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Section of Nutrition, Department of Pediatrics, University of Colorado School of Medicine, Denver, CO 80262
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONREFERENCES |
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KEY WORDS: • iron • zinc • breastfed infants • complementary foods
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONREFERENCES |
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An important consideration for meeting iron requirements in early
infancy is the contribution of human milk. The iron concentration of
iron in human milk is relatively low, 0.2–0.4 mg/L, and concentrations
decline only modestly over the course of lactation. The efficiency of
absorption of iron from human milk is quite high, averaging ~50%
(Dallman 1988
). Total body iron is relatively stable
from birth through ~4 mo of age, but the distribution changes
somewhat as stores are utilized to support growth. Between 4 and 12 mo,
there is a substantial increase in the erythrocyte mass, and to a
lesser extent, in the myoglobin in lean tissue (Dallman 1988
). This increase in iron requirements between 4 and 12 mo
cannot be met readily by the iron available from human milk, and other
sources of dietary iron become increasingly important by midway through
y 1 of life (Institute of Medicine 1993
).
The young infant has a relatively high zinc requirement to support the
very rapid growth of early infancy. Milk zinc concentrations are quite
high in the early weeks postpartum, averaging >3 mg/L at 2 wk, but
then decline sharply over the early weeks of lactation (Krebs et al. 1995
). Although the zinc concentrations in
well-nourished women are relatively resistant to changes in
maternal zinc intake, considerable variability in milk zinc
concentrations exists among women. There is also quite strong
"tracking" of concentrations, with significant correlation between
concentrations in early (2 wk postpartum) and mature milk (5–7 mo)
(Krebs et al. 1995
). Despite increases in volume of milk
intake over the early weeks postpartum, the steep decline in milk zinc
concentrations results in a longitudinal decline in zinc intake as well
(Krebs et al. 1994
) (Fig. 1
). As shown in the figure, factorial estimates of longitudinal
requirements for net absorption of zinc also are highest in the early
weeks of life (Krebs and Hambidge 1986
). The gap between
intake and requirement narrows considerably between 2 wk and 5 mo of
age; by 7 mo, the intake from human milk is nearly equal to estimated
requirements for net absorption (Fig. 1)
. In such circumstances, the
efficiency of absorption would have to be close to 100% to meet
estimated requirements (Krebs and Hambidge 1986
).
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). Other foods, which may have more nutritional
relevance, are less readily accepted. In combination with neophobia,
fear of new things, this often gives parents the impression that the
initial rejection indicates a dislike of a particular food
(Birch and Grimm-Thomas 1996
).
Consideration of past patterns of introduction of complementary foods
indicates an extremely wide range of practice (Fomon 1993
). For centuries, cereal gruels have been commonly
introduced to very young infants. In the early part of this century,
beef broth and meats were fed within y 1 of life, but green vegetables
were withheld until nearly 3 y of age. One of the most radical
sets of recommendations were those in the 1950s that promoted
introduction of foods from all major food groups by 3 wk of age
(Fomon 1993
). Currently, the American Academy of
Pediatrics (AAP) recommends that introduction of solids be delayed for
breastfed infants until ~6 mo of age (American Academy of Pediatrics 1997
). Additional infant feeding guidelines from the
AAP also note that, if complementary foods are withheld until ~6 mo
of age, the order of introduction of specific foods is not critical
(American Academy of Pediatrics 1998
). In a report on
meeting iron needs during infancy, the Institute of Medicine
recommended introduction of solids by 4–6 mo. That report suggested
that iron needs of the infant can be met either by introduction of
iron-fortified cereal (especially concurrently with a source of
vitamin C) or by introduction of meat at this age (Institute of Medicine 1993
).
Current practices for introduction of complementary foods have been
examined recently (Skinner et al. 1997
). Of nearly 100
infants studied at 6 mo of age, cereals were consumed by the greatest
number, followed closely by vegetables, with considerably fewer taking
fruits. Meats were not consumed by any of the infants at 6 mo. By
1 y of age, over half of the infants were consuming cereals and
fruits, but less than half were reported to be consuming meat or meat
mixtures. Only 10% of the infants consumed beef at 12 mo. The authors
commented on the striking popularity and frequency with which certain
foods were consumed at 12 and 16 mo: "bananas, toasted oat cereal,
cheese, chicken, crackers, potatoes and yogurt." These observations
suggest that, as recommended, iron-fortified cereals were consumed
as an early complementary food, and likely contributed importantly to
meeting infants’ iron needs. The calculated nutrient intakes between 6
and 12 mo equaled or exceeded the Recommended Dietary Allowance (RDA)
for iron and most other micronutrients, but with zinc as a notable
exception. Between 12 and 24 mo, mean zinc intake was estimated
consistently at 50–60% of the RDA (NRC 1989
,
Skinner et al. 1997
).
