[Findings on intermediate metabolism in ruminants].
In ruminants, the digestive tract and liver metabolism represents a substantial part of the energy requirements. There is almost no net glucose absorption by the gut although it is an energy fuel for the enterocytes. There is a permanent gluconeogenesis, for which the major substrate is propionate. Propionate plays a major role in glucose synthesis is highly effective and propionate metabolism may affect the utilization of other substrates such as lactate. In addition, amino acid metabolism provides a net supply of carbon for gluconeogenesis. However, there is a maximal sparing of the NH2 group (via glutamate release) by the ruminant liver and the energy cost of the conversion of amino acids into glucose is very high. The actual importance of insulin in the regulation of liver metabolism in the ruminant is not entirely understood but glucagon seems not to be a major factor in the stimulation of gluconeogenesis in the fed ruminant. Propionate has a potent effect on anabolism because of its effects on glycaemia and insulin secretion. When glucose requirements are to be enhanced (pregnancy, lactation), even a minor imbalance in the supply of glucogenic substrates may elicit striking physiological changes such as lipomobilization or ketonaemia. In early lactation, insulin secretion is still small in spite of an enhanced food intake. As a result, glucose, fatty acids and amino acids are increasingly channeled towards udder metabolism. Lipogenesis is depressed whereas lipolysis is stimulated by catecholamines. However, excessive glucose and propionate supply may lead to exagerated anabolism and to a drop in milk fat. Thus, a high milk production requires an optimal supply of the three volatile fatty acids (VFA) and it should be emphasized that an improved nitrogen supply has a stimulatory effect on milk production.