Institute of Pediatrics and Neonatology, Fondazione IRCCS “Ospedale Maggiore Policlinico, Mangiagalli e Regina Elena,” University of Milan, Milan, Italy
Received 31 May, 2007
Accepted 1 November, 2007
Address correspondence and reprint requests to Paola Roggero, Neonatal Intensive Care Unit, University Medical School, Via Commenda 12, 20122 Milano, Italy (e-mail: firstname.lastname@example.org).
The authors report no conflicts of interest.
Preterm infants grow more slowly in the early months after discharge than do healthy term infants (1). Preliminary research findings suggest that preterm infants may benefit from the use of nutrient-enriched postdischarge formula (2). However, data are still lacking on optimal nutrient intakes for the postdischarge treatment of preterm infants (3). Because accurate and noninvasive measurement of an infant's body composition may be useful in evaluating the quality rather than the amount of weight gain and in monitoring nutritional requirements and the efficacy of nutritional interventions (4), the aim of the present study was to evaluate changes in the body composition of preterm infants in relation to protein and energy intakes from term until 3 months of corrected age.
PATIENTS AND METHODS
Among the preterm newborns admitted from January to July 2006 at the Mangiagalli Clinic, Neonatal Intensive Care Unit, Fondazione IRCCS “Ospedale Maggiore Policlinico, Mangiagalli e Regina Elena,” Milan, Italy, 53 consecutive infants were enrolled at term for this study. Inclusion criteria were as follows: birth weight ≤1800 g and gestational age ≤34 weeks. Exclusion criteria were congenital diseases; chromosomal abnormalities; chronic lung disease; cardiac, respiratory, gastrointestinal, or other systemic diseases; severe brain disease, and being breast-fed.
The study design was approved by the Departmental Ethics Committee, and informed consent was obtained from the infants' parents. The infants were prospectively followed until 3 months of corrected age.
The infants' gestational age, sex, and birth weight were recorded at enrollment. Gestational age was based on the mother's last date of menstruation and confirmed by ultrasound examination performed on all of the infants' mothers by the 20th week of gestation. Corrected age was calculated from the chronological age with adjustment for gestational age—that is, for the number of additional weeks from the expected 40 weeks of postconceptional age.
From birth to hospital discharge, all of the infants were fed preterm formula (protein content 2.76 g; energy density 82.90 kcal/100 mL). After hospital discharge, infants were fed a commercially available nutrient-enriched postdischarge formula and were given no other foods. The energy density (kcal) and protein content (g) per 100 mL (per 100 kcal) of the nutrient-enriched postdischarge formula were 75 kcal (100 kcal) and 2 g (2.6 g), respectively. Infants were fed on demand. Parents were instructed to record in a diary the daily quantities of formula consumed by the infants. Average daily energy and protein intakes were calculated at 1, 2, and 3 months of corrected age.
Anthropometric variables (weight, length, and head circumference) and body composition were assessed at term and at 1, 2, and 3 months of corrected age. Infants were weighed naked on an electronic scale accurate to the nearest 0.1 g. Body length was measured to the nearest millimeter on a Harpenden neonatometer (Holtain Ltd, London, UK). Head circumference was measured with a paper insertion tape to the nearest millimeter.
Body composition was assessed by use of the PEA POD Infant Body Composition System (LMI, Concord, CA). A detailed description of the PEA POD physical design, operating principles, and measurement procedures is provided elsewhere (5,6). Briefly, the PEA POD assesses fat mass and lean body mass by the direct measurements of body mass and body volume and the application of whole-body densitometric principles. Infants were assessed nude in the PEA POD. Body mass was measured on the integrated electronic scale of the PEA POD. Body volume was measured in the test chamber of the PEA POD by applying gas laws that relate pressure changes to volumes of air in the enclosed chamber. Each measurement period lasted approximately 5 min, with the mass and volume measurements lasting 5 to 20 seconds and 2 minutes, respectively. Body density was then computed from the study participant's measured mass and volume and automatically inserted into standard formulas to estimate percentage of fat mass according to a classic 2-compartment model. The intraobserver coefficient of variation was 0.3%.
Descriptive data are shown as mean (SD). Differences within study participants in repeated measurements of body fat mass were assessed by analysis of variance. Linear regression analyses were performed to examine the effects on fat mass of energy (kilocalories per kilogram per day) and protein intakes (grams per kilogram per day) per 100 mL formula, with sex and gestational age as covariates. For each of the 2 independent variables (energy and protein intakes per 100 mL formula), 2 separate regression models were developed, 1 using only protein intake (grams per kilogram per day) and the other using only energy intake (kilocalories per kilogram per day). This avoided possible collinearity between these 2 independent variables. For further analysis, infants were categorized in high- (≥3 g) and low- (<3 g) protein-intake (grams per kilogram per day) groups in the first month of corrected age. An independent-samples t test was used to compare weight and lean body mass gain (g) in infants between the high- and the low-protein-intake groups. Statistical significance was set at the 0.05 level, and statistical analysis was performed by use of SPSS (SPSS, version 12, SPSS Inc, Chicago, IL).
