Sex Related Nutritional Outcomes Among Preterm Very Low Birth Weight Infants

Abstract

Objective: Sex-specific differences exist in morbidity and growth yet little is known about other nutritional outcomes in preterm very low birth weight (VLBW) infants. We aimed to determine the effect of sex on days to full enteral feedings and other nutritional outcomes.

Study Design: This was a longitudinal 2-group comparison using secondary analysis of existing data of 314 infants born <32 weeks gestation and weighing <1500g. Information regarding nutritional outcomes was obtained from the infant’s EMR. 

Results: Males had higher direct bilirubin levels (p=0.04), more emesis (p=0.02), and more late-onset sepsis (p=0.04). Males weighed more at birth but not at 6-weeks and while the slope of weight increase over 6-weeks was higher in males (p=0.04), growth velocity was similar between sexes yet tended to be higher in females.  

Conclusion: Sex-specific differences in nutritional outcomes among VLBW infants may exist suggesting personalized nutritional support may improve health outcomes.  

Introduction

Provision of sufficient nutrition to safely achieve appropriate growth and development in infants born preterm and very low birth weight (VLBW) is one of the most challenging aspects of neonatal care. Unfortunately, preterm VLBW infants often experience poor postnatal growth during their hospitalization in the neonatal intensive care unit (NICU).¹ Because impaired nutrition and poor growth are associated with less favorable neurodevelopmental outcomes in this vulnerable population of infants, strategies to promote optimal nutrition and growth are essential.^2,3

Well-established sex-specific differences in mortality, morbidity and neurodevelopmental outcomes exist among VLBW infants, with males being at higher risk.^4,5,6 Male infants are also at greater risk of postnatal growth failure than females and, when growth is sufficient, experience greater neurodevelopmental benefits.^7,8 Furthermore, evidence suggests the effect of diet enhancement on growth and neurodevelopment may be sex-dependent, with increased benefits seen in male infants.^6,8 The higher macronutrient content in human milk produced by mothers of male infants, as well as their production of more significant volumes of milk,⁹ further suggests nutritional requirements may differ depending upon infant sex^10,11 Yet sex-specific nutritional support is rarely addressed when caring for preterm VLBW infants in the NICU. Furthermore, although sex-specific differences exist regarding morbidity, mortality, and growth, little is known regarding potential differences in other nutritional outcomes in infants born preterm and VLBW.

Therefore, this study aimed to determine whether differences exist regarding nutritional outcomes between male and female VLBW infants. The primary aim of this study was to determine the effect of sex on days to the attainment of full enteral feedings. We hypothesized that female infants would reach full enteral feeds earlier than male infants. Secondary objectives aimed to determine the effect of sex on other nutritional outcomes, including days requiring parenteral nutrition and central line access, incidence of parenteral nutrition-associated liver disease (PNALD), level of alkaline phosphatase, days to discharge, and growth as well as episodes of feeding intolerance, necrotizing enterocolitis (NEC) and late-onset sepsis, over the first 6 weeks after birth.

Materials/subjects And Methods

Design

This study was a longitudinal 2-group comparison of existing data from 2 National Institute of Health-funded randomized controlled trials (RTC) conducted between October 2013 and October 2017. Study 1 enrolled 175 infants and their mothers to determine the optimal time to initiate milk expression in mothers of preterm VLBW infants to enhance lactation success and infant consumption of their mother’s own milk (MOM). Study 2 enrolled 139 infants to determine the risks and benefits of omitting pre-feed gastric residuals on nutrition. 

Sample and Setting

Infants were eligible for inclusion if they were born <32 weeks gestation and weighing <1500g. Infants were excluded if they had congenital or chromosomal abnormalities or died before the completion of the 6-week study. All infants were recruited from a 72-bed Level IV NICU, a referral hospital in an urban tertiary care center serving a predominantly rural and semi-rural population. All feeding decisions, including time of initiation, rate of advancement, withholding of feeds, and human milk fortification, were made according to the NICU’s nutritional guidelines. Due to differing protocols of the individual parent studies, 139 infants received only human milk [either mother’s own milk (MOM) or donor human milk (DHM)] while 175 infants received either human milk or formula. Study approval was obtained from the Institutional Review Board at the University of Florida. Informed consent was signed by the mother or father of all infants enrolled in the study.

