Models for nutritional research on the fetus: problems and pitfalls.

The concept of providing nutritional models of fetal development appears superficially to be inappropriate as the nutritional environment of prenatal life is thought to be highly controlled (Battaglia & Meschia, 1978; Bassett & Fletcher, 1982; Meschia, 1982). Nevertheless, the fetus responds to changes in maternal diet, to starvation and to reductions in maternal placental blood flow (Girard et al. 1977; Battaglia & Meschia, 1978; Phillips et al. 1978; Lafeber et al. 1979; Jones, 1982; Rolph & Jones, 1982; Chalk et al. 1983). The responses are usually adaptive (and sometimes precocious) changes in enzyme development and hence in metabolic substrates used by the fetus (Girard et al. 1977; Jones, 1982). These observations are therefore indicative of the fact that the fetus is able to influence its nutritional environment and modify its metabolic capacity accordingly. Such an interpretation is exemplified by the fact that the secretion of fetal hormones in the long-term and short-term is influenced by fetal nutrition (Bassett & Madill, 1974; Girard et al. 1977; Lafeber et al. 1979; Robinson et al. 1980) and by the fact that aciite changes in the fetal plasma concentration of hormones such as insulin have marked effects on fetal substrate utilization. Thus in many respects the fetus behaves as the adult, but with placental rather than alimentary supply of substrate and with a substantially different metabolic capacity. For instance, the fetal tissues have limited or no capacity for glucose synthesis or for fatty acid oxidation and, in some species, for amino acid oxidation (Battaglia & Meschia, 1978; Jones, 1982; Snell, 1982) but have high rates of substrate turnover characteristic of growing cells (Scornik, 1982). Investigation of the nutritional requirements and adaptive responses of prenatal life therefore requires an assessment of the pathways associated with the metabolism of a particular substrate, of the responses of such pathways to changes in nutritional state, of the control of hormone secretion in the fetus and of the inter-relationship between nutrition and fetal growth.