Feeding the Sick Infant, edited by Léo Stem
Nestlé Nutrition Workshop Séries, Vol. II.
Nestec Ltd, Vevey/Raven Press, New York
© 1987.
Protein and Nitrogen Metabolism in
Low-Birth-Weight Infants
Niels Râihâ and Gunther Boehm
Department of Pediatrics, University of Lund, Malmô General Hospital,
Malmo, Sweden S-21401
Feeding of the newborn infant has always been one of the central problems in
clinical pediatrics. However, the emphasis has generally been centered on the nutritional requirements for the normal infant, and much less attention has been given
to the needs of the sick or compromised newborn infant. There has been much
controversy in the pédiatrie literature concerning the protein requirements of lowbirth-weight (LBW) infants, and two main schools hâve developed during récent
years: (a) those who advocate formula feeding using formulas with much higher
protein and calorie content than that présent in human milk, and (b) those who
feel that fresh human milk from the mother or supplemented pooled human milk
at adéquate volume intakes will fulfill most requirements for normal growth and
development. The issue becomes even more complicated when a particular disease
or médical problem is superimposed on the already complicated nutritional situation in the very low birth weight (VLBW) infant. This paper will présent some
récent findings on the metabolic effects of protein intake in LBW infants who are
both appropriate for gestational âge (AGA) and small for gestational âge (SGA)
and in infants with septic bacterial infection.
NUTRITIONALLY AVAILABLE PROTEIN CONTENT OF
HUMAN MILK
The nutritionally available protein content of human milk is lower than has generally been thought. The mean protein concentration of expressed mature human
milk based on the total nitrogen content as analyzed by the Kjeldahl method and
multiplied by the conversion factor of 6.25 is about 1.2 g/dl (1). During the first
weeks of lactation the total protein content is higher mainly because of high colostral immunoglobulin levels (2). There is also a high degree of variation in the protein content of mature human milk both between samples from the same mother
and between samples from différent mothers (3). Mothers of preterm infants hâve
63
64
PROTEIN AND NITROGEN METABOUSM
somewhat higher true protein concentrations for the first weeks of lactation than
mothers of term infants (3,4). This is, however, owing in part to a higher concentration of secretory IgA in preterm milk. The true protein is determined by amino
acid analysis after the précipitation of the protein in breast milk with acid, and has
been found to be about 0.9 g/dl in mature milk (1,5). In preterm milk the overall
mean value for true protein during 12 weeks postpartum is found to be 1.23 g/dl
(4). The différence between total protein content and true protein content in human
milk, about 25%, is attributed to the nonprotein-nitrogen (NPN) fraction. Urea is
the major component of this fraction and comprises about 50% of it. The nutritional significance of the NPN fraction for the healthy infant is probably negligible
(6), since most of the urea is consumed by the intestinal bacterial flora (7).
The protective proteins found in the whey fraction of human milk, lactoferrin,
lysozyme, and secretory IgA are highly résistant to proteolysis and mainly excreted in the stool: They are thus not nutritionally totally available (8,9). Thèse
proteins are relatively abundant in human milk, and their combined concentration
in mature milk is about 0.3 g/dl (1). The nutritionally available protein in mature
human milk may thus be as low as 0.7 g/dl and in preterm milk somewhat higher.
The nutritionally available protein intake in normal exclusively breast-fed infants
during the first months of life has been estimated to about 1.2 g/kg/day (9).
EFFECTS OF BACTERIAL SEPSIS ON NITROGEN METABOLISM IN
LBW INFANTS
The catabolic state of protein metabolism as a resuit of sepsis is a well known
phenomenon in adults (10), but very little is known regarding LBW infants, in
whom bacterial infection still is a common cause of morbidity and mortality during
the neonatal period.
We hâve studied 18 VLBW infants (< 1,500 g) who developed bacterial sepsis
during the first 7 days of life and compared the metabolic responses of thèse infants with those of healthy control infants and infants with idiopathic respiratory
distress syndrome (IRDS). AH infants were enterally fed with fresh human milk
from their own mother with some supplementary glucose infusion depending on
the clinical condition. When full volume of human milk was reached, the total
protein intake was about 2 g/kg/day in ail infants studied.