Demographic Factors

Information including sex, single or multiple birth, exposure to antenatal steroids, birthweight, gestational age, Apgar scores, as well as race and ethnicity were collected from the infant’s electronic medical records (EMR). Feeding regimens including percentage of MOM, DHM, and formula consumed, were also collected from the EMR. 

Study Outcomes 

Nutritional outcomes. All information regarding nutritional outcomes was obtained from the infant’s EMR. The primary outcome for this study was days to reach full enteral feedings, defined as the day of life the infant first received 120 mL/kg/d of formula and/or human milk.  Secondary outcomes included the number of days the infant received any parenteral nutrition (PN), number of days with a central line, evidence of parenteral nutrition-associated liver disease (PNALD; direct bilirubin level > 2 mg/dl), level of direct bilirubin, level of alkaline phosphatase and days to NICU discharge. Direct bilirubin and alkaline phosphate levels were assessed with clinically ordered labs.  

Feeding tolerance. Incidence of feeding intolerance, as evidenced by the number of emeses and episodes of abdominal distention defined as an increase in abdominal circumference by > 2 cm during the first 6-weeks of life was collected from the infant’s EMR. 

Incidence of late-onset sepsis and necrotizing enterocolitis. Episodes of presumed (treated with antibiotics for >5 days) or culture-proven, late-onset sepsis (occurring > 3 days of life) and stage II or greater necrotizing enterocolitis (NEC) were collected from the EMR.  

Growth. Weight, length and head circumference were collected from the EMR at birth and weekly for the first 6-weeks. Weight was measured daily to the nearest gram using an electronic scale, length was measured weekly to the nearest centimeter using a tape measure or length board, and head circumference was measured weekly using a tape measure. In addition, weekly growth velocity was determined by calculating grams gained or lost per kilogram of weight per day. 

Statistical Analysis

This was a secondary analysis of an existing dataset, so an a priori power calculation was not performed. However, with the given sample size of 314 (166 Male, 148 Female) in the existing dataset, we had at least 80% power to detect effect sizes (Cohen’s f) of 0.22 in the gender by time interaction effect.¹²

Descriptive statistics appropriate for measurement level were obtained for each variable to evaluate distributions and identify patterns of missing data. Comparisons of male and female baseline values were produced using Satterthwaite t-tests (continuous variables) and chi squared contingency tables with calculation of exact p values (categorical variables). Where statistical model assumptions were not tenable, two-sample Wilcoxon tests were employed. Changes in weight over time were tested using a two (sex) by six (week) general linear mixed model with an unstructured within covariance matrix. The tenability of statistical model assumptions was examined, and no remedial measures were needed. All analyses were performed with SAS version 9.4.¹³

Results

Subjects. A total of 339 infants were enrolled in the study. Twenty-five infants were withdrawn for death. Thus 314 infants (166 male and 148 female) were included in the final analysis (Figure 1). Male and female infants had mean [SD] gestational ages of 27.4 weeks [+2.3] and 27.6 weeks [+2.1] and birth weights of 1002 grams [+250.3] and 939 grams [+250.8], respectively. Groups were similar in terms of mode of delivery, multiple births, exposure to antenatal steroids, Apgar score, and type of feeding (MOM, DHM and/or formula). However, male infants had a statistically significant higher birthweight, although birth length and birth head circumference were similar (Table 1). 

Nutritional outcomes. We found no statistically significant differences in time to reach full enteral feedings nor days requiring parenteral nutrition or a central line. Male infants reached full feeds two days later (18.0 vs. 15.5; p=0.84), required parenteral nutrition for nearly an additional four days (18.3 vs. 14.4; p=0.40), and had a central line for over three days longer (20.0 vs. 16.3; p=0.70) than female infants. In addition, male infants remained in the NICU three days longer than female infants (78.2 vs. 74.5; p=0.32). Male infants had statistically significantly higher direct bilirubin levels over the 6-week study (1.1mg/dL vs. 0.80mg/dL; p=0.04) yet episodes of parental nutrition-associated liver disease (direct bilirubin > 2 mg/dL did not differ (Table 2). 

Feeding tolerance. Male infants experienced a greater number of emesis episodes (3.2 vs. 2.3; p=0.02), but there was no difference in episodes of abdominal distension (Table 2).  

