Poor in-hospital growth is one of the most frequently seen morbidities in extremely low birth weight (ELBW) infants (1,2). It has been associated with adverse neurodevelopmental outcome, including neurological and sensory impairment, delayed cognitive development, and poor school performance (3–5). Moreover, Barker's hypothesis (4) has raised concerns that extrauterine growth restriction, secondary to suboptimal nutritional status during a critical third trimester of the ex utero preterm infant, would result in later morbidity and risk for adult-onset diseases (6). Optimizing postnatal nutrition in the neonatal intensive care unit (NICU) for these preterm infants to prevent extrauterine growth restriction and its negative consequences is therefore essential, not only in the short-term but also for longer-term health and development (7–9).
Among the strategies to improve growth, delivery of larger amounts of nutrients, particularly immediately after birth (10), is suggested as pivotal. In this context there are reasons to suggest that focusing on protein intake may be particularly important (11,12): in utero, fetuses are supplied with large amounts of amino acids, which are not only used for protein synthesis but also serve as an important fuel source (13). In the postnatal period it is well established that ELBW infants who receive glucose alone lose approximately 1.2 g/kg/d of protein, corresponding to a daily loss of 1% to 2% of their total endogenous body protein stores (14). Several studies have demonstrated the safety of early amino acids as well as the efficacy in reversing the negative nitrogen balance seen in infants receiving glucose alone (15–21). Nevertheless, most of these studies have generally included only modest intakes, far from the intakes that are sufficient to approximate fetal protein accretion rates, and/or have never looked at clinical outcomes beyond the very first days of life. Our aim was to determine whether growth rates of ELBW infants fed with early aggressive protein intake that more closely approach fetal delivery rates were better than those of similar historical controls, fed with standard protein intake and treated with the same main intensive care practice.
PATIENTS AND METHODS
ELBW infants admitted to our NICU from 2001 to 2003 were included in the analysis if they had no major congenital anomalies or severe renal failure (22) and were still hospitalized at 36 weeks' postmenstrual age (PMA).
From January 2001 to August 2002 the infants (standard protein group, SP) received 1.0 g/kg/d of protein in the first day of life and 3.0 g/kg/d as the planned peak intake to be reached on day 7 of life. From September 2002, recommendations about protein intake were changed to increase by 20% the mean total intake during the first 2 weeks of life. As a result, the subsequently admitted infants (high protein group, HP), received an initial protein intake of 2.0 g/kg/d and a planned peak protein intake of 3.5 g/kg/d to be reached on postnatal day 6. Total protein intake was calculated as intravenous protein plus 85% of enteral protein, assuming that only 85% of the enteral intake is absorbed from the intestine (23). Similarly, energy intake was calculated as intravenous energy, including energy for protein, plus 85% of enteral energy units.
No other significant differences were identified between the 2 contiguous periods of protein intake regarding nutrition and main intensive care practice in our NICU (eg, ventilation, medications, blood transfusions). Two separate NICU databases were used as information sources: a nutrition database containing details about parenteral and enteral feeding and growth outcomes until discharge home, and a neonatal database that contains demographic details, antenatal and perinatal history, postdelivery status, neonatal diagnoses, procedures, therapies, complications, and outcomes at discharge. Data from the 2 databases and manual review of medical records were entered into a single study database for retrospective analysis.
Parenteral nutrition (PN), in which amino acids were supplied as trophamine 6% (Baxter), was started for all ELBW infants within the first 24 h of life. Progression of nutrient intakes was regulated using an electronic decision-support program developed by our unit. Caregivers, using a computer in the NICU, could confirm or modify the recommended dosages looking at the single infant. PN was stopped when infants tolerated 125 mL/kg/d of enteral feeding and showed sustained growth, defined by at least 15 g/kg/d during the last 72 h. Enteral feeding was initiated as trophic feeding within the first 72 to 120 h of life with pasteurized pooled premature human milk; subsequently, infants were fed their own mother's milk fortified with Eoprotin (Milupa, Italy) when an intake of 100 mL/kg was well tolerated and never before day 14 of life. Human milk was estimated to contain 1.2 g protein/100 mL (2 g:100 kcal) (24) and 2.2 g protein/100 mL (2.7 g:100 kcal) if fortified. When there were difficulties in obtaining sufficient quantities of human milk or a mother was not able to supply her own milk, these infants were nourished, partially or totally, with a preterm formula having a protein content of 2.4 g/100 mL (3.0 g/100 kcal). Weight was measured daily by nursing personnel using digital scales accurate to 5 g. Additional anthropometric measurements were performed weekly by the same investigator (V.L.) using standardized procedures. Length was measured with a fixed headboard and movable footboard and head circumference, at the maximal occipitofrontal circumference, using a nonstretchable tape accurate to the nearest