Comparing Infant Formulas with Human Milk
ABSTRACT The vast majority of infants in the United States are fed human-milk substitutes by 6 months of age. This food source, although inferior to human milk in multiple respects, promotes more efficient growth, development, and nutrient balance than commercially available cow milk. Manufacturers often add new ingredients to infant formulas in an attempt to mimic the composition or performance of human milk. However the addition of these ingredients is not without risks as a result of a range of complex issues, such as bioavailability, the potential for toxicity, and the practice of feeding formula
and human milk within the same feeding or on the same day. Assessing the safety of ingredients new to infant formulas by comparing the proposed formulas with human milk also presents both regulatory and research issues. From a research standpoint, clinical studies that assess the effects of new ingredients are difficult to design because infants cannot be randomized to consume formulas or human milk. Furthermore, there may be significant non-nutritional confounding variables between the groups, including factors related to which mothers choose to breastfeed. Finally, human-milk composition varies considerably among and within individuals over time, while the content of infant formulas generally remains constant. From a regulatory standpoint, the effect of an ingredient new to infant formulas is usually driven by the manufacturer’s desire to produce a product that mimics the advantages of breastfeeding. This motivation implies that formulas in their current state are less efficacious (e.g., neurologically or immunologically), although not necessarily unsafe, when compared with human milk. Thus the safety of any addition of an ingredient new to infant formulas will need to be judged against two controls: the previous iteration of the formulas without the added ingredient and human milk. Copyright © National Academy of Sciences. All rights reserved. Infant Formula: Evaluating the Safety of New Ingredients http://www.nap.edu/catalog/10935.html 42
INFANT FORMULA: EVALUATING THE SAFETY OF NEW INGREDIENTS BACKGROUND Multiple health organizations, including the World Health Organization (WHO, 2002), the American Academy of Pediatrics (AAP, 1997), the American Academy of Family Physicians (AAFP, 2003), the American Dietetic Association (ADA, 2001), the Institute of Medicine (IOM, 1991), the Life Sciences Research Organization (LSRO, 1998), the U.S. Department of Health and Human Services (HHS/OWH, 2000), Health Canada, and the Canadian Pediatric Society (Canadian Paediatric Society, 1998) endorse breastfeeding as the optimal form of nutrition for infants for the first year of life. Nevertheless the vast majority of infants in the United States are fed human milk substitutes by 6 months of age (Ryan et al., 2002). This food source, although inferior to human milk in multiple respects, promotes more efficient growth, development, and nutrient balance than commercially available cow milk. The American Academy of Pediatrics recommends that infants who are not breastfed should consume iron-fortified infant formulas rather than cow or goat milk until 12 months of age (AAP, 1997). HISTORY OF THE DEVELOPMENT OF INFANT FORMULAS Milk-Based Formulas Human-milk substitutes existed before the modern age of formulas. Because some infants could not be fed by their mothers, humans adopted two methods for substitute feedings. The most obvious was the utilization of a surrogate mother (e.g., wet nurse), who would feed the child human milk. The alternative was to feed the child milk obtained from another mammal. The most frequently used sources were the cow, sheep, and goat (Fomon, 1993). Until the end of the nineteenth century, the use of a wet nurse was by far the safest way to feed infants who could not be breastfed by their mothers. As general sanitation measures improved during the latter part of the nineteenth century, and as differences in composition between human milk and that of other mammals were defined, feeding animal milk became more successful. However few infants survived until infant formulas based on cow milk with added water and carbohydrate were introduced. Box 3-1 lists the main landmarks in the BOX 3-1 History of Commercially Available Infant Formulas in the United States Cow-milk-based formulas 1867 – Formula contained wheat flour, cow milk, malt flour, and potassium bicarbonate 1915 – Formula contained cow milk, lactose, oleo oils, and vegetable oils; powdered form 1935 – Protein content of formula considered 1959 – Iron fortification introduced 1960 – Renal solute load considered; formula as a concentrated liquid 1962 – Whey:casein ratio similar to human milk 1984 – Taurine fortification introduced Late 1990s – Nucleotide fortification introduced Early 2000s – Long-chain polyunsaturated fatty-acid fortification introduced Noncow-milk-based formulas 1929 – Introduction of commercially available soy formula (soy flour) Mid 1960s – Isolated soy protein introduced Copyright © National Academy of Sciences. All rights reserved. Infant Formula: Evaluating the Safety of New Ingredients http://www.nap.edu/catalog/10935.html COMPARING INFANT FORMULAS WITH HUMAN MILK
43 history of the development of infant formulas. Liebig
’
s food for infants was marketed in
1867 and consisted of wheat flour, cow milk, malt flour, and potassium bicarbonate (Fomon, 2001). In 1915 a formula called
“synthetic milk adapted”
was developed with nonfat cow
milk, lactose, oleo oils, and vegetable oils. This was the basis for modern commercially prepared formulas (Fomon, 1993). The limitations of using nonhuman-mammalian milks as substitutes became clear. As early as 1545, people were concerned with the feeding of animal milks to babies. The
Boke of
Chyldren
stated that “
If children be fed the milk of sheep, then their hair will be soft as that of
a lamb, but if they be fed the milk of the goat, the hair will be coarse
”
(Phaire, 1955, P. 18).
