ORIGINAL RESEARCH


https://doi.org/10.5005/jp-journals-11002-0018
Newborn
Volume 1 | Issue 1 | Year 2022

Accretion Rates of Fat and Fat-free Mass in Infants at 30–45 weeks’ Postmenstrual Age

Sreekanth Viswanathan1, Kera M McNelis2, Akhil Maheshwari3, Zaineh Aja’Nini4, Stephanie Merlino5, Marissa Culver6, Marc Collin7, Darlene Calhoun8, Sharon Grow-Wargo9

1,6,8Division of Neonatology, Department of Pediatrics, Nemours Children’s Hospital, University of Central Florida College of Medicine, Orlando, Florida, United States of America

2Division of Neonatology, Department of Pediatrics, Cincinnati Children’s Hospital Medical Center, University of Cincinnati College of Medicine, Cincinnati, OH, United States of America

3Global Newborn Society, Baltimore, Maryland, United States of America

4Division of Neonatology, Department of Pediatrics, Mercy Kids Children’s Hospital, University of Missouri School of Medicine, Springfield, MO, United States of America

5,7,9Division of Neonatology, Department of Pediatrics, MetroHealth Medical Center, Cleveland, OH, United States of America

Corresponding Author: Sreekanth Viswanathan, Division of Neonatology, Department of Pediatrics, Nemours Children’s Hospital, University of Central Florida College of Medicine, Orlando, Florida, United States of America, Phone: +2163152327, e-mail: sreekanth.viswanathan@nemours.org

How to cite this article: Viswanathan S, McNelis KM, Maheshwari A, et al. Accretion Rates of Fat and Fat-free Mass in Infants at 30–45 weeks’ Postmenstrual Age. Newborn 2022;1(1):7–13.

Source of support: Nil

Conflict of interest: None

ABSTRACT

Background: Body composition assessment using noninvasive air displacement plethysmography (ADP) can help determine the quality of postnatal growth in infants. The accretion rates of fat mass (FM) and fat-free mass (FFM), both are known to change in various clinicopathological situations in a discordant fashion, can also help evaluate the short-term impacts of nutritional interventions on body composition.

Objectives: To determine the FM and FFM accretion rates from 30 to 45 weeks’ postmenstrual age (PMA) and whether these rates are different between male and female infants.

Methods: We used previously published normative data with median FM and FFM values for infants at 30–45 weeks’ PMA (Norris et al., 2019). Weekly gains in FM and FFM in g/week and g/kg/week were calculated using early one-point and average two-point methods.

Results: FM and FFM accretion rates calculated by the early one-point method were higher than the average two-point method. Male and female infants had similar FM and FFM accretion rates. Constant accretion rates of FM and FFM were not aligned with individual weekly accretion rates, which showed a twofold–fourfold change. A composite index (FFM/FM accretion rate ratio), which we named the “body composition accretion ratio” (BCAR), was more sensitive than the individual weekly accretion rates and showed a ninefold change during the study period.

Conclusions: Weekly FM and FFM accretion rates can help assess quality of postnatal growth in young infants, but BCAR can be a more useful, sensitive index for early identification of body composition changes and may possibly guide nutritional interventions.

Keywords: Body composition, Postnatal growth, Preterm infants.

INTRODUCTION

The quality and quantity of early postnatal growth can impact the survival, neurodevelopment, and later metabolic function in preterm infants.13 Body composition studies in preterm infants have observed that early body composition, especially with a gain in fat-free mass (FFM), is associated with better long-term neurodevelopmental outcomes.46 Current nutritional practices in neonatal intensive care units (NICUs) that attempt to grow preterm infants along the in utero growth curves using weight-based calorie intake goals are often associated with deficit in fat-free mass (FFM) and increased fat mass (FM) at term-corrected age compared to term-born peers.711 Such disproportionate growth in FM and FFM has raised concerns about weight-based nutritional interventions. With the availability of safe, reliable, noninvasive infant body composition assessment using air displacement plethysmography (ADP, trade name “PEA POD”—an infant version of the ADP device), body composition assessment is increasingly clinically used to determine the quality of postnatal growth in preterm infants.12,13 Also, normative, sex-specific reference charts for body composition for the first 6 months of life to monitor growth are now available (Norris body composition charts).14