Within 24 hr after the first clinical symptoms of bacterial infection had occurred,
the total urinary nitrogen excrétion increased twofold from 15 mmoles/kg/day to
over 30 mmoles/kg/day in the septic infants as compared to the control infants or
to the infants with IRDS (Fig. 1). The increased nitrogen excrétion was accompanied by a significant increase in the concentrations of plasma ot-amino nitrogen
and sérum urea as seen in Fig. 2 and 3. Thèse changes are similar to those seen
in VLBW infants on high protein intakes (5). The results indicate that bacterial
sepsis in VLBW infants rapidly produces a catabolic state accompanied by in-
PROTEIN AND NITROGEN METABOUSM
65
mmole/ kg /d
7 days
F I G . 1 . T o t a l urinary nitrogen excrétion in V L B W infants (control, I R D S , septicemia) during
the first w e e k of life. Ail infants w e r e f e d with o w n mother's milk.
mm oie/1
F I G . 2 . P l a s m a α - a m i n o nitrogen in V L B W infants (control, I R D S , septicemia) during t h e first
w e e k of life. Ail infants w e r e f e d with o w n mother's milk.
7 ims
FIG. 3. Sérum urea in VLBW infants (control, IRDS, septicemia) during the first week of life
Ail infants were fed with own mother's milk.
66
PROTEIN AND NITROGEN METABOUSM
creased urea and amino acid concentrations in the blood. An adéquate calorie intake should be the primary aim in this situation, whereas excessive protein intake
in such infants may further aggravate the metabolic imbalances présent.
EFFECTS OF PROTEIN INTAKE IN SGA, VLBW INFANTS
VLBW infants are especially sensitive to excessive protein intake due to metabolic immaturities of protein and amino acid metabolism (11). We hâve previously
shown that VLBW infants who are AGA hâve increased plasma levels of most
essential amino acids and elevated blood urea concentrations on a high protein intake (5). However, little information is found in the literature on the effects of
protein intake on VLBW infants who are SGA.
We hâve recently studied a group of VLBW infants (< 1,500 g) who were below the lOth percentile on the national intrauterine growth charts and compared
the effects of différent protein intakes to those found in VLBW infants who are
AGA. The infants were fed for the first week either with human milk from their
own mother (protein content ~ 1 . 2 g/dl) or an adapted formula with a protein content of 1.7 g/dl. The volume intake was increased during the first week to full
volume (180 ml/kg/day). Thus, the protein intake on the 8th day was 2 g/kg/day
in the infants on human milk and 3 g/kg/day in the infants on the formula. The
SGA infants had a higher plasma α-amino nitrogen concentration and a higher aamino nitrogen excrétion on the formula than the AGA infants. Figure 4 shows
that when full volume was reached on the 8th day of life, the SGA infants also
had a significantly higher sérum total bile acid concentration on the higher protein
intake. Thèse results would suggest that SGA, VLBW infants are even more sensitive to increased protein intakes than AGA infants of similar birth weight. This
may be attributable to delayed enzyme maturation in the liver and deserves further
investigation.
A|i = 8 <i»s
FIG. 4. Sérum total bile acid concentration in AGA and SGA, VLBW
infants at 8 days of âge who hâve
been fed human milk or adapted formula (protein 1.7 g/dl).
PROTEIN AND NITROGEN METABOUSM
67
PROTEIN INTAKE AND BILE ACID METABOLISM IN
VLBW INFANTS
Our previous studies hâve demonstrated that the bile acid pool size and the intraluminal bile acid concentrations are significantly lower in infants fed formula
when compared with infants fed human milk (12,13). This différence was not owing to an increased bile acid synthesis rate in the human milk fed infants, but
rather to a différence in the enterohepatic circulation of the bile acids (13).