There are, of course, far greater concerns about feeding animal milk to infants, such as folate deficiency (goat milk) and early onset hypocalcemic seizures and azotemia (cow milk). By the early twentieth century it was clear that cow milk was most likely the best animal-milk base to work from, but that certain modifications were needed to make it safe and palatable for human infants. These modifications included:
•
removing animal fat and substituting vegetable oils,
•
diluting the protein content for the newborn’
s relatively immature renal tubular
system, and
•
adding or balancing minerals and vitamins (e.g., adding iron, adjusting the calcium:
phosphorus ratio). The process of modifying cow milk for large-scale production in the 1920s represented the birth of the infant formula industry. Since then new ingredients have been added for a variety of reasons. For example, iron was added in 1959 to reduce the risk of iron deficiency in formula-fed infants (Fomon, 1993), and long-chain polyunsaturated fatty acids (LC-PUFAs) were recently added in an effort to improve infant visual and cognitive development. The protein content of formulas was a consideration from about 1935 onward. Early estimates of human-milk protein levels were higher than is now known, and it was believed that cow-milk protein was far inferior to human-milk protein. Formulas thus included high levels of protein (3.3
–
4.0 g/100 kcal). In the 1960s renal solute load began to be considered
in the design of infant formulas, although infant formula regulations permit higher loads than are currently recommended by expert panels (no greater than 30 mosm/100 kcal) (Fomon, 2001). Based on the recognition that human milk contains a predominance of whey proteins, while in cow milk, caseins are higher, formulas with a whey:casein ratio similar to human milk were introduced in 1962. By 2000 whey-predominant formulas were the most widely used milk-based formulas. These changes were made primarily based on composition rather than on functional measures (Fomon, 2001). In 1984 taurine was added to infant formulas, based on at least a decade of studies that included composition, provisional essentiality, safety, and function in mammals (MacLean and Benson, 1989). Nucleotides were added to formulas with both compositional and efficacy claims in the late 1990s. They may act as growth factors and may have immunomodulating effects on immune defenses (Carver et al., 1991). When considering new ingredients, manufacturers analyze every step in the production process, including raw materials (availability, source, and purity), processing methods, packaging, storage conditions and shelf life, methods of home preparation, and potential for misuse. Chapter 4 provides a discussion of these manufacturing considerations and their relevance to the regulatory process.
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INFANT FORMULA: EVALUATING THE SAFETY OF NEW INGREDIENTS
These considerations continue today as manufacturers attempt to alter infant formulas
to imitate human milk in either composition or performance and to address the nutritional
needs of specific infant populations (e.g., those with cow-milk allergy, metabolic abnormalities,
and prematurity) (Benson and Masor, 1994). This chapter is concerned with infant
formulas that are being altered to mimic composition or performance of human milk; it does
not address the nutritional needs of specific infant populations.
