Aromatic amino acid metabolism of neonatal piglets receiving TPN: effect of tyrosine precursors

N-Acetylcysteine is a Highly Available Precursor for Cysteine in the Neonatal Piglet Receiving Parenteral Nutrition

BACKGROUND

Cysteine (CYS) is accepted as an indispensable amino acid for infants receiving parenteral nutrition (PN), and CYS is unstable in solution. Thus, developing a method to supply CYS in PN for neonates is needed. N-acetyl-L-cysteine (NAC) is stable in solution and safe for use in humans; therefore, NAC may be a means of supplying parenteral CYS.

METHODS

We determined the bioavailability of NAC in intravenously (IV)-fed piglets randomized to 1 of 4 diet treatments, each supplying 0.3 g/kg/d methionine and either 0.2 g/kg/d CYS (CON), 0 NAC (zeroNAC), 0.13 NAC (lowNAC), or 0.27 g/kg/d NAC (highNAC). Piglets (2 days old; 1.8 kg, n = 20) were surgically implanted with femoral and jugular catheters. On day 3 postsurgery, test diets were initiated and continued until day 8. Piglets were weighed daily. Blood was sampled 6 hours before test diet initiation and at 0, 6, 12, 18, 24, 36, 48, 60, 72, 84, 96, 108, and 120 hours. Urine was collected on ice in 24-hour sample periods.

RESULTS

Total mean weight gain was not different between groups; however, average daily gain in the zeroNAC and lowNAC groups declined significantly (p < .05) over the 5-day treatment period. Nitrogen retention was similar between the CON and highNAC groups, both were higher than the lowNAC group, and the zeroNAC treatment produced the lowest nitrogen retention. NAC percent retention was not different between lowNAC and highNAC and was 85.4% and 82.6%, respectively. Plasma NAC was higher in highNAC than lowNAC (p < .05).

CONCLUSIONS

These data demonstrate that NAC is available as a precursor for CYS to support growth and protein (nitrogen) accretion in piglets administered a parenteral solution.

Amino acid needs for early growth and development

Few data exist on amino acid needs in infants and children, mainly because until recently, amino acid requirements were determined using nitrogen balance. The advent of the indicator amino acid oxidation (IAAO) method permits studies to be conducted with minimal adaptation to the test amino acid. In light of the very limited data available for human infants, toddlers, and children, it was proposed that a factorial approach should be used to estimate their essential amino acid requirements. Using amino acid oxidation techniques, dietary essential amino acid requirements in adults have been nearly completed. Data on changes in total body potassium are now available for infants and children. From these data it is possible to calculate protein deposition during growth, and hence, it is now possible to estimate the amino acid requirements in children using a factorial model. However, there has been no independent verification of the model. Recently we determined total branched chain-amino acid requirements for young adults and children, and we can provide data to support the validity of the factorial model. IAAO has been used on children with liver disease as young as 3 y. The minimally invasive IAAO model opens the door for determination of dietary essential amino acid requirements in infants and children during health and disease. For study of preterm neonates, we used a piglet model to show that the amino acid needs for parenteral feeding are markedly reduced for several essential amino acids; this suggests that current commercial total parenteral nutrition amino acid solutions are less than ideal.

Amino acid and energy needs of pediatric patients receiving parenteral nutrition

The technique of parenteral nutrition has become such an established part of modern pediatric care that it is difficult to imagine how pediatricians, as recently as 25 years ago, managed a large group of very difficult patients; however, despite its obvious nutritional advantages, the technique is not without problems. Many of these can be circumvented or controlled by careful attention to all aspects of the technique. Certainly the incidence of these problems can be maintained at a level sufficiently low that the benefits of the technique far outweigh its risks; however, the technique clearly can be further improved. One requirement for doing so is to recognize that the technique is deceptively simple and that it should not be used indiscriminantly without careful consideration of indications and alternative strategies for nutritional management. Additional research also is required. As discussed earlier, the available parenteral amino acid mixtures and lipid emulsions, although considerably improved over earlier versions, remain far from optimal. Some of the actual and theoretic problems that should be addressed in the near future are discussed in the preceding sections; there also are many others.