To calculate the short-term growth in infants being treated in the NICUs, growth velocity calculated over a defined time period (e.g., weekly) is often used and a weight gain of 15–20 g/kg/day is often considered acceptable.1517 This growth velocity rate is usually expected to be similar in both male and female infants, and a sex-specific growth velocity rate for weight is not usually used.16 However, unlike weight gain, there is no readily available data for the FM and FFM accretion rate over a defined time period. These data are important for clinicians to assess the short-term impact on the body composition of various nutritional interventions aimed to optimize the postnatal growth of preterm infants. Also, previous data suggest that male infants have higher FFM, lower FM, and lower body fat% compared to female infants.14 This difference in body composition between male and female infants may be best evaluated with sex-specific FM and FFM accretion rates. The objective of this study was to determine the expected sex-specific weekly FM and FFM accretion rates for infants across the postmenstrual ages (PMAs) 30–45 weeks calculated based on the Norris body composition charts, and we hypothesized that these accretion rates would be different between male and female infants.

METHODS

This study is a secondary analysis of the previously published Norris body composition charts that were created using pooled data from four studies comprising both term and preterm infants (n ~ 1,450 which included 222 preterm infants) from three high-income countries (Ireland, Italy, and the United States).14 Institutional review board approval was not needed for this secondary analysis of data that are available in the public domain. The infants included in these studies were born singleton, medically stable, and without significant perinatal morbidity. Using the combined data, sex-specific centiles for FM (kg), FFM (kg), and body fat percentage were estimated using the lambda-mu-sigma (LMS) method. For each sex and measure (e.g., FM), these charts also include postconceptional charts, ranging from 30.3 to 67.3 weeks’ PMA, which enable serial postnatal monitoring of body composition in preterm infants.

To determine weight growth velocity, the three common calculation methods used are early one-point method, average two-point method, and exponential two-point method.16,17 The exponential model was not suitable in this study because of the assumption that growth occurred at a constant fraction of the previous weight over time; existing data show that FM accumulation usually increases rapidly toward term gestation. As defined earlier, the early one-point method was calculated as [(W2 – W1)/(W1 ÷ 1000]/number of days. The average two-point method was computed as (W2 – W1)/[(W1 + W2) ÷2 ÷ 1000]/number of days.17 First, we calculated the FM and FFM absolute values (g) corresponding to the median (50th percentile) for 30–45 weeks’ PMA according to the Norris body composition charts for male and female infants. Then, the weekly gain in FM and FFM in g/week and g/kg/week (by early one-point method and average two-point method) was calculated. For example, for FM, we used the following formulas for early one-point method [FM week 2–FM week 1/weight at week 1] × 1,000 and average two-point method [FM week 2–FM week 1/(weight week 1 + weight week 2)/2] × 1000.

Statistics

Student t-test was used for parametric continuous variables (FM and FFM gain and weekly accretion rates) to identify the unadjusted differences between male and female infants. All quantitative data were expressed as the mean ± standard deviation (SD). A p ≤0.05 was considered statistically significant. The statistical software IBM SPSS Statistics version 24 (SPSS, Chicago, Illinois) was used for the statistical analysis of the data.

RESULTS

The reference median rates of FM and FFM accretion across 30–45 weeks’ PMA for male and female infants are shown in Tables 1 and 2. Both FM gain and FFM gain (g/week) increase with advancing PMA in both male and female infants (Tables 1 and 2, Figs 1A and D). FM and FFM median weekly accretion rates (g/kg/week) by early one-point method were higher than those by the average two-point method in both male and female infants throughout the gestational age ranges studied (Tables 1 and 2, Figs 1B to F). The FM accretion rates increased gradually with advancing PMA, while FFM accretion rates decreased with advancing PMA, suggesting a relatively higher FM accretion toward later gestational weeks in both male and female infants.

