Comparison of Breast Milk Compositions Among Mothers Delivering Small-for-Gestational Age,Appropriate-for-Gestational Age,and Large-for-Gestational Age Infants

Abstract

Objectives:

The aim of this study was to compare the breast milk compositions of mothers who delivered babies in three different birth weight categories.

Materials and Methods:

We collected 75 breast milk samples from mothers of small-for-gestational age (SGA), appropriate-for-gestational age (AGA), and large-for-gestational age (LGA) term infants (25 per group) on the 11th–28th day postpartum using a manual or electric breast pump. Homogenized samples were analyzed using a mid-infrared human milk analyzer within 2 hours of collection, and protein, carbohydrate (CHO), fat, and energy levels were compared.

Results:

Of 75 participants (median maternal age: 29 years), there was a significant difference in maternal body mass index (BMI), gestational age, and birth weight among the three groups. However, we found no significant variation in breast milk true protein, crude protein, fat, or energy. The CHO composition of breast milk in mothers of SGA infants (7.1 g/100 mL, range: 6.6–7.3 g/100 mL) was significantly lower than that in the AGA (7.3 g/100 mL, range: 7.1–7.5 g/100 mL) and LGA (7.2 g/100 mL, range: 6.7–7.4 g/100 mL) groups. After post hoc multiple comparison, only CHO levels between the SGA and AGA groups were significantly different (p = 0.025), whereas no significant correlation was found between maternal BMI, gestational age, and CHO.

Conclusions:

The CHO component in breast milk from mothers of SGA infants was significantly lower compared with the AGA and LGA groups, but with no clinical significance. Breast milk compositions of mothers of LGA infants were comparable to those of SGA and AGA infants. Therefore, fetal growth status does not significantly influence maternal breast milk nutrients.

Introduction

The nutritional content of breast milk has long been known to be maternally altered to effectively meet each infant's specific needs.¹ Michaelsen et al. reported correlations between breast milk fat content and infant birth weight.² Recently, research on the fat content in breast milk of mothers delivering small-for-gestational age (SGA) babies indicated that fatty acid composition or fat content of human milk was not affected by fetal growth.³

This apparent data discrepancy prompted this study on breast milk nutrients of mothers delivering infants of different birth weights. The results of such a study would be helpful to organize in selecting and possibly modifying milk donations from breast milk banks. Therefore, the objective of this study was to analyze if infant birth weight affects the macronutrients in maternal breast milk.

Materials and Methods

This cross-sectional study was approved by the institutional review board of the Queen Sirikit National Institute of Child Health, Bangkok, Thailand. The study was conducted in neonatal units at the Children's Hospital, Bangkok, Thailand, between June 2016 and October 2017. Lactating mothers of term infants (born between 37 and 41 weeks of gestation), whose babies were classified as SGA, appropriate for gestational age (AGA), or large for gestational age (LGA) (25 from each category making 75 in total), were then enrolled on the study. All participants provided informed written consent and completed a questionnaire including relevant demographic data. Mothers with contraindications for breastfeeding were excluded from the study.

Infants weighing less than two standard deviations (SD) than the mean or below the 10th percentile at birth were classified as SGA, whereas infants weighing between the 10th and 90th percentile at birth and those weighing more than two SDs or greater than the 90th percentile at birth were classified as AGA and LGA, respectively.

Milk samples were collected after lunch (during 12.00–14.00) on the 11th–28th day of postpartum using either a manual or electric breast pump. Mothers were instructed to empty the entire breast on each side. The milk was then homogenized using a sonicator (MIRIS^®, Uppsala, Sweden) for 1 second per 1 mL. The samples were analyzed within 2 hours postexpression using a mid-infrared human milk analyzer (MIRIS). Approximately 10–15 mL of homogenized milk was used for each test (3–5 mL of milk was analyzed three times and the mean values of protein, carbohydrate [CHO], and fat were calculated for each sample). The milk analyzer was calibrated before every test. The results were presented as protein (g/100 mL), CHO (g/100 mL), fat (g/100 mL), and energy (kcal/100 mL).

Statistical analyses

Data analyses were performed using the Statistical Package for the Social Sciences (SPSS) software (IBM SPSS Statistics for Windows, version 16.0). We compared the demographic characteristics of participants in each of the three groups, including age (years), weight gained during pregnancy (kg), body mass index (BMI, kg/m²), day of milk collection, infant's gestational age (weeks), and birth weight (g). These data were presented as median values and interquartile ranges because variables were non-normally distributed.

Compositions of breast milk consisted of protein (g/100 mL), CHO (g/100 mL), fat (g/100 mL), and energy (kcal/100 mL). The Kruskal–Wallis test was used to analyze these data owing to the non-normal distribution of variables. Post hoc analyses were conducted using a Bonferroni correction. For all analyses, p < 0.05 was considered statistically significant.

