STORAGE AND SUPPLY OF FATTY ACIDS BEFORE AND AFTER BIRTH D. Hull FIG. I. Diagrammatic representation of traffic of fatty acids in the fetus Hepatocyte

STORAGE AND SUPPLY OF FATTY ACIDS BEFORE AND AFTER BIRTH

Placenta

D. HULL B.Sc. F.R.C.P. D.C.H. D.Obst.R.C.O.G. Department of Child Health University of Nottingham 1 Placental transfer of fatty acids a Free fatty acid concentrations in maternal and fetal blood b Transfer of labelled fatty acid c Umbilical venous-artenal differences 2 Role of the placenta in fetal Iipid metabolism 3 Source of tnglyceride stored by the fetus a Sites of triglyceride storage in the fetus b Lipogenesis in the fetus 4 Effects of a maternal fast 5 Lipolysis in the adipose tissue of the fetus and newborn 6 Fatty acid oxidation 7 Summary References

i FAi

In 1935, Boyd & Wilson put forward the hypothesis that, as the Iipid concentration in the maternal blood stream increases, the placenta passes more of these substances on to the umbilical blood, from whence they are absorbed in increasing amounts by the fetus. As these workers observed, this theory requires experimental verification that (i) lipids are added to umbilical blood by the placenta, and (ii) lipids are absorbed from the umbilical blood by the fetus. Boyd & Wilson measured arteriovenous differences in the concentrations of phospholipids, cholesterol and neutral fat in human umbilical blood. The blood lipids they did not measure were the albuminbound free fatty acids (FFA). Many years passed before the importance of this small fraction of the total blood lipids was appreciated. FFA provide the major mode of fatty acid transport. This review will begin by examining the evidence relating to the placental transfer of FFA. The body stores chemical energy as triglyceride, mainly in adipose tissue. The tissue may form the fatty acid component of the triglyceride by lipogenesis from glucose or other substrates, or it may obtain the fatty acids directly from the blood stream by extraction and break-down of triglycerides which are packaged as a lipoprotein, i.e., chylomicron and endogenous triglyceride. The source of the stored triglyceride in the fetal adipose tissue will differ from that of the mother, for chylomicrons are unlikely to pass unchanged through the cellular barrier of the placenta. Figure 1 shows traffic of fatty acids in the fetus. The evidence relating to the source of the fatty acids of the triglyceride stored by the fetus in adipose tissue is examined next. The triglyceride in adipose tissue is broken down by lipolysis to fatty acids and glycerol. The FFA released into the circulation are rapidly cleared and in most tissues they may be oxidized to provide chemical energy or be used to form structural lipoprotein or be re-esterified to form a small local store of triglyceride. However, in the liver, excess fatty acids may be re-esterified to triglyceride and released back

ENERGY

Adipocyte Abbreviations: FA: triglyceride FA

FA: fatty acldi into the circulation as endogenous triglyceride, or they may be broken down and released as ketone bodies, another readily available source of cellular energy. Section 6 of this paper will be concerned with evidence relating to the utilization of fatty acids by the fetus and newborn. 1. Placental Transfer of Fatty Adds The transport of fatty acid across the placenta has been deduced from a variety of experimental studies, including those on therelationshipof the maternal to fetal concentrations of fatty acids, the transfer of labelled fatty acids, umbilical blood venous-arterial (V-A) differences, uptake of FFA by fetal tissues, and the profile of the fatty acids in the blood and adipose tissue stores. a. Free Fatty Acid Concentrations in Maternal and Fetal Blood

The maternal blood concentrations of FFA are usually, though not invariably, higher than the fetal blood concentrations. Any interpretation of single blood estimations of FFA must be cautious, for FFA are cleared very rapidly from the 32

Br. Med. Bull. 1975

STORAGE AND SUPPLY OF FATTY ACIDS BEFORE AND AFTER BIRTH D. Hull blood stream of mother and fetus, the concentrations may differ from one part of the circulation to another, and many factors may influence the maternal or fetal concentrations independently. A number of investigators have found that high maternal blood FFA concentrations are associated with high umbilical venous concentrations. To explain this relationship it has been suggested that the stimulus that led to the rise in concentration in the mother also stimulated lipolysis and release of fatty acids by the adipose tissue of the fetus (Shelley & Thalme, 1970; James, Meschia & Battaglia, 1971). However, it is also consistent with our original hypotheses that, as the maternal concentration increases, more maternal fatty acids cross the placenta into the fetal vascular compartment, and, as will become clear, the weight of evidence now strongly supports this alternative.

