The American Academy of Pediatrics (AAP) [1
] and the World Health Organization (WHO) [2
] recommend mother’s own human milk feeding because of immunological benefits which lessen disease in the high-risk neonate [3
]. Epidemiological investigations have determined that feeding mother’s own milk decreases the likelihood of necrotizing enterocolitis and late-onset sepsis in preterm infants [4
], resulting in shorter hospital stays and decreased cost of care [7
]. When mother’s own milk is not available, pasteurized donor milk (PDM) is used as a reasonable alternative for the preterm infant [9
]. To achieve this practice, mother’s own milk is often augmented with pasteurized donor milk [14
]. In fact, recent retrospective work demonstrated cost effectiveness in the intensive care unit that optimized human milk feeding [15
]. However, a major gap in knowledge is the nutritional consequences of feeding infants with low volumes of PDM that is often provided by mothers feeding older term infants who consume much greater quantities of milk and receive food sources at 4–6 months [16
]. Furthermore, whether from the mother or donor, specific human milk components can be influenced by maternal diet [17
]. The reality is that many preterm infants fed even fortified human milk may demonstrate growth, meeting specified weight goals, but 43% are still documented as small for gestational age and are in the lower growth percentiles at discharge—unlike their term counterparts [18
]. Most concerning is that growth failure—particularly linear velocity—is associated with neurodevelopmental morbidities [21
]. In addition, a recent multi-site trial randomly assigning infants to donor milk or preterm formula did not find developmental advantages [24
]. Consequently, understanding the nutrient characteristics of pooled human milk is essential to improving fortification strategies.
The composition of human milk changes throughout the course of lactation as the infant matures, and these changes are consistent with infants consuming increasing quantities of milk and ingesting other food sources [25
]. Consequently, the nutrient composition of most PDM alone is not considered adequate to meet the needs of the preterm or volume-restricted infant due to the late lactational stage of the donor milk, which is nutritionally less concentrated than early milk [27
], and there is consensus that PDM requires fortification [29
]. Furthermore, a recent meta-analysis of term and preterm human milk composition confirmed that human milk is profoundly variable [33
]. There are a multitude of factors that may contribute to this variability, including stage of lactation, diurnal cycles, exposure to environmental contaminants such as cigarette smoke, and the treatment of expressed milk (i.e., storage, containers, or tubing delivery) [33
]. While many of these variables are normalized in mother’s own milk fed consistently around the clock and for months, milk obtained for donation is subject to the circumstances at that time. Expert opinion suggests more research is needed in the area of donor milk and fortification [11
]. As a starting point to address the most basic elements in donor milk composition, we chose to measure fatty acid and amino acid contents. The purpose of this study was to determine the variability in fatty acid and amino acid composition in individual and pooled PDM using current pooling practices in place at milk banks compliant with Human Milk Banking Association of North America (HMBANA) guidelines, specifically for protein (0.7–1.0 g/100 mL) and caloric contents (67–81 kcal/100 mL) [36
Mother’s own human milk is recommended by the AAP as a unique biologic source of nutrition for both term and preterm infants [1
]. When mother’s milk is not available, PDM is a reasonable alternative for the preterm infant; however, PDM does not meet the nutritional needs of infants that are preterm or ingesting low volumes of milk if fed as the sole source of nutrition, and thus requires supplementation [9
]. While women donating to human milk banks undergo extensive medical screening, there are many variables that are not controlled. The milk content of fatty acids of an individual woman may vary substantially depending upon her recent diet and/or other demographic factors. Human milk collection, storage, and fortification and biologic components are often described [40
], but in light of the recent clinical practice of using donor milk, and publications on variability in human milk, the effectiveness of pooling to adequately address the nutritional differences in individual donor milk needs to be evaluated [33
Our data indicate that donor milk protein-to-calorie ratio is within the range for nitrogen retention of 2.6–3.6 [41
]; however, these studies were done with preterm formula, and other factors such as individual variation, heat treatment, absorption, or micronutrition may be limiting. We did find substantial differences between individuals and Banks in the nutrient contents of milk samples; these differences were largely overcome by pooling. These data demonstrate that infants fed DM that has not been pooled may experience dramatic differences from one day to the next in the composition of the milk they are fed—especially if the milk is from different individual donors. While the effects of nutrient variability are largely normalized in pooled PDM, there remain some deficiencies. This is especially important in the preterm population, because poor somatic growth correlates with low mental developmental scores and development of cerebral palsy [42
Milk collected at milk banks in the United States is variable and is not a “single point in time”, but likely to have been collected longitudinally over several months. Each milk bank is pasteurizing different volumes of milk with varying mothers in a pool. For example, the OhioHealth Mothers’ Milk Bank typically pasteurizes 1500–2000 ounces per day from 4 to 10 different mothers, each having varying amounts of milk. The number of moms in a pool is dependent upon the available volume and calorie content of each. Current pooling practices utilizing HMBANA guidelines normalize the composition of PDM such that minimal differences in fatty acids or amino acids are observed among pooled samples and no correlations with lactational stage remain [36
]. This observation may be explained by the fact that the sample size for pooled samples was one third that of the individual samples, and consequently there was less power to detect differences. Another explanation could be that the statistical differences between Banks Centers in the individual samples are largely driven by the outliers (up to five-fold different) and not actual differences from site to site. Despite the normalized contents in pooled samples, the absolute values vary two-to-three-fold and may still constitute a stressor on the infant. These findings would support customized supplementation for infants fed PDM to optimize nutrients.
