Protein intake of premature infants: A reply

March 1977

TheJournalofPEDIATR1CS

507

Protein intake of premature infants." A reply

THE PUBLICATION of the accompanying commentary' and this reply culminates a series of communications between us which span approximately one year. As a result of these communications, the present authors have learned much, and we have come to know our own work -'-~'better. We are grateful both for the considerable scrutiny that has been given to our work and for the general approbation, despite the special interpretive reservations. Fomon and Ziegler have summarized our study concisely and accurately. Some of the questions of interpretation which they raise bring into clear focus conceptual differences of general interest which deserve study by all who are interested in the care of the preterm infant. DESIGN OF STUDY: PROTEIN Q U A N T I T Y VS P R O T E I N Q U A L I T Y The selection of the dietary intake of protein which is questioned by Fomon and Ziegler was based upon the commentary published in ThE JOURNAL by Cox and Filer. '~ In that review on protein requirements in the preterm infant they concluded "daily intakes of protein in the range of 2.25-5.0 Gm per kilogram per day from milk are satisfactory for the growth requirements dependent on this dietary compound." The selection of 4.5 g m / k g / d a y for the high-protein diet, therefore, approached the high end of the best recommendations then available. The reservation of Fomon and Ziegler about the design of our study was related to the data of Babson and BramhaW that suggest "even in the presence of an adequate intake of protein, the relatively low mineral intake from the f o r m u l a s . , , may have been limiting with respect to rate of growth," This reservation is puzzling because Babson and Bramhall found that the mineral content of the diet was limiting for weight gain only and had no demonstrable effect on linear growth. Furthermore, Babson and Bramhall emphasize, throughout their paper, that weight gain and growth are no~ synonymous, e.g. "The measurement of length is suggested to be a better single criterion of growth than is weight." They found that the effect of a high-protein intake (5.25 gm/ kg/day) on weight gain was not demonstrable with a low-

ash content formula (0.45 gm/kg/day), whereas with a high-ash content formula (0.9 gm/kg/day) the highprotein intake resulted in an increased gain in weight. Thus a high-protein and high-ash formula is more likely to result in more rapid weight gain than either a highash:low-protein formula or a low-ash:high-protein formula, because protein itself represents a considerable solute load. ~ Since the ability of the premature infant kidney to excrete this solute load is low, more water would tend to accumulate, as demonstrated by Kagan and coworkers,"

See related articles, pp. 348, 356, and 504.

I

Abbreviation used GFR: glomerular filtration rate

Our basic premise is that the quantity of dietary protein cannot be determined without considering the quality of that protein. This is not really a new nutritional concept, but it is one which has not been thought of great importance in infant nutrition (cf e.g., reference 1 and p 133 of reference 10). What is new about our study is that it is a systematic examination of the quality (60:40 vs 18:82, whey:casein) as well as the quantity (1.5 vs 3.0 gm/ dl) of the protein given preterm infants. Furthermore, mineral content is not a variable among the four synthetic formulas (although it is between the four formulas and pooled human milk). We decided to compare the infants fed the four formulas to a group of infants fed pooled human milk because it was and is the standard clinical practice in most Scandinavian neonatal centers. DATA ANALYSIS AND INTERPRETATION Can weight gain he equaled with growth? Fomon and Ziegler's new suggestion, that we do a two-way analysis of variance, has proven helpful in that it has allowed us, once again, to come to know and to understand our data better. Dr. Harry Smith, Professor and Chairman of the Department of Biostatistics at Mount Sinai School of Medicine