FIG. 2. Specific activity of [l- 14 C]palmitate in the fatty acids (FFA) in the maternal circulation ( • ) and in individual fetuses (o) 1001— Injection of (i- u C]palmitate

I

50

o X

b. Transfer of Labelled Fatty Acid The transport of labelled fatty acids (usually [l-14C]palmitate) across the placenta has been examined in a number of experimental models. In the anaesthetized experimental animal (rabbit: Van Duyne, Havel & Felts, 1962; sheep: Van Duyne, Parker, Havel & Holm, 1960; guinea-pig: Hershfield & Nemeth, 1968; monkey: Portman, Behrman & Soltys, 1969; rat: M. C. Elphick, D. G. Hudson and D. Hull, unpublished observations), injection of labelled fatty acid into the mother led to its rapid appearance in the fetal circulation. The passage of labelled fatty acid is down a maternal-fetal concentration gradient. In the experiment illustrated in fig. 2 the specific activity of [l-14C]palmitate in the fatty acid pool of the fetus was close to that of the mother within minutes of the injection of the labelled material; this suggests that, under the conditions of the experiment, most of the fatty acids in the fetal vascular pool had been derived from the maternal circulation and within a matter of minutes! In the fetuses of anaesthetized rabbits towards term, the specific activity was on average 60% of the maternal value (M. C. Elphick, D. G. Hudson and D. Hull, unpublished observations). Injection of labelled fatty acid into the fetus (monkey: Portman et al. 1969; rabbit: M. C. Elphick, D. G. Hudson and D. Hull, unpublished observations) or into the umbilical artery of the placenta in situ (guinea-pig: Kayden, Dancis & Money, 1969) led to the rapid appearance of [l-1'*C]palmitate in the maternal circulation. An experiment on an anaesthetized rabbit is illustrated in fig. 3. This passage of fatty acid occurs up the maternal-fetal concentration gradient. Transport of FFA from maternal to fetal and from fetal to maternal side of the isolated human placenta has been demonstrated, but the rate of transport in this preparation is low compared with that of smaller molecules, e.g., antipyrine (Szabo, Grimaldi & Jung, 1969; Dancis, Jansen, Kayden, Schneider & Levitz, 1973). It is possible, however, that the transfer of FFA is more dependent on the viable integrity of the cellular components of the placenta barrier.

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Time (min) [l-t4C]Palmltate was injected into the mother. The average fatty acid concentration In the mother was 1.31 mequlv./l, and in the fetuses It was 0.42 mequlv./l This experiment demonstrates that fatty acids rapidly cross the placenta and that, under the conditions of the experiment, most of the fatty acids in the fetal circulation were derived from the mother

maternal blood concentrations were high. A direct correlation between maternal blood concentrations and the umbilical V-A difference has been observed in rabbits (M. C. Elphick, D. G. Hudson and D. Hull, unpublished observations), and in man after natural delivery (Sabata, Wolf & Lausmann, 1968) and during elective Caesarean section (M. C. Elphick, D. Hull and R. R. Saunders, unpublished observations). The average V-A differences of umbilical cord blood samples obtained at term after natural delivery in man have been reported to be 0.056 mequiv./l (Sabata et al. 1968), 0.058 mequiv./l (Persson & Tunell, 1971) and 0.068 mequiv./l (Sheath, Grimwade, Waldron, Bickley, Taft & Wood, 1972). From the magnitude of the V-A difference and a knowledge of the placental blood flow it can be calculated that fatty acids crossing the placenta could be the sole source of the fetal stored triglyceride in both man and rabbits. However, most of these observations were made when the maternal concen-

c. Umbilical Venous-Arterial Differences The perfusing concentration is a major factor determining the rate of fatty acid uptake by the body tissues; thus one might expect that an increase in the fetal circulating concentration would lead to an increase in the fetal tissue uptake, which in turn would lead to an increase in the flux across the placenta. Thus the V-A difference would be greater when the 33

