Carbohydrate metabolism in the foetus and the newly born.

Carbohydrate metabolism in the foetus Glucose crosses the placenta readily, probably by 'facilitated diffusion' ; i.e. it travels in the direction of the diffusion gradient but at a faster rate than can be explained by simple diffusion. Since the foctal blood glucose concentration is usually about half that in the mother, glucose norrnally passes from mother to foetus. Measurements of the rate of glucose uptake in foetal lambs (Alexander, Britton & Nixon, 1966a) and in babies at dclivcry (Wolf, Sabata, Frerichs & Melichar, 1969) suggest that it may be slightly greater than is needed to account for the foetal oxygen consumption. If most of the glucose is, in fact, oxidized, it may be the principal metabolic fuel of the foetus. This belief is strengthened by the relative impermeability of the placenta of diverse species to free fatty acids and ketone bodies, and by the fact that the evidence available suggests that although the foetus can oxidize ketone bodies, it is probably unable to degrade and oxidize fatty acids (Popjhk & Beeckmans, 1950; Koren & Shafrir, 1964; Alexander et al. 1966b, 1969; Shelley, 1969; Wolf et aI. 1969). Maternal amino acids are transported across the placenta against the concentration gradicnt, but again the foetus is unable to oxidize them (Yarnell, Nelson & Wagle, 1966); presumably amino acids are conserved for protein synthesis. But not all the glucose which crosses the placenta is oxidized. Some must be used for lipogenesis; for which foetal liver has a large capacity, and some is stored as glycogen. The amount of lipid stored by the foetus varies enormously, but all species, including man, accumulate glycogen in the foetal liver and skeletal muscles (Table I) . Except in the rabbit, the glycogen concentration in the foetal liver at term is about twice the usual adult concentration, but that in the skeletal muscles varies more widely, from ten times the adult concentration in the pig to four to five times the adult concentration in the sheep, monkey and man, and ncar-adult levels in rodents and the dog. In species with a short gestation (rat, rabbit, dog, guinea-pig) the liver glycogen remains low (<5 mg/g) until a few days before term, but in other species the concentration begins to rise at a much earlier stage of gestation; for instance in man the concentration has already reached 40-50 mglg, the usual adult concentration, by 20 weeks, only halfway through gestation. In all species the glycogen concentration in the skeletal muscles reaches adult levels early in