A Review of the Literature Examining the Benefits and Challenges, Incidence and Duration, and Barriers to Breastfeeding in Preterm Infants

Benefits of Human Milk for Preterm Infants

Feeding human milk to preterm infants provides nutritional,^[20,22] gastrointestinal,^[23,24] immunological,^[25,26] developmental,^[27,28] and psychological benefits^[29,30] that may impact their long-term health and development. Human milk is advocated as the best source of nutrition for preterm infants because it provides substances not supplied in formula.^[31,32]

The amino and fatty acid patterns of human milk confer distinct advantages to preterm infants.^[32] The protein content of human milk is suitable for LBW infants because it contains 30% casein and 70% whey, whereas bovine milk is 82% casein.^[20] The whey proteins in human milk are more appropriate for preterm infants because they are easily digested and promote more rapid gastric emptying.^[33] Whey also contains a-lactalbumin, a nutritional protein more easily digested by preterm infants.^[20] In contrast, bovine milk contains the whey protein, α-lactoglobulin, which is probably responsible for bovine milk protein allergy and colic.^[21,22,34] Human whey proteins also contain lactoferrin, lysozyme, and secretory immunoglobulin A (sIgA) that influence the host defense of the preterm infant.^[21,22] Found in only trace quantities in bovine milk, these proteins are beneficial for preterm infants who are susceptible to nosocomial infections.

Human milk also contains carbohydrates, including lactose and oligosaccharides.^[31,35] Low birthweight infants absorb > 90% of the lactose in human milk.^[36] The presence of small quantities of unabsorbed lactose from human milk feeding creates a softer stool consistency, improves absorption of minerals, and increases beneficial intestinal flora.^[35,37,38] Oligosaccharides prevent bacterial attachment to the host mucosa and assist in preventing systemic infection and necrotizing enterocolitis in the LBW infant.^[31]

The lipids in human milk provide further nutritional benefits.^[20] In addition to providing 50% of the calories in the milk, the pattern and distribution of fatty acids on the triglyceride molecule, and the presence of bile salt-stimulated lipase, allows the LBW infant to absorb greater amounts of fat.^[39—41] Because the lipase disintegrates under heat, superior fat absorption occurs as long as the milk is unprocessed.^[41]

Human milk also contains long-chain fatty acids that are associated with cognition, growth, and vision.^[42,43] These fatty acids-namely, arachidonic acid (20:4n-6) and docosahexaenoic acid (22:6n-3)-are not found in bovine milk.^[44,45] A recent meta-analysis examining dietary essential fatty acids and long-chain polyunsaturated fatty acids in preterm infants concluded that there were significant differences in visual resolution acuity at 2 and 4 months of age in preterm infants fed docosahexaenoic-acid-supplemented formula versus those fed docosahexaenoic-acid-free formula.^[46] In contrast, the Cochrane systematic review examining long-chain polyunsaturated fatty acid supplementation in preterm infants found no differences in visual acuity, development, or growth.^[47]

In summary, human milk is beneficial for preterm infants because of its unique protein structure, its ability to promote fat absorption, and its pattern of fatty acids that promote growth and development. The provision of docosahexaenoic-acid-supplemented formulas to preterm infants remains controversial.

Human milk is also beneficial for the preterm infants' gastrointestinal system because gastric emptying is faster after feeding human milk than commercial bovine formula.^[23] In addition, factors in human milk, including hormones, peptides, amino acids, and glycoproteins, may play a role in the maturation of the small intestine.^[24] Components of human milk that may promote gastrointestinal maturation include epidermal growth factor, nerve growth factor, somatomedin-C, insulin, thyroxine, cortisol, taurine, glutamine, and amino sugars.^[24]