In vitro studies on perfused rat liver and on isolated rat hepatocytes hâve shown
that amino acids that undergo a Na+-dépendent hepatic uptake inhibit bile acid
uptake into the liver cell (14,15). This may explain the well-known cholestatic effect of some amino acid solutions used for parenteral alimentation in early infancy
(16).
In the présent studies we hâve correlated plasma α-amino nitrogen concentration
with sérum and intraluminal total bile acid concentrations in VLBW infants. The
intraluminal bile acid concentration correlated inversely with the sérum bile acid
concentration in the infants studied (Fig. 5).
At 30 days of âge a change from human milk to an adapted formula containing
1.7 g/dl of protein significantly increased the plasma α-amino nitrogen concentra­
tion in VLBW infants as compared with control infants who were continued on
human milk. This change was accompanied by a marked élévation of the sérum
total bile acid concentration with a concomitant decrease in the intraluminal bile
acid concentration (Fig. 6).
Thus, VLBW infants are sensitive to the cholestatic effect of increased enterai
protein and amino acid intake. The high sérum bile acid concentration, the low
bile acid pool size, and intraluminal bile acid concentration in formula-fed VLBW
mmole/l
10-
•
C_3
•
f 5-
•
·
· ·
•
25
•
·
, ·#
50
75
•
t ·
t
100 jimole/l
SERUM TOTAL BILE ACIOS
FIG. 5. Corrélation of intraluminal and sérum total bile acid concentrations in VLBW infants
on différent protein intakes.
68
PROTEIN AND NITROGEN METABOUSM
PLASMA α - Α Μ Ι Ν Ο - Ν
SERUM TOTAL BILE AGIOS
Huiain Milk
mmole/l
HumniMilk
Formule
Formula
Agi = 30 dtys
FIG. 6. Sérum total bile acid concentration and plasma α-amino nitrogen concentration in
VLBW infants at 30 days of âge who were fed either human milk or adapted formula (protein
1.7g/dl).
(12,13) may be because of excessive protein intake producing hyperaminoacidemia, which in turn inhibits bile acid uptake into the liver cell and may thus indirectly affect intestinal lipid absorption.
CONCLUSIONS
The absorbed and nutritionally available protein content of human milk is lower
than generally assumed. Infant formulas on the market today contain a nutritionally available protein concentration that is more than twice that of mature human
milk. The high protein intake of normal infants fed conventional formulas may
hâve unwanted metabolic effects (17).
The VLBW infant is sensitive and vulnérable to protein intakes that exceed the
needs for normal growth because of developmental immaturity of the metabolic
and excretory Systems. Sick LBW infants, such as infants with sepsis and infants
suffering from intrauterine malnutrition, are especially sensitive to the quantity of
protein intake. Protein overloading in such infants produces hyperaminoacidemia,
which may interfère with the enterohepatic circulation of bile acids and affects the
already low intraluminal concentration of bile acids in thèse infants. This may decrease fat absorption.
ACKNOWLEDGMENTS
Thèse studies hâve been supported in part by grants from The Swedish Médical
Research Council B84-19X-6259-0313 and by the Nestlé Nutrition Grant Program.
PRQTEIN AND N1TROGEN METABOUSM
69
REFERENCES
1. Hambraeus L. Proprietary milk versus human breast milk in infant feeding, Pediatr Clin North
Am 1977;24:17-36.
2. Butte NF, Goldblum RM, Fehl LM, et al. Daily ingestion of immunologie components during
the first four months of life. Acta Paediatr Scand 1984;73:296-301.
3. Hibberd CM, Brooke OG, Carter ND, Haug M, Harzer G. Variation in the composition of breast
milk during the first 5 weeks of lactation: implications for the feeding of preterm infants. Arch
Dis Child 1982;57:658-62.
4. Butte NF, Garza C, Johnson CA, O'Brian Smith E, Nichols BL. Longitudinal changes in milk
composition of mothers delivering preterm and term infants. Early Hum Dev 1984;9:153-62.
5. Râihâ NCR, Heinonen K, Rassin DK, Gaull GE. Milk protein quantity and quality: I. Metabolic
responses and effects on growth. Pediatrics 1976;57:659-74.
