Body-composition assessment using air displacement plethysmography in healthy term infants : an observational study : a thesis presented in partial fulfilment of the requirements for the degree of Masters of Science in Nutrition and Dietetics at Massey University, Albany, New Zealand

Abstract

Background: Infant body weight and composition at birth have been recognised to be important indicators of fetal growth, maternal and offspring health, and later health outcomes. While it is well documented that average birth weight varies significantly between New Zealand-born infants of different ethnicities, there is limited evidence on body composition in new-born infants. Ethnic differences in body composition have been reported in New Zealand adults and children and it is currently unknown whether these differences are evident shortly after birth. The aim of this study was to examine the differences in fat mass (FM) and fat free mass (FFM) using Air Displacement Plethysmography (ADP) between NZ European (reference group), Māori, Pacific, Asian and South Asian healthy term infants. Method: Healthy term infants (37 to 42 weeks’ gestation) were recruited from Auckland City Hospital (ACH). Birth parameters were recorded and weight, length, and head circumference and waist circumference were measured using standardised techniques. Air Displacement Plethysmography (ADP) was used to measure fat mass (FM) and fat free mass (FFM) of the infants. Ethnicity of all infants and their mothers was classified using standard ethnicity data protocols. Dummy variable multiple linear regression analysis and t-tests were used to compare FM and FFM of Māori, Pacific, Asian, and South Asian infants with New Zealand European (NZE) infants. Results: Body composition was assessed in 214 healthy term infants at a mean age of 1.7±0.85 days, while adjusting for gender and postnatal age. South Asian infants had significantly lower FFM (2691.7±389.7g vs 2938.6±364.0g, P= 0.006) and weight than NZE infants (3045.5±535.2g vs 3352.3±575.8g, P= 0.014). They also had the smallest head (34.2±1.7cm vs 35.4±1.7cm, P= 0.002) and waist circumference (31.5±3.0cm vs 33.2±2.1cm, P= 0.003). Waist circumference of Asian infants was also significantly smaller than NZ European infants (32.3±2.1cm vs 33.2±2.1cm, P= 0.044). When categorised by gender, males had significantly greater FFM, weight, length and head circumference (P< 0.05). No gender or ethnic difference was noted in FM (g) or %FM. Conclusion: This is the first study in New Zealand to report body composition in healthy term infants using ADP. While no differences in FM were seen between NZE and each of the other ethnicities, the differences noted in FFM and weight between NZE and South Asian infants were comparable to other studies. Longitudinal assessment of changes in FM and FFM is needed to establish the significance of ethnic differences.