Neonatal body composition: crossectional study in healthy term singletons in Germany

The incidence of obesity in children is rising worldwide. Currently, 17.0% of children in the United States are obese and the prevalence of extreme obesity is 5.8% [1]. In a recent study on German children and adolescents aged 3 to 19 years studied in 2014–2017, the prevalence of overweight was 15.4% and of obesity 5.9%, both increased with age [2]. Obesity in children is relevant for public health, because obese children already have elevated blood pressure and abnormal fasting glucose concentrations [3]. Furthermore, obese children are likely to become obese adults with increased risk of obesity-related complications (e.g., type II diabetes and cardiovascular disease) and mortality [4‐6].

Epidemiological studies suggest that inadequate intrauterine supply of nutrients may impact on metabolic health in adulthood [7, 8]. Most studies investigating the relationship between intrauterine growth and later metabolic risk used birth weight alone. It is conceivable, however, that the determination of body composition might be a more sensitive marker for in utero environment and increased neonatal fat mass. It may also be a better indicator of later metabolic risk, as there is considerable variability of neonatal body composition parameters such as fat mass (FM), fat-free mass (FFM) and the proportion of FM divided by total body mass (BF%) in newborns of similar weight and length [9, 10].

There are different methods to determine neonatal body composition (e.g. dual energy x-ray absorptiometry, magnetic resonance imaging or isotope dilution). For about 15 years, air displacement plethysmography (ADP) has been available as a method for rapid, non-invasive, pain-free determination of body composition at relative low cost, providing immediate results without ionizing radiation; thereby making body composition measurements in healthy children acceptable for parents and ethics committees. It has been shown that these measurements are highly reproducible and accurate and therefore suitable even for large epidemiological studies [11, 12]. ADP calculates BF%, FM and FFM according to the two compartment model based on measurements of the infants’ weight and volume.

Reference data for healthy newborns are important as a basis to identify deviations in body composition from the reference standard in special patient groups (e.g., small for gestational age or preterm infants) and to plan intervention studies aiming at improving modifiable pre- and postnatal factors affecting long-term health. For example, the aim of nutritional care of preterm infants is to achieve similar growth as in utero. Due to improvements in nutritional care of preterm infants, weight-gain as in utero is now often achieved [13], but body composition at term equivalent age continues to differ from values found in term-born infants [14]

Differences in body composition between populations of different ethnic and/or socioeconomic background have been reported in adults and children [15, 16] and neonates [12, 17].

We aimed to generate reference data for German Caucasian population for BF%, FM and FFM at birth in healthy, term, singleton infants and to investigate factors influencing body composition.

The aims of this cross-sectional observational study were to establish reference data for body composition by air displacement plethysmography in healthy term singletons for Germany and to investigate various factors potentially influencing body composition.

The infants studied here showed values for median BF% (10.8%) similar to those from other European countries such as Portugal (11.3%) [22], the Netherlands (10.3%) [23], and Ireland (11.1%) [24], but higher values than infants from Australia (BF% 9.2) [12] and Ethiopia (BF% 7.8) [9]. Differences in total body fat between populations of different ethnic background have already been reported in adults and children [15, 16], but little data based on air displacement plethysmography exist in neonates. Furthermore, an Australian study reported that infants of Caucasian mothers showed higher BF% and birthweight in comparison to infants of Asian mothers [12]. While it remains unclear whether the observed differences are truly ethnic (i.e., genetic) or rather economic or nutritional, these factors seem to have an influence on neonatal body composition.

In our study population a significantly higher median BF% (11.2% vs. 9.6%) and lower median FFM (2786 g vs. 2977 g) were found in female infants compared to male infants, and gender was significantly associated with BF%, FM and FFM on multiple regression analysis. Consistent with this, the SCOPE pregnancy study, a large population-based study in Ireland, including 786 term infants confirmed differences in body composition between female and male neonates with girls having a higher BF% and lower FFM [25]. In a cross-sectional Australian study including 599 term infants, gender showed the strongest association with neonatal BF%, followed by maternal ethnicity [12]. Gender is known to be a major determinant of body composition throughout life; girls and adult women also have a higher BF% and lower FFM than their male counterparts [26, 27].

Besides ethnic factors and gender, increasing gestational age is associated with differences in body composition: Hawkes et al. described a significant and linear increase in BF% with increasing gestational age [25], but this was not confirmed in this German population. In this cohort of term infants, we found that BF% showed no significant difference between gestational age groups, and gestational age was only significantly associated with FM and FFM on multivariate analysis.