6. Heine W, Plath C, Richter I, Wutzke K, Towe J. '5N-tracer investigations into the nitrogen metabolism of preterm infants fed mother's milk and a formula diet. J Pediatr Gastroenterol Nutr
1983;2:606-12.
7. Heine W, Tiess M, Stolpe HJ, Wutzke K. Urea utilization by the intestinal flora of infants fed
mother's milk and a formula diet, as measured with the ,5N-tracer technique. J Pediatr Gastroenterol Nutr 1984;3:709-12.
8. Ogra SS, Weintraub D, Ogra PL. Immunologie aspects of human colostrum and milk. III. Fate
and absorption of cellular and soluble components in the gastrointestinal tract of the newbom. J
Immunol 1977;119:245-8.
9. Râihâ NCR. Nutritional proteins in milk and the protein requirement of normal infants. Pediatrics
1985;75(Suppl):136-41.
10. Mizeck B. Septic shock. A metabolic perspective. Arch Intern Med 1984;144:579-85.
11. Râihâ NCR. Protein in the nutrition of the preterm infant. Biochemical and nutritional considérations. Adv Nutr Res 1980;3:176-206.
12. Jàrvenpââ AL, Rassin DK, Kuitunen P, Gaull GE, Râihâ NCR. Feeding the low-birth-weight
infant. III. Diet influences bile acid metabolism. Pediatrics 1983;72:677-84.
13. Watkins JB, Jàrvenpââ AL, Szczepanik-van Leeuwen P, et al. Feeding the low-birth-weight infant. V. Effects of taurine, cholestérol and human milk on bile acid kinetics. Gastroenterology
1983;85:793-800.
14. Blitzer BL, Ratoosh SL, Donovan CB. Amino acid inhibition of bile acid uptake by isolated rat
hepatocytes: relationship to dissipation of transmembrane Na + gradient. Am J Physiol
1983;245:G399-G403.
15. Preisig R, Rennert O. Biliary transport and cholestatic effects of amino acids (AA) [Abstract].
Gastroenterology 1977;73:A-42/1240.
16. Dahms BB, Halpin TC. Sériai liver biopsies in parenteral nutrition-associated cholestasis of early
infancy. Gastroenterology 1981;81:136-44.
17. Moro G, Minoli I, Râihâ N. Milk protein quantity in the term infant: metabolic responses and
effects on growth [Abstract]. Pediatr Res 1984;18:1051.
DISCUSSION
Dr. Hamosh: I wonder if you would like to comment on Ginsberg's data on the increased
insulin sécrétion in formula-fed babies? Could this be the reason for the higher percentage
of obesity, of fatter babies?
Dr. Ràihà: I do not think I dare comment on obesity, but I would like to say that as far
as the frequency of diabètes is concemed, an interesting paper from Denmark was recently
published in Lancet (1) showing that the frequency of diabètes type 1, in children, was
indirectly related to the frequency of breast feeding. In a longitudinal study they were able
to show a very nice inverse corrélation between the frequency of breast feeding and the
number of infants with diabètes: With more breast feeding, there was less diabètes and vice
versa. As regards obesity, I do not know.
Dr. Guesry: In your présentation, you assume that an increase of the glomerular fïltration
70
PROTEIN AND NITROGEN
METABOLISM
rate in the newborn may be deleterious; I do not see any reason for this. On the contrary,
one could also argue without any more proof than you hâve, that soon the baby will be
confronted with a more diversified diet with more protein and that early adaptation to a
différent diet may be useful and should take place as early as possible.
Dr. Ràiha: It is clear that the protein intake of artificially fed infants is much higher
than the protein intake of breast-fed infants, and that the différence increases further once
supplementary foods, that consist of vegetable and fruit purées, and, between 5 and 6
months of âge, fish and méat purées, are introduced. I am not sure this is a good thing.
We do not really know what harm it can cause, but it may be an unnecessary stress on the
kidney function and also on the pancreatic function. Why give them more than they need
when they grow and develop just as well with less.