1Iol. 90, No. 3, pp. 507-510

508

Editor's column

of the City University of New York, has performed a twoway analysis of co-variance on the data. ~ Birth weight is the co-variable. In this analysis the following results were obtained: There is a statistically significant difference at the 5% level in the average gain in weight per week (from time of regained birth weight to time of discharge) between the group fed pooled human milk and all of those fed the other formulas, with the infants fed the other formulas demonstrating a higher rate of weight gain. The differences in weight gain among the four formula groups did not reach the 5% level of significance. The differences in crown-rump length between the infants fed formulas and those fed pooled human milk did not reach the 5% level of significance, but differences in crown-rump length of the formula fed groups were significantly greater at the 10% level, a comparison that most investigators would not consider conclusive. There was no evidence of differences in femoral length among or between infants fed the four formulas or between this group and those fed pooled human milk even at the 10% level. A complete report of these data and a general discussion of the statistical treatment of growth data in such feeding studies is in preparation. Although it remains as impossible as ever to prove a negative, we would still interpret our evidence as insufficient to demonstrate a clinically significant growth advantage for infants who receive the formulas. If one equates weight gain with growth, as Fomon and Ziegler seem to do in their two most recent commentaries," '~ then the formulas might offer a small growth advantage. We would agree with Babson and BramhalU however, that the best single measure of growth is linear growth. Our new interpretation of the growth data,-' based on the results of this more powerful statistical tool, is that the increased gain in weight of all the formula-fed infants could be the result of their increased solute load, i.e., this increased solute could give rise to increased water retention in the premature infant, who is known to have a reduced ability to excrete a high-solute load ~- 1'-' (cf also p 245 ff of reference 10). In support of this interpretation it should be noted that we used two measures of linear growth. The only one that was different was crown-rump length, and this difference was present only at the 10% level of confidence. It is conceivable that this questionable difference was also the result of increased fluid retention, because crown-rump length is influenced by the size of the thighs and buttocks. The measure of linear growth unbiased by possible fluid retention, femoral length, was not significantly different amongst the various groups. Fomon and Ziegter argue that increased gain in weight may be worth the metabolic cost if a significant impairment in functional outcome from the high metabolic cost cannot be demonstrated. We argue that even a potentially

77re Jolo'nal o[' Pedtatric~ r 1977

dangerous metabolic cost should not be paid for by a small increase in weight gain. Is there an~ demonstrable advantage for the infant to grow bigger, faster, i.e., to maintain an intrauterine rate of weight ~ain in lhc extrauterine situation? It has been known for som~. time that formula-fed, term infants will gain weight faster than breast-fed, term infants; yet few would argue that this faster rate of weight gain is an advantage. In fact, there is considerable evidence that the reduced safety margins for solute loads found early ~ and obesity found laicr '~ arc outcomes of formula feeding that are patently unde~,ir able. Implicit in our studies is the idea that the "ideal" feeding regimen, whatever it is, must be designed with an eye to the limitations in the biochemical development of the preterm infant, In this regard, our study demonstrates that there is no justification for the continued use of casein-predominant formulas in feeding preterm infants. In considering whether or not human milk requires supplementation with a protein more nearly resembling human milk proteins, i.e., whether the preterm human infant requires a greater protein intake than the human infant born at term, the time after birth of initiating supplementation is also an important consideration. Although we do not interpret our data as showing that more protein than is present in pooled human milk is desirable in the first few weeks after birth, we do not exclude the possibility that such supplements might be desirable later, when a greater degree of enzymatic maturation has been attained. Investigation of this possibility would be of great importance. What is a normal serum albumin concentration in the preterm infant? A major interpretive issue which is raised

by Fomon and Ziegler is that we should "at least entertain the possibility that the lower serum concentration of albumin of the infants fed human milk reflected less satisfactory protein nutritional status." This possibility was not ignored. ~ Our purpose, rather, was to suggest some less obvious alternative interpretations. We still insist that no one really knows what the serum albumin should be. There is some justification for not thinking that "more is better" in this instance. In addition to arguments raised previously,~. :~ the following should be considered: (1) It is known that the glomerular filtration rate of the preterm infant is low? ~ Since GFR = hydrostatic pressure - oncostatic pressure and since oncostatic pressure is determined almost entirely by serum albumin,' ..... a higher serum albumin would act to decrease further the filtration pressure. Indeed, the high values of blood urea nitrogen in the infants fed high-protein formulas (cf reference 2, Fig. 1) may be the result, in part, of a reduction in GFR. The lower serum albumin concentrations in the preterm infants fed pooled human milk could

Volume 90 Number 3

Editor's column

be viewed as salutary or compensatory in that they would tend to increase the GFR. (2) Albumin has been found to inhibit the uptake of tryptophan by rat brain, 'G because of its ability to bind tryptophan, an important neurotransmitter precursor.

