The influence of maternal nutrition on fetal nutrient supply

The associations between maternal prepregnancy weight, pregnancy weight gain and birth weight are well known. Birth weight increases with increasing maternal prepregnancy size,6 and has also been associated with maternal weight gain in pregnancy, particularly with increases in maternal fat mass.7 Similarly, poor maternal weight gain in all trimesters of pregnancy has been associated with lower birth weight,8-10 although there is some disagreement about which period of pregnancy is most crucial. Customized growth charts have been developed which take into account maternal height, weight, parity and ethnic group,11 which may assist in the detection of babies that are not growing appropriately.12,13 However, these factors account for, at best, 15% of the variability in fetal growth,14 with the best predictor of birth weight being the mother's own growth in utero. This is true in both developed and developing nations.14,15

The fact that less than 15% of the variability in birth weight is accounted for by markers of maternal nutrition before and during pregnancy may explain why maternal nutrient supplementation during pregnancy has little effect on birth weight. A meta-analysis of studies of maternal balanced protein/energy supplementation does demonstrate a reduction in the incidence of small-for-gestation-age (SGA) babies, but not a significant effect on birth weight.16 However there is increasing evidence that some aspects of maternal diet may have stronger effects on birth weight. High protein supplements during pregnancy actually had a negative effect on birth weight.17 A study from Southampton found that placental and fetal weights

Neonatal Nutrition and Metabolism. Second Edition, ed. P. Thureen and W. Hay. Published by Cambridge University Press. © Cambridge University Press 2006.

Maternal Nutrient Metabolism

diabetes adolescent pregnancy placental vascular disease maternal diabetes inherited disease maternal phenylketonuria diabetes adolescent pregnancy placental vascular disease maternal diabetes inherited disease maternal phenylketonuria

Figure 1.1. The fetal supply line. Nutrients ingested by the mother must pass along a supply line before being utilized by the fetus. The amount of any given nutrient finally utilized by the fetus for growth or metabolism may be affected at any point along this supply line. A few examples are given. Phe = phenylalanine, Tyr = tyrosine, TTTS = twin-twin transfusion syndrome.

were related to the balance of energy obtained from protein and carbohydrate at different times in pregnancy, with high carbohydrate intake in early pregnancy being related to smaller placentae and lower birth weights, and higher protein intake in late pregnancy being related to increased birth weights.18 This study, and others, have also reported that as many as 40% of pregnant women fail to reach the recommended daily intake for many nutrients.18-20 Another recent study from the UK found no association between the intake of any macronutrient and birth weight,21 but a significant, although small, increase in birth weight with increasing vitamin C intake. The potential role of micronutrients in fetal growth is also supported by a recent study in rural Nepal which demonstrated an increase in birth weight with folic acid and iron supplementation and with supplementation with multiple micronutrients.22 Rural Indian women who consumed more green vegetables and milk, rich sources of several micronutrients including folate, also gave birth to bigger babies.22 Thus although total maternal energy consumption may have little effect on size at birth, the balance of macronutri-ents and the micronutrient content of the mother's diet may have an important influence on the nutrition of her baby.

One explanation for the lack of effect of maternal dietary supplementation may be the fact that the fetus lies at the end of a long "supply line" involving maternal metabolic and hormonal status, uterine and umbilical blood flows, placental size and transport capacity and the fetal metabolic and hormonal status. Maternal nutrient supply to the fetus may be affected at any point along this supply line, for example by placental disease, variations in uterine blood flow (e.g. smoking) etc., thus influencing fetal nutrition (Figure 1.1). An alternative, or additional, explanation may be that certain micronutrients are deficient, or borderline deficient. If one particular micronu-trient is supplemented, this may simply lead to the next most marginal nutrient becoming limiting, so that there is little overall effect on fetal growth. Analogous situations may occur in conditions of excess of certain nutrients, e.g. phenylalanine in phenylketonuria, when the high amounts of phenylalanine may saturate placental amino acid transporters and prevent transport of other essential amino acids such as tyrosine and tryptophan (Figure 1.1).23,24 Another example maybe diets high in methionine, which requires glycine for detoxification via transulphura-tion. Excess methionine in a diet already marginal in glycine may lead to glycine availability to the fetus becoming limiting.25

The potential role of folate has been mentioned above. Folate cannot be synthesized by humans, yet is an essential vitamin for many cellular processes including the recycling of methionine and homocysteine, steps in the formation of purines and pyrimidines, and for the formation of glycine from serine. Intake of green leafy vegetables, a rich source of folate, has been found to be strongly associated with birth weight in a rural Indian community.26 Erythrocyte folate concentrations were also independently associated with birth weight and with intake of green leafy vegetables in this study. It has also been proposed that the intrauterine growth restriction (IUGR) seen in millions of low birth weight babies born to mothers infected with malaria may be, in part, due to disturbances within the folate pathway.27 Malaria increases folate demand secondary to hemolysis and to a functional folate deficiency caused in part by hyperhomocysteinaemia and also the coexisting deficiency of other vitamins such as B12. A secondary effect of folate deficiency, or of functional disruption of the folate cycle, is glycine deficiency. Glycine is considered to be a conditionally essential amino acid for the fetus and neonate.28 During growth demands for glycine are high, and it is used in many metabolic processes essential for growth such as purine and porphyrin synthesis, interconversion with serine and also for the production of the free radical scavenger glutathione from a-glutamylcysteine. Glycine is also necessaryfor the detoxification of excess methionine. Up to 90% of fetal glycine is produced from serine by the placenta, and folate is essential for this interconversion.29-31 Urinary excretion of 5-L-oxoproline has been used as a measure of glycine insufficiency.32 5-L-oxoprolinuria increases throughout pregnancy, and has been found to be higher in women with a poorer diet compared with better-nourished women,33 suggesting that glycine may be relatively deficient in pregnant women.

Another amino acid that has received attention recently is taurine, the most abundant free amino acid in the body. Taurine is involved in cholesterol degradation, is a neurotransmitter, an osmoregulator and an antioxidant. Reduced activity of placental taurine transporters has been reported following maternal undernutrition and in IUGR in rats.34,35 Recent interest, however, has focussed upon the role of taurine inpancreatic beta cell development. Rats fed alowpro-tein diet have reduced circulating taurine levels, as do their fetuses.36,37 Pups from mothers fed a low protein diet have reduced f cell mass and reduced islet area, and this persists into adult life. Supplementation of the mothers' drinking water with 2.5% taurine reversed the IUGR, restored a normal balance of proliferation and apoptosis in pancreatic islets38 and restored insulin secretion in vitro to normal.36 Furthermore, fetal plasma insulin levels were significantly correlated with fetal taurine concentrations.37

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