Dr. Guesry: If we use nonhuman milk protein, more particularly soy protein, unmodified
cow's milk protein, or even modified cow's milk protein with a whey-to-casein ratio of
60:40 and give 10 g protein per liter, the quantities of some essential amino acids, like
tryptophan or phenylalanine that the infant receives, will be less than in human milk. This
means that in giving 10 g per liter of cow's milk protein, even adapted, we shall induce
some amino acid deficiencies. Do you not think that we need to be quite cautious and
proceed gradually in reducing the level of protein; maybe one should not drop the level to
10 g per liter as you advocate?
Dr. Ràiha: Most amino acids are very much higher in formulas, even in low-protein
formulas, than in human milk. Now we must remember that the protein concentrations in
human milk include ail the whey proteins, including those that are not well absorbed. If
we would consider only the nutritional proteins in human milk and the concentration of the
nutritional amino acids, then we would still get lower figures. I think we can go down in
formulas to about 12 or maybe 11 g protein per liter and still be safe and hâve a very good
margin to human milk. I agrée that cysteine levels are quite low, but normal babies are
able to synthesize cysteine from methionine so that in the normal baby, cysteine and methionine may be considered together. Ail I can say, to conclude, is that babies who are fed
with formulas containing 1.1 g protein per 100 ml grow very well, hâve normal plasma
levels of amino acids and hâve no problems whatsoever.
Dr. Schwartz: I would like to comment on the interprétation of C-peptide. I wish to point
out that the processing of insulin begins in the beta cell with the formation of proinsulin
that is then split into C-peptide and insulin, which are released in equimolar amounts. The
insulin is extracted to varying degrees, from 40% to 60%, by the liver, the C-peptide presumably is not extracted by the liver. Thus, C-peptide has been considered a measure of
insulin sécrétion. It turns out that C-peptide is excreted in the urine and, therefore, summation of the C-peptide in the urine presumably refiects what has been going on with insulin
sécrétion. Now it is complicated because ail of us secrète proinsulin in addition to insulin
and C-peptide. We hâve up to 25% proinsulin in our circulation. Proinsulin, C-peptide, and
insulin hâve différent half-lives. Proinsulin is also excreted in the urine. It also tums out
that the immunochemical methods that we use for measuring thèse substances hâve crossreactivity. In the assay of insulin, for example, you get cross-reaction between insulin and
proinsulin, and there are beautiful data from Sweden (2) showing that in a newborn term
infant, as much as 100% of the insulin immunoreactivity can be accounted for by proinsulin. Because of that kind of cross-reactivity, until we do some gel filtration and some other
sophisticated analyses, I would just urge you to take thèse data as qualitative. Further work
needs to be done before we can go quite as far as you would like us to; you may well be
right, but I think we hâve to be a little bit cautious.
Dr. Maggioni: Dr. Ràiha, do you think that the nucleotides in human milk may hâve any
PROTEIN AND NITROGEN
METABOLISM
71
influence on the protein synthesis? You know that there are infant formulas, for instance, in
Spain, with added nucleotides.
Dr. Râihà: Yes, I hâve seen the studies from Spain in which they added nucleotides. I
do not think there was any effect on the growth of the infants. Whether nucleotides in formulas hâve any influence on protein synthesis, I cannot answer.
Dr. Heim: Why did you use 6.38 as a multiplication factor for nitrogen calculated in the
protein and not 6.25? Because of the WHO recommendation and everybody is taking the average nitrogen concentration of the animal protein and using 6.25? Did you analyze the amino
acid content and nitrogen content of both breast milk and formula?
Dr. Râihà: We used 6.25.
Dr. Heim: You just presented Heine's studies (3) on human milk and formula. Did I
understand correctly that he gave 2 g/kg/day protein in human milk and 3 g in formula?
Did the two groups hâve différent protein intakes?
Dr. Râihà: Yes, he gave human milk at an intake of 180 ml/kg/day, and gave the same
volume of formula, which contained 1.7 g of protein/100 ml, so that the intake was about
twice as high in the formula group.
