The metabolism of iron in the anemia of premature infants

T H E M E T A B O L I S M OF IRON I N T H E A N E M I A OF PREMATURE INFANTS CHARLES E . SNELLING,*

M.B.

TORONTO, CANADA

I N F A N T S born p r e m a t u r e l y , the e r y t h r o c y t e count and hemo~ I Nglobin fall to a low level and r e t u r n to normal during the first six months of life. Finkelstein 1 states that p r e m a t u r e l y born infants develop anemia in spite of t r e a t m e n t and recover in due course of time without special t h e r a p e u t i c measures other t h a n the t r e a t m e n t r e q u i r e d b e c a u s e of t h e i r immaturity. In this clinic 2 it has been shown t h a t in general an anemia de.velo,ps in p r e m a t u r e infants up. to about ten weeks to twelve weeks of age, a f t e r which the blood tends to r e t u r n to normal, spontaneously. I t has been no.ted also that the amo.unt of reduction in red blood cells and hemoglob.in va.ries inversely as the length or weight of the i n f a n t at birth, or, in other words'., directly wi~h the degree of prematur~ty. The red cells and hemoglobin have a coincident fa]i, and the color index remains close to unity. 1Vfany investigators believe t h a t p r e m a t u r e infants are lacking in the mineral elements. Mineral balance studies 3, 4, ~ made on p r e g n a n t mothers have shown t h a t d u r i n g the last three months of i n t r a u t e r i n e life a large p r o p o r t i o n of the various inorganic salts is retained, and it is assumed t h a t these are all utilized b y the fetus. In contrast to this, analyses of fetuses 6, 7 have shown t h a t t h r o u g h o u t p r e g n a n c y there is the same relative amount of the various inorganic elements as are f o u n d in the f u l l y m a t u r e fetus. The l a r g e r positive balance in the last t h r e e months of p r e g n a n c y is t h o u g h t to be due to the rapid gain in fetal weight d u r i n g t h a t period. Quantitative analyses f o r iron have shown t h a t fetuses and newborn infants contain a p p r o x i m a t e l y three times as much of this metal in p r o p o r t i o n to b o d y weight as does the child of one y e a r or the adult, s In other words, t h e r e is presumably enough iron reserve in the newborn infant, w h e t h e r born prem a t u r e l y or at term, to allow f o r a t r i p l i n g of the b i r t h weight without the r e q u i r e m e n t of additional iron f r o m the food, p r o v i d i n g there has not been a loss b y excretion or hemorrhage. The development of anemia in p r e m a t u r e infants with subsequent spontaneous r e c o v e r y has not been a d e q u a t e l y explained, although *National Research ,Council Fellow, Toronto, Canada. F r o m the D e p a r t m e n t of Pediatrics, H a r v a r d 1V[edical School, and the I n f a n t s ' and Children's Hospitals, Boston. 546

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various theories have been advanced. I n o r d e r to secure f u r t h e r inf o r m a t i o n concerning the p h e n o m e n o n of the d e v e l o p m e n t of anemia in p r e m a t u r e infants, iron b a l a n c e studies were c a r r i e d out f o r twelve periods, of t h r e e to six days each, on seven p r e m a t u r e i n f a n t s of v a r y ing ages. The p i g m e n t excretion of these i n f a n t s was r e c o r d e d t h r o u g h o u t t h e i r s t a y in the hospital. ]Y[ETHODS The infants were placed on metabolism beds designed by Dr. Gamble. The urine and stools were collected in porcelain containers. The urine and stool specimens from male infants were collected in separate: containers. At first the attempt was made to keep the specimens from females separate, but this was found to be impossible in small infants. Granite ware containers were not used because chipping of the container with consequent rust formation might produce considerable error. Spatulae and all crucible and evaporating dishes were of porcelain. The only metallic articles used in any of the processes were platinum crucibles for tile ashing. Carmine was given to mark the stools at the beginnings and end of the observation period. Each patient was kept on a constant twenty-four-hour diet throughout the experimental period. Aliquot samples of the diet, breast milk or formulas, were analyzed and the intake was computed in this manner. The amount of iron was determined by the amyl alcohol method described by Josephs,9 which is a modification of that given by Marriott and Wolf.lo This method was found to give results agreeing within 2 per cent on theoretical solutions as well as yielding good duplicate vMues on unknowns. The method of following the pigment excretion is described in another communication~ and need not be given here. RESULTS