509

changes in plasma occur later, are progressive, and are less striking than they are in urine are compatible with our earlier finding that rat kidney conserves taurine by decreasing renal clearance.'" FINAL COMMENT

METABOLISM CONTAINING

OF SULFURAMINO ACIDS

We agree with Fomon and Ziegler I that our data do not show that the preterm infant is "unable [sic] to convert cystathionine to cysteine." What we stated' was: "The present data provide further support [sic] to our earlier suggestion that the capacity of the preterm infant to convert methionine to cystine is limited [sic]..... " Moreover, we think their expectation that the urinary excretion of cystathionine, the substrate of cystathionase, should account for a larger percentage of the methionine intake is unreasonable in light of experience with enzymatic deficiencies in inborn errors of amino acid metabolism, In a patient with homocystinuria due to absence of cystathionine synthase activity studied by us (G.G. and D.R.), the excretion of homocystine in the urine was equivalent to 1,030 #moles of methionine/day on a calculated methionine intake of 8 mmoles/day. This patient had no measurable hepatic cystathionine synthase activity, yet one can account for only 12% of the intake of methionine by urinary excretion of homocystine, the substrate of the missing enzyme. It is not surprising, therefore, that with a partial cystathionase deficiency (some of infants could have developed full cystathionase activity early) one can account, on the average, for so little of the methionine intake as cystathionine. The fact remains that the higher excretions of cystathionine and the higher incidence of cystathioninemia in the infants fed the formulas 4 give further support for the postulated slower postnatal development of cystathionase in preterm infants; this is all that we claimed? Fomon and Ziegler use the obverse argument with regard to taurine excretion. How can an infant excrete more taurine than it received in the diet unless it can synthesize substantial amounts of taurine from precursors? (1) We did not argue that there is no capacity to synthesize taurine, and we specifically pointed out the possibility of alternative or latent pathways for taurine synthesis. (2) The argument that urinary excretion cannot exceed dietary intake assumes that there are no storage pools in the body. There is considerable evidence that such pools exist in the newborn infant (cf. reference 17 for review) and it is for this reason that the early neonatal period is characterized by taurinuria. TM What we showed is that neonatal taurinuria is less in infants fed synthetic formulas, especially casein-predominant forniulas, than it is in infants fed pooled human milk. The facts that these

The real issue, as we see it, is that the premature infant is a new biological entity and that standards of normal growth and normal biochemical measurements are uncertain. We suggest that the assumption of maintenance of the intrauterine rate of growth in the extrauterine situation as the proper goal of neonatal nutrition is facile and potentially dangerous. The large, apparently satisfactory, clinical experience with the use of human milk for preterm infants in Finland and elsewhere now provides a safe, if not optimal, basis for the nutrition of the preterm infant during the early neonatal period. Future studies on whether, when, and how human milk requires supplementation must take into account the schedule of biochemical development in the preterm infant. Even if a slight growth advantage in terms of gain in weight or length could be achieved by formulas containing more protein than that found in human milk, one can question whether or not such an increased growth rate would be desirable, if it also produces potentially dangerous metabolic imbalances. GeraM E. Gaull, M.D. David K. Rassin, Ph.D., Niels C. R. Ri~ihfi, M.D. Department o f Pediatric Research New York State Institute for Basic Research in Mental Retardation Staten Island, N. Y. 10314 Mount Sinai School o f Medicine o f the City University o f New York New York, N Y Children's Hospital University o f Helsinki HelsinkL Finland REFERENCES

1. Fomon SJ, and Ziegler EE: Protein intake of premature infants, J PEDIATR90:504, 1977. 2. R~iih~iNCR, Heinonen K, Rassin DK, and Gaull GE: Milk protein quantity and quality in low-birth-weight infants: 1. Metabolic responses and effects on growth, Pediatrics 57:659, 1976. 3. Rassin DK, Gault GE, Heinonen K, and R~iiha NCR: Milk protein quantity and quality in low-birth-weight infants, ll. Effects on selected essential and non-essential amino acids in plasma and urine, Pediatrics 59:407, 1977. 4. Gaull GE, Rassin DK, R~iih/iNCR, and Heinonen K: Milk protein quantity and quality in low-birth-weight infants. Ill. Effects on sulfur amino acids in plasma and urine, J PEDIATR90:348, 1977.