Results

A total of 75 human milk samples were collected from 75 mothers subdivided into three groups based on the delivery weight of their babies: SGA, AGA, and LGA. The median maternal age was 29 years (range: 24–34 years), with a median gestational age of 38 weeks (range: 38–39 weeks). Of these, 16 participants (21%) were aged 35 years and older and 13 (17%) were overweight (BMI >25 kg/m²). The median maternal weight gained during pregnancy was 12.5 kg (range: 10.5–15.5 kg), and the median birth weight of infants was 3,172 g (range: 2,380–3,968 g) (Table 1).

Table 1.

Baseline Characteristics of Mother and Infant Study Participants

Characteristic	All subjects (n = 75)
Maternal age (years)	29 (24–34)
Maternal age ≥35 years, n (%)	16 (21)
BMI ≥25 kg/m², n (%)	13 (17)
Maternal weight gained (kg)	12.5 (11–15.5)
Day of milk collection	16 (15–18)
Gestational age (weeks)	38 (38–39)
Birth weight (g)	3,172 (2,380–3,968)

Data are reported as median values (ranges) except for maternal age ≥35 years and BMI ≥25 kg/m² [expressed as n (%)].

BMI, body mass index.

Birth weight, gestational age, and maternal BMI were significantly different between these three groups (p < 0.001) (Table 2). The differences in birth weights were significantly related to the classification of SGA, AGA, and LGA infants.

Table 2.

Comparison of Baseline Characteristics of Mother and Infant Study Participants in All Groups

Characteristic	SGA group (n = 25)	AGA group (n = 25)	LGA group (n = 25)	p
Maternal age (years)	25 (22–32)	29 (24–36)	30 (26–34)	0.067
Maternal weight gained (kg)	12.5 (10.5–15.5)	12.0 (8.7–16.0)	14.0 (12.0–17.5)	0.34
Maternal BMI (kg/m²)	19.1 (17.4–21.8)	21.7 (18.6–23.6)	24.5 (22.2–27.4)	<0.001
Day of milk collection	15 (11–18)	17 (15–18)	16 (15–19)	0.067
Gestational age (weeks)	38 (37–38)	39 (38–40)	39 (38–39)	0.001
Birth weight (g)	2,312 (1,982–2,388)	3,172 (3,015–3,327)	4,026 (3,968–4,199)	<0.001

Data are reported as median values (ranges).

AGA, appropriate-for-gestational age; LGA, large-for-gestational age; SGA, small-for-gestational age.

The maternal BMI of the SGA group was 19.1 kg/m² (range: 17.4–21.8 kg/m²), which is significantly lower than the mothers of the AGA group (21.7 kg/m², range: 18.6–23.6 kg/m²) and the LGA group (24.5 kg/m², range: 22.2–27.4 kg/m²). The median of gestational ages of the SGA, AGA, and LGA groups were 38 weeks (range: 37–38 weeks), 39 weeks (range: 38–40 weeks), and 39 weeks (range: 38–39 weeks), respectively (p = 0.001). However, the maternal age, maternal weight gained, and day of milk collection were not significantly different among the three groups.

Breast milk compositions in each group are shown in Table 3. There were no significant differences in true protein, crude protein, fat, and energy. However, the median CHO composition of breast milk from mothers of the SGA group (7.1 g/100 mL, range: 6.6–7.3 g/100 mL) was significantly lower than the AGA mothers (7.3 g/100 mL, range: 7.1–7.5 g/100 mL) and LGA mothers (7.2 g/100 mL, range: 6.7–7.4 g/100 mL). Post hoc Bonferroni adjustment for multiple comparisons was used to identify pairs of groups with significant composition differences. Only CHO in the SGA and AGA groups was found to be significantly different (p = 0.025) (Table 4). No significant correlation was observed for maternal BMI, gestational age, and CHO.

Table 3.

Breast Milk Compositions of Small-for-Gestational Age, Appropriate-for-Gestational Age and Large-for-Gestational Age Groups

Variable	SGA group (n = 25)	AGA group (n = 25)	LGA group (n = 25)	p
True protein (g/100 mL)	1.2 (1.2–1.5)	1.2 (1.0–1.3)	1.3 (1.1–1.4)	0.149
Crude protein (g/100 mL)	1.6 (1.5–1.9)	1.5 (1.3–1.7)	1.7 (1.3–1.8)	0.231
Carbohydrate (g/100 mL)	7.1 (6.6–7.3)	7.3 (7.1–7.5)	7.2 (6.7–7.4)	0.029
Fat (g/100 mL)	3.7 (2.9–5.4)	3.7 (2.9–4.9)	4.0 (2.8–5.0)	0.969
Energy (kcal/100 mL)	71.0 (64.0–82.5)	69.0 (63.0–82.0)	71.0 (61.3–86.0)	0.946

Data are expressed as median values (ranges).