The d a t a on iron balance a n d blood findings are r e c o r d e d in Table I. Nine of the twelve d e t e ~ n i n a t i o n s show a positive iron b a l a n c e a n d three, a v e r y small n e g a t i v e balance. The a v e r a g e balance p e r d a y is +0.10 mg. F e . The t h r e e d e t e r m i n a t i o n s giving a n e g a t i v e iron balance show a difference of i n t a k e and o u t p u t of such slight degT'ee t h a t f o r all practical p u r p o s e s t h e y m a y be t a k e n as r e p r e s e n t i n g equilibrium. P a t i e n t 6, weighing 4-5 pounds, h a d a v e r y high positive balance. The breast m i l k h a d a n e x c e p t i o n a l l y high iron content, a n d the i n f a n t was evidently utilizing most of it. All the i n f a n t s at the t i m e of the test periods showed some anemia as evidenced b y a decrease of both the red blood cells and the hemoglobin. The color indices w e r e high and a p p r o a c h e d unity. The anemia was more p r o n o u n c e d in the infants who were smaller at b i r t h and increased with age in these p a t i e n t s as long as t h e y were followed in this series. The d a t a on p i g m e n t excretion are f o u n d in Table II. The stool urobilin a n d bilirubin e x c r e t i o n was not excessive as c o m p a r e d with m y f o r m e r studies. ~1 As in the f o r m e r investigation, the p i g m e n t ex-

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TABLE DAILY

CASE

I 2 3 14 5 8 9 I0

549

IRON

II

fl~VERAGE ivOR ]~IVE-DAY PERIODS Ol~ PIGMENT EXCRETION EXPR,ESSED AS 1rIG. UlCOBILIN PEI~ D&Y

IN STOOLS~

PER,IODS I

1.8 6.6 3.3 0.3 0.3 0.6

II

3.8 1.2 1.5 0.5 0.3' 0.4

iii 1.3 Trace 3.2 1.0 6.0 0.4 0.3

gI

VII

VIII

IX

x

0.5

V 1.2 0.9 0.6

0.3 0.5 0X

0.4 0.6 2.8

0.3 2.1 0.6

1.2 1.4

1.0

2.7 0.3

0.2

4.0

4.6

2.2

2.2

1.6

IV i.i

* T h e v a l u e I r e p r e s e n t s a s m a l l e x c r e t i o n of b i ! i ~ u b i n . t H i g h e x c r e t i o n d u e to c o l l e c t i o n s s t a r t i n g a t s e v e n t h o r e i g h t h d a y o f life.

ereted in the stools of these infants d u r i n g the first few weeks of life was urobilin in some and bilir~lbin in others. A f t e r the i n f a n t s r e a c h e d two months of age the only p i g m e n t f o u n d on e x t r a c t i o n of the stools was urobilin. DISCUSSION

.These results differ f r o m those of L i c h t e n s t e l n 12 who c a r r i e d out iron balance studies on p r e m a t u r e l y b o r n infants. H e f o u n d t h a t t h e r e was a n e g a t i v e iron b a l a n c e unless excessive a m o u n t s of iron w e r e fed. Our m e t h o d s of analysis are diff'erent but coincide in g i v i n g appr'oxim a t e l y the same values f o r iron in b r e a s t milk. As t h e p r e m a t u r e l y b o r n i n f a n t develops a n e m i a in spite of t r e a t m e n t , a n d recovers spontaneously, it does not seem reasonable t h a t a n e g a t i v e iron balance could occur f o r a n y considerable period. The da~a on p i g m e n t excretion do not indicate excessive hemolysis and, t h o u g h .Whipple 16 states t h a t urobilin excretion is not quantitat i v e l y r e l a t e d to h e m o g l o b i n destruction, still t h e r e is a definite prop o r t i o n a l relation b e t w e e n the two in conditions with excessive blood d e s t r u c t i o n such as h e m o l y t i c jaundice and sickle cell anemia. The course of the anemia, the if'on retention, a n d the low stool pigm e n t excretion in p r e m a t u r e i n f a n t s indicate t h a t the condition is not the result of blood destruction. I n a d e q u a c y of the m i l k feeding as a source of iron has been conside r e d an i m p o r t a n t factor. E x p e r i m e n t a l l y a m i l k diet has been shown to be effective in p r o d u c i n g anemia in rats. 13, 1~ I n rats, however, at the age which p e r m i t s the use of an e x p e r i m e n t a l diet, the r e s e r v e supp l y of iron is not so high as in infants. I t has been shown in the newb o r n animals t h a t the iron r e s e r v e s are directly p r o p o r t i o n a l to the usual d u r a t i o n of the s u c k l i n g period, 8, 15 a n d consequently animals m a i n t a i n e d on a m i l k diet b e y o n d the suckling period wili in time become anemic. This p h e n o m e n o n is seen in the h u m a n b e i n g as well as in the e x p e r i m e n t a l animal.