5I0

Editor's column

5. Rassin DK, Gaull GE, R~iiha NCR, and Heinonen K: Milk protein quantity and quality in iow-birth-weight infants. IV. Effects on tyrosine and phenylaianine in plasma and urine, J PEDIATR9tt:356, 1977. 6. Cox WM, and Filer LJ: Protein intake for low-birth-weight infants, J PEDIAXt~74:1016, 1969, 7. Babson SS, and Bramhall JL: Diet and growth in the premature infant. The effect of different dietary intakes of ash-electrolyte artd protein on weight gain and linear growth, J PEDIATR74:890, 1969. 8. Edelmann CM, and Barnett HL: Role of the kidney in water metabolism in young infants. Physiologic and clinical considerations, J PEDIATR56"154, 1960. 9. Kagan BM, Stanincova V, Felix NS, Hodgman J, and Kalman K: Body composition of premature infants: relation to nutrition, Am J Clin Nutr 25:1165, 1972. 10. Fomon SJ: Infant nutrition, ed 2, Philadelphia, 1967, WB Saunders Company. 11. Fomon SJ, Ziegler EE, and Vazquez HD: Human Milk and the small premature infant, Am J Dis Child (in press).

The Journal of Pediatrics March 1977

12. Davies DP: Problems of osmolality and obesity. Protein intake, osmolality homoeostasis and renal function in infancy, Postgrad Med J 51:26, 1975. 13. Lloyd JK: Obesity in infancy, Postgrad Med J 51:35, 1975. 14. Starling EH: On the absorption of fluids from the connective tissue spaces, J Physiol (Lond) 19:312, 1896. 15. Starling EH: The glomerular functions of the kidney, J Physiol (Lond) 24:317, 1899. 16. Etienne P, Young SN, and Sourkes TL: Inhibition by albumin of tryptophan uptake by rat brain, Nature 262:144. 1976. 17. Sturman JA, Rassin DK, and Gaull GE: Taurine in development, Life Sci (in press). 18. Lindblad BS, and Baldesten A: Time studies on free amino acid levels of venous plasma during the neonatal period, Acta Pediatr Scand 58:252, 1969. 19. Sturman JA, Cohen PA, and Gaull GE: Effects of deficiency of vitamin B~ on transsulfuration, Biochem Med 3:244, 1969.

Pollutants in breast milk

CERTAIN CHEMICAL POLLUTANTS in the food chain are excreted i n the fat of breast milk. Their biologic impact on infants i n the general population is unknown, but in a heavy accidental exposure to polychlorinated 9 biphenyls in Japan, disease was produced in the breastfed infant. PCBsAND

COLA-COLORED BABIES

In 1968 about 1300 people in Kyushu, Japan, developed chloracne from cooking oil contaminated with polychlorinated biphenyls.' This heat-transfer agent, normally contained in pipes, had leaked through pin-hole erosions into cooking oil during its manufacture.'-' Nine newborn infants, whose mothers had ingested the oil, were small for dates and had cola-colored skin and ocular discharge, among other abnormalities. 1-:~ The darkened skin cleared in a few months, but in adults chloracne has persisted. Polychlorinated biphenyls, used commercially for more than 40 years, are now known to be a worldwide poilutant. ~In the United States, chicken feed was contaminated during its manufacture through erosion of pipes, just as in Japan. The problem was discovered and the birds and Vol. 90. No. 3, pp. 510-512

their eggs were destroyed but not before some had been marketed." Fish have been contaminated by polluted waters," as in the Hudson River and the Great Lakes. PCBs were discharged into the H u d s o n as waste from factories where they were used in electrical capacitors and transformers. The chemical is nonbiodegradable and is stored in the body fat. It is cleared slowly over a period of years, especially through the fat in breast milk. Abbreviations used PCB: polychlorinated biphenyl PBB: polybrominated biphenyl In 1974 and 1975 the Environmental Protection Agency collected about 1,600 samples of h u m a n breast milk to evaluate the pesticide content. These samples are now being used to measure PCB levels. Among the first 80 studied, 16 had milk-fat levels of 2.5 ppm or higher (up to 10.6 ppm), when measured once at some time between 16 and 540 days after delivery.; The dose that might be harmful to the breast-fed infant is unknown. In the Japanese epidemic, the PCBs contained impurities that