Table 4.

Pairwise Comparisons of Carbohydrate Composition in Breast Milk from Small-for-Gestational Age, Appropriate-for-Gestational Age, and Large-for-Gestational Age Groups

Group	p
AGA–LGA	0.838
AGA–SGA	0.025
LGA–SGA	0.355

Discussion

Breast milk is a highly complex and variable biofluid. The composition of human milk changes in response to many factors and is also specifically adjusted by each mother to precisely reflect their baby's requirements.^4–6

Our results showed that there was no significant difference in true protein, crude protein, fat, or energy between the maternal breast milk from the SGA, AGA, and LGA groups. However, the CHO level in the SGA group was significantly lower than that in the AGA and LGA groups. After post hoc multiple comparison, only CHO in maternal breast milk between the SGA and AGA groups was found to be significantly different (p = 0.025).

Many studies have examined the influences of maternal characteristics on breast milk compositions. Breast milk composition is modified by many factors and its lipid content is the most highly variable macronutrient. Some important factors known to affect breast milk compositions are the time of day, the nursing process stage, lactation length, diurnal variation, the age of the mother, the quality of the mother's diet, and maternal weight gain during pregnancy.^4,7–11 Despite the differences in baseline characteristics between the three groups in our study, including birth weight, gestational age, and maternal BMI, no significant effects were identified on the overall CHO level following correlation analyses.

Despite maternal age, maternal weight gained during pregnancy, and expression timing are known to affect breast milk compositions,^8–11 there was no significant difference in milk macronutrient levels among the three groups of mothers in our study. Therefore, these factors should have no influence on our results.

Michaelsen et al. showed an association of increasing breast milk fat content among mothers of lowest and highest birth weight infants. However, no data on the lengths of gestation were reported.² Despite this, recent studies on maternal breast milk composition associated with SGA infants have suggested that there was no difference in breast milk fat compositions in these sample groups.^2,3,5 The latter results correspond with our findings.

Consistent with previous studies, our results reveal a significant reduction in CHO components in breast milk from mothers delivering SGA infants compared with breast milk of those delivering AGA and LGA infants. However, in clinical terms, a CHO difference of only 0.2 g/100 mL is unlikely to have much effect on infant growth. In particular, the total energy per 100 mL of milk is also not significantly different, unless a baby is consuming a substantial volume of milk.

A strength of this study is our use of a rigorous standardized protocol for breast milk collection. Milk samples were collected on the 11th–28th day postpartum, which is defined as the mature milk stage.¹² Our protocol was designed to reduce variation caused by different milk stages because colostrum and immature (transitional) milk (which follows colostrum on the 4th–10th day following childbirth) possess high fat and high energy. Therefore, these milk would have considerably affected the overall breast milk macronutrients. In addition, we used consistent milk collection times (after lunch and between 12.00 and 14.00) to decrease diurnal variation of milk fat concentration and to reflect the changes in breast milk that occur over the course of a single day. Furthermore, all mothers expressed their milk using either manual or electrical pumps at the hospital, where they could be closely monitored.

To ensure that our measurements of breast milk compositions in this study accurately represent the actual macronutrients of human milk (which are not predominant in foremilk or hind milk), we requested that all mothers empty the entirety of each breast and used milk from either side for our analyses. In addition, all milk samples were equivalently homogenized before testing.

To summarize, this is the first study to report a comparison of breast milk compositions between the SGA, AGA, and LGA groups. The results confirmed that, in clinical terms, there were no significant differences in either maternal milk macronutrients or energy between the three groups. Another important aspect to consider is that routine breast milk analysis of all mothers of healthy term infants may not be necessary if the nutrients in their breast milk are comparable, unless those infants do not grow properly, have health problems, or if healthcare providers express concerns that a particular breastfed baby is not receiving adequate nutrition.

One limitation of our study is that we collected milk samples only once from each mother. We believe that our results could be built upon if samples were obtained from each subject more than once, were collected at different times of day, or focused upon the different milk stages in turn.

Conclusions

We have revealed that the CHO component in breast milk from mothers of SGA infants is significantly lower compared with mothers of AGA and LGA infants. However, we believe that this observation has no clinical significance. Breast milk macronutrient compositions from mothers of LGA term infants are equivalent to those of SGA and AGA infants. Therefore, we conclude that fetal growth status does not significantly influence the concentration of maternal breast milk nutrients components.

Footnotes

Acknowledgment

The authors wish to acknowledge the support of Sukhontha Siri, PhD, for the biostatistics and bioinformatics.

Disclosure Statement

No competing financial interests exist.

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