The use of human milk provides preterm infants with optimal immune defense. The lactoferrin, lysozymes, sIgA, and interferon found in preterm human milk may help to protect the infant from infection, including sepsis and meningitis.^[25] Infants fed human milk also have a lower incidence of necrotizing enterocolitis (NEC), a potentially fatal condition causing necrosis of the bowel.^[26,48] A large (N = 926) prospective, multicenter, randomized controlled study of hospitalized preterm infants (BW < 1850 g) examined the incidence of NEC in infants fed human milk or formula.^[26] Necrotizing enterocolitis was significantly lower in infants fed human milk, either exclusively or partially, than in infants fed only formula ( P < 0.001).^[26] Although the precise role of human milk in preventing NEC remains unclear, studies have shown the protective role of IgA in the gut lumen.^[49,50]

Another randomized, multicenter, controlled trial of 777 infants with a BW of < 1850 g, feeding human milk versus formula, found a reduction in the development of allergic symptoms in preterm infants fed human milk, with a strong family history of atopy ( P < 0.01).^[51]

Preterm infants may also be protected from infection through the enteromammary immune system. The mother produces sIgA antibodies when she is exposed to foreign antigens. The infant receives passive immunity by ingesting the milk containing the specific sIgA antibody. While this system is active in full-term infants, the extent to which the enteromammary system functions in the LBW-infant-mother dyad is unknown.^[52] Nursery policies that advocate early skin-to-skin contact aid in activating the host defense system of the preterm infant by exposing mothers to the pathogens in the infant's immediate environment.^[32]

Children who were born at VLBW have poorer cognitive function and academic performance than normal-birthweight controls.^[53,54] The long-term effect of nutrition in early infancy on later neurodevelopmental outcomes remains controversial. Possible explanations for the role of human milk in supporting neurodevelopment include the high concentration of long-chain polyunsaturates such as arachidonic and docosahexaenoic acid.^[43]

Most human studies on the effects of nutritional deprivation during critical periods of rapid brain growth were conducted on malnourished infants in developing countries.^[55—57] A prospective, nonrandomized study (n = 300 former preterm infants; BW < 1850 g) compared IQ scores in children 7.5 to 8 years of age who had received either human milk or formula during their hospitalization.^[27] After adjusting for specific social and educational factors, the children who received human milk had significantly higher scores (an 8.3 IQ advantage, P < 0.0001) on the Weschler Intelligence Scales for Children-Revised;^[27] a dose-response relationship between the proportion of mother's milk and intelligence was also noted.^[27] Another prospective cohort study of 137 VLBW infants examined the effect of human milk consumption on neurodevelopmental outcomes at 6 and 12 months of age.^[28] The volume of human milk intake was prospectively recorded by mothers participating in the study. After controlling for sociodemographic and infant variables, the study showed no statistically significant effect of predominantly breastfeeding compared to formula feeding ( P > 0.05).^[28]

A meta-analysis of 20 studies indicated that LBW infants had greater benefits in cognitive development from being fed human milk (n = 1294) compared to those fed formula (n = 751).^[58] For LBW infants, the combined analysis of 6 studies showed a significant ( P < 0.001) advantage of 5.18 points in the cognitive development score in infants fed human milk compared to formula.^[58] A limitation of this analysis was the difference in measured and unmeasured cofactors in the studies reviewed, which may account for 2.4 points in the developmental score.^[58] Because nutritional studies of VLBW infants are fraught with confounding variables, further research is required to identify factors specific to VLBW infants, who are most vulnerable to negative developmental sequelae.

Another important aspect of human milk feeding is the potential for greater mother-infant attachment. For the mother whose infant is critically ill, and who can provide little or none of her infant's care in the first few weeks after birth, the ability to supply human milk may be an important tangible contribution. The importance of this psychological benefit cannot be underestimated and is consistently documented in the literature.^[29,30] Breastfeeding mothers have also reported feelings of attachment, maternal empowerment, and confidence.^[4]

Overall, previous studies indicate that feeding human milk to preterm infants confers nutritional, gastrointestinal, immunological, developmental, and psychological benefits. These benefits positively impact the infant's long-term health and development.

Adv Neonatal Care. 2005;5(2):72-88. © 2005 W.B. Saunders

Cite this: A Review of the Literature Examining the Benefits and Challenges, Incidence and Duration, and Barriers to Breastfeeding in Preterm Infants - Medscape - Apr 01, 2005.