Late-onset sepsis and NEC. While there were no differences in the incidence of stage II or greater NEC between sexes, male infants were more likely to be diagnosed with presumed or culture-proven, late-onset sepsis than female infants (62.3 vs. 50.7%; p=0.04) (Table 2).

Growth. Male infants weighed more at birth (1002 vs. 939 grams; p=0.03) but not at 6-weeks of life (1562.8 vs. 1516 grams; p=0.31). There were no differences in length or head circumference at either birth or 6-weeks. In addition, the slope of weight increase from birth to 6-weeks was higher in males (p=0.04) but not for length or head circumference (Figure 2). Finally, over the first 6 weeks after birth, growth velocity was similar between males and females, with females tending to have a higher growth velocity over the 6-week study (11.8 ± 0.22 grams/kg/day vs. 11.2 ± 0.21 grams/kg/day; p=0.07) (Figure 3). 

Discussion

In this secondary analysis of data from two randomized controlled trials, we found sex-specific differences in nutritional outcomes over the first 6 weeks following birth among preterm VLBW infants. While previous research has examined sex-related growth differences, to our knowledge this is the first to examine nutritional outcomes in addition to growth. Because early nutrition correlates with risk of chronic disease in adulthood and improved neurodevelopmental outcomes, strategies to optimize the nutritional support of infants born preterm are essential and nutritional interventions may need personalization based upon infant sex.^2,3,14 

Males reached full feedings two days later, required PN almost 4 days and a central line more than 3 days longer than females which may be clinically significant. Although the incidence of direct hyperbilirubinemia was similar, male infants had statistically significantly higher direct bilirubin levels. Because the most common cause of increased bilirubin is PN cholestasis, it is possible that the longer PN requirement in males led to higher bilirubin levels.¹⁵

Male infants experienced significantly more episodes of emesis than females, although there were no differences in abdominal distension between sexes. It is possible that concern for emesis led clinicians to prescribe a slower advancement of feedings which delayed time to full feeds. Due to prematurity-related intestinal dysfunction, feeding intolerance is common in infants born preterm and VLBW and it is possible that intestinal immaturity is more significant in males.¹⁶

Consistent with previous research, we found a higher incidence of late-onset sepsis in male infants.¹⁷ Because a longer duration of central line access is known to increase the risk of late-onset sepsis, it is possible that the longer time male infants required a central line may have increased their risk of LOS.¹⁸ Moreover, sex-related differences in systemic immunity may increase the susceptibility of male infants to infection. Full term male infants have been found to exhibit weaker innate and adaptive immunity, reduced vaccine response and poorer pathogen clearance than females, and these sex related differences may begin in-utero as evidenced by greater pro-inflammatory response found in cord blood.^19-21 

Although male infants were heavier at birth, weight at 6-weeks was similar between groups. In addition, while the slope of weight increase from birth to 6-weeks was higher in males, female infants tended to have a higher growth velocity, consistent with previous research suggesting that female infants have more adequate growth during their NICU hospitalization.^7,22   In addition, while fat mass is known to be higher in female infants born term, this is not true among preterm infants suggesting inadequacies in the nutrition provided to male infants in the NICU.^23,24 

While male infants were heavier at birth, there were no differences between sexes regarding length or head circumference at either time point or in weight at 6-weeks. The slope of weight increase from birth to 6 weeks was higher in males (p=0.02) but not length (p=0.66) or head circumference (p=0.82). This was an unexpected finding since head circumference and length at birth are greater in male infants born at term.²⁵  Because appropriate head growth is necessary for optimal neurodevelopmental outcomes and provision of enhanced nutrition has been shown to improve head growth at 18 months, particularly in male infants, this may be especially important.^26,27 The higher risk of poor neurodevelopmental outcomes in males and findings that enhanced nutrition may be more important in the neurodevelopment of males emphasizes the need to optimize nutrition in preterm male infants.^4,6,17 

While the exact cause for the higher risk of poor growth in male infants is unknown, it may be related to hormonal differences both in-utero and after birth. Males have higher testosterone levels beginning in-utero, increasing over the first few weeks of life and enhancing lean body mass.^28,29 Furthermore, protein and lipid levels are higher in the milk from mothers of male infants which may partially explain the increased growth rates in male infants.³⁰

Limitations of this research included a lack of control for clinical acuity and the possibility that collecting data from two separate studies could have affected outcomes. In addition, we defined as attainment of full feeds as reaching feeds of 120 mL/kg/d but did not consider whether infants were sustained on this feeding volume. 