millimeter. To avoid errors associated with week-to-week observer variability and the errors associated with repeated measures analysis, rates of weight, head circumference and length gain were calculated by fitting a linear regression model to each participant's data (25). Weight, length and head circumference were compared with intrauterine reference values (26) using z scores (zsW, zsHCzsL). Every 3 d during the first week, twice in the second week and once 1 week later, blood samples (0.5 mL) were drawn from the participants for determination of serum glucose, blood urea nitrogen (BUN), serum creatinine, electrolytes, albumin, calcium, phosphorus, magnesium, triglycerides, cholesterol and bicarbonate. Growth performance was measured by calculating the z score change from birth to discharge. Postnatal weight change, days to birth weight regain, growth velocity (g/kg/d and cm/week), prevalence of early catch-up, defined by a z score at discharge greater than that at birth, and number of subjects with z score <−2 at discharge were also calculated. Tolerance to protein intake was monitored with BUN and bicarbonate levels. Among the non-nutritional factors likely to influence anthropometric outcomes case mix, medical practices and complications were carefully evaluated. Baseline newborn characteristics included gestational age, birth weight, exposure to prenatal steroids, gender and Apgar scores. Illness severity was measured using the Clinical Risk Index for Babies (CRIB) score. Small-for-gestational-age was defined as a weight z score <−2 based on Italian intrauterine reference values. Bronchopulmonary dysplasia was defined as receipt of supplemental oxygen at 36 weeks' PMA. Severe intracranial hemorrhage was defined as grade III or grade IV using the Papile classification. Periventricular leukomalacia was defined as cerebral ultrasound findings of increased echogenicity and cystic lesions in the periventricular white matter. Severe retinopathy of prematurity was defined as receipt of laser or cryotherapy for treatment of retinopathy of prematurity. Severe necrotizing enterocolitis was defined as Bell stage II or III. Patent ductus arteriosus was documented by echocardiography. Sepsis was diagnosed if an infant had severe deterioration of its general condition associated with a positive blood culture. Use of postnatal steroids, surfactant, diuretics, methylxanthines, blood transfusions and length of hospital stay was also documented.
Data for trial groups were compared using Student t test for parametric data and the Mann-Whitney U test for nonparametric data. Categorical variables were compared using Yates' continuity corrected χ2 test or Fisher exact test. The level of significance was set at P < 0.05. Multiple linear regression analysis was used to identify significant predictors of growth performances. All statistical analyses were performed on a per-protocol analysis, including infants who were fully treated according to protocol. In addition, statistical analyses were performed on an intention-to-treat basis including also those infants excluded before 36 weeks' PMA.
During the whole study period, 56 of the 71 admitted ELBW infants were included in the analysis, 31 in the SP group and 25 in the HP group. Thirteen infants were excluded because of death (6 SP, 4 HP), mainly within the first week of life (107.7 ± 89.4 h of life), or discharge before 36 weeks' PMA (1 SP, 2 HP), 1 (SP) because of renal failure and 1 (HP) because of missing data. The demographic and clinical characteristics are listed in Table 1 and showed no baseline differences between SP and HP groups. No differences were found between the 2 groups in main nutritional and clinical outcomes (Table 2). The actual mean protein intake during the first 14 d of life (P14) was 21.5% greater in the HP group. No difference was found in nonprotein energy, total fluid and enteral intakes. Mean BUN and bicarbonate levels were similar as was the number of infants with a low base deficit or with BUN >25 mg/dL. The prevalence of hyperglycemia was similar between the 2 groups, but mean serum glucose levels were significantly lower in the HP group (Table 3). HP group infants had a lower mean postnatal weight loss (−3.1%; 95% confidence interval [CI] −5.9, −0.2,) and an earlier regain of birth weight (−4.1 d; 95% CI −6.6, −1.7). In SP group infants the risk for postnatal weight loss >15% was doubled (odds ratio 2.1; 95% CI 0.5, 9.3), and the risk of birth weight regained after the first 2 weeks of life was significantly higher (odds ratio 15.2; 95% CI 1.8, 127.2) (Table 4). Multiple linear regression analysis (r 2 0.45) identified P14 as an independent predictor (coeff. B – 6.22, P = 0.001) for time to regain birth weight, together with weight zs W at birth (coeff. B 2.91, P = 0.012) and mean fluid intake during the first 14 days of life (coeff. B 0.26, P = 0.001). In HP infants the z score decrease (Table 5) was significantly less pronounced in both weight (−0.57; 95% CI −1.01, −0.12) and length (−0.51; 95% CI −0.97, −0.05). In the multiple linear regression model (Table 6) P14 was 1 of the independent predictors of Δzs W and of Δzs L. Moreover, during the NICU stay, only a small percentage of HP infants, in comparison to SP infants, had a z score fall >1 in weight (16.0 vs 48.4%), length (20.0 vs 45.2%) and head circumference (12.0 vs 25.8%). As a consequence, a greater number of the HP group infants showed catch-up growth in weight, length and head circumference and this resulted in a lower proportion of growth-delayed infants at discharge (Table 7). The intention-to-treat analysis resulted in similar results (data not shown).
