Exploring the fat mass and fat free mass of term and moderate to late preterm infants : an observational study : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Nutrition and Dietetics at Massey University, Albany, New Zealand

Abstract

Background: Moderate to late preterm infants (32-36+6 weeks’ gestation) make up 83% of preterm births in New Zealand. Preterm birth is associated with having a higher risk of obesity, diabetes and cardiovascular disease in later life. Preterm infants demonstrate postnatal growth restriction followed by a period of accelerated growth. When compared to term infants at equivalent ages, preterm infants have been found to have a higher % fat mass. Nutrition is a modifiable factor contributing to the growth of preterm infants. While the goal is for these infants to be breast-fed, this is often not possible in early postnatal life due to delayed maternal milk supply and immaturity of the infant. Evidence is limited for the optimal feeding strategy for these infants until full breast-feeds can be established and there is great variability in practice. Aims: To measure the % fat mass and fat free mass of moderate to late preterm infants and term infants after birth at Auckland City Hospital; to explore the relationship between feeding strategies currently used within Auckland City Hospital and the acquisition of fat and fat free mass in moderate to late preterm infants after birth. Methods: Moderate to late preterm infants and term infants were recruited from Auckland City Hospital. Air displacement plethysmography (ADP) was used to measure the fat mass and fat free mass of the infants. Preterm infants were measured once they were medically stable and term infants were measured within 72 hr of birth. Eleven preterm infants were measured a second time prior to discharge from hospital. Information regarding what the preterm infants were fed within the first five days after birth was collected from their medical notes and the infants were prospectively grouped according to which feeding strategy they predominantly received, either: breast-milk, infant formula, 10% dextrose, or parenteral nutrition. Statistical analysis was performed using independent t-tests, Pearson’s Chi square tests, Mann-Whitney tests, paired sample t-tests, one-way ANOVA, and Kruskal-Wallis tests. Results: Forty seven preterm moderate to late preterm infants and sixty nine term infants were recruited. Term infants had a mean ± SD % FM of 10.9 ± 4.2%, when broken down by sex males had 9.4 ± 3.5% and females had 12.2 ± 4.8% (P = 0.018). Preterm infants measured within the first week of birth (n = 25) had a mean ± SD % FM of 8.7 ± 4.4% and those measured within the second week of birth (n = 19) had a mean ± SD % FM of 8.1 ± 2.9% (P = 0.6). Twenty two preterm infants were measured at ≥ 36 weeks’ postmenstrual age and had a mean ± SD % FM of 10.9 ± 5.0%, which did not differ from the % FM of term infants measured after birth (P = 0.98). Eleven preterm infants were measured twice during their hospital admission and their mean ± SD % FM increased from 8.5 ± 3.5% to 15.0 ± 4.2% (P <0.001). The median [25th-75th quartiles] % increase in FM for infants with two measurements was 98.9 [70.1, 114.9] %. One-way ANOVA revealed significant differences in FFM (P = 0.004), weight (P = 0.013), and length (P = 0.036) between the feeding groups. Post hoc analysis showed that infants in the parenteral nutrition group had significantly less FFM than infants in the formula group (P = 0.008) and were lighter and shorter than the breast-milk group (P = 0.013, P = 0.036). Conclusions: Moderate to late preterm infants experience a rapid increase in FM during hospital admission and reach the % FM of a term infant before term corrected age. Moderate to late preterm infants in the parenteral nutrition group were the lightest, shortest and had the least FFM. This study also highlights considerations to be made for future research using ADP in the Newborn Intensive Care Unit at Auckland City Hospital.