Preterm VLBW infants are hospitalized in the NICU during a critical period of growth and development, during which provision of insufficient nutrition may adversely affect long-term growth and development. While sex-based growth differences are known to occur in term infants and human milk composition differs based on infant sex, sex-based differences in nutritional requirements are not considered in the NICU, which may be an essential nutritional strategy.   Understanding potential sex-related differences in nutritional outcomes in preterm infants may lead to more targeted therapies and interventions to lessen potential sex disparities for male infants in the NICU.

Declarations

Funding Source: This work was not supported by any funding

Conflict of Interest: None of the authors have conflict of interests. 

Author Contributions: LAP designed the study, interpreted the data and drafted the manuscript. DD helped interpret the data and critically revised the manuscript. MW analyzed the data. JN contributed to study design, interpretation of data, and critically revised the manuscript. 

References

1. Williamson AL, Derado J, Barney BJ, et al. Longitudinal BMI Growth Curves for Surviving Preterm NICU Infants Based on a Large US Sample. Pediatrics. 2018;142(3).
2. Hickey L, Burnett A, Spittle AJ, et al. Extreme prematurity, growth and neurodevelopment at 8 years: a cohort study. Archives of disease in childhood. 2021;106(2):160–166.
3. Simon L, Théveniaut C, Flamant C, Frondas-Chauty A, Darmaun D, Rozé JC. In Preterm Infants, Length Growth below Expected Growth during Hospital Stay Predicts Poor Neurodevelopment at 2 Years. Neonatology. 2018;114(2):135–141.
4. van den Akker CH, te Braake FW, Weisglas-Kuperus N, van Goudoever JB. Observational outcome results following a randomized controlled trial of early amino acid administration in preterm infants. Journal of pediatric gastroenterology and nutrition. 2014;59(6):714–719.
5. Bæk O, Cilieborg MS, Nguyen DN, Bering SB, Thymann T, Sangild PT. Sex-Specific Survival, Growth, Immunity and Organ Development in Preterm Pigs as Models for Immature Newborns. Frontiers in pediatrics. 2021;9:626101.
6. Christmann V, Roeleveld N, Visser R, et al. The early postnatal nutritional intake of preterm infants affected neurodevelopmental outcomes differently in boys and girls at 24 months. Acta paediatrica. 2017;106(2):242–249.
7. Frondas-Chauty A, Simon L, Branger B, et al. Early growth and neurodevelopmental outcome in very preterm infants: impact of gender. Archives of disease in childhood. Fetal and neonatal edition. 2014;99(5):F366-372.
8. Isaacs EB, Fischl BR, Quinn BT, Chong WK, Gadian DG, Lucas A. Impact of breast milk on intelligence quotient, brain size, and white matter development. Pediatric research. 2010;67(4):357–362.
9. Engstrom JL, Meier PP, Jegier B, Motykowski JE, Zuleger JL. Comparison of milk output from the right and left breasts during simultaneous pumping in mothers of very low birthweight infants. Breastfeeding medicine: the official journal of the Academy of Breastfeeding Medicine. 2007;2(2):83–91.
10. Correia L, Cardoso M, Papoila AL, et al. Does Fetal Growth Adequacy Affect the Nutritional Composition of Mothers' Milk?: A Historical Cohort Study. American journal of perinatology. 2021.
11. Galante L, Milan AM, Reynolds CM, Cameron-Smith D, Vickers MH, Pundir S. Sex-Specific Human Milk Composition: The Role of Infant Sex in Determining Early Life Nutrition. Nutrients. 2018;10(9).
12. PASS 15 Power Analysis and Sample Size Software. 2017.
13. Littell R, Milliken G, Stroup W, Wolfinger R, O. S. SAS for mixed models. In: Inc SISI, ed. SAS/STAT® 14.3 User’s Guide. 2nd ed. Cary NC2017.
14. Lapillonne A, Griffin IJ. Feeding preterm infants today for later metabolic and cardiovascular outcomes. The Journal of pediatrics. 2013;162(3 Suppl):S7-16.