DHA is a long chain poly-unsaturated omega-3 fatty acid that is a fundamental lipid in the human cortex and gray matter, and is correlated to developmental scores in preterm infants [43
]. Furthermore, low concentrations of whole blood DHA have been correlated to late-onset sepsis and chronic lung disease, indicating the importance of DHA in overall infant health [45
]. Throughout the last trimester of pregnancy, DHA is accreted at a rate of 50–75 mg/kg/day [46
]. Infants born before the last trimester miss this accretion period and current DHA concentrations typically fed in the neonatal intensive care unit (NICU) fall short of the fetal accretion levels. Depending on diet, human milk concentrations of DHA can range from 0.2 to 2 mol % [28
]. In the current study, measured levels of DHA in the regional PDM samples ranged between 2.22 and 20.20 mg/100 mL (equivalent to 0.08 and 0.67 mol %). This demonstrates that DHA levels in PDM are insufficient to consistently provide intrauterine accretion levels for preterm infants. In a previous study, supplementation of donors with 1 g of DHA per day resulted in human milk DHA concentrations that reached intrauterine accretion levels for the infant (0.8 mol %) [48
]. These data suggest that mothers donating milk—especially milk to be used for preterm infants—should be advised to increase their daily intake of DHA, or direct supplementation of the PDM milk may be required.
Typical clinical practice is to increase total protein intake to improve growth without consideration of the concentrations of specific amino acids. Most requirements for amino acids are established for parenteral nutrition, and few address the needs for the enterally-fed infant [49
]. Lysine is an essential amino acid used in protein synthesis and along with methionine is a precursor for carnitine synthesis, which is essential for fatty acid metabolism. After total protein, lysine is the amino acid most rate-limiting for growth and is essential to the biological value of food sources [49
]. Huang et al. has demonstrated that infants require 130 mg/kg/day of lysine for optimal growth [50
]. We observed substantial variability across Banks among amino acid contents in individual milk samples, but this variability was no longer evident in pooled milk samples. Several of the individual amino acids were lower than that recommended for parental nutrition (even in the pooled samples), but the relevance for enteral feeding is unknown (tyrosine, 74 mg/k/day; methionine, 49 mg/k/day; threonine, 33 mg/kg/day) [49
]. Most importantly, lysine concentrations ranged from 52.1 to 66.3 mg/150 mL (the amount consumed by a healthy preterm per day), which is lower than the recommended 130 mg/kg/day assuming that most preterm infants weigh between 0.5 and 1 kg. Since many essential amino acids have defined functions, supplementing PDM with a specific quality of protein to ensure these amino acids are available may be as important as increasing total protein content.
One limitation of our study is that donated milk was from a wide range of lactational stages and included newborn milk to 12 months post-birth. Our study captured a median of 1 to 5.5 months, which can vary widely in protein and lipid content. A targeted donor process could perhaps improve the amino acid and fatty acid profile. The strength of our study is that it included a cross-sectional prospective examination of many regions in the United States to mimic current clinical practice to evaluate baseline nutrition in the donor milk.