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Anemia due to Iow iron intake, such as is seen in a wholly milk diet, has certain characteristics. It is present until the diet is corrected by the addition of iron or' iron-containing' foods. I t also shows a distinctive blood picture. The red cells are only slightly affected in r e g a r d to numbers, but the hemoglobin is m a r k e d l y reduced, and as a consequence there is a hypoehromie anemia with a low color index. In eontr'ast to this picture, the anemia of p r e m a t u r i t y presents equal diminution of red blood cells and hemoglobin with a color index approaching unity. This f o r m of anemia also develops in spite of iron t h e r a p y and recovers spontaneously. F r o m these two f a c t s ' i t appears probable t h a t anemia of p r e m a t u r i t y is not a r e s u l t of inadequate iron supply in the milk. The positive iron balances r e c o r d e d are not high, a n d the average figure of +0.10 rag. per day equals 30 rag. of hemoglobin (100, gm. of hemoglobin contain 0.34 gin. of iron). 18 This would not be sufficient to maintain new hemoglobin f o r m a t i o n to make up for' the iner'eased blood volume coincident with growth. 17 However, the high iron content of fetuses and n e w b o r n infants should ~, 7, s p r o v i d e adequate reserve to meet this r e q u i r e m e n t . The findings e n u m e r a t e d above support the following points : (1) the iron balance tends t o be positive or at equilibrium; (2) the pigment excretion is not excessive; (3) the anemia p r e s u m a b l y is not hemolytic in origin; (4) it is a p p a r e n t l y not dietary in t y p e ; (5) there are available iron r e s e r v e s ; (6) r e c o v e r y is spontaneous. Assuming t h a t the anemia of p r e m a t u r i t y is not due to some of the more usual causes of anemia, indulgence in speculation concerning its cause may be permissible. There are at least two possibilities immediately presenting : (1) The anemia m a y be a direct outcome of the physiologic immat u r i t y of the bone marrow. This is an all inclusive hypothesis and cannot be confirmed or disproved with our present knowledge. (2) The second possible f a c t o r in the p r o d u c t i o n of the condition is a more specific b r a n c h of the first. As has been shown above, the anemia is not due to the w a n t of available building stones for' hemoglobin p r o d u c t i o n but, rather, to failure of t h e i r utilization. It may be that this failure of utilization occurs becaxtse there is insufficient bone m a r r o w to p r o d u c e enough red blood cells. W i t h g r o w t h this incap a c i t y is overcome. T h a t this m a y be the case is suggested by the anatomic development of hematopoiesis in the fetus. Blood f o r m a t i o n begins in the liver and spleen in early f e t a l life. F o r a time in the fetus, these organs are the only source of blood formation, but later, as the bones form f r o m the original cartileges, m e d u l l a r y space is produced. As this m e d u l l a r y space enlarges, t h e p r o d u c t i o n of blood cells increases in this site and decreases in the liver; so normally in the full-term i n f a n t hemato-