Table 1: Reference weekly fat mass (FM) and fat-free mass (FFM) median accretion rate (50th percentile) in male infants from 30 to 45 weeks’ postmenstrual age (PMA) according to the calculation method based on the Norris body composition chart
Males
PMA, weeks FM (g) gain/week FM (g) gain/kg/week—early one-point method FM (g) gain/kg/week—average two-point method FFM (g) gain/week FFM (g) gain/kg/week—early one-point method FFM (g) gain/kg/week—average two-point method
30–31  7.5 5.0 3.3 76.3 51.1 33.4
31–32 14.8 9.4 6.1 143.2 90.8 58.6
32–33 17.2 9.9 6.4 154.6 89.1 57.5
33–34 19.9 10.4 6.7 165.5 86.8 56.0
34–35 22.5 10.8 7.0 173.0 82.7 53.5
35–36 25.4 11.1 7.2 176.4 77.1 49.9
36–37 28.1 11.3 7.3 175.9 70.6 45.8
37–38 32.0 11.9 7.7 170.7 63.4 41.2
38–39 37.8 13.1 8.5 164.3 56.7 37.0
39–40 46.9 15.1 9.9 158.7 51.2 33.4
40–41 60.1 18.2 11.9 156.0 47.2 30.8
41–42 64.2 18.2 11.9 160.3 45.5 29.7
42–43 116.2 31.0 20.2 164.4 43.9 28.6
43–44 113.9 28.3 18.4 166.0 41.2 26.9
44–45 119.8 27.8 18.2 164.0 38.1 24.8
45–46 116.2 25.3 16.6 159.0 34.6 22.6
Table 2: Reference weekly fat mass (FM) and fat-free mass (FFM) median accretion rate (50th percentile) in female infants from 30 to 45 weeks’ postmenstrual age (PMA) according to the calculation method based on the Norris body composition chart
Females
PMA, weeks FM (g) gain/week FM (g) gain/kg/week—early one-point method FM (g) gain/kg/week—average two-point method FFM (g) gain/week FFM (g) gain/kg/week—early one-point method FFM (g) gain/kg/week—average two-point method
30–31 8.2 6.0 6.0 72.5 52.8 34.5
31–32 17.1 11.7 11.7 142.5 98.0 63.0
32–33 19.7 12.2 12.2 156.0 96.6 62.2
33–34 22.2 12.4 12.4 167.0 93.3 60.1
34–35 24.8 12.5 12.5 175.5 88.7 57.2
35–36 27.5 12.6 12.6 179.0 82.1 53.1
36–37 30.2 12.7 12.7 177.0 74.2 48.1
37–38 33.5 12.9 12.9 170.0 65.6 42.6
38–39 38.3 13.7 13.7 160.0 57.2 37.3
39–40 45.0 15.0 15.0 146.9 49.0 32.0
40–41 54.4 17.1 17.1 137.1 43.0 28.1
41–42 67.6 20.0 20.0 132.5 39.2 25.6
42–43 80.4 22.5 22.5 132.5 37.0 24.2
43–44 91.3 24.1 24.1 133.8 35.3 23.1
44–45 98.1 24.4 24.4 134.0 33.4 21.8
45–46 99.9 23.5 23.5 132.2 31.1 20.4

Figs 1A to F: Postnatal fat accretion during 30-45 weeks’ postmenstrual age, compared by gender. (Panels A, D), Total FM and FFM gain/week (data from males in A, females in D); (Panels B, E), FM median accretion rate per unit weight (g/kg/week; data from males in B, females in E); and (Panels C, F), FFM median accretion rate per unit weight (g/kg/week; data from males in C, females in F). The figures show the temporal evolution of these indices in the Norris body composition charts. FM, fat mass; FFM, fat-free mass; PMA, postmenstrual age

The cumulative average FM and FFM gain and accretion rates in males and female infants are given in Table 3. The cumulative average FM accretion rate (g/kg/week) across the whole 30–45 weeks’ PMA for males and females was 16.1 and 15.8 by early one-point and 10.4 and 10.3 each by average two-point method, respectively. Similarly, the cumulative average FFM accretion rate (g/kg/week) across the whole 30–45 weeks’ PMA for males and females was 60.6 and 61.0 by early one-point and 39.4 and 39.6 by average two-point method, respectively (Table 3). The similar trend in FM and FFM accretion rates in male and female infants is shown in Figure 2.