Nonmilk-Based Formulas
Soy-based formulas were developed for infants perceived to be intolerant of cow-milk
protein. The first soy formulas were commercially available in 1929 (Abt, 1965). These
formulas were made with soy flour and were not well accepted by parents, who complained
of loose, malodorous stools, diaper rash, and stained clothing. In the mid-1960s isolated soy
protein was introduced into formulas. These formulas were much more like milk-based
formulas in appearance and acceptance. However the preparation of isolated soy protein
resulted in the elimination of most of the vitamin K in the soy, and a few cases of vitamin K
deficiency were reported. The occurrence of nutrient deficiencies in infants fed milk-free
formulas contributed to the development of federal regulations concerning the nutrient
content of formulas (Fomon, 1993). Soy formulas now account for about 40 percent of
formula sales in the United States. Some parents want to avoid cow-milk protein in the diet
and thus wean directly to soy without any reported intolerance to cow-milk formulas. While
formulas containing extensively hydrolyzed protein have long been available for infants with
allergy to intact cow-milk protein, formulas with protein that is not as completely hydrolyzed
have recently been introduced for normal-term infants.
CHALLENGES OF MATCHING HUMAN-MILK COMPOSITION AND
BREASTFEEDING PERFORMANCE
Infant formula manufacturers have made changes to formulas in order to match either
human milk composition or breastfeeding performance (Benson and Masor, 1994). The
term
“breastfeeding performance”
is used because, with the exception of one study of
preterm infants (Lucas et al., 1994), all other studies comparing human milk with formulas involved breastfeeding
—
not providing human milk from a bottle.
Matching Human-Milk Composition
Historically one approach to match human-milk composition is to add new ingredients
(see Appendix B for the composition of formulas and human milk). This turns out to be a
quixotic quest since human milk is a complex body fluid that is variable not only among
individuals, but within an individual over time. In addition, it contains components, such as
live cells and bioactive compounds, that either cannot be added to formulas or cannot
survive a shelf life. Finally, not all human-milk constituents are essential; some, like LCPUFAs,
docosahexaenoic acid (DHA), and arachidonic acid (ARA), can be synthesized by
term and preterm infants born at 33 weeks gestation (Uauy et al., 2000).
Manufacturers who wish to add some, but not all, ingredients found in human milk may
defeat the purpose of the added nutrients or may potentiate negative interactions. Examples
include the deleterious effect on growth when eicosapentaenoic acid is added without adequate
DHA (Carlson et al., 1996) and the potential negative effect of adding polyunsatu
Copyright
© National Academy of Sciences. All rights reserved. Infant Formula: Evaluating the Safety of New Ingredients http://www.nap.edu/catalog/10935.html
COMPARING INFANT FORMULAS WITH HUMAN MILK
45
rated fats and large amounts of iron without adding adequate antioxidants (Halliwell and
Chirico, 1993; McCord, 1996).
The issue of the context or matrix in which nutrients are provided in milk remains a
challenge to infant formula manufacturers as they try to match human-milk composition
and breastfeeding performance (Benson and Masor, 1994). The matrix can highly influence
the bioavailability of nutrients. In the simplest example, nutrients that are present in both
milks may be present in different ratios. For many nutrients that do not interact chemically
or compete for enzymatic or receptor binding sites, the relative amounts may not be important.