Vol. 31 No. 1

into the mother

STORAGE AND SUPPLY OF FATTY ACIDS BEFORE AND AFTER BIRTH D. HuU 2. Ride of the Placenta in Fetal LipU Metabolism

FlG. 3. Curve indicating the rise in radioactivity in the fatty acids of the maternal circulation following an intravenous injection of [I-14C]palmitate into a fetal rabbit 600

It is possible that the umbilical V-A difference is influenced as much by placenta lipid metabolism as by the maternal blood concentrations. The placenta stores triglyceride, which it can form either from fatty acids or by lipogenesis from glucose and other substrates. Does it behave as other tissues and keep a local store of triglyceride for its own use ? Or can it behave like the liver and release endogenous triglyceride and ketones into maternal or fetal circulations or both? Or does it behave like adipose tissue and release the triglyceride as glycerol and FFA 7 The role of the placenta as an organ of fat storage, particularly in the fetus before adipose tissue is formed, needs further investigation.

Injection of |i-uC]palmitate into the fetus

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400

The net uptake by fetal tissues will be influenced by the prevailing ability of the maternal system to supply fatty acid and also by the setting of the fetal systems to receive fatty acid. Theoretically, a number of factors could influence the rate of fatty acid utilization or storage. One is the presence and capacity of the fat-storing tissues, and the setting of these tissues towards storage. Another is the rate of lipogenesis within the liver and fat-storing tissues. A third is the need for fatty acids either to form structural lipids or to provide cellular energy and this will depend on alternative sources. For essential fatty acids there are no alternatives. But there arc other factors, including the endocrine agents, which could influence the cellular transport of energy substrates across the placenta and in and out of the fetal cells.

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a. Sites of Triglyceride Storage in the Fetus Phylogenetically and ontogenetically, adipose tissue is one of the last major body tissues to appear. In most mammals it develops in the last third of gestation; but in some it may lay down very little triglyceride before birth. Many newborn mammals, but not all, have two distinct forms of adipose tissue, one of which, brown adipose tissue1>2, has an important role of thermoregulatory heat production in the newborn (Hull, 1974). These two forms of adipose tissue have different growth patterns. The liver is a major fuel store in lower animals without adipose tissue and it could obviously perform this role for the fetus in early and mid-gestation. However, it is difficult to think of a situation in which it might be called upon to complement the supply from the placenta in mid-pregnancy, although its role as a source of fuel immediately before, during and after birth could be vital. The placenta, too, could be a temporary holding stage (see section 2).

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Time after injection (min) The average maternal FFA concentration was 1.25 mequlv./l, and that of the fetus was 0.52 mequlv./l This experiment demonstrates that fatty acids are rapidly exchanged from the fetus to the mother and that this occurs up a maternal-fetal gradient

trations were high, i.e., during administration of an anaesthetic or after normal labour. The rate of lipolysis in the mother increases towards the end of pregnancy, partly because of the action of placental hormones but also because of a behavioural change in the mother's feeding pattern. This increased lipolysis would favour an increase in lipid transfer to the fetus (Felig, 1973; Knopp, Saudek, Arky & O'Sullivan, 1973). On the simple hypothesis that maternal blood concentrations determine the V-A difference, and with the knowledge that fatty acids rapidly pass from fetus to mother, it is conceivable that, when the maternal concentrations fall, fatty acids may be discharged back into the mother or that fatty acids made by the fetus may be lost into the maternal circulation. Negative V-A differences have been recorded in the presence of low maternal fatty acid concentrations. More information in this area is required.

b. Lipogenesis in the Fetus The fetal liver and adipose tissues have a considerable capacity to form fatty acids by lipogenesis from glucose and other substrates, i.e., acetate, ketones and amino acids (Jones, 1973). The capacities of the enzymes of fatty acid synthesis in liver and brown adipose tissue of rabbits fall rapidly after birth (Iliffe, Knight & Myant, 1973). Clearly, glucose could be a major source of fetal triglyceride, but the extent to which it is an important source under physiological conditions at different 1 See Hall, Br. Med. Bull. 1966,22, 92-96.—ED. » See Afctander, pp. 62-68 of tiii Bulletin.—ED.