In our study with measurement of body composition within the first 96 h, postnatal age at measurement was not associated with BF%, FM or FFM. In agreement with our results, there was no association of BF% with postnatal age in the SCOPE study [25], whereas Roggero et al. [28] in a longitudinal study on 28 breastfed, term infants described a higher loss of BF% during the initial weight loss period of the first 5 days after birth compared to the FFM.

BF% and FM were significantly associated with pre-pregnancy maternal BMI. Our results are consistent with previous studies using ADP. In a large pre-birth cohort study, the Healthy Start study (Colorado, USA), Starling et al. showed positive and independent associations of pre-pregnancy BMI with neonatal adiposity measures [29]. Pereira-da-Silva et al. also found that pre-pregnancy overweight was positively associated with offspring weight, BMI and FFM; additionally in male infants also with fat mass [22]. Contrary to this, Eriksson et al. could not find any change in body composition in relation to pre-pregnancy BMI, but weight and BMI of the infants correlated with maternal BMI before pregnancy.

In our study, there was no correlation between absolute maternal gestational weight gain and body composition but a positive association between excessive weight gain by IOM recommendations and neonatal BF%. Additionally, inappropriately low weight gain was associated with decreased FFM in this German cohort. An altered body composition in infants born to mothers with excessive gestational weight gain was also found in the Healthy Start study [29]: gestational weight gain exceeding recommendations was associated with higher neonatal FM and FFM but not BF% compared to adequate weight gain during pregnancy [29].

The association of body composition parameters with pre-pregnancy BMI and gestational weight gain category reflect the impact of intrauterine environment on offspring obesity risk.

There is evidence that obese infants and children are more likely to become obese adults, with an increased risk of characteristic complications (e.g. diabetes and cardiovascular disease) and increased mortality [4‐6]. Body composition shortly after birth is not influenced by postnatal factors (i.e., nutritional) and may therefore enables to study the effects of intrauterine environment and targeted interventions (e.g. maternal diet or physical activity before and during pregnancy).

In this study, only 3% (n = 8) of mothers reported smoking during pregnancy, nevertheless multivariate analyses showed a significant association with lower BF% and FM, the magnitude of the effect of smoking exceeding that of gender. The apparent lack of impact on FFM may be due to the small numbers. The KiGGS Wave 2 study reported that 10.9% of German mothers smoked during pregnancy and that the proportion of pregnant women who smoke has declined over the last two decades [30]. Furthermore, they found a distinct social gradient in maternal smoking: the higher the social status, the less likely are pregnant women to smoke (1.6% in high social status vs. 27.2% in low social status) [30]. Unfortunately, socio-economic status is difficult to assess reliably and was not documented in this study, however the population of Tuebingen, a town dominated by its university, is generally characterized by a high educational level. It is well known that prenatal smoking increases the risk of intrauterine growth restriction and thus possibly also body composition. The Healthy Start study found a significant effect of exposure to prenatal smoking with a lower FM and FFM at delivery. In that study, 7% of mothers reported smoking during pregnancy [31] and the exposure to prenatal smoking was also associated with a significantly more rapid postnatal growth and increased FFM in the first 5 months of life, possibly explaining the increased risk of metabolic disease in exposed infants.

Strengths of this study are the rather large sample size which was representative for all neonates born during the study period, and the recruitment rate was 55%, which is within the usual range for studies in neonates shortly after birth. Furthermore, inclusion criteria for the study aimed to minimize confounding factors by investigating singleton, healthy, term newborn infants. Using ADP, a reliable, validated and non-invasive method was used for measuring body composition. Based on our experience with participants in neonatal clinical studies at our institution, we assume that a relatively homogeneous population in terms of socioeconomic, ethnic, and cultural background was studied, but it is a weakness of our study that this was not well documented.

In retrospect, additional information on maternal dietary intake, maternal physical activity during pregnancy, as well as placental weight would have been helpful to further identify important factors associated with body composition at birth.

This cross-sectional study on body composition of healthy term singletons indicates a median (Q1-Q3) BF% of 10.8% (7.7–13.4%) as reference for contemporary neonates for a German Caucasian population. These data are thus similar to those reported for other European countries. Factors associated with body composition in this study were gender, parity, smoking during pregnancy, pre-pregnancy BMI, gestational age at birth and gestational weight gain category, this information will be helpful for the design of future studies.