Table 3: The cumulative average weight, fat mass (FM), and fat-free mass (FFM) gain and weekly accretion rates in males and female infants across 30–45 weeks’ postmenstrual age (PMA) according to the calculation method based on the Norris body composition chart
Characteristics Males Females p value
Weight (g) 3070.7 ± 1036.5 2904.4 ± 960.8 0.64
FM (g) 360.7 ± 262.5 379.3 ± 234.9 0.83
FFM (g) 2710.0 ± 786.2 2525.0 ± 735.9 0.50
FM gain (g)/week 52.7 ± 41.0 47.4 ± 30.7 0.68
FM (g) gain/kg/week—early one-point method 16.1 ± 7.9 15.8 ± 5.5 0.93
FM (g) gain/kg/week—average two-point method 10.4 ± 5.2 10.3 ± 3.6 0.93
FFM gain (g)/week 158.0 ± 23.4 146.8 ± 26.5 0.21
FFM (g) gain/kg/week—early one-point method 60.6 ± 19.5 61.0 ± 24.5 0.96
FFM (g) gain/kg/week—average two-point method 39.4 ± 12.5 39.6 ± 15.6 0.97

Figs 2A and B: Weekly FM (A) and FFM (B) Median accretion rate (g/kg/week) across the 30–45 weeks’ PMA calculated based on early one-point and average two-point method compared between male and female infants. FM, fat mass; FFM, fat-free mass; PMA, postmenstrual age

Since there were no significant differences in FM and FFM accretion rates between male and female infants, an average of these was taken to calculate the constant FM accretion rate for the 30–45 weeks’ PMA by early one-point (16.0) and average two-point (10.4) and constant FFM accretion rate by early one-point (61.0) and average two-point (39.5), respectively. The constant FM and FFM rates were then superimposed over the weekly reference FM and FFM accretion rates to see the overlap (Fig. 3). The concordance for reference and constant accretion rates for both FM and FFM were limited to a short range of PMA weeks (Fig. 3). For FM, the constant average two-point rate (10.4) was more aligned with the reference before 37-week PMA, and the constant early one-point rate (16.0) after 37 weeks’ PMA. On the contrary, for FFM, the constant early one-point rate (61.4) was more aligned with the reference before 37 weeks’ PMA, and the constant average two-point rate (39.4) after 37 weeks’ PMA.

Figs 3A and B: Weekly FM (A) and FFM (B) Median accretion rate (g/kg/week) across the 30–45 weeks’ PMA calculated based on early one-point and average two-point method compared to the constant rate. FM, fat mass; FFM, fat-free mass; PMA, postmenstrual age

We did not find the constant accretion rates to simulate the reference accretion rates. Hence, we also computed individual weekly normative FM and FFM accretion rates by averaging the median accretion rates of male and female infants for 30–45 weeks’ PMA (Table 4). We also calculated a composite index comprised of the ratio of weekly median FFM/FM accretion rates, which we named as the “body composition accretion ratios” (BCAR) based on early-1 point and average two-point methods (Table 4). Irrespective of the calculation method used, BCAR were similar across the studied PMA. The BCAR ratios trended downwards with advancing gestation suggesting a relatively higher proportional FM accretion, compared to FFM (Fig. 4). Compared to the individual weekly accretion rates which showed a 2–4-fold change during the study period, the BCAR showed a 9-fold change, suggesting that it is a more sensitive metric for weekly body composition changes.

Table 4: Normative weekly FM and FFM median accretion rate (50th percentile) and FFM/FM weekly accretion rate ratio (BCAR) in infants from 30 to 45 weeks’ PMA according to the calculation method based on the Norris body composition chart
PMA, weeks FM (g) gain/kg/week—early one-point method FM (g) gain/kg/week—average two-point method FFM (g) gain/kg/week—early one-point method FFM (g) gain/kg/week—average two-point method FFM/FM accretion rate ratio—early one-point method (BCAR) FFM/FM accretion rate ratio—average two-point method (BCAR)
30–31 5.5 3.6 51.9 34.0 9.4 9.4
31–32 10.6 6.8 94.4 60.8 8.9 8.9
32–33 11.0 7.1 92.9 59.8 8.4 8.4
33–34 11.4 7.4 90.0 58.1 7.9 7.9
34–35 11.6 7.5 85.7 55.3 7.4 7.4
35–36 11.9 7.7 79.6 51.5 6.7 6.7
36–37 12.0 7.8 72.4 47.0 6.0 6.0
37–38 12.4 8.1 64.5 41.9 5.2 5.2
38–39 13.4 8.7 57.0 37.1 4.3 4.3
39–40 15.1 9.8 50.1 32.7 3.3 3.3
40–41 17.6 11.5 45.1 29.5 2.6 2.6
41–42 19.1 12.5 42.4 27.7 2.2 2.2
42–43 26.7 17.4 40.5 26.4 1.5 1.5
43–44 26.2 17.1 38.3 25.0 1.5 1.5
44–45 26.1 17.1 35.7 23.3 1.4 1.4
45–46 24.4 16.0 32.9 21.5 1.3 1.3
FM, fat mass; FFM, fat-free mass; BCAR, body composition accretion ratio; PMA, postmenstrual age

