6 mo did not influence growth (7) or any of the other indices of micronutrient status. The definition of exclusive breast-feeding allows for the use of vitamin-mineral supplements when warranted (30). Given the advantages of exclusive breast-feeding for 6 mo for both infants and mothers (1,2), particularly the risks of diarrheal disease associated with contaminated complementary foods in developing countries, it would be safer to ensure adequate iron status of LBW infants through the use of medicinal iron drops rather than iron-fortified complementary foods before 6 mo. The infants in this study showed little evidence of zinc deficiency, but in other populations of LBW infants, there may be combined iron and zinc deficiency, in which case use of medicinal preparations containing both iron and zinc may be warranted. Because it is possible that both iron and zinc supplements may adversely affect copper status (17,31), inclusion of copper in such preparations should be considered. In populations in which maternal vitamin status is questionable, improving maternal diets or providing supplements to the mothers should prevent vitamin deficiencies in the infants (with the possible exception of vitamin D in some situations). Therefore, exclusive breast-feeding for 6 mo, with iron (and possibly zinc and copper) supplementation, can be recommended for term, LBW infants.

	ACKNOWLEDGMENTS

 
We are grateful to the Honduran research team for their dedication and to the mothers of San Pedro Sula for their willingness to participate in the study. We acknowledge the collaboration of Leonardo Landa Rivera of Medicina Infantil, and Liga de la Lactancia Materna, Honduras, the local institutions involved in the study, and the Ministry of Health, Honduras, for assistance with the project. We thank the Centro de Investigaciones Medicas in San Pedro Sula for the on-site laboratory analyses, and Janet Peerson for her expert statistical advice.

	FOOTNOTES

 
¹ Supported by UNICEF and Wellstart International (through the Health Services Division, Office of Health, Bureau of Global Programs, Field Support and Research Development, U.S. Agency for International Development, under the terms of Cooperative Agreement no. DPE-5966-A-00–1-45–00). The opinions expressed herein are those of the authors and do not necessarily reflect the views of the U.S. Agency for International Development or Wellstart International.

³ Abbreviations used: EBF, exclusively breast-fed group; Hb, hemoglobin; Hct, hematocrit; LBW, low birth weight; MCV, mean corpuscular volume; SF, solid foods group.

Manuscript received 2 September 2003. Initial review completed 16 October 2003. Revision accepted 12 February 2004.

	LITERATURE CITED

TOP ABSTRACT SUBJECTS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 

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