34

Br. Med. Bull. 1975

STORAGE AND SUPPLY OF FATTY ACIDS BEFORE AND AFTER BIRTH D. Hull rate of lipolysis in rabbit fetal brown adipose tissue (Harding & Ralph, 1970). Asphyxia during labour may lead to higher concentrations of glycerol and FFA in blood of the newborn; this suggests that lipolysis takes place in adipose tissue (Sabata, Stembera & Hodr, 1964; Persson & Tunell, 1971). However, a rise in circulating FFA concentration may not only indicate an increased rate of lipolysis but also a decreased rate of utilization; in the fetus it may also reflect changes in maternal concentrations and placenta! flux. The pattern of the concentrations of different FFAs in the fetal blood stream does not match that of the mother; this fact also supports the view that the fetal fat stores release fatty acids into the circulation (King et al. 1971). Fatty acid traffic in the newborn is represented diagrammaticaily in fig. 4. After birth the concentration of FFA in the circulation rises in man (Persson & Tunell, 1971), sheep (Van Duyne et al. 1960; Alexander & Mills, 1968) and rabbits (Elphick, 1971). It is interesting that the rise in rabbits was less if the animals were born by rapid Caesarean section and kept from birth in a thermoneutral environment (Hardman, Hull & Mflner, 1971). The associated rise in glycerol and ketone bodies and the increasing accumulation of lipid in the liver and other tissues, with the fall in respiratory quotient, suggest that the rise in circulating concentrations is brought about by an increase in

stages of pregnancy remains to be demonstrated. Evidence has shown that it is not a major source in anaesthetized animals and during labour. The conversion of glucose to fatty acids may be one mechanism by which the fetus discharges excess glucose back to the mother! The triglyceride in lipid stores of the fetus has a different fatty acid profile from that of maternal lipid stores and from that of the FFA in the maternal circulation. In particular, the fetal lipids have a higher percentage of the Cl6 fatty acid, palmitate, a fatty acid preferentially formed by lipogenesis from glucose; this supports the view that at least some of the triglyceride is formed by lipogenesis from glucose (King, Adam, Laskowski & Schwartz, 1971). However, the fatty acids received by the fetus from the maternal circulation will reflect those not of the maternal stores, but of the maternal blood, FFA and possibly triglycerides, over the last trimester of pregnancy, provided of course that they are not processed by the placenta. Studies in the perfused placenta did not suggest that the different fatty acids are selectively transported (Kayden et al. 1969). Changes in the fatty acid composition of the diet of the mother during pregnancy result in altered fatty acid composition of the fetus (Soderhjelm, 1953). 4. Effects of a Maternal Fast Many years ago in experimental animals it was observed that when the mother was starved there was an increase in the fat content of the liver of the mother and her fetus. Our recent investigations (J. L. Edson, D. G. Hudson and D. Hull, unpublished observations) have shown that when the pregnant rabbit was fasted for two days the lipid content of the fetal stores—liver and adipose tissue—increased by 80 %. When the mother was fasted, the maternal blood sugar fell and fatty acids were mobilized from her adipose tissue stores, also the circulating concentration of FFA increased. In the fetuses of starved does, the evidence strongly suggests that at least 40% of the stored fetal fatty acid is derived from the maternal circulating FFA. Similar observations have been made on guinea-pigs (E. Widdowson, personal communication). In summary, on evidence available it is not possible to state whether the triglycerides of fetal stores are derived principally from maternal FFA or glucose or from any other substrate. It may be that the fetal stored triglyceride is in dynamic balance with the fetal and maternal vascular pools and that the source of the stored fatty acids varies from hour to hour according to the state of maternal nutrition.

FlG. 4. Diagrammatic representation of traffic of fatty acids in the newborn Hepatocyte

Endogenous triglyceride

ENERGY

5. Lipolysis In the Adipose Tissue of the Fetus and Newborn Does the fetus contribute to its own circulating FFA? Under usual circumstances one would not expect the fetus to be subject to intermittent nutrition or starvation, nor to experience excitement, fear, cold or many of the other stresses that activate the sympathetic nervous system and lipolysis. However, experimental studies have shown that fetal adipose tissue in vitroreleasesglycerol and fatty acids into the medium, and the rate of release is accelerated by lipolytic agents, e.g. noradrenaline (M. C. FJphick, D. G. Hudson and D. Hull, unpublished observations). Similarly, infusion of catecholamincs into the fetal sheep and rabbit increases the circulating concentrations of glycerol or FFA (Dawkins, 1964; James et al. 1971; Comline & Silver, 1972), suggesting that the rate of lipolysis has been increased. Chronic hypoxia increased the