15. Fawaz R, Baumann U, Ekong U, et al. Guideline for the Evaluation of Cholestatic Jaundice in Infants: Joint Recommendations of the North American Society for Pediatric Gastroenterology, Hepatology, and Nutrition and the European Society for Pediatric Gastroenterology, Hepatology, and Nutrition. Journal of pediatric gastroenterology and nutrition. 2017;64(1):154–168.
16. Weeks CL, Marino LV, Johnson MJ. A systematic review of the definitions and prevalence of feeding intolerance in preterm infants. Clinical nutrition. 2021;40(11):5576–5586.
17. O'Driscoll DN, McGovern M, Greene CM, Molloy EJ. Gender disparities in preterm neonatal outcomes. Acta paediatrica. 2018; On line ahead of print
18. Köstlin-Gille N, Härtel C, Haug C, et al. Epidemiology of Early and Late Onset Neonatal Sepsis in Very Low Birthweight Infants: Data From the German Neonatal Network. The Pediatric infectious disease journal. 2021;40(3):255–259.
19. Klein SL, Flanagan KL. Sex differences in immune responses. Nature reviews. Immunology. 2016;16(10):626–638.
20. Muenchhoff M, Goulder PJ. Sex differences in pediatric infectious diseases. The Journal of infectious diseases. 2014;209 Suppl 3(Suppl 3):S120-126.
21. Kim-Fine S, Regnault TR, Lee JS, et al. Male gender promotes an increased inflammatory response to lipopolysaccharide in umbilical vein blood. The journal of maternal-fetal & neonatal medicine: the official journal of the European Association of Perinatal Medicine, the Federation of Asia and Oceania Perinatal Societies, the International Society of Perinatal Obstet. 2012;25(11):2470–2474.
22. Chou FS, Yeh HW. Sex differences in postnatal weight gain trajectories of extremely preterm newborns. Journal of perinatology: official journal of the California Perinatal Association. 2021;41(8):1835–1844.
23. Simon L, Borrego P, Darmaun D, Legrand A, Rozé JC, Chauty-Frondas A. Effect of sex and gestational age on neonatal body composition. The British journal of nutrition. 2013;109(6):1105–1108.
24. Hawkes CP, Hourihane JO, Kenny LC, Irvine AD, Kiely M, Murray DM. Gender- and gestational age-specific body fat percentage at birth. Pediatrics. 2011;128(3):e645-651.
25. Villar J, Cheikh Ismail L, Victora CG, et al. International standards for newborn weight, length, and head circumference by gestational age and sex: the Newborn Cross-Sectional Study of the INTERGROWTH-21st Project. Lancet. 2014;384(9946):857–868.
26. Cheong JL, Hunt RW, Anderson PJ, et al. Head growth in preterm infants: correlation with magnetic resonance imaging and neurodevelopmental outcome. Pediatrics. 2008;121(6):e1534-1540.
27. Poindexter BB, Langer JC, Dusick AM, Ehrenkranz RA. Early provision of parenteral amino acids in extremely low birth weight infants: relation to growth and neurodevelopmental outcome. The Journal of pediatrics. 2006;148(3):300–305.
28. Goldenberg RL, Andrews WW, Faye-Petersen OM, Goepfert AR, Cliver SP, Hauth JC. The Alabama Preterm Birth Study: intrauterine infection and placental histologic findings in preterm births of males and females less than 32 weeks. American journal of obstetrics and gynecology. 2006;195(6):1533–1537.
29. Kuiri-Hänninen T, Seuri R, Tyrväinen E, et al. Increased activity of the hypothalamic-pituitary-testicular axis in infancy results in increased androgen action in premature boys. The Journal of clinical endocrinology and metabolism. 2011;96(1):98–105.
30. Thakkar SK, Giuffrida F, Cristina CH, et al. Dynamics of human milk nutrient composition of women from Singapore with a special focus on lipids. American journal of human biology: the official journal of the Human Biology Council. 2013;25(6):770–779.

Tables

Table 1: Baseline infant characteristics, providing mean (SD) or frequency (%) n=314

Table 2. Nutritional outcomes providing mean (SD) or frequency (%) n=314

NEC: necrotizing enterocolitis; LOS: late onset sepsis; HC: head circumference

¹ Wilcoxon test