Fig. 4: Weekly FFM/FM accretion rate ratio (body composition accretion ratio, BCAR) across the 30–45 weeks’ PMA

DISCUSSION

There is increasing emphasis on improving the quality of postnatal growth in preterm infants as cumulative data suggest that the trajectory of postnatal growth in terms of FM and FFM accretion rate and proportion can influence short-term and long-term clinical outcomes.5,6,13,1820 Noninvasive body composition assessment using ADP is a validated technique in infants, and many NICUs are using this tool for quality improvement initiatives as well as an objective way of assessing the impact of nutritional interventions in routine NICU clinical practice.12 Our data reported here on the expected weekly changes in FM and FFM accretion rates and BCAR across the gestational age range of 30–45 weeks’ PMA can be a clinically meaningful, readily available practical guide to monitor and compare the quality of postnatal growth and nutrition in NICU infants.

Body composition data during the early postnatal weeks in preterm infants suggest that absolute body weight, FM, and FFM increase with advancing gestation.14 However, our data show that FM and FFM accretion rates follow a contrasting trend. FM accretion rates increases, while FFM accretion rate decreases with advancing postnatal age because of the relatively higher FM deposition resulting in a higher percentage of body fat. The progressively higher fat deposition with advancing gestation in the first 3–4 months of life is secondary to the physiological adaptation process to extrauterine life from the age-related changes in total body water and its compartments—as the total body water decreases with gradual contraction of extracellular fluid, the body fat proportionately increases.21 Recent data suggest that energy intake, appetite, and energy expenditure in young infants are strongly associated with metabolically active, energy-using tissues in FFM, but not FM.22 However, the status quo, as it pertains to nutritional practices in the NICU, is guided by weight gain trends and not based on the quality of growth (i.e., changes in body composition). Our attempts to meet the one-size-fits-all conventional weight-based feeding volume targets (150–160 mL/kg/day) do not account for the possible variations in underlying body composition. It is possible that feeding and milk intake volume in convalescent NICU infants may be determined by FFM volume and proportion. Further studies are needed to test the hypothesis that whether optimizing milk intake volume and nutrition based on serial body composition will improve the growth and clinical outcomes in NICU infants.

Because of FM and FFM accretion rates are similar in both male and female infants across the gestational weeks studied, it is reasonable to use a common FM and FFM accretion rates for both sexes that is calculated by averaging the median accretion rates for male and female infants for each individual week (Table 4). The sex difference in body composition is appreciated even in early life with probable contribution from sex steroids (testosterone and estrogen).23 Longitudinal body composition studies from birth to 5 months have shown that male infants accumulate 17 g/week additional FFM, compared to female infants.23 For practical purposes, a common FM and FFM accretion rates for male and females during the 30–45 weeks’ PMA is reasonable, but it is possible that the difference in accretion rates may become significant with further period of growth, such that sex-specific accretion rates may become necessary.

Compared to the constant weight velocity of 15–20 g/kg/day, the constant FM and FFM accretion rates were covering only a limited PMA ranges, suggesting that FM growth and FFM growth are not constant during this period of growth. Therefore, for more accurate results, it is better to use the individual weekly FM and FFM accretion rates as given in Table 4. We believe that the early one-point accretion rate can be used to predict the expected gain in FM and FFM (g) over the next week when at least one ADP measurement is available, while the average two-point method can be used to compare the FM and FFM accretion rate (g/kg/week) when ADP data are available at two time points. Similar to the growth velocity calculation for weight, we apply caution while using early one-point method to summarize the FM and FFM accretion rate of more than one week duration because of the use of smaller denominator (using early body weight), thereby producing a larger estimate than average two-point method.16 When trying to calculate the FM and FFM accretion rate over a longer period (e.g., 2–4 weeks apart), we suggest using the constant average two-point method accretion rate by averaging the weekly accretion rate of each individual week included in the defined time period.