Of the 53 infants recruited for the study, follow-up data through 3 months of corrected age were collected from 48 (23 boys). Of the 5 infants who dropped out of the study, 1 moved away and 4 failed to attend the scheduled study visits. None of the infants recruited for the study died.
The mean (SD) gestational age was 30.8 (2.56) weeks. Mean birth weight, length, and head circumference were 1357 (376) g, 40.6 (4.1) cm, and 28.7 (1.96) cm, respectively. Table 1 shows body composition and intakes of protein and energy at each month.
The mean (SD) percentage of fat mass increased significantly from 14.8 (4.3) at term to 19.2 (4.1) at 1 month, 23.2 (3.7) at 2 months, and 23.6 (6.1) at 3 months of corrected age (P < 0.0001).
Protein intake (grams per kilogram per day) per 100 mL formula was negatively associated with percentage of fat mass at 1 month of corrected age (R2 0.51; P = 0.002); whereas energy intake (kilocalories per kilogram per day) was found to have no effect on percentage of fat mass at 1, 2, and 3 months. No association was found between gestational age, sex, and percentage of fat mass. The mean daily volume intakes (milliliters per kilogram per day) progressively decreased during follow-up from 147 mL · kg−1 · day−1 at 1 month of corrected age to 107 mL · kg−1 · day−1 at 3 months of corrected age. In infants belonging to the high-protein-intake group, the mean protein intake (grams per kilogram per day) per 100 kcal was 2.6 g · kg−1 · day−1, whereas that of infants belonging to the low-protein-intake group was 2.1 g · kg−1 · day−1.
The high-protein-intake group (n = 26) showed significantly lower weight gain (g) [946.7 (375.2) vs 1238 (407), P < 0.05] than did the low-protein-intake group (n = 22) within the first month of corrected age. However, as shown in Fig. 1, the delta in lean body mass as a percentage of weight gain between term and the first month of corrected age was significantly higher in the high-protein-intake group than in the low-protein-intake group.
In the present study, high protein intake (≥3 g · kg−1 · day−1) during the first month of corrected age resulted in a significantly higher lean body mass gain when compared with low protein intake (<3 g · kg−1 · day−1). The reason for choosing a cutoff of 3 g · kg−1 · day−1 of protein intake to classify the groups was based on the estimated needs for protein intake in premature infants at the time of discharge, which are in the range of 2.5 to 3.5 g · kg−1 · day−1 (7).
This study adds to the current understanding of the effect of protein intake on body composition changes in preterm infants during the first year of life and furthermore posits that high protein intake in the early postdischarge period promotes lean mass gain in the first month of corrected age. Although De Curtis et al (8) and Embleton and Cooke (9) did not find any clear advantage in lean mass gain in the first 3 months of corrected age as a result of a protein intake of either 2.4 g/100 kcal or higher than 2.7 g/100 kcal, respectively, Koo and Hockman (10) showed that a protein intake of 2.6 g/100 kcal resulted in a slightly greater lean mass percentage 12 months after discharge compared with a protein intake of 2.1 g/100 kcal (69.1% vs 68.3%). Consistent with the findings from Koo and Hockman (10) and the present study, Cooke et al (11) found a significantly higher lean body mass (7224 ± 717 g vs 6764 ± 605 g) as a result of the use of a protein intake of 2.7 g/100 kcal compared with 1 of 2.1 g/100 kcal in boys, evaluated at 12 months of corrected age.
Inasmuch as the period in which critical changes in body composition may occur (12) is still not well defined, we designed this study to assess body composition monthly to clarify the changes in weight gain composition that occur in the early postdischarge period. Breast-fed infants were excluded from this study to avoid inaccuracy in the estimation of protein/energy intakes.
The absence of an effect on body composition in the second and third month of corrected age similar to that observed in the first month could have been due to a spontaneous decrease in protein intake, inasmuch as the infants, being fed on demand, regulated their milk intake to their individual dietary requirements. Indeed, other authors have already reported that a protein intake of 2.1 g/100 kcal did not promote an increase in lean body mass (10,11). Mean gains in body weight, body length, and head circumference during follow-up were satisfactory, and growth was adequate according to the WHO Child Growth Standards (13). One limitation of the study was the calculation of energy and protein intakes, inasmuch as these quantities were based on parents' records of daily quantities of milk consumed by the infants.
In conclusion, the present study suggests that high protein intake, with an adequate energy supply, results in higher lean mass gain in the first month of corrected age when compared with low protein intake. Additional randomized controlled studies may clarify the potential role of high protein intake in promoting better quality of weight gain composition in the early months after term.
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