Dr. Heim: So the basic différence between the two groups was that their protein intake
was less from human milk than from formula, and in spite of this he found the same synthesis, the same breakdown, the same déposition, and a lower excrétion. Did he do balance
studies?
Dr. Râihà: It was a complète balance; he measured stable isotope in stool and urine as
well.
Dr. Heim: Thèse results surprise me. If protein intake was double and everything the
same except for a small différence in rénal nitrogen excrétion, where are the other proteins
going? An important question hère, which we need to examine very critically, concerns the
validity of the 15 N-technique. Ail thèse calculations are based on the plateau principle that
you can never achieve if the amino acid that you use is continuously diluted. Now, glycine
is a nonessential amino acid which can be synthesized from glucose; this dilutes the whole
pool. The newborn infant is so labile in this respect that you can never rely on thèse results.
We therefore hâve to find new markers, essential amino acids, or mixtures of protein, and
re-investigate ail thèse questions which were investigated with 15 N-glycine. We hâve to
be very critical.
Dr. Stern: Dr. Râihâ, when you make the statement that there is no impairment of
growth, what do you actually mean, length and weight, or do you mean something more
discrète in terms of qualitative growth of différent organ Systems?
Dr. Ràihà: I mentioned that only in relation to the full-term infant study that we did on
human milk ad libitum versus two formulas, ad libitum as well: a test formula and a conventional formula that contained 1.2 and 1.6 g protein/100 ml, respectively. We measured
the intakes and we measured the following growth parameters: weight, length, skinfold
thickness, and head circumference. There was no différence. We also carried out a followup study on thèse infants at 1 year of âge and as far as neurological development is concerned, there was no différence either.
Dr. Stern: Ail thèse éléments are fine except that they are fairly discrète. One of the
problems of using length, weight, etc., is that there are constitutional factors that govern
them. Some people grow up to be taller than the rest of us, no matter what you feed them.
I suggest that those parameters may not be sufficient. What I am getting at is that if we
talk in terms of nutrition, as we become more sophisticated> we may be in a better position,
for example, to evaluate incorporation of spécifie amino acids into spécifie Systems. One
of our problems regarding the central nervous System is that we really do not hâve too
72
PROTEIN AND NITROGEN
METABOLISM
many good measures of early cognitive development and just to use head size as a function
of central nervous System integrity is too crude. I do not, however, hâve any ready solution. I think maybe the question ought to be left open, and what you are entitled to say is
that for the moment, at least in terms of growth parameters, you see no difficulty with a
relatively low protein diet.
Dr. Rûihà: I think we should still agrée that a normal baby, fed ad libitum with mother's
milk, should be the norm.
Dr. Stern: When you were developing the argument about protein inhibition of bile sait
excrétion, I assume from what you were saying that the effect is an inhibition on the hepatic
ability to clear bile acids. Is that right?
Dr. Ràihà: By giving too much protein and increasing amino acid concentrations in
plasma, you may interfère with the entrance of bile salts into the liver cell.
Dr. Stern: That présupposes a fairly constant enterohepatic absorption of bile salts from
the gut. Would it be possible that there is a différence in the reabsorption that is related to
the lipid content of whatever you happen to be feeding, and whether the différence in the
bile sait concentration could at least in part be owing to that as well as to the inhibition of
its excrétion?
Dr. Ràihà: Yes, this is certainly possible, and there could also be a factor in human milk
that would enhance the development of the intestine so that absorption would be more favorable. There may be many factors; I am just saying that this may be one.
Dr. Rubaltelli: You touched upon the relationship between cholestasis and the amino
acid intake and the amino acid levels. It seems to me that this is a very good point because
we know that with a similar intake of amino acids, infants on total parenteral nutrition hâve
higher plasma levels of amino acids and that thèse high plasma levels are very well correlated with cholestasis and with jaundice in this group of infants.