However in situations where there is competition for enzymes (e.g., among
n-3 and n
-
6 PUFAs) (Brenner, 1974) or receptor binding sites in the intestine (e.g., for zinc, iron, and copper), the relative proportions may have biological significance. Manufacturers must also consider the form of the molecule in which a nutrient is presented to the intestine and its bioavailability. For example, the high bioavailability of iron from lactoferrin in human milk allows for a much lower concentration of iron in human milk (0.2
–0.4 mg/L) compared with infant formulas (4.0–
12 mg/L) and thereby decreases
competition between iron and other divalent cations, such as copper and zinc (Lonnerdal and Hernell, 1994). In the case of LC-PUFAs, care must be taken to ensure no toxicity from these compounds. Manufacturers must study the effects of fats, minerals, enzymes, or other factors on LC
–
PUFA bioavailability and processing. For example, newborn fat absorption can be
highly variable because of the immaturity of several lipases, including pancreatic lipase (for review, see Hamosh, 1988). Human milk contains lipases that compensate for the lack of pancreatic lipases. Thus human-milk fat is more bioavailable than the vegetable oils found in infant formulas. Finally, manufacturers must examine the effects of infant formulas in the context of mixed feedings (Ryan et al., 2002). Throughout the course of the day, an infant in the United States may consume both human milk and infant formulas in any number of combinations. For example, some infants of working mothers are breastfed during the morning and evening and fed formula during the day by a caregiver. Here the nutrients and their respective matrixes are kept quite separate and less interaction may be expected than in the situation where an infant is supplemented with formula directly after each nursing. In the latter case there is a theoretical concern that certain nutrients found in high concentration in infant formulas (e.g., iron) may interfere with the intended matrix delivery system found in human milk (e.g., lactoferrin). The nutritional consequence of mixed-feeding paradigms has not been adequately investigated, but should be targeted in future studies of the performance of infant formulas.
Matching Breastfeeding Performance
The alternative to matching human-milk composition is to match breastfeeding performance
(Benson and Masor, 1994). Initially the goal of infant formulas was to match the
growth rate of the breastfed infant. However over time it was recognized that breastfeeding
may confer several other potential advantages to the infant (for review, see AAP, 1997),
including:
•
prevention of infectious diseases (Beaudry et al., 1995; Dewey et al., 1995),
•
neurodevelopment (Mortensen et al., 2002), and
•
protection from chronic diseases in childhood (Saarinen and Kajosaari, 1995; Shu et
al., 1995).
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INFANT FORMULA: EVALUATING THE SAFETY OF NEW INGREDIENTS
These perceived and potential advantages of breastfeeding are the impetus behind many
of the proposed addition of ingredients to infant formulas. Not all of these advantages are
necessarily attributable to the nutritional content of human milk. Advantages resulting from
a fundamentally different interaction between the nursing mother and her infant or to a
selection bias of mothers who choose to breastfeed cannot be matched by simply adding
nutrients to cow milk. It has been difficult to sort out which of the performance factors of
breastfeeding are due to nutritional components and which are accounted for by social and
psychological factors. Obviously, randomized trials assigning infants to breastfeed or formula
feed are not ethically feasible.
Breastfeeding also confers certain risks to the developing infant, including potential
nutrient deficiencies (Kreiter et al., 2000; Pisacane et al., 1995) and exposure to toxins
secreted by the mother into her milk. Advantages and risks are discussed in detail below.
PERFORMANCE ADVANTAGES OF BREASTFEEDING
Breastfed infants have different growth characteristics compared with formula-fed infants.
They grow at slightly different rates and have a different body composition (Butte et
al., 1990; Heinig et al., 1993) and may have a lower risk for later obesity (Gillman et al.,
2001; Singhal et al., 2002). (These characteristics are discussed in greater detail in Chapter
6.) Given the great interest in the effect of early nutrition on metabolic setpoints that may
affect the child
’
s risk for adult diseases (e.g., the early origins of chronic disease hypothesis)
(Barker et al., 2002) and the increasing incidence of early insulin resistance, obesity, and type II diabetes in teenagers, future research should concentrate on whether breastfeeding is protective. As discussed earlier, breastfed infants absorb fat better than formula-fed infants due to the presence of lipases in human milk that are not present in cow milk (Hamosh, 1988). The healthy breastfed infant consumes less milk (approximately 85 kcal/kg body weight/day) during the first months of life than the same infant given ad libitum infant formula (100 kcal/kg/day; Heinig et al., 1993). The breastfed infant continues to consume approximately 10 fewer kcal/kg/body weight calories than the formula-fed infant. The breastfed infant has a lower total energy expenditure (Butte et al., 1990) and a slower growth rate (Butte et al., 1990; Heinig et al., 1993). In addition, there is less gastro-esophageal reflux in breastfed infants, most likely due to a more rapid gastric emptying time, resulting in less loss of intake. Some of the trophic and metabolic factors that promote the characteristic nutrient handling and growth of the breastfed infant are listed in Table 3-1. Breastfed infants, compared with formula-fed infants, have a lower incidence of infectious diseases, such as diarrhea (Popkin et al., 1990), otitis media (Duncan et al., 1993), and lower respiratory tract illness (Wright et al., 1989). The effect is particularly profound in the developing world, but studies show clear advantages in the developed world as well (Wright et al., 1989). The effect extends beyond breastfeeding itself to when human milk is administered without the infant nursing from the mother. For example, preterm infants fed human milk by nasogastric tube in the newborn intensive care unit have a lower rate of necrotizing enterocolitis (Lucas and Cole, 1990). Moreover, the presence of the close contact between the mother and child stimulates the mother to make antibodies against bacteria colonized in the infant and to secrete these antibodies in her milk. Human milk has multiple components that likely mediate this anti-infectious, immunologically enhancing effect. These include secretory immunoglobulin A, lactoferrin, lysozymes, intact cellular components, and oligosaccharides. A comprehensive list of compounds found in human milk by class of ingredient is shown in Table 3-2.