Most conventional indices in FM and FFM showed twofold–fourfold changes over the 30–45 weeks’ PMA study period. Because the accretion rates of FM and FFM changed in different trajectories during the studied period of postmenstrual age, we sought to determine whether a composite index of the ratio of FFM/FM accretion rates could be useful. We named this index the body composition accretion ratio (BCAR; Table 4). Interestingly, this index showed a ninefold change during the study period, indicating that the BCAR could be a much more sensitive measure of the accretion of lean body mass than the direct measurements of FFM and FM. Clearly, these observations suggest a need for further investigation in this area. Because the accretion of FFM is related to better clinical outcomes, a higher BCAR may be preferable and could be a useful measure of the accretion efficiency of nutrition being currently provided.

We acknowledge that our study has both strengths and limitations. The ADP technique is sensitive enough to detect a 30–45 g change in FFM in measurements taken one week apart and was therefore appropriate for these studies.20 One limitation of this, and all studies focused on the utility of nutritional interventions, is in the limitations of the normative reference charts. The Norris reference charts included only 222 preterm infants, which is a considerably smaller cohort than the ones used to construct the anthropometric reference growth charts commonly used by clinicians.14,24,25 This may limit the applicability at the extremes of growth, below the 10th percentile and greater than the 90th percentile. Additionally, the limited cohort of preterm infants in the Norris reference who were recruited in the US cohort included may not represent a more global population. There may be other population-based factors that alter the growth potential including genetic and ethnic differences. Similar to the Fenton growth charts, the Norris growth charts are a reference intrauterine growth chart instead of a “prescriptive” postnatal curve. Still, despite all shortcomings, these findings suggest that the ADP technique may be a useful bedside tool for clinicians to interpret body composition trajectory in NICU infants and merit further evaluation of these findings in a larger, multicentric cohort.26

ABBREVIATIONS

ADP: Air displacement plethysmography

FM: Fat mass

FFM: Fat-free mass

NICU: Neonatal intensive care unit

PMA: Postmenstrual age

REFERENCES

1. Ong KK, Kennedy K, Castañeda-Gutiérrez E, et al. Postnatal growth in preterm infants and later health outcomes: a systematic review. Acta Paediatr 2015;104(10):974–986. DOI: 10.1111/apa.13128.

2. Chan SH, Johnson MJ, Leaf AA, et al. Nutrition and neurodevelopmental outcomes in preterm infants: a systematic review. Acta Paediatr 2016;105(6):587–599. DOI: 10.1111/apa.13344.

3. Hortensius LM, van Elburg RM, Nijboer CH, et al. Postnatal nutrition to improve brain development in the preterm infant: a systematic review from bench to bedside. Front Physiol 2019;10:961. DOI: 10.3389/fphys.2019.00961.

4. Pfister KM, Zhang L, Miller NC, et al. Early body composition changes are associated with neurodevelopmental and metabolic outcomes at 4 years of age in very preterm infants. Pediatr Res 2018;84(5):713–718. DOI: 10.1038/s41390-018-0158-x.

5. Bua J, Risso FM, Bin M, et al. Association between body composition at term equivalent age and Bayley scores at 2 years in preterm infants. J Perinatol 2021;41(8):1852–1858. DOI: 10.1038/s41372-021-01074-x.

6. Frondas-Chauty A, Simon L, Flamant C, et al. Deficit of fat free mass in very preterm infants at discharge is associated with neurological impairment at age 2 years. J Pediatr 2018;196:301–304. DOI: 10.1016/j.jpeds.2017.12.017.

7. Johnson MJ, Wootton SA, Leaf AA, et al. Preterm birth and body composition at term equivalent age: a systematic review and meta-analysis. Pediatrics 2012;130(3):e640–e649. DOI: 10.1542/peds.2011-3379.

8. Ramel SE, Gray HL, Ode KL, et al. Body composition changes in preterm infants following hospital discharge: comparison with term infants. J Pediatr Gastroenterol Nutr 2011;53(3):333–338. DOI: 10.1097/MPG.0b013e3182243aa7.

9. Ramel SE, Demerath EW, Gray HL, et al. The relationship of poor linear growth velocity with neonatal illness and two-year neurodevelopment in preterm infants. Neonatology 2012;102(1):19–24. DOI: 10.1159/000336127.

10. Bruckner M, Khan Z, Binder C, et al. Extremely preterm infants have a higher fat mass percentage in comparison to very preterm infants at term-equivalent age. Front Pediatr 2020;8:61. DOI: 10.3389/fped.2020.00061.