Adipocyte

For abbreviation*, tee below fig. I

35 Vol. 31 No. 1

STORAGE AND SUPPLY OF FATTY ACIDS BEFORE AND AFTER BIRTH D. Hull the rate of lipolysis that is greater than the increase in the rate of utilization. The dynamic exchange of FFA between mother and fetus raises the possibility that the rise might be caused in part by an interruption of the placenta!flux;and the rate of rise will be higher in those set to a higher rate of lipolysis before birth. This might explain the observation that the rate of rise is greater in "small-for-dates" infants (Melichar, Novak, Sabata, Hahn &Koldovsky, 1965) and less in infants of diabetic mothers(Chen, Adam, Laskowski.McCann &Schwartz, 1965).

this respect is brown adipose tissue. Within minutes of birth, this tissue can consume oxygen at twice the rate of the rest of the body, and the fuel used is the fat stored within brown adipose cells (Hardman, Hey & Hull, 1969). At this time, brown adipose tissue releases little or none of its stored fatty acid into the circulation for utilization elsewhere. Because fatty acids pass rapidly from fetus to mother, it may be that only after birth does the liver become concerned with clearing excess fatty acids. Evidence suggests that most of the ketones in the fetal circulation are derived from the mother. After birth the circulating concentrations rise. These extra ketones must be produced by the infant and they may well be an important source of energy to the developing brain (Dahlquist, Persson & Persson, 1972; Kraus, Schlenker & Schwedesky, 1974).

6. Fatty Add Oxidation Fetal tissue in vitro can convert labelled palmitate to stored fats and structural lipids and to a small extent such tissue will also break them down to carbon dioxide (Roux, Yoshioka & Myers, 1970; Yoshioka & Roux, 1972). Thus the fetus could use fatty acids as a source of fuel. However, from studies on monkey fetal tissue in vitro, Roux & Myers (1974) concluded that the conversion of palmitate to carbon dioxide in the fetus was small. The activity of the systems for fatty acid oxidation is low in fetal tissues but it rises rapidly after birth (Wolf, Stave, Novak & Monkus, 1974), although there may be some delay in the utilization of FFA as a major source of cellular energy. This might explain in part the fall in blood sugar in thefirst24 hours of life in the face of a rising metabolic rate. However, one tissue that appears to have no problem in

7. Summary It is well established that fatty acids are an important source of cellular energy in the newborn. However, the generally held view that glucose is invariably the main source of both the metabolized and stored energy in the fetus may not be correct There appears to be a dynamic exchange of fatty acid between the maternal circulation, the placenta, the fetal circulation and fetal tissues. Fetal tissues, including the brain, are able to use fatty acids and their by-product, ketones, to form structural lipids and, to some extent, to provide cellular energy.