Dr. Senterre: Dr. Râihâ, you observed, in preterm infants fed human milk, a urinary
excrétion of creatinine of 15 to 16 mg per day in the first days of life; at 7 weeks of âge,
it was still only 17 mg/day. In formula-fed infants, the figures were 23 and 26 mg/day, at
birth and at 7 weeks of âge, respectively. Creatinine cornes from protein turnover, chiefly
in muscles. In my expérience, urinary creatinine excrétion is rather stable when it is expressed per kg body weight per day, but absolute figures increase from week to week, because the weight is increasing. I think creatinine excrétion should be expressed per kg body
weight per day. In my expérience, urinary creatinine excrétion per kg body weight per day
in preterm infants receiving a slightly higher protein intake than in those receiving human
milk, is a bit higher, around 10 to 12 mg versus 8 to 9 mg/kg/body weight/day in the latter.
My interprétation is that thèse figures reflect the lean body mass, which develops faster in
those who receive the higher protein intake. Do you hâve other data showing that thèse
différences are chiefly owing to a différence in glomerular filtration rate?
Dr. Ràihà: No, we do not hâve data to prove that. We do not hâve sérum creatinine
levels, and we cannot therefore really make exact calculations but the best explanation is
that there is an increased filtration rate, which would stress the kidney.
Dr. Rubaltelli: What was the gestational and the postnatal âge of the children in Heine's
study that showed comparable protein synthesis in formula- and breast-fed infants? Other
studies on prématuré babies below 32 weeks of gestation seem to demonstrate that infants
fed banked human milk ad libitum tolerate more milk than formula-fed infants but that they
receive fewer calories and grow less in weight as well as in length and head circumference.
Thèse data are very stimulating, but I wonder if they were obtained in prématuré babies
below 32 weeks of gestation?
Dr. Ràihà: Heine's study (3) was done on babies over 32 weeks of gestation, so they
PROTEIN AND NITROGEN
METABOUSM
73
were not VLBW infants. I fully agrée with you that this kind of study also has to be done
on VLBW infants, and I certainly agrée with you and with Dr. Senterre that babies with a
birth weight around 1,000 g hâve higher protein requirements than thèse infants.
Dr. Rubaltelli: What should be the référence value for normal plasma levels of amino
acids since it is known that thèse levels are higher in the fétus than in preterm and fullterm babies, even if fed high protein formulas?
Dr. Râihà: Our hypothesis is that a protein intake in VLBW infants which produces intrauterine growth rates should give plasma amino acid levels equal to those found in breastfed term infants. Our récent studies indicate that this is true and we think this should be
the model or référence value.
Dr. Marini: Regarding amino acid levels, what is the relationship between plasma and
tissue levels. Also, do you not think it could be useful to evaluate ratios of some amino
acid concentrations, rather than absolute concentrations of individual amino acids?
Dr. Râihà: Thèse are very important questions. Dr. Heard at Columbia University in
New York is the only one who has studied plasma levels versus tissue levels. He has done
studies on puppies with différent protein intakes and has shown that a small change in the
plasma level may induce much higher changes in the tissues. So it would be very important
to measure tissue levels but of course we cannot measure them in humans. As far as bile
salts are concerned, in in vitro studies methionine especially and some other amino acids
were very powerful inhibitors of their transport. It may very well be that an increase of a
certain amino acid has more effect than maybe an increase of another.
Dr. Vidailhet: May I hâve your opinion about taurine supplementation of formulas that
we now see in Europe?
Dr. Ràiha: My feeling is that if we add taurine to formula in order to make it closer to
human milk, then it is a good argument. I do not think we hâve really very much évidence
that it would make any drastic différence if it was not added but again, as Dr. Stern says,
we do not hâve very sophisticated methods of measuring thèse différences. We can measure
weight, length, development, and so on, but there may be advantages or disadvantages by
adding taurine that we cannot measure. The only thing I can say is that we hâve added
taurine to formula in our studies and by doing so hâve achieved the same plasma levels as
when babies are fed human milk, both in preterm and term babies; we did not see any acute
différence as far as growth, well-being, and so on are concerned. So, I think if the reason
to add it is that you want to make formula more like human milk, and you want to hâve
the same levels of taurine in the baby as you hâve when you feed human milk, then it is
quite acceptable.