Early postnatal growth of very preterm infants should be considered not only as an outcome variable to be measured in NICU but also as a predictive variable in long-term follow-up studies. Poor weight gain is associated with longer lengths of hospital stay and directly related to poor growth at 1 and 3 years of age and poor neurodevelopmental outcome (3–5). Poor growth of ELBW infants results from a complex interaction of many factors, including their baseline characteristics, morbidities affecting nutrient requirements and/or absorption and administration of drugs that influence nutrient metabolism. However, inadequate nutrition is 1 of the major factors and could explain ≈50% of the growth variation (12,23,27). More specifically, analysis of growth data from birth to discharge suggests that the growth deficit in ELBW infants is mainly the result of nutritional restriction, especially of protein (12), during the early weeks of life (28). It has been estimated that attaining intrauterine rates of protein deposition may require as much as 3.85 g/kg of amino acid intake in the smallest ELBW infants (29), but clinical evidence concerning protein intake immediately after birth that approaches the fetal amino acid delivery is rare (19) or limited to extremely short-time metabolic outcomes, rarely beyond the first 72 h of life (18). In our study higher mean protein intakes during the first 2 weeks of life, mainly via the intravenous route, was associated with lower early weight loss, earlier regain of birth weight, lower z score fall from birth to discharge and better z scores at discharge, without a significant change of BUN or serum bicarbonate concentrations when compared with infants receiving standard protein intake. These clinical results are in line with the main metabolic role of protein intake in promoting nitrogen retention and increasing protein synthesis (30). Early amino acid administration could also increase insulin secretion (19) and contribute to greater protein deposition and better glycemic control. Our data showed lower serum glucose levels (−21.7 mg/dL; 95% CI −41.9 to −1.5 mg/dL) in the HP group during the first 2 weeks of life, as already shown by previous observations consistent with a relationship between early amino acid intake and better glucose tolerance (20,31). On average, all of the infants studied experienced a significant drop in their weight and length z scores from birth to discharge. This well-known phenomenon, defined as extrauterine growth retardation, could be mainly conditioned by the postnatal weight loss proportion. Postnatal weight loss depends upon the reduction of the extracellular space accounting for about half of the initial weight loss, but the remainder represents either failure to accrete lean mass, fat, or both at a reasonable rate or actual loss of lean mass, fat, or both (28). Both postnatal weight loss and time to regain birth weight could therefore represent reliable markers of early nutrition and at the same time good predictors of poor growth risk. Furthermore, as recently reported (32), the effect of the early protein intake seems sustained over discharge time as shown by the actual mean protein intake during the first 14 d of life, resulting in 1 of the independent predictors of weight and length z score changes. We identified BUN and bicarbonate levels as indexes of protein toxicity mainly because of their bedside availability. Nevertheless, reliable and sensitive markers of amino acid toxicity that are also readily available are still lacking. Acidosis is not a sensitive indicator of amino acid intolerance because premature infants can be acidotic for several reasons; normal ammonia levels in premature infants are not yet clearly established and could be fully influenced by the sample methodology (33). Blood urea nitrogen depends also on hydration status and renal function and increases with use of amino acids as energy substrate (34). Amino acids serve as energy substrates for the fetus, and there is no reason why they should not play this role in the preterm infant. One solution can be represented by a wider use of bedside amino acid analysis; many studies have already shown total and/or essential amino acids levels in normal ranges after early protein intakes (19), but data are few and inconclusive In a pilot study a standardized feeding advancement protocol targeting a dose of 3.8 g/kg amino acids/protein during the first week of life in ELBW infants caused hyperaminoacidaemia in 46% of the samples (35).
Intention-to-treat analysis did not substantially change the results obtained with the per-protocol analysis. There may be at least 2 reasons for this: the major portion of the excluded infants died very early, at a time when it is difficult to speculate about relationships with protein intake, and the number of those infants excluded later is too small to influence outcomes.
Our study has 3 limitations. First, we used historical controls and we cannot exclude the important influence of the different types of care provided during the 2 study periods, which were not considered in the comparison between the 2 groups or not included in the multiple regression models. Second, we did not evaluate the quality of growth and other indicators of protein metabolism such as amino acid profiles or nitrogen balance. Third, the early protein intake provided to the SP group was greater than that usually defined as standard (12), so that some clinical and metabolic differences with the HP group could have been blunted. However, our results suggest that early high protein intake could be associated with improved short-term growth outcomes, highlighting the potential benefits of high protein intake immediately after birth. Higher protein intake could partially prevent the early postnatal malnutrition commonly seen with current nutritional strategies that supply the intakes to approximate intrauterine growth rates, but are not enough to allow the infants to catch up (27). There are still few well-controlled, prospective investigations that validate this aggressive approach (36), so we believe there is an urgent need for large randomized trials to test the long-term safety and efficacy of early aggressive protein intake in very preterm infants.
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Keywords:© 2007 Lippincott Williams & Wilkins, Inc.
Extremely low birth weight infants; Protein intake; Growth; Parenteral nutrition