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COMPARING INFANT FORMULAS WITH HUMAN MILK
47
TABLE 3-1
Unique Factors in Human Milk That Positively Affect Nutritional Status
and Somatic Growth
Ingredient Class of Ingredient Function Reference
Amylase Enzyme Polysaccharide digestion Howell et al., 1986
Epidermal growth Growth factor/hormone Gastrointestinal growth/ Donovan and Odle,
factor differentiation 1994; Dvorak et al.,
2003; Howell et al.,
1986
Erythropoietin Growth factor/hormone Red cell production; possible Kling, 2002
growth factor for gut and
central nervous system
Insulin Growth factor/hormone Anabolic hormone promotes Donovan and Odle,
carbohydrate, protein, and 1994
fat accretion
Insulin-like growth Growth factor/hormone Primary growth hormone of Donovan and Odle,
factor-I late fetal/neonatal period 1994
Insulin-like growth Growth factor/hormone Unknown function; thought Donovan and Odle,
factor-II to be weak growth hormone 1994
Lactoferrin Carrier protein Increases efficiency of iron Howell et al., 1986
delivery
Lipase Enzyme Triglyceride hydrolysis Howell et al., 1986
Nerve growth factor Growth factor/hormone Neuronal growth/ Donovan and Odle,
differentiation 1994
Proteases Enzyme Unknown if active in protein Howell et al., 1986
hydrolysis
Relaxin Growth factor/hormone Regulates morphological Donovan and Odle,
development of the nipple 1994
Transforming growth Growth factor/hormone Gastrointestinal growth Donovan and Odle,
factor-alpha 1994; Dvorak et al.,
2003
TABLE 3-2
Unique Factors in Human Milk with Anti-Infective or Immunological
Properties
Class of
Ingredient Ingredient Function Reference
Antiproteases (e.g., Enzyme Inhibits breakdown of anti- Howell et al., 1986;
secretary immuno- infective immunoglobulins IOM, 1991
globulin A and and enzymes
trypsin inhibitor)
Arylsulfatase Enzyme Degrades leukotrienes Hanson et al., 1988
Catalase Enzyme Degrades hydrogen peroxide; Lindmark-Mansson
protects against bacterial and Akesson, 2000
breeches of intestinal barrier
Fibronectin Opsonin May present debris to macrophages IOM, 1991;
Mestecky et al.,
1990
Free fatty acids Lipids Antiviral (coronavirus); antiparasitic Mestecky et al., 1990
(
Giardia, Entamoeba
)
Granulocyte-colony Cytokine Causes endothelial cell migration Wallace et al., 1997 stimulating factor and proliferation Hemagglutinin inhibitor Opsonin Prevents bacterial adherence Neeser et al., 1988
Continued
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INFANT FORMULA: EVALUATING THE SAFETY OF NEW INGREDIENTS
Histaminase Enzyme Degrades histamine Hanson et al.,1988
Immunoglobulin G Immunoglobin Immune protection Howell et al., 1986;
IOM, 1991
Interleukin-1-beta Cytokine Activates T-cells Mestecky et al., 1990
Interleukin-6 Cytokine Enhances immunoglobulin A Mestecky et al., 1990
and C-reactive protein production
Interleukin-8 Cytokine Chemotaxis Maheshwari et al.,
2002
Interleukin-10 Cytokine Decreases inflammatory cytokine Goldman et al., 1996
synthesis
Lactadherin Protein Prevents rotavirus binding Peterson et al., 2001
Lactoferrin Carrier Anti-infective; may prevent iron Howell et al., 1986;
from being bioavailable to IOM, 1991
microbes
Leukocytes Live cell Cytokine production by T-cells; IOM, 1991;
direct in vivo roles of B-cells, Mestecky et al.,
macrophages, and neutrophils 1990
Lipases Enzyme Releases bacteriostatic and Howell et al., 1986;
bacteriocidal free fatty acids IOM, 1991
Lysozyme Enzyme Bactericidal Howell et al., 1986;
IOM, 1991
Macrophage colony Cytokine Macrophage proliferation Goldman et al., 1986
stimulating factor
Mucin Protein Inhibits
E. coli
binding Peterson et al., 2001
to gut epithelium Oligosaccharides, Carbohydrates, Receptor analogs block Coppa et al., 1999; polysaccharides, glycoconjugates binding of enteric bacteria; IOM, 1991; gangliosides growth promoters for Rivero-Urgell and