King, K. C , Adam, P. A. J., Laskowski, D. E. & Schwartz, R. (1971) Pediatrics (Springfield) 47,192-198 Knopp, R. H., Saudck, C. D., Arky, R. A. & O'Sullivan, J. B. (1973) Endocrinology, 92,984-988 Kraus, H., Schlenker, S. & Schwedesky, D. (1974) Hoppe-Seyler's Z. Physiol. Chem. 355,164-170 Melichar, V., Novak, M., Sabata, V., Hahn, P. & Koldovsky, O. (1965) Physiol. Bohemoslov. 14, 553-558 Myant, N. B. (1970) In: Philipp, E. E., Barnes, J. & Newton, M., ed. Scientific foundations of obstetrics and gynaecology, pp. 354-371. Heinemann Medical, London Persson, B. & TuneU, R. (1971) Ada Pediatr. Scand. 60, 385398 Portman, O. W., Behrman, R. E. & Soltys, P. (1969) Am. J. Physiol. 216,143-147 Robertson, A. F. & Sprecher, H. (1968) Ada Paedlatr. Scand. suppl. no. 183 Roux, J. F. & Myers, R. E. (1974) Am. J. Obstet. Gynecol. 118, 385-392 Roux, J. F. & Yoshioka, T. (1970) Clin. Obstet. Gynecol. 13, 595-620 Roux, J. F., Yoshioka, T. & Myers, R. E. (1970) Nature {Lond.) 22H, 963-964 [Letter] Sabata, V., Stembera, Z. K. & Hodr, J. (1964) Ceskoslov. Gynekol. 29, 509-512 Sabata, V., Wolf, H. & Lausmann, S. (1968) Biologia Neonat. 13, 7-17 Sheath, J., Grimwade, J., Waldron, K., Bkkley, M., Taft, P. & Wood, C. (1972) Am. J. Obstet. Gynecol. 113, 358-362 Shelley, H. J. & Thalme, B. (1970) In: Joppich, G. & Wolf, H , ed. Metabolism of the newborn, pp. 178-202. Hippokrates, Stuttgart Soderhjelm, L. (1953) Ada Soc. Med. Ups. 58, 239-243 Szabo, A. J., Grimaldi, R. D. & Jung, W. F. (1969) Metabolism, 18,406415 Van Duyne, C. M., Havel, R. J. & Felts, J. M. (1962) Am. J. Obstet. Gynecol. 84,1069-1074 Van Duyne, C. M., Parker, H. R., Havel, R. J. & Holm, L. W. (1960) Am. J. Physiol. 199, 987-990 Wolf, H , Stave, U., Novak, M. & Monkus, E. F. (1974) J.Perinat. Med. 2, 75-87 Yoshioka, T. & Roux, J. F. (1972) Pediatr. Res. 6, 675-681

REFERENCES

Readers interested in other aspects of lipid metabolism in the perinatal period are referred to the excellent review articles by Robertson & Sprecher (1968), Roux & Yoshioka (1970), Myant (1970) and Harding (1971). Alexander, G. & Mais, S. C. (1968) Biologia Neonat. 13, 53-61 Boyd, E. M. & Wilson, K. M. (1935) / . Clin. Invest. 14, 7-15 Chen, C. H., Adam, P. A. J., Laskowski, D. E., McCann, M. L. & Schwartz, R. (1965) Pediatrics (Springfield) 36, 843-855 Comline, R. S. & Silver, M. (1972) / . Physiol. (Lond.) 222, 233-256 Dahlquist, G., Persson, U. & Persson, B. (1972) Biol. Neonate, 20,40-50 Dancis, J., Jansen, V., Kayden, J., Schneider, H. & Levitz, M. (1973) Pediatr. Res. 7,192-197 Dawkins, M. J. R. (1964) Biologia Neonat. 7,160-166 Elphick, M. C. (1971) Biol. Neonate, 17, 410-419 Felig, P. (1973) Am. J. Clin. Nutr. 26, 998-1005 Harding, P. G. R. (1971) Clin. Obstet. Gynecol. 14, 685-709 Harding, P. G. R. & Ralph, E. D. (1970) Am. J. Obstet. Gynecol. 106, 907-912 Hardman, M. J., Hey, E. N. & Hull, D. (1969)/. Physiol. (Lond.) 205,39-50 Hardman, M. J., Hull, D. & Milner, A. D. (1971) / . Physiol. {Lond.) 213,175-183 Hershfield, M. S. & Nemeth, A. M. (1968) / . Ltptd Res. 9, 460468 Hull, D. (1974) In: Davis, J. A. & Dobbing, J., ed. Scientific foundations of paediatrics, pp. 440-455. Heinemann Medical, London Iliffe, J., Knight, B. L. & Myant, N. B. (1973) Biochem. J. 134, 341-343 James, E., Meschia, G. & Battaglia, F. C. (1971) Proc. Soc. Exp. Biol. Med. 138, 823-826 Jones, C. T. (1973) In: Comline, R. S., Cross, K. W., Dawes, G. S. & Nathanielsz, P. W., ed. Foetal and neonatal physiology, pp. 403-409 (Proceedings of The Sir Joseph Barcroft Centenary Symposium held at Cambridge, 25-27 July 1972). Cambridge University Press, London Kayden, H. J., Dancis, J. & Money, W. L. (1969) Am. J. Obstet. Gynecol. 104, 564-572

36 Br. Med. Bull. 1915