Dr. Tsang: In your studies on bile salts and bile sait pools, am I right in thinking that
you used pure human milk versus formula, which means that there were many différences
between the two: fat blend, taurine, and many other things?
Dr. Ràiha: That is right, although in the first study by Dr. Jârvenpàa, in which we
looked at the intraluminal bile sait concentration, we also compared a formula with taurine
and without taurine. There were no statistically significant différences in the total bile sait
concentration in either instance. But in any case, infants on formula always had lower total
bile sait concentrations than infants fed human milk. However, there was clearly a distinct
différence in the taurine-to-glycine ratio with and without taurine.
Dr. DeLuca: I am not sure where the statement came from that SGA babies require more
protein or more nitrogen per kg per day. Is there any information to substantiate that?
Dr. Ràiha: No, not really; as far as I know, there are no controlled clinical studies on
that.
Dr. DeLuca: I ask this question because the way we hâve been trying to abate and elimi-
74
PROTEIN AND NITROGEN
METABOUSM
nate the cholestasis that we hâve seen with total parenteral nutrition, is that we do not administer more than 1 to 1.5 g/kg protein per day. By doing so, we hâve been able to eliminate the cholestatic picture that we had seen 5 to 6 years ago.
Dr. Râihâ: We hâve data showing that SGA babies are probably more sensitive as far
as the increase in the sérum bile acid concentration is concerned in relation to protein intake. We had a higher increase in the sérum bile acid concentration in SGA infants than in
AGA prématurés. There must be a différence, but there is not a good study on how much
protein SGA infants need.
Dr. Stern: The assumption that SGA infants require more of anything was based on the
original oxygen consumption measurements that were calculated per kg body weight; because those infants had mostly active tissue mass and very little muscle, the oxygen consumptions were one and a half to two times as high. But, when those were recalculated on the
basis of square meters or surface area, they were the same so that the argument that you
need more of either protein, fat, or carbohydrate for an SGA infant is probably not valid.
Dr. Heim: The oxygen consumption may not be higher in the SGA infant if you calculate
it on the basis of body surface. It may be true, but anyway, the higher oxygen consumption
of the SGA infant is probably due to the higher oxygen consumption of the brain, which
is proven to be larger in the SGA infant than in the AGA infant. But the protein requirement has nothing to do with it because the protein contribution to energy metabolism is
very small, 7% to 8%, in any infant with any intake. The higher protein requirement of the
SGA infant is probably owing to the less efficient absorption of protein from breast milk
or formula. This has been published by us (4), and it has been confirmed and is in print
by Oliver Brookes in London. Thèse are studies on large séries of pair-matched AGA-SGA
infants. Protein absorption is significantly less in SGA infants. If you want to achieve the
same déposition, you must probably give a little more protein.
Dr. DeLuca: You are absolutely correct, but this applies actually only to enterally fed
infants. I was referring to the cholestasis that has been reported with parenteral nutrition
during which the amino acids are directly deposited into the circulation and therefore almost
100% absorbed. In addition, the gastrointestinal tract of prématuré babies hâve other problème that interfère with absorption.
Dr. Heim: You are right. If your question was specifically related to SGA infants on
total parenteral nutrition, then I agrée with you that they may not need more protein.
REFERENCES
1. Borch-Johnsen K, Mandrup-Poulsen T, Zachau-Christiansen B, et al. Relation between breast-feeding and incidence rates of insulin-dependent diabètes mellitus. Lancet 1984;ii: 1083-6.
2. Heding LG, Persson B, Stangenberg M. β Cell function in newborn infants of diabetic mothers.
Diabetologia 1980;19:427-32.
3. Heine WL, Plath C, Richter I, Wutzke K, Tôwe J. "N-tracer investigations into the nitrogen metabolism of preterm infants fed mother's milk and a formula diet. J Pediatr Gastoenterol Nutr
1983;2:606-12.
4. Chessex P, Reichman B, Verellen G, et al. Metabolic conséquences of intra-uterine growth retardation in very low birth weight infants. Pediatr Res 1984;18:709-13.