Lactobacillus
Santamaria-
Orleans, 2001 Peroxidases Enzyme Bactericidal Howell et al., 1986; IOM, 1991 Platelet activating Enzyme Catabolizes platelet Furukawa et al., acetyl hydrolase activator factor 1993 factor Prostaglandin E2, Prostaglandin Intestinal cytoprotection Howell et al., 1986 F2-alpha Ribonuclease Enzyme Prevents viral replication Nevinsky and Buneva, 2002 Secretory Immunoglobulin Immune protection (broad Howell et al., 1986; immunoglobulin A spectrum antiviral, antibacterial, IOM, 1991 antiparasitic) Soluble intracellular Cytokine Alters adhesion of viral or Xyni et al., 2000 adhesion molecule-1 other molecules to intestinal epithelium Transforming growth Cytokine Produces immunoglobulin A Bottcher et al., 2000 factor-beta and activates B-cells Tumor necrosis Cytokine Mobilizes amino acids Mestecky et al., 1990 factor-alpha Uric acid Small molecular- Antioxidant Van Zoeren-Grobben weight et al., 1994 nitrogenous compound
TABLE 3-2
continued
Class of
Ingredient Ingredient Function Reference
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COMPARING INFANT FORMULAS WITH HUMAN MILK
49
The neurodevelopmental advantages of breastfeeding or supplying infants with human
milk have received significant amounts of attention (Lucas et al., 1998; Morrow-Tlucak et
al., 1988; Mortensen et al., 2002; Wang and Wu, 1996). Indeed, the primary impetus for
adding LC
–
PUFAs to infant formulas is their postulated effect on brain development. The
general research on breastfeeding, human milk, and neurodevelopment is fraught with confounding variables that have prevented pinpointing specific nutrients that are responsible for the putative effect. Overall it appears that breastfed infants have modest improvements in cognitive, motor, and visual status up to the age of 8 years, but it is unclear whether any early effects disappear over time (for review, see Grantham-McGregor et al., 1999). The degree of neurodevelopmental advantage is directly related to duration of breastfeeding (Mortensen et al., 2002). However critics of the literature point out that there may be fundamental differences not only between mothers who do or do not choose to breastfeed, but also between those who choose to breastfeed for a longer rather than shorter time period. These selection biases may be based on characteristics (e.g., maternal IQ, education, and socioeconomic status) that may confer independent positive effects on the neurodevelopment of the infant. Furthermore, patterns of parent-child interactions may be different in those who breastfeed longer; these interactions may have effects on development. Just as it is difficult to separate out the confounding social factors among those who do and do not choose to breastfeed, it is also difficult to isolate the role of nutrition alone in the assessment of the positive effects. This is because very few individuals bottle-feed their infants human milk and, when this is done, it is frequently for medically extenuating circumstances (e.g., prematurity). Thus one cannot expect to rely on randomized trials of breastfeeding versus formula feeding or breastfeeding versus bottle feeding of human milk to sort out the nutritional effects of human milk on the developing brain. The only trial that approached this issue was conducted by Lucas and coworkers (1994), where preterm infants received either human milk or term infant formula by gavage tube during their early weeks. Infants fed bottled human milk had higher mental and psychomotor development indices 18 months after hospital discharge. However it should be reiterated that these were premature infants and that they were not randomized to their particular diets. Nevertheless there are reasons to think that the provision of human milk, based on its composition, is good for the human brain. Human milk contains LC
–