The nutritional implications of different choices of complementary
foods are quite striking. The protein, iron and zinc content of fruits,
iron-fortified rice cereal, beef and poultry are shown in
Figure 2
(Pennington 1989
). Although fruits are commonly
introduced as an early "solid," they contribute very little
protein, iron or zinc; they do provide fiber and certain vitamins.
Iron-fortified infant cereals obviously contribute substantial
amounts of iron. Beef and chicken contribute several-fold more
protein, moderately more zinc and much less iron, compared with the
cereal. It can also be seen from the figure that although both meat and
poultry are good sources of protein, beef is considerably higher than
chicken in zinc and approximately equivalent in iron (Pennington 1989
).
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.
An intervention study examined the effect of greater intake of meat as
a complementary food (Engelmann et al. 1998b
). In this
study, 8-mo-old partially breastfed infants were randomly assigned to
either a low (10 g/d) or high (27 g/d) meat group for a 2-mo
intervention period. The meat puree contained beef, lamb, poultry or
fish; the remainder of energy intake (milk, cereal, bread, fruit) was
chosen freely by parents. Iron-fortified formula intake and
iron-fortified gruels were not controlled. The results after the
intervention indicated that protein and iron intakes were greater in
the high meat group. Total iron and zinc intakes were not significantly
different between groups, a finding attributed to the low meat group
having consumed more iron-fortified gruel, breads and cereals.
Comparison of biochemical indices of iron and zinc status at the end of
the intervention indicated that for those in the low meat group,
hemoglobin declined significantly, whereas for the "high" meat
group, hemoglobin was minimally lower. There were no differences in
ferritin or plasma zinc between the two groups. Given that intakes of
iron were not different, the authors concluded that the iron in meat
was better absorbed and resulted in improved hematologic status
(Engelmann et al. 1998b
).
These investigators also examined the effect of the addition of meat on
nonheme iron absorption from a vegetable puree in a subgroup of
10-mo-old infants. Iron absorption was studied by iron stable isotope
incorporation into erythrocytes. Nonheme iron absorption was
significantly higher from the vegetable puree containing meat; addition
of 25 g meat to 100 g vegetable puree led to a 2.7-fold
increase in total iron absorbed. The amount of iron absorbed from the
meat-enriched vegetable puree was ~30% of the daily iron
requirement; the amount of iron absorbed from the vegetable puree was
~12% of the daily iron requirement. No significant correlations
between nonheme iron absorption and other outcome measures were found,
although the small number of subjects in the absorption studies may
have precluded detection of such relationships (Engelmann et al. 1998a
).
We have taken a somewhat different approach to testing the effects of introduction of meat into infants’ diets. We examined the effects of beef vs. iron-fortified cereal as first complementary food on growth, zinc and iron status, development and absorption of zinc. Subjects included exclusively breastfed infants, enrolled at 3–4 mo of age, who were randomly assigned to either pureed beef or iron-fortified rice cereal as a first complementary food, to be introduced between 5 and 7 mo. Intake of the other test food was not allowed until after 7 mo, but no restrictions were placed on intake of foods low in iron and zinc, such as pureed fruits and vegetables. The infants were followed through 1 y of age.
The intervention resulted in significantly higher dietary intakes of
protein and zinc in the beef group from 5 through 7 mo, but these
differences disappeared at the 9- and 12-mo follow-ups. Iron intake
was significantly higher in the cereal group at 7 mo, but also was not
different thereafter. The mothers were also asked to record during each
diet record period the baby’s "acceptance" of the assigned food.
There was no difference between the cereal and beef groups with respect
to acceptance. No effect of the intervention was found on growth,
development or biochemical indices of iron and zinc status at 9 mo
(Westcott et al. 1998
).
Zinc absorption studies were undertaken in a subset of infants at 7 mo
of age. For these studies, we administered zinc stable isotopes on two
consecutive days, one with the assigned complementary food alone (beef
or cereal), and on the following day with the assigned complementary
food plus human milk. Although fractional absorption of zinc was
slightly higher in the beef group than in the cereal group, the
difference was not significant. Absorption for both groups averaged
~40%. Comparison of zinc intake from the test meal and the amount
absorbed (intake x fraction absorption) yielded significantly
greater absorbed zinc from the beef test meal compared with the cereal
meal (P = 0.001). Had the differences in intake of zinc
persisted beyond 7 mo, it seems likely that the enhanced quantity of
intake of a highly bioavailable source of zinc would result in improved
zinc status. This tentative conclusion is based on the measurement of
the exchangeable zinc pool, which was correlated with zinc intake
(P < 0.05) (Jalla et al. 1998
).
