Hormonal control of glucose homoeostasis in ruminants.

Little, if any, glucose is absorbed from the alimentary tract of the grazing ruminant and it appears that significant absorption of glucose only occurs in ruminants consuming relatively large amounts of grain (Bergman, 1973 ; Lindsay, 1978). In spite of this, ruminants have an absolute requirement for glucose which is similar to that of nonruminants. Certainly glucose is an essential metabolite for the brain as there is no oxidation of ketones in the brain of the ruminant (Lindsay, 1980). Moreover, glucose is required for turnover and synthesis of fat, as a precursor of muscle glycogen and in pregnant and lactating ruminants, glucose is an essential metabolite. Indeed the glucose requirements of late-pregnant and lactating ruminants increase dramatically beyond that required for maintenance (Bergman, 1973, see also Table 2, p. 164). It is not surprising that volatile fatty acids derived from rumen fermentation of carbohydrate provide some 70% of the energy requirements of the ruminant (Bergman, 1973). Even so, gluconeogenesis and the maintenance of glucose homoeostasis are critical processes in view of the absolute requirements for