PUFAs (e.g., DHA and
ARA) that are important for synaptogenesis in the visual system. However studies assessing the addition of these ingredients to cow-milk formula have not resulted in consistent effects. Some demonstrated enhanced visual acuity and speed of processing in infants fed the supplemented formulas (Uauy et al., 1990; for review, see Uauy-Dagach and Mena, 1995). The positive effects on visual acuity have been found most often in premature infants, who are arguably more deficient of these fats. There may be effects on cognitive outcome, although the effects are inconsistent, particularly in term infants (Auestad et al., 2001; Wroble et al., 2002). The reason for these inconsistent effects might be that these compounds do not work alone; rather the matrix of human milk includes general growth factors and specific neural growth factors (see Table 3-3). If there is a positive effect on neurodevelopment, it is likely that these factors work in concert with each other. Finally, there is epidemiological evidence that breastfeeding protects infants from certain childhood diseases at older ages, including atopy/allergy (Kull et al., 2002; Saarinen and Kajosaari, 1995), obesity (Gillman et al., 2001; Singhal et al., 2002), and childhood leukemia/ lymphoma (Shu et al., 1995). The biological mechanisms of the positive effects are not always clear, but may relate to avoidance of exposure to antigenic proteins found in cow milk, particularly in relation to allergy. The lack of clear biological mechanisms makes it more difficult to resolve conflicting results, such as those recently indicating an increased
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INFANT FORMULA: EVALUATING THE SAFETY OF NEW INGREDIENTS
risk of atopy (Sears et al., 2002) and eczema (Bergmann et al., 2002) in large cohorts of
breastfed infants.
RISKS OF BREASTFEEDING
Breastfeeding is not without potential nutritional risks. The best documented risks
include iron deficiency (Duncan et al., 1985; Pisacane et al., 1995), vitamin D deficiency
(Kreiter et al., 2000), and exposure to environmental toxins. The inability to sustain growth
due to the low energy density of milk is relatively rare in the first 4 months of life in the
breastfed infant. However there is great variability in the protein-energy density of human
milk. Energy values may range from 15 to 24 kcal/oz. Most infants can overcome a lowerdensity
milk by consuming a greater volume.
Iron deficiency is approximately twice as common in breastfed infants; up to 30 percent
have iron deficiency anemia, and more than 60 percent of the anemic infants are also iron
deficient at 12 months of age (Pisacane et al., 1995), although the etiology is unclear. The
iron content of human milk is low: 0.5 mg/L compared with 10 to 12 mg/L in supplemented
cow-milk formulas. The absorption rate, however, is considerably higher. Breastfed infants
absorb up to 50 percent of consumed iron, compared with a 7- to 12-percent absorption rate
for formula-fed infants (Fomon et al., 1993). The risk of iron deficiency increases after 4
months of age since most full-term infants are born with adequate iron stores to support
hemoglobin synthesis through the first 4 months after birth.
There have been increasing reports of nutritional rickets in breastfed infants, particularly
in northern climates (Kreiter et al., 2000). This is likely due to lack of sunlight exposure,
which is increasingly common with the use of sunscreens and the tendency to cover
infants for health or cultural reasons. Human milk, like cow milk, is very low in vitamin D,
with average concentrations of 24 to 68 IU/L. Since infants consume less than 0.5 L of milk/
day in the first months of life, breastfed infants have vitamin D intake well below the
Adequate Intake of 200 IU/day. With sun exposure this is not likely to be a problem.