11. Hamatschek C, Yousuf EI, Mollers LS, et al. Fat and fat-free mass of preterm and term infants from birth to six months: a review of current evidence. Nutrients 2020;12(2):288. DOI: 10.3390/nu12020288.

12. Alja’nini Z, McNelis KM, Viswanathan S, et al. Infant body composition assessment in the neonatal intensive care unit (NICU) using air displacement plethysmography: strategies for implementation into clinical workflow. Clin Nutr ESPEN 06 2021;43:212–222. DOI: 10.1016/j.clnesp.2021.04.014.

13. Nagel E, Hickey M, Teigen L, et al. Clinical application of body composition methods in premature infants. JPEN J Parenter Enteral Nutr 2020;44(5):785–795. DOI: 10.1002/jpen.1803.

14. Norris T, Ramel SE, Catalano P, et al. New charts for the assessment of body composition, according to air-displacement plethysmography, at birth and across the first 6 mo of life. Am J Clin Nutr 2019;109(5):1353–1360. DOI: 10.1093/ajcn/nqy377.

15. Patel AL, Engstrom JL, Meier PP, et al. Calculating postnatal growth velocity in very low birth weight (VLBW) premature infants. J Perinatol 2009;29(9):618–622. DOI: 10.1038/jp.2009.55.

16. Fenton TR, Anderson D, Groh-Wargo S, et al. An attempt to standardize the calculation of growth velocity of preterm infants-evaluation of practical bedside methods. J Pediatr 2018;196:77–83. DOI: 10.1016/j.jpeds.2017.10.005.

17. Fenton TR, Chan HT, Madhu A, et al. Preterm infant growth velocity calculations: a systematic review. Pediatrics 2017;139(3):e20162045. DOI: 10.1542/peds.2016-2045.

18. Bell KA, Matthews LG, Cherkerzian S, et al. Associations of growth and body composition with brain size in preterm infants. J Pediatr 2019;214:20–26.e22. DOI: 10.1016/j.jpeds.2019.06.062.

19. Paviotti G, De Cunto A, Zennaro F, et al. Higher growth, fat and fat-free masses correlate with larger cerebellar volumes in preterm infants at term. Acta Paediatr 2017;106(6):918–925. DOI: 10.1111/apa.13829.

20. Ramel SE, Gray HL, Christiansen E, et al. Greater early gains in fat-free mass, but not fat mass, are associated with improved neurodevelopment at 1 year corrected age for prematurity in very low birth weight preterm infants. J Pediatr 2016;173:108–115. DOI: 10.1016/j.jpeds.2016.03.003.

21. Fusch C, Jochum F. Water, sodium, potassium and chloride. In: Koletzko B, Poindexter B, Uauy R, editors. Nutritional care of preterm infants: scientific basis and practical guidelines. World Rev Nutr Diet, vol. 110. Basel: Karger; 2014. p. 99–120.

22. Wells JC, Davies PS, Hopkins M, et al. The “drive to eat” hypothesis: energy expenditure and fat-free mass but not adiposity are associated with milk intake and energy intake in 12 week infants. Am J Clin Nutr 2021;114(2):505–514. DOI: 10.1093/ajcn/nqab067.

23. Davis SM, Kaar JL, Ringham BM, et al. Sex differences in infant body composition emerge in the first 5 months of life. J Pediatr Endocrinol Metab 2019;32(11):1235–1239. DOI: 10.1515/jpem-2019-0243.

24. Fenton TR, Kim JH. A systematic review and meta-analysis to revise the Fenton growth chart for preterm infants. BMC Pediatr 2013;13(1):59. DOI: 10.1186/1471-2431-13-59.

25. Olsen IE, Groveman SA, Lawson ML, et al. New intrauterine growth curves based on United States data. Pediatrics 2010;125(2):e214–e224. DOI: 10.1542/peds.2009-0913.

26. Cormack BE, Embleton ND, van Goudoever JB, et al. Comparing apples with apples: it is time for standardized reporting of neonatal nutrition and growth studies. Pediatr Res 2016;79(6):810–820. DOI: 10.1038/pr.2016.26.

________________________
© The Author(s). 2022 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by-nc/4.0/), which permits unrestricted use, distribution, and non-commercial reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.