In summary, these investigations found no difference in
acceptance by breastfed infants of meat, including pureed beef,
compared with other complementary foods (Engelmann et al. 1998b
, Westcott et al. 1998
). This is consistent
with research that indicates that repeated exposure to new foods,
especially those that are not sweet, is critical for acceptance
(Birch and Grimm-Thomas 1996
). Intake of meat was
associated with higher intakes of protein and zinc, but without an
effect on energy intake from complementary foods (Westcott et al. 1998
). Intake of meat is associated with enhanced
absorption of nonheme iron (Engelmann et al. 1998a
).
Although zinc absorption was not significantly greater in beef compared
with cereal, the greater zinc content of the meat resulted in
significantly more absorbed zinc from beef (Jalla et al. 1998
). Meat intake was associated with hematological status
that was at least equivalent (Westcott et al. 1998
), or
possibly improved (Engelmann et al. 1998b
), and with
equivalent growth and development (Westcott et al. 1998
). These surveillance and intervention studies were
undertaken in populations of infants from middle-income homes and
in relatively protected environments. It was thus likely that food
availability and knowledge of appropriate feeding practices were
favorable. Such may not be the same for more marginalized environments,
particularly those with more infectious morbidity and less choice in
complementary foods. These results suggest that introducing meat
products as a component of the diet may benefit the micronutrient
status of the weanling infant in such environments.
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FOOTNOTES |
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2 Supported by the National Cattlemen’s Beef Association and The Gerber Products Company; additional research support was provided by The University of Colorado Pediatric Clinical Research Center, National Institutes of Health grant MO1RR00069 NCRR and NIH grant DK02240.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONREFERENCES |
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1. Dallman P. R. Nutritional anemia of infancy: iron, folic acid, and vitamin B12. Tsang R. C. Nichols B. L. eds. Nutrition during Infancy 1988:216-235 Henley and Belfus Philadelphia, PA.
2. American Academy of Pediatrics Committee on Nutrition. Kleinman R. E. eds. Pediatric Nutrition Handbook 4th ed. 1998 American Academy of Pediatrics Elk Grove Village, IL.
3.
American Academy of Pediatrics Work Group on Breastfeeding. Breastfeeding and the use of human milk. Pediatrics 1997;100:1035-1039
4. Beauchamp G. K., Moran M. Dietary experience and sweet taste preference in human infants. Appetite 1982;3:139-152[Medline]
5. Birch L. L., Grimm-Thomas K. Food acceptance patterns: children learn what they live. Pediatr. Basics 1996;75:2-6
6. Engelmann M. D., Davidsson L., Sanstrom B., Walczyk T., Hurrell R. F., Michaelsen K. F. The influence of meat on nonheme iron absorption in infants. Pediatr. Res. 1998a;43:768-773[Medline]
7. Engelmann M. D., Sandstrom B., Michaelsen K. F. Meat intake and iron status in late infancy: an intervention study. J. Pediatr. Gastroenterol. Nutr. 1998b;26:26-33[Medline]
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9. Institute of Medicine Earl R. Woteki C. E. eds. Recommended Guidelines for the Prevention, Detection, and Management of Iron Deficiency Anemia among U.S. Children and Women of Childbearing Age 1993:48-51 National Academy Press Washington, DC.
10. Jalla S., Steirn M. E., Miller L. V., Krebs N. F. Comparison of zinc absorption from beef vs iron fortified rice cereal in breastfed infants. FASEB J 1998;12:A346(abs.)
11.
Krebs N. F., Hambidge K. M. Zinc requirements and zinc intakes of breast fed infants. Am. J. Clin. Nutr. 1986;43:288-292
12.
Krebs N. F., Reidinger C. J., Hartley S., Robertson A. D., Hambidge K. M. Zinc supplementation during lactation: effects on maternal status and milk zinc concentrations. Am. J. Clin. Nutr. 1995;61:1030-1036
13. Krebs N. F., Reidinger C., Robertson A. D., Hambidge K. M. Growth and intakes of energy and zinc in infants fed human milk. J. Pediatr. 1994;124:32-39[Medline]
14. Michaelsen K. F., Milman N., Samuelson G. A longitudinal study of iron status in healthy Danish infants: effects of early iron status, growth velocity and dietary factors. Acta Paediatr 1995;84:1035-1044[Medline]
15. National Research Council Recommended Dietary Allowances 10th ed. 1989 National Academy Press Washington, DC.
16. Pennington J. A. Bowes’ and Church Food Values of Portions Commonly Used 15th ed. 1989 J. Lippincott Philadelphia, PA.
17. Skinner J. D., Carruth B. R., Houck K. S., Coletta F., Cotter R., Ott D., McLeod M. Longitudinal study of nutrient and food intakes of infants aged 2 to 24 months. J. Am. Diet. Assoc. 1997;97:496-504[Medline]
18. Westcott J. L., Simon N. B., Krebs N. F. Growth, zinc and iron status, and development of exclusively breastfed infants fed meat vs cereal as a first weaning food. FASEB J 1998;12:A847(abs.)
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