However infants born to mothers with vitamin D deficiency are at increased risk for rickets,
as are those who are not exposed to the sun. The American Academy of Pediatrics and the
Canadian Paediatric Society recently recommended supplementing all breastfed infants with
200 IU of vitamin D by 2 months of age (AAP, 2003; Canadian Paediatric Society, 1998).
TABLE 3-3
Unique Factors in Human Milk That May Positively Affect
Neurodevelopment
Ingredient Class of Ingredient Function Reference
Choline Amino acid Neurotransmitter Zeisel et al., 1986
synthesis
Insulin-like growth factor-1 Growth factor/hormone Neuronal growth/ Cheng et al., 2003;
differentiation Donovan and Odle,
1994
Long-chain polyunsaturated Essential/semiessential fat Visual acuity Uauy-Dagach and
fatty acids Mena, 1995
Nerve growth factor Growth factor/hormone Neuronal growth/ Donovan and Odle, 1994
differentiation
Oligosaccharides (fucose, Carbohydrates Neuronal cell-cell Hynes et al., 1989
mannose,
n
-acetylglucosa- communication
mine, sialic acid)
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COMPARING INFANT FORMULAS WITH HUMAN MILK
51
In addition to transplacental passage of environmental allergens and dietary antigens, it
is possible that susceptible infants may be sensitized to such agents by exposure to maternal
milk. Although dietary antigens have been recovered in human milk, and allergen-specific
IgE antibodies have been demonstrated in cord blood (F
ä
lth-Magnusson, 1995; Lilja et al,
1988), available evidence suggests little or no role for breastmilk-associated food antigens in the development of food allergy (Businco et al., 1983; F
älth-Magnusson, 1995; Fä
lth-
Magnusson and Kjellman, 1987). Breastfed infants can be exposed to environmental toxins (e.g., lead and polychlorinated biphenyls), legal and illegal drugs, and infectious pathogens that the mother may harbor (e.g., Human Immunodeficiency Virus [HIV]). A discussion of all of the potential environmental toxins, drugs, and infectious agents is beyond the scope of this chapter. However it is important to note the effect of increasing rates of HIV infection worldwide and the potential for human milk to be both a vector of transmission of the virus from mother to infant and to contain protective anti-infective factors that may decrease the risk of vertical transmission. These risks and benefits must be weighed against the potential risks of formula feeding, not the least of which is preparation of formula with water contaminated with infectious agents (Humphrey and Iliff, 2001; Mbori-Ngacha et al., 2001; WHO, 1992).
SUMMARY
This chapter affirms that breastfeeding is the standard by which all other infant-feeding
methods should be judged. This position has been taken by numerous professional bodies
and reflects the fact that human milk is species specific and thus uniquely suited for human
infant nutrition. It must be recognized, however, that using a human-milk composition or
breastfeeding performance standard presents both regulatory and research issues when assessing
the addition of ingredients new to infant formulas.
From a research standpoint, clinical studies that assess the effects of new ingredients will
be difficult to design because infants cannot be randomized to be formula fed or breastfed.
Furthermore, there may be significant non-nutritional confounding variables between the
groups, including, but not limited to, factors related to which mothers breastfeed. Finally,
human-milk composition varies considerably among individuals and within individuals over
time, while infant formula content remains constant.
The committee anticipates that manufacturers will wish to add both ingredients that are
currently contained in human milk, but not in formulas (e.g., LC-PUFAs), and those not
found in human milk (e.g., prebiotics) to enhance the performance of formulas to a level at
or nearer to human milk. Thus a breastfed control group should be part of experimental
designs to assess the addition of ingredients new to infant formulas in order to provide a
performance standard.
From a regulatory standpoint, the effect of an ingredient new to infant formulas is
usually driven by a manufacturer
’
s desire to produce products that mimic the advantages of
breastfeeding. This motivation implies that formula in its current state is inferior (e.g., relatively neurologically or immunologically less beneficial, although not necessarily unsafe) when compared with human milk. Thus the safety (and efficacy) of any addition of an ingredient new to infant formulas will need to be judged against two control groups: one fed the previous iteration of the formula without the added ingredient, and one breastfed.
