The present study is the first to show that protein anabolism, an important target of nutritional support in critically ill infants, can be achieved within the first days after admission to the PICU by increasing enteral protein and energy intakes above dietary reference levels using a protein-energy enriched formula. This target was not achieved with a standard infant formula. The higher protein balance resulted from stimulated protein synthesis exceeding the rate of concomitant stimulated protein breakdown. Nitrogen balance data confirmed our phenylalanine results.
Our findings of increased protein synthesis and protein balance are in agreement with several studies in premature and term neonates evaluating the effects of amino acid supplementation.10–13 29–33
This is also true for protein breakdown which was either increased33
or not affected by amino acid supplementation.11 13 29 31
has also reported suppression of proteolysis, this was in healthy instead of critically ill infants, receiving short term supplementation. Our finding of both increased protein synthesis and protein breakdown with higher protein and energy intakes is probably due to overall stimulation of protein turnover, as shown by the increased whole body protein turnover rate in the PE-group.34
Increased protein intake promotes protein anabolism, but may lead to increased amino acid oxidation with urea formation as seen in neonates with increasing amino acid supplementation,11 13 31
when exceeding needs. However, in the present study, neither phenylalanine hydroxylation nor TUN excretion (both reflecting amino acid oxidation), nor plasma urea concentrations (as described in our previous report)22
differed significantly between groups, suggesting that protein intake up to, and probably above, 3.1 g/kg/day does not exceed these infants' needs.
We are aware that using a PE-formula makes it difficult to discern the influences of separate macronutrients on protein metabolism. However, studies in adults and children have shown that protein is the major dietary determinant of WbPM as long as energy intake is sufficient.35
Additionally, supporting this hypothesis, the finding of a positive relationship between plasma EAAs and protein synthesis and balance suggests that EAA availability plays a crucial role in increasing protein synthesis and protein balance. It also agrees with previous observations in healthy adults indicating that (essential) amino acids are the primary stimulus for (muscle) protein synthesis.36
In these critically ill infants, receiving large amounts of intravenous fluids and medications, 120 ml/kg/day was the maximum achievable nutritional volume intake. Despite these fluid restrictions, an anabolic state was obtained within 5 days after admission using a protein-energy enriched formula, thereby limiting delay of growth and neurodevelopment during critical illness as much as possible. We have previously reported that the PE-formula is safe, well tolerated and improves nitrogen and energy balance at days 1–5 after admission.22
This type of formula is thus preferable to standard formulas to achieve adequate nutrition in comparable clinical settings. Since the subjects were a typical sample of infants with respiratory insufficiency due to viral bronchiolitis, we suggest that the results apply to the general population of these critically ill infants.
Our study is also the first to report values of first pass SPEPhe
in continuously enterally fed critically ill infants. In this population, first pass SPEPhe
did not differ between groups with an average of 46.8%. Comparable values have been described in healthy adults after a meal21
and in enterally fed piglets.37
There is discussion about correcting protein intake for SPE in calculations of WbPBal, since these retained amino acids are used for constitutive or secreted (glyco-)proteins in the gut,38 39
which is then considered part of WbPS. We have therefore also calculated the data without correction for SPE (not shown) and found that protein breakdown was 15–19% lower and protein balance 2.7–3.9 times higher. Only the absolute values are affected by this calculation, and the main conclusion of the study is not affected.
There are several limitations to this study. Despite using a randomised design, gestational age was significantly lower in the group receiving protein-energy enriched formula. This might have biased our results of protein metabolism as protein turnover decreases with increased (post-)conceptional age.40
Furthermore, the proportion of female subjects was relatively high. Protein deposition has been shown to be similar for healthy male and female children prior to adolescence and it is recommended that estimates of protein requirements for healthy children are calculated for both sexes combined.25
However, in children with burns (8 years of age on average), females had a less negative net muscle protein balance compared to males, and females gained lean body mass whereas males lost lean body mass. These differences were possibly due to the observed attenuated hypermetabolic response in females.41
Assuming that the same differences are true for critically ill infants, this would mean that the achievement of protein anabolism in the first days after admission in our study population could have been biased by the high proportion of females. However, gender differences in protein kinetics have not been described for critically ill infants. Moreover, our study population of infants with a viral infection is distinctly different from children with burns, who are subject to an extended hypermetabolic stress response with high inflammation.41
Also, when comparing the female with the male subjects within the PE- and S-groups of our study, the only notable difference was a non-significant trend towards higher protein turnover, synthesis and breakdown in the females compared to the males within the PE-group, but resulting in similar protein balances in both sexes. Therefore it seems unlikely that our results were affected by gender differences, despite the high proportion of females. Since the female subjects were equally distributed among both groups in our study, neither did it influence the comparison of groups.
Another limitation is that protein synthesis and protein breakdown were derived by extrapolating phenylalanine metabolism, which in fact only reflects the effects on the kinetics of this particular EAA. Other amino acid tracers may have shown different patterns, although the phenylalanine/tyrosine and leucine methods are considered to be reference methods to obtain reliable estimates of whole-body protein metabolism in most physiological conditions.26
The present study was not designed to establish exact protein and energy needs in critically ill infants. Neither was it adequately powered to detect clinical effects. Dose–response studies and studies into the clinical effects of improved protein balance in larger groups of critically ill infants are therefore necessary.
In conclusion, protein anabolism in critically ill infants with viral bronchiolitis can be achieved in the first days after admission by increasing protein and energy intakes above reference levels. Since protein anabolism is an important goal of nutritional support in these infants, increased protein and energy intakes should be preferred over standard intakes.