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poiesis has practically all been t a k e n over by the bone marrow. Analyses of newborn infants have shown t h a t the bone m a r r o w is approxim a t e l y 2.3 per cent of the body weight. 19 The p r e m a t u r e l y born i n f a n t requires e x t r a m e d u l l a r y hematopoiesis to supply an adequate q u a n t i t y of blood cells, and e x t r a m e d u l l a r y hematopoiesis m a y be d e m o n s t r a t e d in infants delivered at term. I t is p r o n o u n c e d in p r e m a t u r e l y b o r n infants and is directly p r o p o r t i o n a l to the degree of prematurit'y, being much g r e a t e r in the smaller infants. In a p r e m a t u r e i n f a n t t h a t survives, the ductus vcnosus closes and the liver becomes the r o u t e of the portal blood. The liver discontinues the function of hematopoiesis and assumes the usual e x t r a u t e r i n e functions. Thus the p r e m a t u r e i n f a n t is deprived of this source of hematopoiesis which would otherwise, as in utero, sufficiently a u g m e n t the bone m a r r o w so t h a t blood cell p r o d u c t i o n would be adequate. It also seems likely t h a t the actual amount of available bone marrow space is an i m p o r t a n t f a c t o r in the development of anemia in prem a t u r e l y born infants. I n adults, blood formation, suftleient to maintain a Constant level u n d e r normal conditions, is confined chiefly to the flat bones and the metaphyses o f the long bones. The diaphyses of the long bones contain f a t t y , inactive m a r r o w which is capable o f active blood f o r m a t i o n in response to various stimuli. On the other hand, in infants all of the available bone m a r r o w space is utilized to maintain the n o r m a l levels. The bones of a p r e m a t u r e infant are necessarily smalIer t h a n those of a normal full-term infant and have proportionately smaller bone m a r r o w cavities. As the i n f a n t grows these spaces enlarge; and, if this f a c t o r of available space had a n y influence on the adequate p r o d u c t i o n of blood cells, the anemia should r e c o v e r spontaneously w h e n the i n f a n t ' s size increases, with the concomitant increase in the bone m a r r o w space. That the lack of bone m a r r o w space m a y conceivably be one of the factors in the p r o d u c t i o n of the anemia of p r e m a t u r i t y is an a t t r a c t i v e hypothesis, although u n s u p p o r t e d b y any direct observations. I f this hypothesis is correct, it offers an explanation f o r the relation of the anemia to the degree of p r e m a t u r i t y , for the t y p e of the anemia and f o r the spontaneous recovery. CONCLUSIONS

The results of studies of iron balance and pigment excretion in a series of p r e m a t u r e infants t e n d to establish certain f e a t u r e s of this condition and lead to possible explanations of the disorder. 1. P r e m a t u r e infants t e n d to have a positive iron balance when fed according" to the m e t h o d commonly employed. 2. The pigment excretion is not excessive. 3. The anemia affects the red blood cells and hemoglobin equally, develops in spite of t r e a t m e n t , and improves spontaneously.

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4. The anemia appears to be due to failure to utilize the available supplies rather than to lack of the substances which compose hemoglobin. 5. An etiologic hypothesis which would explain many of the observed phenomena portrays failure of utilization of the substances which compose hemoglobin due to inadequate medullary space coincident with the premature cessation of hematopoietic activity in the liver. REFEREN

1. Finkelstein:

CES

Textbook, Sauglings Krankheiten, Berlin, 1924, Julius Springer. 2. Diamond, L. I{., Blaekfan, K. D., and Baty, J. IV[.: Oxford lV[onographs, Vol. IX, Chap. XXV, 1930. 3. Coons, C. 1VI.,. and Blunt, 14:.: J. Biol. Chem. 86: 1, 1930. 4. ~[acy, I. G., et al.: J. Bio~. Chem. 86: 17, 1930. 5. Itoffstrom, O.: Skandinav. Arch. f. Physiol. 23: 326~ 1910. 6. I-Iugounenq, L.: J. de physiol, et de path, g6n. 2: 509, 1900. 7. Camerer, W.: ZItschr. f. Biol. 43: 1, 1902. 8. Sherman, H. C.: Chemistry of Food and Nutrition, New York, Ed. I I I , 1926, The Macmillan Co. 9. Josephs, H. W.: Personal communication. 10. Marriott, W. 1VI., and Wolf: J. Biol. Chem. 1: 451, 1905-1906. 11. Shelling, C . E . : Ieterus Neonatorum (to be published). 12. Lichtenstein, A.: Aeta Pediatriea 1: 194, 1921. 13. Miller, R. C., Forbes, E. B., and Smyth, C . V . : J. Nutrition 1: 217, 1928. 14. Miller, 1%. C, and Forbes, E . B . : J. Nutrition 4: 483, 1931. 15. Bunge: Physiological and Pathological Chemistry, Philadelphia, 1902, P. Blakiston's Son & Company. 16. Whipple, G. H , and l~obscheit-l~obbins, F. S.: Am. J. Physiol. 83: 60, 1927. 17. )/[eIntosh, R.: J. Clin. Investigation 7: 203, 1929. 18. Peters, J , and Van Slyke, D . D . : Quantitative Clinical Chemistry, Interpre ~ tations~ Baltimore, 1931, p. 522, Williams & Wilkins Co. 19. Toppieh, G.: (Arch. f. Anat., 1914) quoted from Ny% 1%. N.: Prec. Soc. Exper. Biol. & l~Ied. 29: 34, 1931. HOSPITAL ~0~ ~ICK CHIL])I~EN~ TOROI~•